soil conservation technical handbook

Soil Conservation

Technical Handbook

Disclaimer

This Handbook has been prepared by the Ministry for the Environment together with the parties listed in the acknowledgments and published by theNew Zealand Association of Resource Management under contract from the Ministry for the Environment. The Handbook represents best currentpractice in the field of soil conservation in New Zealand as at June 2001. The Ministry for the Environment and the New Zealand Association of ResourceManagement note however that there may be unacknowledged science work that has been completed or is being undertaken which is pertinent to theHandbook, but which has not been included, mainly due to it not being commonly available as public domain information at the time of publication.

Any representation, statement, opinion, or advice expressed in this publication is made in good faith but on the basis that the Ministry for theEnvironment and the New Zealand Association of Resource Management are not liable to any person for any damage or loss which has occurred or mayoccur in relation to that person taking or not taking action in respect of any representation, statement, opinion, or advice referred to above.

Editors

Doug H Hicks

Tabitha Anthony

Principal Authors

Ian Cairns

Brian Handyside

Murray Harris

Nick Lambreschtsen

Norm Ngapo

Technical Editing

Mary Cresswell

Technical Working Group

Bob Cathcart

Garth Eyles

Dex Knowles

Contribution Authors

Doug Hicks

Don Miller

Map Preparation (Appendix II)

Andy Manderson

Layout

Zac Guja

ZED Communications Ltd

Publishing

The Ministry for the Environment

P O Box 10362

Wellington

New Zealand

Production Team for the Soil Conservation Handbook

Soil Conservation Technical Handbook

June 2001

ISBN: 0-478-24033-3ME: 404

Soil Conservation

Technical Handbook

❹❸

❷

❶Cover Photography

❶ Slip erosion. Photo: H Cairns.

❷ Mount Hikurangi and poplars, Central North Island. Photo: Guy Vickers.

❸ A graded bank and space planted trees on an earthflow. Photo: Don Miller.

❹ Pole planting. Photo: H Cairns.

New Zealand has a history of land management that has resulted in soil erosion;silted-up streams rivers and estuaries; algal blooms and dense growth of aquatic weedsfrom nutrient-rich runoff. While considerable progress has been made in achievingmore sustainable land production, significant problems still remain.

National awareness of soil erosion on hill country in New Zealand was polarised bystorm events in the 1930s and 1940s, mostly in the Esk Valley, the Wanganuicatchment, Marlborough and the Waipaoa basin. These and subsequent stormsinitiated soil erosion on a massive scale on the then recently developed pastoral hillcountry.

The strong northwest winds on the dry plains on the east coasts of the North andSouth Islands have caused tremendous soil loss in the past. It is only after the controlof rabbits and the establishment of wind breaks and vegetative cover on the bare earththat this form of erosion has been bought under control.

It was primarily because of the fears that low-lying communities would be inundated ifsoil erosion continued unchecked, that the Soil Conservation and Rivers Control Actwas passed in 1941. This Act established catchment boards and enabled theappropriation of government and local body funds to assist with the construction oferosion control measures and flood protection works.

Why are we so interested in sustainable land management and promoting the work inthe Soil Conservation Technical Handbook? I can list four key reasons.

• Soil erosion affects downstream communities and the natural environment byincreasing the risk of flooding and causing damage through silt depositions.

• Our reputation for “quality goods from a quality environment” depends onsustainable land use practices. Soil erosion is a very visible negative impact ofpastoral land use. That image may affect our “clean green” image and trade in thefuture.

• We care about future generations, and that means ensuring ample soil remainsavailable to sustain agricultural production capacity.

• We want to protect our biodiversity. Our terrestrial biodiversity is dependent on thequality of our soils and its carrying capacity and we want to protect our freshwaterand estuarine aquatic biodiversity.

Soil conservators have long been a corner-stone of sustainable land management inNew Zealand. Techniques have been developed over the years based on engineeringand scientific principles, as well as some trial and error and time-honoured Kiwiingenuity that adapts techniques to regional conditions.

For the first time in New Zealand we have in one place, a comprehensive range ofpractical field-tested soil conservation practices and techniques. This handbook hasbeen written by experienced soil conservators drawing on their own and theircolleagues practical experience, under-pinned by the science undertaken on thesubject for the last 50 years.

Another important area of land management, namely riparian management, is notcovered in depth in this publication. Instead, readers are referred to the MfEpublication “Managing Waterways on Farms”. This publication is complementary to theHandbook, as they are both based on the same principle of gathering together field-tested techniques and allowing for additional information to be added.

Preface

This Handbook is a valuable resource for regional council land management officers. It is also useful for anyone interested in understanding erosion processes and wantingto learn more about preventing and rehabilitating sites. Readers can expect to gain aworking knowledge of erosion types and practical recommendations for preventionand rehabilitation.

Finally I want to acknowledge the stewardship role of the New Zealand Association ofResource Management (NZARM), who represent one of the main groups ofpractitioners who will be using the information in this Handbook. This Associationhas demonstrated its support for the project by undertaking the publishing andprinting of this document. I commend NZARM for its foresight. It is this sort ofinitiative that will ensure that the Handbook will be used by its members and mostimportantly be updated when new information and practices come to hand.

Denise F ChurchChief ExecutiveMinistry for the EnvironmentJune 2001

The Soil Conservation Technical Handbook is a comprehensive collection of know howabout soil conservation in New Zealand. Information has been gathered fromindividual knowledge and personal notes along with past often unpublished, or scarcecopies of, printed material. Some of this material is derived from internal regionalcouncil manuals and papers.

The main forms of erosion in New Zealand are covered in this Technical Handbook.

• Mass movement erosion – which occurs when heavy rain or earthquakes causewhole slopes to slump, slip or landslide. Most hill slopes steeper than 15 degreesare susceptible to mass movement, and those steeper than 28 degrees generallyhave severe potential. Storms are the primary triggers. This is the most commonform of erosion in the hill country.

• Fluvial erosion – which occurs when running water gouges shallow channels ordeeper gullies into the soil. On sloping land the gullies can cut deep into thesubsoil or undermine surrounding soils.

• Surface erosion – which occurs when wind, rain or frost detach soil particles fromthe surface, allowing them to be washed or blown off the paddock. Surface erosioncan occur on any land which is exposed to wind and rain but occurs largely outsidethe hill country.

• Sediment erosion – activities involved in earthworks, plantation forests, croplandand pasture management may all result in significant sediment loads beingmobilised and often entering watercourses.

The Handbook is divided into two main parts. Part A covers the forms and processes ofthe main types of erosion in New Zealand. Readers can expect to gain a workingunderstanding of erosion types, their typical geographic locations, as well as theoverall principles of control. Descriptions are written from a practical point of view.More technical descriptions can be found in the references provided. A table at theend of each chapter summaries the main control technique headings and is cross-referenced to Part B.

Part B describes the control techniques. In most cases the Handbook will provideenough information for the practitioner to put together a soil conservationprogramme. Where more details (mostly engineering) are needed readers are referredto more specialised text. Often techniques apply to more than one erosion type andsituation, for example maintaining good pasture cover is important in manysituations. In these cases, general guidelines are given, as it is recognised that regionalconditions will often dictate specific actions required.

The Handbook has been formatted so additional information can be added at anytime.It is anticipated that additional written material will be prepared in the future onassociated land management topics and issues (for example indigenous biodiversity) asthe demand requires. Space has also been provided for personal notes.

Introduction

Part A

Principles

iSoil Conservation A

1 Wind Erosion 1•11.1 Introduction 1•1

1.1.1 Impacts on land use 1•1

1.2 Forms, processes and extent of wind erosion 1•2

1.2.1 Forms and processes 1•2

1.2.2 Wind erosion research 1•3

1.2.3 Extent in New Zealand 1•4

1.3 Principles of control 1•5

1.4 Appropriate control practices 1•5

1.5 References 1•6

2 Sheet and rill erosion 2•12.1 Introduction 2•1

2.1.1 History of sheet/rill erosion research 2•1

2.1.2 Impacts on land use 2•1

2.2 Types, processes and extent 2•2

2.2.1 Extent 2•3


2.4 Appropriate Soil Conservation Practices 2•4


3 Shallow Mass Movement – slip erosion 3•13.1 Introduction 3•1

3.1.1 History of slip erosion control 3•1

3.2 Types, processes 3•1

3.2.1 Falls 3•2

3.2.2 Slides 3•2

3.2.3 Flows 3•2

3.2.4 Shallow mass movement in New Zealand 3•2

3.2.5 Extent of shallow mass movement 3•3


3.3.1 Preventive works 3•3



Contents – Part A

ii Soil Conservation A

4 Deep-seated Mass Movement

– slides, slumps and flows 4•14.1 Introduction 4•1


4.2.1 Slides (block glides) 4•1

4.2.2 Slumps 4•1

4.2.3 Flows 4•2

4.2.4 Extent of deep-seated mass movements 4•2

4.3 Conservation measures to control erosion on

deep-seated mass movements 4•2

4.4 Appropriate control techniques 4•3


5 Gully Erosion 5•15.1 Introduction 5•1

5.1.1 History of gully control 5•1


5.2.1 U-shaped gullies 5•2

5.2.2 Extent of U-shaped gullies 5•2

5.2.3 Deep-seated gullies (V-shaped gullies) 5•2

5.2.4 Extent of deep-seated gullies 5•3

5.2.5 Tunnel gullies 5•3

5.2.6 Extent of tunnel gullies 5•4

5.2.7 Mountain gullies 5•4


5.3.1 U-shaped gullies 5•4

5.3.2 Tunnel gullies 5•5

5.3.3 The key principles to control V shaped gullies include 5•6

5.3.4 Others 5•6



6 Mountain Lands Erosion 6•16.1 Introduction 6•1

6.1.1 History of research 6•1

6.2 Types, processes and extent of erosion in the mountains 6•2

6.2.1 Frost heave (needle ice formation) 6•2

6.2.2 Sheet erosion 6•2

6.2.3 Wind erosion 6•2

iiiSoil Conservation A

6.2.4 Periglacial erosion 6•3

6.2.5 Debris avalanches 6•3

6.2.6 Debris flows 6•3

6.2.7 Mountain gullies 6•4

6.2.8 Torrents 6•4

6.2.9 Conservation measures to control natural erosion in mountains 6•4

6.2.10 Erosion rates in mountain lands 6•4




7 Sand Dune Erosion 7•17.1 Introduction 7•1

7.1.1 History of sand erosion control 7•1

7.1.2 The “natural character” of sand dunes 7•1

7.1.3 Preventive works 7•2

7.1.4 The Coastal Dune Vegetation Network 7•2

7.2 Forms, processes, extent 7•2



7.5 Bibliography 7•5

8 Earthworks 8•18.1 Introduction 8•1

8.1.1 Impact of earthworks erosion on the environment 8•1

8.2 Types, processes and extent of erosion on earthworks 8•1

8.3 Principles of Erosion Control on Earthworks 8•2

8.4 Appropriate control techniques 8•3

8.5 Bibliography 8•4

9 Sediment Control – Non Earthworks 9•19.1 Introduction 9•1

9.2 Sediment control from surface erosion in plantation forests 9•1

9.2.1 Principles of erosion and sediment control 9•2

9.2.2 Forest planning 9•3

9.3 Sediment control from surface erosion in croplands 9•3

9.3.1 Market gardening and grain cropping on granular loams 9•3

9.3.2 Principles of control on granular loams 9•4

9.3.3 Grain cropping 9•5

iv Soil Conservation A

9.3.4 Principles of control 9•5

9.3.5 Orchards and vineyards 9•5


9.3.7 Practices 9•6

9.4 Sediment control from surface erosion in pasture 9•6


9.4.2 Practices 9•7

9.5 Surface erosion control for other sources of sediment 9•8

9.5.1 Sediment from channel erosion 9•8

9.6 The effect of urban development on channel sediment 9•9


Tables

Table 1.1 Some physical and economic effects of wind erosion

(after Wilson and Cooke, 1980) 1•1

Table 1.2 Erosion rankings, North and South Islands

(after Eyles, 1983) 1•4

Table 1.3 Summary of management practices for control

of wind erosion. 1•5


of sheet and rill erosion 2•4

Table 4.1 Control techniques for deep-seated mass

movements on different geomorphology. 4•3

Table 5.1 Control techniques for different gully forms. 5•7

Table 6.1 Summary of mountain erosion types and

management practice 6•5

Table 8.1 Annual soil loss in the Auckland region Auckland

Proposed Regional Plan: Sediment Control, September 1995) 8•1

Table 8.2 Control techniques for erosion resulting

from earthworks. 8•3

Chapter 1 Wind Erosion

1.1 Introduction

Wind erosion is the detachment andtransportation of soil particles by windwhen the airstream passing over asurface generates sufficient lift and dragto overcome the forces of gravity, frictionand cohesion. Once a particle has beendislodged from the surface, it may betransported in suspension or by saltationor by surface creep. In high country, soilparticles are often dislodged by needleice formation and then removed by wind.

“Wind erosion, unlike soil erosion byrunning water, is often a rather subtleprocess, despite the quantity of sedimentremoved. Physical indicators of winderosion, such as sand shadows behindobstructions and sediment ripples infields, tend to be less obvious to theobserver than the rill channels or largegullies cut by fluvial processes. Loss oftopsoil by wind erosion over a relativelyshort time period can significantlydecrease soil fertility and crop yield.Plant nutrients bound to soil colloids

and organic matter can be removed fromfields within small soil units (aggregates<0.2 mm diameter) that are readilytransported by wind.” (Nickling andFitzSimons, 1985)

Wind erosion can also downgrade airquality and contribute to air pollution.Dust particles in the range of 0.25 to10.0 microns aerodynamic diameter canoriginate from farms and be carried up to10,000 metres elevation in the regionalairmass. The table below gives a usefuloverview of some of the effects of winderosion.

1.1.1 Impacts on land use

For wind to erode soil, the followingconditions need to be present:

• poorly aggregated soils

1•1Soil Conservation A 1•1

Physical Effects

1 Fine material, including organic matter, may be

removed by sorting, leaving a coarse lag.

2 Soil structures may be degraded.

3 Fertilisers and herbicides may be lost or redistributed.

1, 2, 3 Long-term losses of fertility give lower returns per

hectare.

3 Replacement costs of fertilisers and herbicides.

1 The crop may be covered by deposited material.

2 Sandblasting may cut down plants or damage the

foliage.

3 Seeds and seedlings may be blown away and

deposited in hedges or other fields.

4 Fertiliser redistributed into large concentrations can

be harmful.

5 Soil borne disease may be spread to other fields.

6 Rabbits and other pests may inhabit dunes trapped in

hedges and feed on the crops.

1-6 Yield losses give lower returns.

1-4 Replacement costs, and yield losses due to lost

growing season.

5 Increased herbicide costs.

6 Increased pest control costs

1 Soil is deposited in ditches, hedges, and on roads.

2 Fine material is deposited in houses, on washing and

cars, etc.

3 Farm machinery, windscreens etc. may be abraded,

and machinery clogged.

4 Farm work may be held up by the unpleasant

conditions during a blow.

1 Costs of removal and redistribution.

2, 3 Cleaning costs.

Soil Damage

Crop Damage

Other Damage

4 Loss of working hours and hence productivity

declines.

Economic Consequences

Table 1.1 Some physical and economic

effects of wind erosion (after Wilson and

Cooke, 1980)

CHAPTER 1

Wind Erosion

• surface soils that are loose, dry andfinely divided

• wind that is strong enough to movesoil.

In general, the finer and drier the soil,the easier it is eroded by wind whenexposed through lack of vegetative cover.In New Zealand, wind erosion can occurfrom seashore to mountaintops. Areasexposed to either coastal winds(prevailing westerly) or föhn winds thenor’westers), have been known to erodeseverely while soils were exposed. Basherand Painter (1997) cite background rates<0.2 t/ha/yr, extreme storm event losses>3000t/ha, and “average” storm lossesranging from 20–125 t/ha

These losses result in soil qualitydeterioration. In South Canterbury, Cuff(1981) estimated the following losses if 5mm of soil depth is lost; this would beequivalent to 50 t/ha:

• nitrogen – 175 kg

• phosphorus – 45 kg

• potassium – 500 kg

• magnesium – 325 kg

• calcium – 525 kg

Since shallow soils retain less moisturethan deeper soils, the cost of erosionmay also be expressed in long-termproduction losses. Hayward (1969) gives

examples of chou moellier crop yields inNorth Otago on soils of different depths:

• for topsoil depth of 300–375 mm,yields averaged 55 t/ha.

• for shallower soils near hilltops withdepths of 75–225 mm, yieldsaveraged 20 t/ha.

Benny and Stephens 1985 reported soillosses of 11-41 tonnes/ha/year onunprotected soils in South Canterbury,compared with 1.5 – 1.8 tones whereconservation measures were practised.Wheat yields on the unprotected sitesaveraged 3.1 tonnes/hectare comparedwith 3.7 tonnes on the protected sites.Basher (1997) provides more detailedfigures on erosion rates and areal extentfor all erosion types occurring in theCanterbury Region, including wind.

1.2 Forms, processes and extent

of wind erosion

1.2.1 Forms and processes

When wind blows across bare ground,grains of soil or rock are eroded if thestress exerted on them by air turbulenceexceeds the forces that resist movement –that is, gravitational stress, together withfriction and cohesion amongst thesurface particles. Once entrained in theflowing air, grains are transported so longas the stress exerted on them by windexceeds the force exerted by gravity. Ifwind stress declines – due to a drop inwind velocity – or if grains’ momentumreduces – due to particle collision, orpassage through vegetation – they aredeposited.

The physics of the process constrainswind erosion to places where velocityand turbulence are high: exposed coasts;plains or broad valleys; and mountainridges.

Wind erosion occurs only in placeswhere soil is susceptible. Clay is rarelyeroded by wind. Although its particlesare extremely small, they have highcohesion. At the other extreme, gravel orstone fragments are rarely eroded.Although they lack cohesion, theirparticles are sufficiently large for theforce of gravity to outweigh all but themost extreme wind stress.

Silt grains are easily eroded from topsoilby wind, because their particle size is

1•21•2 Soil Conservation A

Active wind erosion, Hakataramea. Photo: MWD.

very small and their cohesion ismoderate. Sand grains are also erodedfrom topsoil, from beaches and fromriverbeds, because their particle size isstill sufficiently small to be entrained bywind, and their cohesion is low.

There are three characteristic forms ofwind erosion:

• Wind blow: Just a millimetre or twoof silty or sandy topsoil is strippedaway, leaving no perceptible erosionscar. The transport process is visible asa cloud of dust. Silt or sandeventually settles downwind, manykilometres away. If repeated andpersistent, deposits accumulatearound the stems of growingvegetation as a thick even blanket ofnew soil.

• Wind scabs (deflation surfaces):Several centimetres of topsoil arestripped away by repeated wind blow,from a patch of ground several squaremetres in extent. The surface of thepatch may be more resistant subsoilor a “lag” of gravel and stones leftbehind after fine particles have beenblown from the topsoil. When largevolumes of soil are eroded from scabs,much may be deposited a shortdistance downwind, forming drifts upto a metre high piled againstobstructions like fences, shrubbyvegetation, or hill slopes.

• Dunes: Many metres depth of sand isblown across the landscape leeward ofbeaches, as a result of alternateerosion of grains from the dune’swindward side, of transport across itscrest, and of deposition on the dune’sleeward side. Different forms ofcoastal sand dune are described inChapter 7, Sand Dune Stabilisation.At some places in New Zealand’sinland landscape, thick deposits ofriver silt or volcanic ash have alsobeen shaped into “fields” of smalldunes, individually a few metres highand long.

For greater detail, see Bagnold (1941) fora definitive account of the physics ofwind erosion, Painter (1978) for anaccount of the New Zealand landscape’ssusceptibility to wind erosion, and Cookeand Warren (1973) for comprehensivedescriptions of the geomorphology.

1.2.2 Wind erosion research

Wind erosion has been the subject ofresearch in the USA since the 1930s (seeFryrear et al’s 2000 study). The first WindErosion Equation (WEQ) to quantifywind erosion was published by Woodruffand Siddoway in 1965. In 1996, Shao etal developed a physically based WindErosion Assessment Model (WEAM) toestimate erosion event movement acrosscontinents. A more process-orientedWind Erosion Prediction System (WEPS)is under development (Hagen, 1991).Fryrear and colleagues describe a RevisedWind Erosion Equation (RWEQ) basedon the parameters wind force, surfaceroughness, soil wetness, and crop residueor crop canopy.

Fifty years of wind erosion research bythe USDA, Agricultural Research Serviceat Kansas State University was celebratedwith a symposium in 1997, theproceedings of which may be accessed atwww.weru.ksu.edu/symposium/proceed.htm.

In New Zealand, the problem wasrecognised as early as 1858 by Ferdinandvon Hochstetter in his book, NewZealand. Its physical geography, Geologyand Natural History. The first attempt toquantify soil losses resulting from winderosion was that by Campbell (1945),who used a portable blower on a numberof North Island east coast soils. He variedthe speed of the blower to simulate soilsunder pasture and cultivation. The valuesof soil loss ranged from nil to 1668t/ha/yr.

The forms and processes of wind werefirst described by Cockayne (1913) forsand country, and by Cumberland (1944)for arable land and depleted tussockrangeland. Much of the research intoforms and processes has been undertakenin sand country; well-known workincludes that of Brothers (1954), Cowie(1967), Hicks (1975) and Hesp (1990).Some research into the geomorphologyof wind erosion on arable land iscontained in unpublished reports andtheses by Painter (1977, 1978) and hisstudents. More detailed investigationshave been carried out in recent years byBasher et al (1997, 2000).

A major review of erosion risk on arablesoils (Ross et al, 2000) has been preparedfor Environment Canterbury and is anessential reference on this topic.

In New Zealand’s environment, themajor factors affecting the occurrence of


wind erosion, and its extent and severity,are:

• climate (wind patterns, precipitation,frost action)

• soil (texture, moisture, structure,organic matter content)

• topography (exposure, elevation,terrain roughness, localisedfunnelling of wind)

• cultural practices (cultivation,vegetation depletion).

Looking at these factors, it is clear thatthe near-surface soil water content is animportant influence on wind erosionrates in New Zealand, where theevaporative energy supply can be veryhigh and can rapidly reduce the near-surface water content. The soil’sresistance to evaporative flux, combinedwith soil erodibility (for thepredominantly light textured soils ofNew Zealand) determines whether a soilwill erode.

1.2.3 Extent in New Zealand

New Zealand Land Resource Inventory(NZLRI) survey assessed wind erosion(NWASCO, 1974-79). Data from thissurvey were summarised by Eyles (1983)and are reproduced in the followingtable. The distribution of wind erosion asmapped in the same survey is shown inAppendix II.

In 1984, Salter used NZLRI data toundertake a more detailed review of thedistribution and severity of wind erosionin New Zealand. He found thatapproximately 13 percent (3.4 millionhectares) was affected. Wind erosionaffects 4.6 percent of the North Island,mainly in three environments:

• the mobile, coastal dunes on the westcoast of Northland and theRangitikei-Manawatu that have notyet been afforested or have poor grasscover

• high-altitude (>700 m), volcanic ash-mantled, slopes in the central NorthIsland that have poor vegetationcover due to frequent strong windsand cool temperatures

• low-altitude, loess-mantled argillitehill country and alluvial terraces inthe eastern North Island with low-fertility soils and severe seasonal soilmoisture deficits.

In the South Island, wind erosion affects19 percent of the land, mostly on loess-mantled terraces and slopes in low-rainfall, seasonally dry eastern regionssubject to frequent strong föhn winds:

• extensive alluvial plains inCanterbury, Marlborough andSouthland with silty alluvial soils onyounger terraces and loess soils onolder terraces, that are susceptible towind erosion when cultivated

• loess-mantled downlands inCanterbury and North Otago, subjectto wind erosion when cultivated

• large inland basins in central Otago,Canterbury and Southland with loess-mantled terraces and moraines whereextensive grazing by sheep andrabbits has led to severe vegetationdepletion

• the steep, dry mountain lands ofCanterbury, Otago and Marlboroughwith severe summer soil moisturedeficits and widespread vegetationdepletion

• hill country with shallow soils,discontinuous loess cover, severesummer soil moisture deficits andlocalised vegetation depletion inNorth Canterbury


Erosion Ranking Area (ha) Area (%)

1 slight 255,500 48.5

2 moderate 131,700 25.0

3 severe 67,400 12.8

4 very severe 16,500 3.1

5 extreme 55,800 10.6

1 slight 1,620,900 77.9

2 moderate 388,600 18.7

3 severe 58,000 2.8

4 very severe 12,000 0.6

5 extreme 1,500 trace

* Areas are those of map units in which wind erosion was

recorded, with the given ranking.

** Areas are those of map units in which wind erosion was

the dominant type.

No

rth

Is

lan

d*

So

uth

Is

lan

d *

*

Table 1.2 Erosion rankings, North and South Islands

(after Eyles, 1983)

• the exposed rolling uplands of Otagowhere vegetation is depleted.

According to the NZLRI survey, the areapotentially subject to wind erosion ismuch larger than the area alreadyaffected. Approximately 27 percent ofNew Zealand being susceptible: 7.1million hectares of the North Island, and5.8 million hectares of the South Island.

1.3 Principles of control

The techniques available to control winderosion have one or more objectives incommon:

• reduce the cause of erosion (eg,reduce local windspeeds)

• enhance resistance to erosion (eg,reduce soil erodibility)

• reduce the undesirable consequencesof wind erosion (eg trapping saltatingsoil, reducing physiological damageto plants).

In 1997, Basher and Painter review theextent to which these objectives havebeen attained. They note that winderosion control in New Zealand has beendominated by the use of vegetative coverto reduce entrainment of soil particles orto provide a barrier which reduces windvelocity. There has been little use oftillage to increase surface roughness andproduce non-erodible aggregates, in spiteof advocacy by soil conservators.

Baker (cited by Ross et al. 2000) similarlyemphasised the various options nowavailable to increase surface roughnessand retain soil. Basher and Painter (1997)also consider that there is greateropportunity to use irrigation to increasesurface soil moisture content, and reducethe hazard at times when soil wouldotherwise be susceptible to wind erosion.Mitigation of wind-borne dust emission,usually by spraying water or waste oil,from rural roads, construction sites,quarries and other industrial locations isincreasingly practised.

1.4 Appropriate control practices

Table 3 summarises wind erosion controlpractices currently used on arable land,and at susceptible sites in the hillcountry and mountain lands. Controltechniques include:

• shelterbelts

• pasture species and management

• cultivation and drilling practices

• stubble management

• adhesives and tackifiers

• irrigation.

For wind erosion control practices onsand country, refer to Chapter 5


Control Factors to Consider/ Refer to Part B forPractice Technique Practice Description

Shelterbelt Siting 14.1

Design 14.2

Establishment 14.3

Design description (various) 14.4

Pasture Revegetation – Lowlands 4.1

management – Hill country 4.2

& species – Mountain lands 4.3

Pasture management – Lowlands 2.1

– Hill country 2.2

– Tussock 2.3

Cultivation & Minimum tillage 1.3

drilling practice Direct drilling 1.4

Stubble management Stubble retention 1.5

Others

– adhesives & tackifier

– irrigation


of wind erosion.

1.5 References

Anon. 1971 Wind erosion control. Soil &Water 8:23-27 and 29

Bagnold, R. A. 1979: Sediment transportby wind and water. Nordic Hydrology,v10, pp309-322.

Baker, CJ and Ross, CW 2000: Thepractical issues of increasing theadoption of no tillage in Canterbury,pp 114-119 in “Management ofErosion Risk on Arable Soils. AReview”. Report No. U00/49, preparedfor Environment Canterbury.

Basher, LR and Painter, DJ 1997 WindErosion in New Zealand. Paperpresented at the conference “50 Yearsof Wind Erosion Research by USDA –ARS at Kansas State University”. Seewww.weru.ksu.edu/symposium/proceed.htm

Basher, LR 1997: A review of erosionstudies in Canterbury. Reportproduced for Canterbury RegionalCouncil.

Basher LR, Hicks DM, Handyside B andRoss CW, 1997: Erosion and sedimenttransport from the market gardeninglands at Pukekohe, Auckland, NewZealand. Journal of Hydrology (NZ)36(1) p 73-75.

Benny, L.A., and Stephens, P.R., 1985:The feasibility of determining theinfluence of arable land managementon topsoil depth. Soil ConservationCentre, Aokautere, Publication No. 7.

Brothers, N. 1954: Geology of SouthKaipara Head. Unpublished Ph. D.thesis, University of Auckland.

Campbell, D. A., 1945:Soil ConservationStudies applied to farming in HawkesBay Part 1 Investigations into runoffand soil loss, NZ J. of Science andTechnology 26(6): 301-332.

Cockayne, L. 1909: Report on the sanddunes of New Zealand. AppendixJournal House of Representatives, C-11.

Cooke, R. U. and Warren, A. 1973:Geomorphology in Deserts. London.Batsford.

Cowie, J. D. et al 1967: Soils of theManawatu-Rangitikei sand country.Bulletin 29, Soil Bureau, DSIR.

Cumberland 1944 Soil Erosion in NewZealand – A GeographicReconnaissance. Publ, Whitcombe &Tombs Ltd., New Zealand. SecondEdition 1947.

Eyles, G.O., 1983: The distribution andseverity of present soil erosion in NZ,NZ Geographer 39(1):1-28

Fryrear D W, Bilbro, JD, Saleh, A,Schomberg, H, Stout, JE and Zobeck,TM 2000: RWEQ: Improved WindErosion Technology. J Soil and WaterConservation, 55 (2) 183-189.

Hayward, J. A., 1969: The use offractional acre plots to predict soilloss from a mountain Catchment.Lincoln Papers in Water Resources 7,New Zealand Agricultural EngineeringInstitute, Lincoln College 93 p

Nickling, W. G. and FitzSimons, J. G.(1985): Relationship of soil type andagricultural systems to wind erosionin Southwestern Ontario.

NWASCA Miscellaneous Publication No.59 (Ed:. Sturrock 1984).

Painter, D. J. 1977: Wind erosivenesssummaries for New Zealand sites.Report PR15, NZ AgriculturalEngineering Institute, Lincoln.

Painter, D.J., 1978: The Pastoral Scene.Soil Erosion Rates on New ZealandFarm Land. Proceedings of theConference on Erosion Assessmentand Control in New Zealand.

Ross, CW, L Basher, CJ Baker 2000:Management of erosion risk on arablesoils: a review. Landcare ResearchReport LC9900/090 prepared forEnvironment Canterbury regionalCouncil. 119 p.

Salter, RT 1984: Wind Erosion. Pp 206-248 in Speden, I. And Crozier, MJ(compilers) Natural Hazards in NewZealand. New Zealand NationalCommission for UNESCO,Wellington.

Soil Conservation and Rivers ControlCouncil Circular SC 1971/16

Wilson, SJ and Cooke, RU (1980): WindErosion, pp 217-251 in Kirby, MJ andMorgan, RPC (Eds) Soil Erosion, Publ,J Wiley & Sons Ltd.


Chapter 2 Sheet and rill erosion

2.1 Introduction

Running water removes soil and organicmatter from gentle sloping land andsteeper sites by a variety of processeswhich often start off as splash erosionleading to sheet wash and as theconditions change this will lead into rillerosion which in time on the deeper soils– that is, loess or volcanic material – maydevelop gullies. This chapter willconcentrate on sheet and rill erosion ofsoil by overland flow of water, on arableflat-rolling land, downlands and lowerfoothills. Sheet erosion may often co-exist with rill erosion, and one may leadinto the other.

2.1.1 History of sheet/rill erosion

research

Despite the concern expressed about themagnitude of erosion and its impact onproductivity, there have been fewquantitative studies of surface erosion onarable land in New Zealand. Campbell(1945) showed that surface runoff as apercentage of rain under good grazedpasture was 42.2 percent compared to 0.3percent for forest. Benny and Stephens(1985) estimated South Canterbury soillosses on a 12-degree slope of 40 t /ha/yrunder annual cultivation, 23 t/ha/yr withploughing one year in three, 3 t / ha/yrwith continuous cropping and stubblemulching, and 0.4 t / ha / yr underpermanent pasture.

Hunter and Lynn (1988, 1990) mademeasurements after storms inCanterbury. Basher (1997) reviewed thesoil losses in Canterbury and identifiedhigh rates during storms of up to 11 mmthrough sheet and rill erosion. Forcropping paddocks, there wasconsiderable redistribution of the soilwithin paddocks as the soil was initiallymoved off slopes by rill erosion anddeposited in swales, leaving a greatvariation of topsoil depths. There was agreater percentage of shallow topsoilsunder cropping than pasture, sufficientto impact on crop yields.

Basher et al (1997) published a summaryof soil loss measurements at Pukekohe.This study showed that of the soil erodedwithin a paddock, a significant

proportion was redeposited on lowerslopes while a smaller proportion wasdelivered to streams. (Thompson andBasher 1999) identified some interestingfactors due to an intense storm event atPukekohe on 21 January 1999 whereover 135 mm rainfall fell in four hours(exceeding the 1 in 100 year event). Thesoil was prone to downslope movementwhere slopes exceeded 4°. Paddockssteeper than 2° average slope which hadseedbed preparation (eg, by rotaryhoeing) had erosion rills, predominantlyalong wheel tracks, leading to morecompound rill and sheet erosion furtherdown the paddock as the runoff waterincreased in velocity. Erosion rates insome fields reached values as high as 600t/ha. This meant an average loss of about5 cm over the field, mainly due to rillerosion (sheet was considered only minorform); however, most soil was redepositedin nearby hollows or roadside drains.

2.1.2 Impacts on land use

Surface erosion can cause reductions ofone to three-fifths in crop yield, two tofour-fifths in pasture growth and up tonine-tenths in tussock biomass. Most ofthese production losses are reversiblewith appropriate soil conservationmanagement.

In dryland areas like North Otago,Canterbury and Marlborough, soil losseson cultivated paddocks is still a regularoccurrence. In North Otago betweenJanuary and March 1986, it wasestimated that 240 ha on 24 propertieslost an average of 50 t/ha of fertile soil (5mm depth of topsoil). This equates to12,000 t of topsoil, which would requirefertiliser estimated to cost $300,000(1986 figures) to replace the majornutrients lost (Otago Regional Council,1989).

Cuff (1978) estimated that typicalnutrients lost after 10 mm topsoil iseroded equated to:

Total Nitrogen 350kg/ha Total Phosphorus 90 kg/ha Total Potassium 1000 kg/ha Total Magnesium 650 kg/ha Total Calcium 1050 kg/ha


Sheet and rill erosion

CHAPTER 2

2.2 Types, processes and extent

When rainwater runs off across bareground, particles of soil or rock areeroded if three conditions exist:

• The soil’s infiltration capacity isexceeded, so that water can build upinstead of soaking into the soil. Thiscondition may be met on dry soilduring intense rain, but it morecommonly occurs where infiltrationcapacity has been reduced to zero bywater soaking in and saturating thesoil beforehand. A related and equallycommon situation, is where waterseeps from saturated soil and flowsacross its surface – that is, infiltrationcapacity turns negative.

• The water layer is sufficiently thickfor gravitational stress on it to exceedcapillary forces holding water to thesoil’s surface, so that water starts toflow instead of merely ponding.

• The stress exerted on soil particles bylaminar or turbulent flow of water, isgreater than the forces resistingmovement – gravity, together withfriction and cohesion amongst thesoil particles.

Once entrained by water, particles aretransported so long as the force offlowing water is sufficient to keep themclear of the surface (suspension) orbounce them along it (saltation). If waterstress declines, for instance due to a dropin velocity where slope gradient lessens,or due to a drop in mass where somewater soaks into the ground, thenparticles are deposited. This also happensif particles’ momentum is reduced, forinstance where they encountervegetation at the bare ground’s edge.

The physics of the process determineswhere surface erosion is found in nature:

• areas subject to intense rainfall

• soils with low infiltration capacity orwhich quickly saturate

• depressions or breaks in slope, wheresoil water can converge and seep out.

Surface runoff rarely has sufficient massor velocity to transport boulder orstones. Transportation of gravel occursonly if runoff is voluminous and fast.Sands and silts are most commonlytransported by surface runoff, as they areeasily entrained by a low volume ofrunoff moving at a slow velocity. Claysare less commonly entrained, as theircohesion gives greater resistance; butonce in motion, their small particle sizeensures they are transported a longdistance before settling.

Three forms are left behind:

• Sheet wash is the almost-imperceptible removal of a fewmillimetres of topsoil at a time fromparts of a slope. The process is visibleas discontinuous sheets of surfacerunoff. A thin layer of soil is usuallydeposited a short distance away,where runoff slows and soaks in.

• Scabs are distinct scars where severalcentimetres of topsoil are removedfrom patches of slope several squaremetres in extent, by thin broad sheetsof coalescing surface runoff (sheetflow). A distinct layer of soil isdeposited if surface runoff encountersan obstacle, for instance dense grassor rising ground; but if not, soil istransported off the slope. It may bedeposited at the slope foot if gradientlessens sheet flow’s velocitysufficiently for particles to settle; ifnot, eroded soil will be delivered intothe nearest permanent watercourse.

• Rills are long narrow miniaturechannels, where anything from 10 to50 cm of topsoil is removed bysurface runoff concentrated into thicknarrow threads. Its volume andvelocity being greater than sheet flow,rill flow usually transports eroded soiloff slopes and into streams. Repeatederosion of the same rill may developit into a gully (see Chapter 4, GullyErosion).


Fan deposited at slope foot, below rilled wheel tracks,

Pukekohe. Photo: D Hicks.

Refer to Bagnold 1966 for a definitiveaccount of the physics of surface erosionby water; Henderson 1983 for an accountof the New Zealand landscape’ssusceptibility; and Leopold, Wolman andMiller 1964 (Chapters 8 and 10) for acomprehensive description of thegeomorphology.

2.2.1 Extent

Surface erosion ie including sheet andwind (note it is difficult to separate soilloss from sheet and wind erosion) isfound throughout NZ. NZLRI identifiedthat 23.3 percent of the North Island and74 percent of the South Island issusceptible to surface erosion. Thefollowing percentages indicateagricultural land that is susceptible tosurface erosion processes:

• Otago 88 percent

• Canterbury 86 percent

• Nelson-Marlborough 77 percent

• Southland 59 percent

• Northland 41 percent

• Hawkes Bay 30 percent.

The trend drops to 6-9 percent in regionssuch as Taranaki, East Coast andGisborne. where mass movement erosionpredominates (Clough and Hicks 1993).

In the South Island, particularly east ofthe Southern Alps, loess is the mostextensive soil-forming material. Itunderlies 7.45 million hectares – justover 51 percent of the soils (Raeside,1964). However in some areas theamount of loess may be small; the stonysoils on the Canterbury Plains are anexample. In the North Island, the soilsmost susceptible are those formed fromloess, volcanic ash, and pumice.


There are a number of interrelatedfactors, which can initiate and acceleratesurface wash and removal of soilparticles. Some common examples inNew Zealand are:

• overgrazing by domestic farm animals(eg, due to large blocks with sunnyand shady aspects)

• overgrazing by animal pests such asrabbits, hares, goats and feral animals

• burning of native grassland/tussockand inter-tussock species

• burning of scrub and plant pests (egmatagouri, gorse and broom)

• burning of cereal crop stubble andremoval of beneficial organic matter

• cultivation up- and down-slope andcultivating slopes over 20°

• cultivation at regular intervalsgrowing similar crops/pasture

• compaction by stock and machineryand the formation of compactionpans in the soil

• which reduces normal waterinfiltration and increases overlandflow

• cultivation of soil leaving a very finetilth (ie, with a high percentage ofparticles smaller than 5-10 mm).

• forest harvesting with skidder haulingmachinery

• dozing steep slopes

Maintaining and improving groundcover to ensure that all the surface layerhas living or decomposing vegetativematerial on it will help protect soilparticles from rainfall events. Controlmeasures become more critical on steeperland ie greater than 12° slope where soilsare loessial, of volcanic origin or sandyin nature.

Principles of control fall into threegroups:

• Firstly, there is a need to carry outwater management to ensure thatsurface runoff water is removed safelyand does not speed up or concentrateso earthworks such as graded banks,


Localised rill erosion West Otago. Photo: Otago Regional Council.

Type of erosion

Splash and sheet erosion(collectively called sheeterosion)

Rill erosion

Primary Soil Conservation Principle

Ensure effective groundcover, soil structure andhealth be maintained andimproved.

Create a stable seedbed.

Ensure effective groundcover, soil structure and soilhealth be maintained andimproved. Create a stableseedbed.

Factors to Consider

Rainfall, storm frequency,infiltration capacity

Topography and aspect

Slope angle, slope length

Geology, regolith (loess,ash)

Soil type (depth, texture,structure, percentage oforganic matter)

Past tillage and vegetationtype and rotations

Land use – pastoral,cropping, horticultural,forestry or urban use

Stock type

As above (often on steeperslopes)

Examples of Practices*

Water control

Conservation tillage

Drainage and aeration

Earthworks

Stubble mulching

Rotational cropping

Pasture sward maintenance

Matching crop and pasturespecies to the site

Timing cultivation when soilconditions are right (low riskof erosion)

Adjusting grazing pressureto soil conditions i.e. avoidheavy stocking when risk oferosion is high

As above,

contour furrows, strip cropping andfloodbanks are constructed.

• Secondly, it is important to ensurethat a healthy, vegetative groundcover exists over the soil for as longas possible. Practices that maintain orimprove the soil fertility, soil organicmatter, soil flora and fauna as well asutilising the best plant species for thesite (eg, drought grasses for drylanddistricts and water-adapted plants forwetter sites) should be adopted. Thiswould include practices such ascultivation management,

conservation tillage, soil drainage andaeration and Land Use Capabilitymanagement.

• Thirdly, it is important to ensure thatongoing soil, plant and farmmanagement practices are maintainedor improved on all susceptible sites.

2.4 Appropriate Soil Conservation

Practices

See Table 2.1 below.


Table 2.1 Summary of management practices for control of sheet and rill erosion

*May use any one or a combination of them

Descriptions of the various practices for control of surface erosion are found in sections of Part B:

1. Cropland Management for Surface Erosion Control

2. Pasture Management for Surface Erosion Control

3. Fencing Management for Surface Erosion Control

4. Pasture Revegetation Practices

9. Runoff Control Practices – Lowland

2.5 References

Bagnold, R. A. 1979: Sediment transportby wind and water. Nordic Hydrology,v10, pp309-322.

Basher, L. R. et al 1997: Erosion andsediment transport from the marketgardening lands at Pukekohe,Auckland. NZ Jour. Hyd., v36, pp73-95.

Basher, L.R., et al 1999: Assessment oferosion damage at Pukekohe duringthe storm of 21st January 1999,NZARM Broadsheet July 1999pages30-31

Basher L.R., et al 1995:Surface ErosionAssessment in the South CanterburyDownlands, New Zealand using 137Cs Distribution, Australian JournalResearch, 33, 787-803

Benny, L.A., and Stephens, P.R., 1985:The feasibility of determining theinfluence of arable land managementon topsoil depth. Soil ConservationCentre, Aokautere, Publication No. 7.

Campbell, D. A., 1945:Soil ConservationStudies applied to farming in HawkesBay Part 1 Investigations into runoffand soil loss, NZ J. of Science andTechnology 26(6): 301-332.

Clough, P. and Hicks, D.L.,1993: SoilConservation And The ResourceManagement Act Summary, MafPolicy Technical Paper 93/2 for MafPolicy Wellington

Eyles, G.O., 1983: The distribution andseverity of present soil erosion in NZ,NZ Geographer 39(1):1-28

Haynes, R.J., 1995: Soil StructuralBreakdown and Compaction in NewZealand Soils MAF Policy TechnicalPaper 95/5

Henderson, R. 1983: Values of therainfall and runoff factor® of theuniversal soil loss equation for NewZealand. Internal Report 844,Hydrology Centre, Water and SoilDivision, MWD.

Hicks, D.L., 1995: Control of Soil Erosionon Farmland, MAF Policy TechnicalPaper 54/4

Hunter, G. G. and Lynn, I. H. 1990:Storm induced soil losses from SouthCanterbury and North Otagodownlands. DSIR Land ResourcesTechnical Record 22, 7p.

Leopold, L. B., Wolman, G. M., andMiller, J. P. 1964: Fluvial processes ingeomorphology. San Francisco. W. H.Freeman.

North Otago Sustainable LandManagement Group 1995: NorthOtago Sustainable Land ManagementGuidelines, North Otago SustainableLand Management Group inassociation with the Otago RegionalCouncil

North Otago Volcanic SoilsEnvironmental Group 1996:Sustainable Land ManagementGuidelines for the Waiareka Soils,North Otago Volcanic SoilsEnvironmental Group in associationwith the Otago Regional Council.

NWSCA 1985: Soil Erosion in NewZealand, National Water and SoilConservation Authority Publicationno 49

NWSCA 1969: Land Use CapabilitySurvey Handbook, Water and SoilDivision Ministry of Works,Wellington, New Zealand

Painter, D.J., 1978: The Pastoral Scene.Soil Erosion Rates on New ZealandFarm Land. Proceedings of theConference on Erosion Assessmentand Control in New Zealand.

Raeside, J.D., 1964: Loess Deposits of theSouth Island, New Zealand, and SoilsFormed on them. N.Z. Journal ofGeology and Geophysics. 7: 8V-38

Shepherd, T.G., et al 2000: SoilManagement guidelines forSustainable Cropping, LandcareResearch New Zealand

Thompson, T.,1999:Pukekohe StormEvent 21st January 1999, NZARMBroadsheet March 1999 pages 51-54

Watt, J.P.C., 1972: Loess soils andproblems of land use on thedownlands of the South Island NewZealand, Otago Catchment Boardpublication No 4


Chapter 3 Shallow Mass Movement – slip erosion

3.1 Introduction

Slip erosion is a shallow and rapid slidingor flowing movement of the soil andsubsoil, exposing a slip surface which isapproximately parallel to the slope.Debris comes to rest in the area from thebase of the exposed slip, to the toe of theslope. There can be some rotationalmovement, leaving a concave “slipsurface” or failure plane. The failureplane is normally less than 1m (butsometime up to 2m) below the originalsurface.

It is most evident during or immediatelyafter heavy rain. The basic reason for thisis saturation of the soil, which increasesthe weight of the soil mass, lubricates thefailure plane, and turns pore waterpressure positive. The resisting forces areshear strength of the soil, cohesion ofthe material, and tensile strength ofplant roots in the soil. The large scaleclearing of forest cover from the 1870son the hill country in New Zealandeventually resulted in widespread andaccelerated slip erosion as well as otherforms of mass movement.

This type of erosion is widespread on hillcountry throughout New Zealand. It is anatural process that occurs on all typesof terrain and under all types ofvegetation. Accelerated slip erosion mayoccur on unstable terrain when nativeforests are removed and replaced withgrass, or between rotations of plantationforestry. Most slip erosion occurs duringhigh intensity rainfall or prolonged wetperiods when the regolith becomessaturated, pore pressures rise, and slopefailure occurs.

Slip erosion is classified as either soil slipor earth slip in the NZLRI. (Eyles 1985)Treatment and recovery tend to bedifferent for each type.

3.1.1 History of slip erosion control

Severe storm events like the Esk Valleyfloods in 1938 left extensive areas of hillcountry covered in shallow slips. Farmersrealised that the poor quality of thepastures may have contributed to theslipping as well as the loss of forest cover.Rabbits were widespread; pasture

management was almost non existentwith few fences and almost no fertiliserhaving been applied. In the late 1940sexperiments were made in several placesthroughout New Zealand with aerialseeding and the application of fertilisers.This eventually led to hugeimprovements in hill country pasturesand a reduction in erosion (McCaskill,1973).

However, it was shown that, in unstableareas, better pastures alone would notsolve soil erosion. Poplars and willowswere planted as unrooted poles intofarmland in an effort to reduce soil lossand hundreds of thousands of poles wereplanted on hillsides in the following 50years. Poplars were shown to have anextensive rooting system, and they coulddry out the soil by transpiring moistureduring spring and summertime. In recentyears block planting with seedling treeshas become more extensive. Pines, gumsand wattles have been shown to havesimilar qualities in binding the soiltogether as well as being able to removemoisture from the soil throughout mostof the year.

Heavy stock grazing on hill landsusceptible to slip erosion can lead topugging in wet weather, with rainwaterstored in hundreds of depressions madeby hooves. Continued saturation of thesurface may lead to failure of the slope.Improved livestock management bysubdivision and other grazing practicescan help reduce the risk slip erosion.

The effective control of slip erosionrequires good planning and recognitionof areas where slips may occur. Oftenfarmers will actively plant poles on sitesthat have already slipped. It is usuallybest to plant on sites that have apotential to slip and careful examinationof an area can identify these places. Landuse capability surveys are a key toidentifying areas where preventativeplanting should be established.

3.2 Types, processes

Mass movements of soil or rock occurwhen stresses (downslope component ofgravity pulling soil down the slope, pore


CHAPTER 3

Shallow Mass Movement

– slip erosion

water pressure, loading by vegetation,seismic waves propagating through thesoil) exceed resistances (in-slopecomponent of gravity holding soil to theslope, friction and cohesion of soilparticles, reinforcement by vegetationroots). Slope failure occurs by threeprocesses, acting singly or incombination: falls, slides, and flows.

The operation of three processes invarying combinations; differences in thesoil properties that aid or resist failure;and the variety of triggering mechanisms– physical or chemical weathering,changes in pore water pressure duringrainstorms or wet weather, undercuttingor overloading of slopes, passage ofearthquake waves – produce manydifferent forms of mass movement. Thereis no standard nomenclature (despiteseveral attempts at internationalstandardisation).

3.2.1 Falls

Falls are a sudden rupture, followed byfree fall of soil rock fragments, away

from a near-vertical slope. The rupture ismanifest as a fresh shear surface (failureplane) on the bluff or scarp, with a pileof loose debris (talus cone) beneath.

3.2.2 Slides

Slides result from elastic deformation,followed by sudden rupture and slidingof a soil or rock mass, down a steepslope. If movement is slight, the ruptureis visible as tension cracks around thehead and sides of a soil block that isotherwise intact. If movement is greater,the rupture is exposed as a sloping shearsurface in the upper slope, with adisintegrated soil block (debris tail)farther downslope.

3.2.3 Flows

Plastic deformation, followed by inter-particle shear and flowage of soil or rockmass, down a low-angle slope, forms aflow. If flow is slow, it manifests itself assurface rumpling of an internallydeformed soil mass. If it becomes rapid,it is manifest as a disintegrated jumble ofsoil fragments (flow debris) depositedwithin downslope of the failure zone. Afailure plane separates the soil mass orflow debris from intact regolith, but israrely exposed. A flow usually inducessome slide-type failures in the more rigidsoil around its margins; these becomemore-or-less intact soil blocks raftedalong in the flow debris.

3.2.4 Shallow mass movement in New

Zealand

In New Zealand, a practical distinction isdrawn on the basis of failure depth –whether the mass movement is shallow(in the soil) or deep (in underlyingregolith). This separates mass movementswhich may be easily stabilised androutinely undertaken, from those wherestabilisation is difficult but may need tobe attempted (Chapter 4, Deep-seatedMass Movement- Slides, Slumps andFlows).

Four forms of shallow mass movementare common in the New Zealandlandscape. The distinctions are againpractical, differentiating two formswhich can be controlled by vegetativetechniques alone from two for whichengineering measures are usually needed:

• Slips – These are shallow landslides insoil or weathered regolith on steepslopes (Figure 1 – photo). They arealso found on low-angle slopes where


Soil slip, Taranaki. Photo: M. Tuohy.

Earthslip erosion, Te Pohue. Photo: Hawke’s Bay Regional Council.

regolith is susceptible to rupture anddeeply-weathered rock with a highclay content. Occasionally foundwhere steep slopes (otherwise stable)are mantled by volcanic ash or loess.

• Earth flows – These are shallow flowson low-angle slopes (Figure 2 –photo), where regolith is susceptibleto plastic deformation, notablyweathered mudstones which containswelling clay minerals such asbentonite or montmorillonite. Earth flows are also found incolluvium (slip debris) on footslopesand occasionally found where low-angle slopes (otherwise stable) aremantled by weathered volcanic ash,loess or till.

• Debris avalanches are shallow, rapidlandslides in regolith on uppermountain slopes (see Chapter 6 (PartA), Mountain Lands Erosion fordescription).

• Debris flows – shallow, rapid flows incolluvium (avalanche debris) onlower mountain slopes (see Chapter 6for description)

Crozier 1986 gives detailed descriptionsof the four forms, illustrated by manyNew Zealand examples. The physics ofslope failure is complex, but essential tounderstanding the circumstances underwhich any control technique can (orcannot) work. Readers are referred toCarson and Kirkby (1972) (Chapters 5 to7) for a definitive discussion of the topic.Scheidegger (1984) also provides a usefulsummary of the physics.

3.2.5 Extent of shallow mass movement

The NZLRI identified 7,351,600 hectaresof land as susceptible to soil slip or earthslip erosion (refer to Appendix II).1,044,800 hectares were identified assusceptible to earth flows (refer toAppendix II); most of this is terrainaffected by shallow flows, though somedeep-seated flows may be included.2,821,900 hectares of land wereidentified as susceptible to debrisavalanches from debris flows, AppendixII be taken as indication extent of thesealso.


Revegetation trials in Gisborne (Quilter,1993) and in the Wairarapa (Lambert,1993) have shown that it takes up to 20

years to restore productive pastures onerosion scars and this may only be 70-80percent of dry matter production fromuneroded ground. The successfulintroduction of pasture plants ontoeroded sites requires retirement fromgrazing, annual fertiliser applications andcareful stock management. The cost ofremedial work may be uneconomiccompared with other opportunities forfarm investment. Protecting land fromerosion is therefore a more attractiveinvestment than repairing erosion afterthe event.

3.3.1 Preventive works

The prevention of soil slip erosionusually requires a modification topastoral farming practices. Susceptibleareas on farms are readily identified by asuccession of slip scars dating back to thetime of development, and in manyplaces even before forest clearance.

There are several options for reducingthe risk of soil slip erosion. Improvingpasture vigour by introducing deeperrooting species and regular applicationsof fertiliser will no suffice. It must beaccompanied by:

• Spaced planting of trees in pasture, toprovide the soil with rootreinforcement, or

• Retirement from grazing, followed byplanting with commercial timbertrees, or

• Reversion to native scrub cover

3•33•3Soil Conservation A

Earthflow, Hawke’s Bay. Photo: Water & Soil Division, MWD.



Descriptions of the various practices for control of shallow mass movement are found inchapters/sections of Part B:

3.2 Fencing Management Practices for Erosion Control – Hill Country

4.2. Pasture Re-Establishment – Hill Country

8.2. (8.3) Managed Reversion of Retired Land – Hill Country

12. Pole Planting

3.5 References

Carson, M. A. and Kirkby, M. J. 1972:Hillslope form and process.Cambridge University Press.

Crozier, M. J. 1986: Landslides. 212pp,Croon Helm, London.

Eyles GO 1985: The New Zealand LandResource Inventory ErosionClassification. Miscellaneous Waterand Soil Publication Number 85,National Water and Soil ConservationAuthority.

Lambert, MG, Costall, DA, Foote, AG,Trustrum, NA 1993 Proceedings of theNew Zealand Grasslands Association55, 177-181

McCaskill, LW 1973 Hold This Land, AH& AW Reed Ltd.

Quilter, S.J., Korte, C.J., Smithe DR. 1993)Proceeding of the New ZealandGrasslands Association 55, 187-191

Scheidegger, A. E. 1984: A review ofrecent work on mass movements onslopes and rockfalls. Earth ScienceReviews, v21, pp225-249.

3•5Soil Conservation A

Chapter 4 Deep-seated Mass Movement

– slides, slumps and flows

4.1 Introduction

New Zealand has some spectacularexamples of deep-seated massmovements. Lake Waikaremoana andLake Tutira in Northern Hawkes Bay wereboth formed by massive collapses ofearth and rock into deep gorges. Inrecent times, the Abbotsford block glidein Dunedin demonstrated what happenswhen unstable slopes are modified forurban development. Less spectacular, butsimilar movements of soil, rock andvegetation regularly damage roads,railways, even houses, in many parts ofthe country. Huge investments inafforestation or structural works havebeen undertaken to reduce damage fromsuch events.

Geology, water and gravity are the mainfactors in deep seated mass movement.The softer rocks of the Tertiary period,experience slumping and earth flows inmany forms. Up to 41 percent offarmland in the Gisborne District issusceptible to earthflow erosion. Otherexamples of deep seated movementinclude, collapses of coastal cliffs, largeslumps which occasionally form atemporary blockages in rivers andstreams, and regional landslides wheregeological formations slowly collapsethrough an area several square kilometresin extent.


The processes by which deep-seated massmovement occur are the same as thosealready described for shallow massmovement in Chapter 3. The distinctionis one of scale – in addition to failing atdepth in the regolith, deep-seated massmovements extend over many hectaresof land; in extreme cases, several squarekilometres. Their impact on land use issimilarly large-scale. New instances offailure are uncommon, but the NewZealand landscape is dotted with historic(and prehistoric) failures where ongoingmovement restricts use of the land forfarming and forestry or threatens roadsand railways.

Deep mass movements may bedifferentiated into three forms: slides (or

block glides), slumps, and flows. Thedistinctions relate to feasibility ofstabilisation, and what types of measureare worth attempting. Crozier (1986)gives further information about the threeforms, illustrated by examples from theNew Zealand landscape.

4.2.1 Slides (block glides)

A slide is a large block of soil andregolith, which moves sideways along asloping shear plane in underlying rock.Most of the moving mass stays intact,though its edges break up into smallerblocks and fragments. The best-knownexample in recent years has been theAbbotsford slide, which left housesseverely damaged at its upper edge wherethe sliding block separated, but more-or-less intact on other parts of its surface.Elsewhere in the country, it is notunknown for farming to continue onslowly moving block glides, so long asfences and tracks can be adjusted acrosstheir edges.

4.2.2 Slumps

Slumps are made up of several blocks ofsoil and regolith, which move in an arcabove a curved shear surface (rotationalfailure plane) in underlying rock. Themoving mass is separated from the slopebehind by a high vertical rupture or


CHAPTER 4

Deep-seated Mass Movement

– slides, slumps and flows

Slump during Cyclone Bola. Photo: Landcare Research.

“headwall”. It breaks into an unevensurface of backward-tilted segmentsseparated by lower ruptures or “scarps”.Drainage is disrupted, and swamps orpond develop in depressions betweeneach segment and scarp. A well-knownexample is the Ruru slump at Tinui inthe Wairarapa, which is slow-moving andhas been farmed for over 60 years.

4.2.3 Flows

Flows are made up of soil and regolith,which moves by inter-particle or inter-layer shear above a failure plane inunderlying rock. The failure plane maybe either planar or curved. The surfacebreaks into hundreds of hummocks,roughly aligned as arcuate ridgestransverse to the direction of flow, andseparated by tension cracks which formlow scarps. Drainage is extremelydisrupted. A typical example is thelandscape either side of Highway 1 atKilmog Hill, north of Dunedin.

4.2.4 Extent of deep-seated mass

movements

The initial failure of a deep-seated slide,slump or flow is rapid and catastrophic.Many are triggered by earthquakes,although heavy rainfall or basalundercutting by streams are othercommon triggers. The mass movementdebris semi-stabilises and revegetates,sufficiently for indigenous forest toestablish within several hundred years.However, underlying regolith anddrainage remain disrupted, creating theright conditions for secondary failure.Secondary failure usually happens at aslow rate, on patches of the debris ratherthan all of it, in some years but not everyyear.

Deep-seated slides, slumps and flows arewidespread in New Zealand’s tectonicallydisturbed landscape. The 109,900

hectares of terrain recorded as susceptibleto slump erosion by NZLRI (Appendix II)is at best a partial indication of theirextent; mapping recorded slumps on softmarine sediments but excluded deep-seated mass movements on the hardergreywackes and metamorphic rocks.Institute of Geological and NuclearSciences (IGNS) has recently compiled anationwide list of deep-seated massmovements greater that 1,000,000 cubicmetres in volumes. The data used for thelist have come from geological maps andinspection of aerial photographs(McSaveney et al 1998). The IGNS maps,though currently unpublished, areavailable on request.

4.3 Conservation measures to

control erosion on deep-seated

mass movements

Secondary failures, on the surface ofexisting deep-seated mass movements,may be stabilised by a mix of vegetative,soil drainage, and runoff controlmeasures.

Stabilisation of primary failures isdifficult and sometimes impossible.Engineering techniques are essential tosuccess and have rarely been attemptedin New Zealand’s sparsely populatedlandscape – only at a few places, wheredeep-seated mass movements threatenlines of communication or urban areas.Consequently, there are no standardpractices available for inclusion in Part B.

Practices are better-developed in severalcountries overseas, where the landscapeis susceptible to deep-seated slopefailures, and dense populations need tobe protected. New Zealand’s geotechnicalengineers generally adapt the overseaspractices, as and where needed here.Interested readers are advised to obtaininformation from Japan, Taiwan, HongKong, the United Kingdom, France,Canada or the United States; these beingthe countries where relevant engineeringpractice appears most advanced.


Deep earthflow, hill country cleared of bush, near Karitane,

Otago. Photo: D Hicks.


Table 4.1 Control techniques for deep-seated

mass movements on different

geomorphology.

4.5 References

Crozier, M. J. 1986: Landslides. 212pp,Croon Helm, London.

Hicks, D. L. 1995: Control of Soil Erosionon Farmland, MAF Policy TechnicalPaper 95/4

Maunsell, J.R., Slump Control, FP45,Unpublished report held byWellington Regional Council,Masterton

Painter, D J 1973 Prevention and Controlof Mass Movement and GullyControl, NWASCO 1973

Smith, R K. 1973 Prevention and Controlof Mass Movement and GullyControl, NWASCO 1973


Erosion TypeGeomorphology

Crushed Argillite Other Sedimentary Rocks

Erosion control forestry Pole planting

Flows Dewatering Dewatering

Debris dams

Dewatering Dewatering

Slides Pole planting Pole planting

Erosion control forestry Erosion control forestry

Slumps Dewatering Dewatering

Pole watering

Descriptions of the various practices for control of deep-seated mass movement are found in chapters/sections of Part B:

3.2 Fencing Management for Erosion Control – Hill Country


10 Dewatering Techniques for Deep-seated Mass Movements

12 Pole Planting

13 Erosion Control Forestry

Chapter 5 Gully Erosion

5.1 Introduction

Gully erosion is “the removal of soil orsoft rock material by water, formingdistinct narrow channels, larger thanrills, which usually carry water onlyduring and immediately after rains”(Bates and Jackson 1980).

Tunnel gully erosion is “a compounderosion form initiated by the subsurfaceconcentration and flow of water,resulting in scouring and the formationof narrow conduits, tunnels or pipes.Soluble, dispersive or low strengthmaterial is removed, ultimately resultingin collapses, visible either as ‘holes’ inthe land surface, or as gullies developedby the collapse of pipes – followed bycontinued erosion” (Eyles 1985). Tunnelgullies are also called “tomos’”or “under-runners”.

5.1.1 History of gully control

Many gullies occur naturally in the NewZealand landscape. They are oftenformed as a result of geologicalphenomena such as volcanic eruption orfaulting and crushing of marinesediments. Over long periods of time,these natural gullies often becomestabilised as the channel reaches a stablegradient, the side walls of the gullyachieve a stable slope, and the naturalvegetation becomes well established.

During the nineteenth and twentiethcenturies, when large areas of land werecleared for pastoral development, thechanges in runoff patterns and areas ofexposed ground often resulted in oldgullies reactivating, and new gulliesbeing initiated. Sometimes the erosionoccurred quickly, immediately followingthe development phase. In othersituations, a number of years or evendecades passed before the erosionbecame evident.

Following the enactment of the SoilConservation and Rivers Control Act1941, the Soil Conservation and RiversControl Council (Soil Council) wasestablished, and catchment boards wereset up around New Zealand to promotesoil conservation, prevent or mitigate soilerosion, and prevent damage by floods.

“In the early days of the Soil Council,soil conservation in the southern half ofthe North Island meant gully control”(McCaskill, 1973).

While the principles of gully controlwere reasonably well understood, theearly soil conservators had to developspecialised practices for different parts ofNew Zealand (often by trial and error) toachieve success. From the outset, it wasrecognised that vegetative protection(trees and ground cover) provided themain long term method of control.

A historical account of gully controlpractices, initially developed in SoilConservation Reserves before being usedfor subsidised soil conservation work onprivate land, is given by McCaskill (1973)


A gully is a distinct channel, carved intoa hillslope or valley bottom byintermittent or ephemeral runoff. Suchchannels are carved where the forceexerted by flowing water – a function ofits mass and velocity – exceeds thesubsoil’s resistance – a function of gravity(holding it to the slope) together withproperties of its surface particles (bindingthem together, and resisting the tractivestress of water). (A definitive account ofthe physical processes which form gulliescan be found in Leopold et al, (1964),Chapter 10-11.)

The processes – removal, transport anddeposit of particles by flowing water – areexactly the same as for surface erosionand stream erosion. What makes gullyerosion distinct is the scale andfrequency at which processes operate:

• sheetwash or rilling – in topsoil;ephemeral during rain

• gullying – in subsoil; intermittentduring and after rain

• stream bed scour and bank collapse –in alluvium or weathered rock,continuous but slight during lowflows; intermittent but severe duringfloods.


CHAPTER 5

Gully Erosion

There are four main forms of gully,although some gullies may be acombination, gully erosion often occurseither in association with one or moreother erosion types. These types are U-shaped, deep-seated, tunnel andmountain gullies.

5.2.1 U-shaped gullies

U-shaped gullies are formed in loose tonon-cohesive, uncompacted lithologies.They are initiated by channel gullyingand exacerbated by waterfall gullyingand even. They tend to form a box or Ushape.

U-shaped gullies develop in clearlyidentified stages. When overland flowoccurs during or following rainfall,stormwater runoff is concentrated intodepressions. Rill erosion is initiated whenthere is downward scour of the topsoil.This may occur where the surface is lessresistant to particle detachment or whereflow velocity is increased by an abruptgradient change. Rills deeper than 60 cmdeep and wider than 30 cm are classifiedas gullies (Brice, 1966).

As the downward cutting continues, agully head forms and recedes upstream;as the gully deepens and widens there islateral erosion of the gully sides. Awaterfall may develop at the gully headwhere overland stormwater flow plungesfrom the ground surface into the erodingchannel. Sometimes there are a series ofsmall waterfalls in the channel itself. The

depth to which a gully will erodedepends on the subsoil. Downwardcutting will continue until a moreresistant layer is reached.

5.2.2 Extent of U-shaped gullies

U-shaped gullies occur on weatheredrecent sands, on unconsolidated(alluvial) Pleistocene sands, onQuaternary tephra deposits, and ongravels. This form of erosion iswidespread throughout New Zealandwhere soils and soil structure aresusceptible to fluvial erosion. It will oftenoccur when surface vegetation is in poorcondition and stormwater runoff isconcentrated.

U-shaped gullies commonly occur onloose, non-cohesive or uncompactedsoils such as loess, pumice, volcanic ash,alluvial sands and gravels. U-shapedgullies may form on very flat slopes oncethe gully process is initiated by a smallabrupt gradient change or nick point.Trials at Rerewhakaaitu havedemonstrated that the gully erosionprocess can be initiated onunconsolidated exposed ash soils whereslopes are greater than 7°.

5.2.3 Deep-seated gullies (V-shaped

gullies)

Deep-seated gullies form where hillslopewatercourses cut through surficialregolith into unstable rock beneath. Theyare common on three kinds of hillcountry:

Soft marine sediments: Here theprocess is entirely physical. Thesediments are at best weakly cementedby calcite and are often just indurated(compacted). Where vegetation isremoved from an ephemeral watercourse– for instance by land clearance forfarming or forestry – there is no obstacleto entrainment of sediment grains byrunning water. The watercourse cuts adeep trench headwards and its over-steepened sides collapse. Sediment isdelivered into the permanent stream atthe slope foot and re-worked along itscourse through the valley bottom. If thequantity of sediment is large, it aggradesas a fan (at the gully mouth) or a terrace(in the valley bottom). Such gulliesusually stabilise as narrow, deep slits in ahillslope. Occasionally they develop intobranching networks that dissect theentire slope.


U shaped gully on alluvial tephra deposits. Photo: Environment BOP.

Cemented marine sediments: Thisprocess is both physical and chemical.Alternate wetting and dryingdisaggregates weathered rock exposed inthe bed of an ephemeral watercourse.Calcite cement is leached by weakly acidsoil water, percolating towards the bed.Once calcite bonding is lost, any swellingclay minerals (bentonite andmontmorillonite) expand. Leaching andswelling may be accelerated if pyrite(small quantities are present in somemarine sediments) renders soil watermore acid than normal. The watercoursebegins cutting into the loosened rock; asit cuts down, fresh rock is exposed tophysical and chemical weathering.Downcutting often triggers slips orearthflows around the gully’s sides andhead.

If such a gully is stabilised by vegetationwhile incipient, it remains narrow and V-shaped. If not, it develops into abranching network of channels incisedtens of metres into “badland” terrain,several hectares in extent.

Crushed rocks: Where rocks have beencrushed by movement along faults or byintense folding during uplift, soil watercan percolate deep into the crushed rockand weathering is more rapid. If awatercourse crosses such a crush zone, itis eroded much more easily than thehard rock to either side.

Such a gully may remain small, deep andnarrow where hard rock downslope barsdowncutting or hard rock upslopeconstricts backcutting. Where a crushzone extends through a slope, the gullymay extend over several hectares andcoalesce with a similar feature on theother side of a ridge.

There is no single New Zealandpublication which gives a comprehensiveaccount of deep-seated gullies. Somerelevant descriptions are found in reportsby Gage and Black (1979), De Rose et al(1998), and Phillips, Marden and Miller(2000)

5.2.4 Extent of deep-seated gullies

These gullies occur throughout NewZealand on steep hill country or colluvialslopes. In the North Island, they areconfined largely to crushed argillite,jointed mudstone, and jointed finesiltstone in association with earthflowand soil slip erosion forms. In the SouthIsland, most deep-seated gully erosion ison greywacke and sub-schists on

colluvial slopes, in association with sheetand scree erosion.

V shaped gullies can occur in thefollowing situations;

• as finger gullies in Tertiary and lowerCretaceous deposits

• on mudstone hill country inassociation with soil slip, slump, andearthflow erosion

• on crushed argillite hill country asbifurcated or amphitheatre gullies

• On greywacke or schist hill countryas shallow talus colluvial material,deeper greywacke which may havebeen structurally altered or have ahigh argillite content, or as weatheredgreywacke.

5.2.5 Tunnel gullies

These gullies are formed in variablelithologies from a concentration ofunderground water flow above a lesspermeable soil layer. The gully process isinitiated by underground ‘piping’ aslateral flow of soil water entrainsparticles. The pipes enlarge to formtunnels that may ultimately collapse toform gullies.

Tunnel gullies form in situations wherethere is a variation in permeabilitywithin the soil profile, and water is ableto infiltrate into the subsurface layers.Subsurface water is concentrated wherethere is non-cohesive or soft lithologyoverlying more resistant layers. The flowof water along the resistant layer initiates‘piping’, where the soils are soluble,dispersive or prone to fluvial erosion.The ‘piping’ erosion process formstunnels. As the tunnels enlarge, they cancollapse to form holes or continuousgullies as surface erosion features.


V shaped gullies on crushed argillite country.Photo: Gisborne DC.

Following collapse of the tunnel gully,the normal gully erosion processcontinues.

5.2.6 Extent of tunnel gullies

Tunnel gully erosion occurs onsusceptible soils and lithologythroughout various parts of NewZealand. Going by the NZLRI data,tunnel gully erosion is more extensive inthe North Island, and less extensive butmore severe in the South Island.

Gully erosion is found in the NorthIsland on the strongly weatheredsandstone downland and hill country ofNorthland and the colluvial slopes of thetertiary hill country. Tunnel gullies havebeen recorded in Northland,Coromandel, Waikato Basin CentralVolcanic Plateau, Manawatu-Wanganuihill country, Hawkes Bay and theWairarapa. It occurs on the followingrock types:

• In soils derived from stronglyweathered sedimentary rocks (humidclimate)

• On tephra deposits (cool humidclimate)

• On colluvial footslope deposits onsedimentary hill country (humidclimate)

In the South Island, tunnel gully erosionoccurs mainly on loess hill country soils,and has been noted in Northeast

Marlborough, coastal North Canterbury,Banks Peninsula, SouthCanterbury/North Otago Downlands,Otago Peninsula and Mid-Clutha Valley.It occurs on loess-mantled slopes andmixed loess/colluvium slopes withyellow-grey earth soil groups (pallic soils)in sub-humid to semi-arid climate.

5.2.7 Mountain gullies

These form where debris avalanche scarsand debris flow deposits are eroded byrunoff. They are found in the greywackeand schist mountain lands of bothislands; and are present in low forestedranges (below 1000 metres a.s.l.), as wellas the alpine tussock lands.

Chapter 6, Mountain Lands Erosion,describes mountain gullies, noting thatthey are a natural phenomenon andnearly impossible to control.

The NZLRI map does not show deep-seated gullies’ location separately fromthe other forms. They are concentratedin distinct regions, and on distinctgeological formations:

• soft marine sediments: sand of theKaipara and Manukau barriers; sandsand silts of the dissected terracecountry extending from Hawera toPohangina; pockets of sandy gravelmarginal to the hills of Wairarapa,Marlborough and North Canterbury;dissected glaciofluvial gravels ininland Canterbury and Otago

• cemented marine sediments:mudstone formations containingswelling clay, throughout Gisborne,Hawkes Bay and Wairarapa; also smallpockets in Marlborough and NorthCanterbury.

• crushed rocks: argillites of lateCretaceous and early Tertiary age,marginal to the Raukumara Range(Gisborne), Urewera Range (northernHawkes Bay) and Haurangi Range(eastern Wairarapa).


5.3.1 U-shaped gullies

• Control stormwater runoff over thegully head

• Control stormwater runoff throughthe gully floor


Tunnel gully. Photo: M. Tuohy.

• Stabilise the gully head

• Stabilise the gully floor

• Stabilise the gully sides.

The following practices are used incontrolling U-shaped gullies:

• Control of runoff over gully head oraway from the gully head. There is awide range of options includingdiversions, flumes, pipes and dropstructures.

• Reduction in peak runoff rates incombination with stabilising gullyhead and controlling runoff overgully head.

• Planting of gully head (small gulliesonly)

• Planting of strong points at criticallocations.

• Retirement of gullies in associationwith runoff control.

• Ground contouring to “smooth out”small gullies on low terraces (incombination with runoff control andsurface vegetation).

Where the soils or geology is susceptibleto u shaped gully erosion, the followingmatters should be considered to ensurethat gully erosion is avoided:

• Avoid exposure of bare ground,particularly on overland flow paths

• Avoid concentration of stormwaterrunoff as far as practicable

• Avoid digging drains to dischargepounded water where the soils arenon-cohesive and susceptible tofluvial erosion, and the dischargepoint is uncontrolled

• Avoid uncontrolled discharge ofstormwater over steep drop-offs

• Reduce peak flood flows withincatchments by using flood detentionstructures and vegetation as far aspracticable

• Control stormwater runoff so thatoverland flow paths are well stabilised

5.3.2 Tunnel gullies

• Divert and control overland flow tocontrolled surface outlets

• Retain a dense ground cover onoverland flow paths

• Stabilise by planting poles directly incollapse holes, and also in betweenthem.

When tunnel gully erosion problems areonly slight to moderate, and occur asholes or depressions, remedial works canbe undertaken by pole planting usingoversized willow poles planted directly inthe tunnel gully hole. Where the erosionfeature has formed collapsed gullies, pairpole or staggered pole planting can becarried out. In some cases, seedling treeswithin protectors may be used as analternative to poles.

In more severe situations where thetunnel gullies become totally collapsedand exposed, and the gully erosioncontinues to worsen, more extensiveworks are required to remedy theproblem. In these situations, worksinclude diversion and control ofoverland flow to controlled surfaceoutlets, and revegetation of dense stableground cover. Contouring to infill gulliesin association with runoff controls mayalso be an option. Other revegetationoptions are also likely to be required.These include tree planting (includingspace planting or afforestation) andtemporary or permanent retirement ofthe affected area from grazing.

Attempting to control tunnel gullies byripping and compacting the tunnels, willonly provide temporary relief, as the soilsare inherently susceptible to tunnelgullying, and this method of treatmentonly addresses the symptoms of theproblem, not the cause.

Preventative works to control tunnelgully erosion are normally onlyemployed when there is evidence oftunnel gully erosion occurring. In thesesituations, space planting of willow orpoplar poles, or protected seedling treesare common methods used to addressthe problem. Planting can be carried outfollowing the line of the gully erosion ifit is evident. In Northland, coral tree(Erythrina spp.) poles can be planted asan alternative to willow poles.

In areas where the soils are known to besusceptible to tunnel gully erosion,


normal sustainable farming practicessuch as maintaining a dense pasturesward, controlling animal and plantpests, minimising bare ground etc shouldbe used to reduce the risk of tunnel gullyerosion occurring.

5.3.3 The key principles to control V

shaped gullies include

• Control stormwater runoff from thesurrounding catchment

• Maintain negative pore water pressurein soil and rock, around the gully’shead and sides (de-watering)

• Increase shear strength insurrounding soil and rock (tree rootreinforcement)

• Stabilise the gully floor (debris damsor live dams).

Control of V-shaped gullies and theassociated erosion that normally occurswith the gully erosion, is largelydependent on control of the erodingchannel floor. Once control of this hasbeen achieved, then other works tocontrol erosion on the sides of the V-shaped gully can be used. This oftenrequires using a number of practices inconjunction with each other, or insuccession. The main approaches, inorder of importance, are stabilising thegully floor, stabilising the sides of thegully system or reducing the peak runoffthrough the gully system

The Ministry of Works have successfullydemonstrated at Kaitangata Station(Gisborne District) that gully control canbe achieved by using runoff control asthe initial control measure, inconjunction with a range of otherpractices (Hall 1970).

In almost all situations, long-termcontrol will rely on stabilisation of theunstable land mass. This is generallyachieved through some form of treeplanting. There are a number of plantingoptions that may be used. Tree plantingoptions include pole planting of theimmediate gully toe slopes using poplaror willow pole material, wide spaceplanting of trees on the gully andadjacent slopes with grazing underneath,or afforestation over a reasonableproportion of the gully catchment.

Because V-shaped gullies normally occurin conjunction with other types of

erosion, preventative works are aimed toaddress a range of erosion forms on theland susceptible to gullying.

Many of the V-shaped gullies have beenformed following the removal ofindigenous vegetation on steep land andthe conversion of the land to pasture. Apastoral land use may not be sustainablewithout intensive soil conservationplanting, and often some form ofafforestation may be the only sustainableland use option to avoid or mitigateerosion problems.

When these susceptible areas areafforested, the tree roots help bind theland, and evapotranspiration from thetrees pumps water from the soil toreduce the volume and rate ofstormwater runoff. A dense vegetativeground cover (particularly on the gullyfloor) helps to ensure a stable channel,which also assists in reducing the risk ofgully erosion.

Therefore, the key to prevention orstabilisation of V shaped gullies is theestablishment of vegetation, particularlytrees.

5.3.4 Others

Stabilisation of debris avalanche/flowgullies requires engineering measures.Their cost is high, and can only bejustified where roads or railways passthrough the ranges. Even here, suchmeasures are rarely attempted. Elsewherein the mountains, the gullies are simplyleft to revert; a process which will takedecades if not hundreds of years.

Ground contouring is sometimes used inconjunction with runoff control andsurface revegetation on U-shaped gulliesand tunnel gullies. While the costs ofrecontouring can be high, the land canbe brought back into a more productiveuse. The contouring operations alsoprovide an opportunity to directstormwater runoff to a safe controlledoutlet.



Table 5.1 Control techniques for different

gully forms.


Gully Types Control Principles

U Shaped Gullies Runoff Control Systems

Tree & Pole Planting

Surface Protection: Vegetative and Non-Vegetative

Mechanical Infilling or Contouring

V Shaped Gullies Runoff Control Systems

Structures to Stabilise Gully Head/Floor



Tunnel Gullies Runoff Control Systems



Mechanical Infilling or Contouring

Descriptions of the various practices for control of deep seated Gully Erosion are found in chapters/sections of Part B:

3.2 Fencing Management for Erosion Control – Hill Country

3.3 Fencing Management for Erosion Control – Mountain Land


11 Gully Control Structures

12 Pole Planting

13 Erosion Control Forestry

5.5 References

Bartleet JR 1965: The Role of Contouringin the Conservation Programme ofNorth Auckland. Soil conservationpaper from the ConservationSubsection of the 11th New ZealandScience Congress, Auckland, 11-17February 1965.

Bates RL; Jackson JA 1980: Glossary ofGeology 2nd Edition. AmericanGeological Institute, USA.

Bennett HH 1947: Elements of SoilConservation 1st Edition. New Yorkand London, McGraw-Hill BookCompany Inc.

Borlase OM 1967: Gully Control byCheck Dams – Poverty BayCatchment Board Report No.1805,First Revision 1967.

Brice JC 1966: Erosion and deposition inthe loess-mantled Great Plains,Medicine Creek Drainage Basin,Nebraska. Geological SurveyProfessional Paper 352H. UnitedStates Department of the Interior.

Cathcart RW 1998: Soil ConservationFact Sheet 2; Gully Control inNorthland. Unpublished fact sheetfrom Northland Regional Council.

De Rose, R. et al 1998: Gully erosion inMangatu Forest, New Zealand,estimated from digital elevationmodels. Earth Surface Processes andLandforms, v23, pp1045-1053.

Environment BOP 1998: SustainableOptions Fact Sheet SC10/98.Detention Dams and Drop Structures.

Eyles GO 1983: The distribution andseverity of present soil erosion in NewZealand. New Zealand Geographer39(1):12-28.

Eyles GO 1985: The New Zealand LandResource Inventory ErosionClassification. Miscellaneous Waterand Soil Publication Number 85,National Water and Soil ConservationAuthority.

Eyles GO 1993: Gully erosion controltechniques for pumice lands. ManaakiWhenua Landcare Research NewZealand Limited, Lincoln NewZealand in association withEnvironment Waikato.

Eyles GO: Towards a New ZealandErosion Form Classification.Unpublished notes on types of gullyerosion.

Gage, M. and Black, R. 1979: Slopestability investigations at MangatuState Forest. Technical Publication 66,FRI.

Gair J 1973: Pole and netting dams. Apaper presented as part of a seminar“Prevention and Control of MassMovement and Gully Erosion”,arranged by Ministry of Works inconjunction with New ZealandAssociation of Soil Conservators andPoverty Bay Catchment Board, 15-19April 1973.

Hall, J. de Z. 1986: Water control works,Gisborne-East Cape region, 1969-1979. Unpublished report, Water andSoil Division, MWD.

Howie WR 1968: A Design Procedure forGully Control by Check Dams, Waterand Soil Division, Ministry of Works,New Zealand May 1968.

Humes Industries Limited: DrainageProducts Technical Manual

Leopold, L. B., Wolman, M. G. andMiller, J. P. 1964: Fluvial processes ingeomorphology. 522pp, Freeman &Co, San Francisco.

Lynn IH; Eyles GO 1984: Distributionand severity of tunnel gully erosionin New Zealand. New Zealand Journalof Science Vol.27 175-186.

McCaskill LW 1973: Hold This Land, AHistory of Soil Conservation in NewZealand. Published by AH & AWReed, Wellington, Sydney, London.

NWASCO 1973: Prevention and Controlof Mass Movement and GullyErosion. Proceedings of a seminararranged by Ministry of Works inconjunction with New ZealandAssociation of Soil Conservators andPoverty Bay Catchment Board, 15-19April 1973.

NWASCO 1975-79: ‘New Zealand LandResource Inventory Worksheets’,1:63,360. National Water and SoilConservation Organisation,Wellington, New Zealand.

Phillips, C., Marden, M. and Miller, D. E.2000: Review of plant performancefor erosion control in the east CoastRegion. Contract Report LC9900/111to MAF, Landcare Research.


Poverty Bay Catchment Board 1978:Report of Land Use Planning andDevelopment Study for Erosion ProneLand of the East Cape Region: Section1 The East Coast (Red Report), May1978.

Wilkie DR: Restoration of Hill Countryby Conservation Farming, WitherReserve, Blenheim. Bulletin No.8, SoilConservation and Rivers ControlCouncil.


Chapter 6 Mountain Lands Erosion

6.1 Introduction

Mountain land erosion includes needleice (frost heave), sheet wash, wind blow,periglacial creep of soil and rockfragments, together with erosion ofgullies by debris avalanches, debris flowsand torrents.

Natural or geological erosion has beendefined as the gradual wearing away ofthe land by water, wind and gravity. It issometimes called “normal” erosion butthis term is open to criticism on thegrounds that in nature normality isdifficult to define. In mountain landsnatural erosion becomes increasinglyprevalent at higher altitudes.

“Induced” (or “accelerated”) erosionresults from the disturbance of theenvironment by human activities.(“Induced” is the better term.) Formanagement, it is important todistinguish between natural and induced,but there are many situations wherethere is no clear answer as to whetherthe erosion is induced or part of thenatural cycle.

Spectacular mountain land erosion is nota recent development in New Zealand.Strange (in 1850) and Colenso (in 1884)climbed to various high points on bothislands and described various landslidesand severe erosion.

The short tussock grasslands of the highcountry are a recent vegetation type.Three thousand years ago the highcountry of both island was a mosaic offorest, scrub and tall tussock. Shorttussock was present as a successionalplant community on small areas burntby natural fires, such as those resultingfrom lightning strike or volcaniceruption. There was a sudden increase infire frequency 900 years ago when theMaori arrived, and this continued, inparticular, when Europeans arrived.Because of the fires, much of the lowlandand montane forests was replaced byshort tussock grassland.

In the 1850s and 1860s, settlers broughtherbivores such as sheep, cattle, deer,chamois, thar, goats and rabbits to this

grassland. Regular fires continued as partof the European land managementpractice and although regulated by 1913legislation, periodic burning continuedunabated until the Soil Conservation andRiver Control Act in 1941 enforced firecontrol. From the 1870s through the1940s, rabbits proliferated through thetussock. Too-frequent burning, followedby too-heavy stocking, followed byrabbits, depleted the short tussockgrasslands; and to a lesser extent, thehigh-altitude tall tussock. Topsoil wasexposed to surface erosion by ice, waterand wind.

6.1.1 History of research

One of the earliest accounts of soilerosion in the high country (Gibbs andRaeside, 1945) showed that a quarter ofthe land was extremely eroded, with lessthan 50 percent of the topsoil remaining,and that only 20 percent of the area hadslight or no accelerated erosion. Gibbsand Raeside calculated that the surveyedsoil depletion amounted to a total loss ofabout 1.5 billion tonnes of soil. Researchin the last four decades has increased ourunderstanding of the types, magnitude,frequency and triggers of erosion in theNew Zealand mountain lands. Much ofthe work, however, has been into the


CHAPTER 6

Mountain Lands Erosion

Screes and gullies on grazed tussock rangeland, Torlesse

Range, Canterbury. Photo: D Hicks.

larger discrete forms like debrisavalanches and mountain torrents,rather than the more extensive formsproduced by frost heave and wind(Whitehouse 1984).


of erosion in the mountains

6.2.1 Frost heave (needle ice formation)

Needle ice formation will be defined hereas a separate erosion process whichenables particles to be removed later bysheet and wind erosion process. Theneedle ice process is more severe wherebare ground exists and is found in allmountain areas especially higher than900 m above sea level. Ice pulverises thetop of exposed soils and on drying thematerial is easily removed by wind orrain.

Gradwell (1960) and Soons andGreenland (1970) observed needle iceand noted that the growth of ice needlesdepends mainly on the rate of heat lossand the availability of water. Haywardand Barton (1969) used a movie film torecord the disruptive action of needle iceand showed 22 freeze-thaw sequencesmoving surface material up to one metredown slope. Subsequent advances in ourknowledge of this slow erosion,

operating in the Southern Alps have notbeen extensive (Whitehouse, 1984).

O`Loughlin (1984) reports slopewashrates of 150 t/km2/yr measured on baresubsoil surfaces exposed to frost heave inHut Creek, Craigieburn Range. Followingplanting of Pinus contorta on the site,soil loss declined rapidly from tree age 4,until at tree age 10 soil loss wasnegligible.

Altitude and climatic zonation plays asignificant role in the major process ofneedle ice formation where we find frostheave occurring in the semi arid inlandbasins of Otago and Canterbury from aslow as 300-350 m above sea level,particularly where winter frosts andpermafrost occur. Frost heave commonlyoccurs near Alexandra and theintermountain basins of Canterbury –that is, the Mackenzie Basin, wherealtitude can vary from 350 m (flats andterraces) to 1500 m, or to the tops of themountains.

6.2.2 Sheet erosion

Sheet erosion is the removal of a thinsurface layer of topsoil or subsoil by alayer of water flowing over the land. Theimpact of raindrops on the exposed soilbreaks up the surface layer and loosensparticles, which can then be moved. If itcontinues for some time an infertilestone pavement results.

Soil, defined as particles less than 2 mmin diameter in Hayward’s study (1969),appears to be eroded from surfacescovered with rock particles (scree anderosion pavement) at about (0.012 +/-0.004 kgm2/yr. This is some four timesfaster than from totally vegetated sites(0.0028 + 0.0005 kg/m2/yr and somethree times less than from non-vegetatedsurfaces other than scree.

6.2.3 Wind erosion

In the mountain lands wind erosion iswidespread on exposed tops, sunny faces,and areas that have had the vegetativecover denuded. It is possibly morepronounced in the South Island due tothe persistent and often prolonged NorthWest foehn winds.

Wind moves soil particles by theprocesses of deflation and abrasion. (The chapter on wind erosion gives afuller description of these processes.) The amount of soil in transit in the windhas been measured at six sites in the


Fig 6.1 Diagram of common forms of

mountain land erosion in New Zealand (after

Cunningham, 1978)

Rakaia, Waimakariri and AshburtonBasins by Butterfield (1971), using 7-cmdiameter traps located at various heightsabove the ground. Over the year ofmeasurement, wind erosion was greatestin early spring to early summer with thelowest levels of erosion occurring duringmid summer (January) and in winter.Because of the difficulty, wind erosionrates have not been measured in thehigh country since Butterfield’s study.

Recent research work using caesium-137(Hewitt, 1996) into surface soil losses inCentral Otago on the lower foothills andterraces near Alexandra (300–320 mabove sea level) has shown that a sunnyside slope lost 50 percent of its expectedcaesium-137 input through erosion oftop soils. This was equivalent to a loss of3.4cm of soil over 40 years. Shady sideslopes and foot slope sites gainedcaesium-137 through net soil depositionmainly from the northwest winds. Thisinformation indicates that the surfacesoil losses are not as severe as firstthought (even though bare ground maybe significant at certain times, such asafter a drought or rabbit infestation).

Frost heave, sheet erosion and winderosion are the only mountain landerosion processes for which soilconservation would normally beattempted. While natural, they havebeen accelerated at low to middlealtitudes by 150 years of vegetationdisturbance – burning of tussockgrassland, followed by sheep grazing,followed by rabbit infestation, followedby weed colonisation. In these zones, theerosion can be reduced by avoidance orrestriction of burning, revegetation ofdepleted tussock, lenient grazingmanagement, and rigorous control ofpests and weeds.

Other erosion processes occur on alpineslopes and ridges, where soilconservation would not normally beattempted. Techniques for their controlare beyond the scope of this handbook.They are described here just to remindreaders that these processes areencountered in the mountains; arenatural; and are difficult to control.

6.2.4 Periglacial erosion

At high altitudes, soil in the mountainsis extensively disturbed by periglacialerosion processes. Frost heave, alreadydescribed, is one of these. Repeatedfreeze-thaw cycles sort the skeletal soilinto “patterned ground”, with stone

rings or stone stripes separating patchesof silty or sandy soil.

Soil creep is a slow, plastic flow ofsurficial soil and rock debris downslope,creating solifluction terraces orsolifluction lobes. At high altitudes it isdriven by changes in stress andresistance, as the soil alternately freezesand thaws. Scree creep is a relatedphenomenon, occurring on talus slopesbeneath bluffs. The talus initiallyaccumulates from rockfall or debrisavalanches. Where vegetation canestablish, it stabilises; but where itcannot (or if it is breached), theshattered pieces of rock may be slowlyshifted downslope by freeze and thaw ofwater in the cracks between.

6.2.5 Debris avalanches

Debris avalanches are slide-type massmovements of soil or rock on steepmountain slopes. They are triggered bythe same mechanism as shallowlandslides (soil slips) – see Chapter 4 –and start in a similar position, a hollowwhich concentrates soil water drainingfrom farther upslope. Once triggered, themountain slope’s steepness ensures thatlandslide debris, instead of coming torest on the slope, keeps moving. Slopelength enables the debris to build upmomentum sufficient to start scouringaway vegetation and soil in its path. Theresult is a long, narrow scar stretchingfrom upper slope to slope foot, with ajumbled deposit of soil, rock and timberat the base. Repeated debris avalanchingmay build up a talus cone in the lowerpart of the scar.

6.2.6 Debris flows

A debris avalanche may transform into adebris flow as it descends. This happensif the debris incorporates sufficient wateror air in its downward passage to turn itto fluid. The fluidised debris keepsmoving across the valley floor instead ofcoming to rest at the slope foot. If thefloor is wide, repeated debris flows willbuild up an alluvial fan, tapering fromand angle of 20° or more at its apex,down to 5 or 6° at its foot where thedebris gives out or settles. If the valleyfloor is narrow, the debris will turn andflow downstream for distances of severalhundred metres until it gives out. Thiswill be at a point where channel gradientis about 5 to 6°, and a graded deposit ofalluvium interspersed with boulders andtimber will be left.


6.2.7 Mountain gullies

A debris avalanche’s linear scar is anatural conduit for runoff from theupper mountain slope above and aroundit. Subsoil is exposed in its sides, andshattered rock in its base. Many suchscars turn into gullies, but it is importantto remember that such gullies aremaintained as much by occasional debrisavalanches during storms as by runoffduring lesser rainfalls. The same holdstrue for gullies in the fans and valley-bottom deposits left behind by debrisflows.

6.2.8 Torrents

A torrent is a permanently flowingstream going down a mountain orflowing along a mountain valley bottom.It differs from normal streams in that itsgradient is steep, typically exceeding 5°and occasionally in excess of 20,interspersed with rapids and waterfalls.

A torrent’s steep gradient, combined withfrequent passage of short sharp floods,and a sediment load of coarse gravel andsmall boulders, give it great erosiveenergy. The bed and banks are eroded inalmost every flood; the small,discontinuous pockets of coarse alluviumare rapidly worked downstream, andsupplied to larger streams and riverswhich exit from the mountains. Whereslopes are susceptible to debrisavalanches and the avalanches can reachthe channels, alpine torrents are alsosubject to passage of debris flows. Thesecause even more erosion and depositionthan the floods.

The definitive studies of mountainerosion processes are published inJapanese, Chinese, German or Italian. Foran English-language summary, refer toScheidegger (1988). Susceptibility of theNew Zealand landscape is summarised inChapters 6 and 19–22 of Soons and Selby(1992).

6.2.9 Conservation measures to control

natural erosion in mountains

Engineering measures cannot prevent theprocesses described above, but they cancontrol their impact; either stopping ordiverting them before they causedamage. There are no standard practicesfor the control of debris avalanches,debris flows or mountain torrents in NewZealand as few highways or railways passthrough the mountains and controlmeasures are rarely tried. Readers are

advised to seek information from one ofthe countries where such measures areextensively implemented due tosettlement and infrastructure in themountains. Overseas practice appears tobe most advanced in Japan, Taiwan,Switzerland, Austria and Italy.

6.2.10 Erosion rates in mountain lands

The most significant aspect of mountainland erosion is the large quantities of siltand material removed from catchmentsand deposited downstream into lakesonto alluvial flood plains, or transportedout to sea. Sediment yields from somebasins in the Southern Alps are amongstthe highest in the world. Yields varyfrom about 100 t/km2/yr for basin in thedry intermountain areas (Twizel andForks Rivers) to about 15,000 t/km2/yr forbasins in the western part of theSouthern Alps (Hokitika, Cleddau andHaast Rivers) (Hicks D.M. et al 1996).

The most accurate measurement ofsediments in Otago comes from FallsDam, an irrigation reservoir on the upperManuherikia River. Six hundredthousand tonnes of silt, clay and sandhave accumulated behind the dam since1935. For an assumed reservoir trap-efficiency of 80 percent, this is removalof 44 t/km2/y, from the area of drainagebasin above the dam (Bishop et al, 1984).This is a denudation rate of 0.02 mm/yrin the driest part of the mountains. Thehighest figures, from basins in thewestern part of the Southern Alps, have alarge margin of errors (Hicks et al 1996).Even after allowing for this, theydemonstrate the enormity of erosionwhere rainfall is between 3000 and10,000 mm a year.


Much of the surficial soil erosion isinsidious in nature, although debris flowsand the like may come only after a majorstorm. The following factors contributeto erosion in the mountains:

• geology and soil type

• altitude, slope and aspect

• climatic influence related to abovemicrotopography, freeze/thaw regimeand growing season

• burning native grassland (especiallysnow tussock and associated species)


and not spelling from stock or a veryhot fire

• overgrazing after a burn

• depleted, open ground cover subjectto the severe climate and otherelements

• low fertility soils with a poor carbonto nitrogen ratio and low organicmatter content

• animal pest population increases byrabbits, hares, possums, goats, thar,chamois and feral deer

• plant pest population ingression(eg,gorse, briar, matagouri and Hieraciumspecies)

The principles of control for surficialerosion are very similar to that for sheetand rill erosion on the lower plains anddownlands but occur in a more diverseclimatic environment. Control measuresoften must be taken at higher altitudes,where plant-growing days are very short.

The primary principle is to manage theland in its present form by maintainingor improving the vegetative protectivecover and the soil health status especiallythe organic matter content. Success inerosion control on these high altitudeproperties can often only be made bychanging land management (such asburning and grazing management)systems over the entire farm unit andnot on a paddock (or block) basis.

There has been considerable researchwork carried out into soil conservationmanagement on the mountain lands inNew Zealand by various organisationssince the 1941 Soil Conservation Act waspassed. This included work by the ForestResearch Institute at Craigieburn and inthe North Island on the Ruahines, byDSIR Grasslands Division, Ministry ofAgriculture, AgResearch, MWD SoilConservation Centres, LandcareResearch, Tussock Grasslands andMountain Lands Institute, The HellabyTrust, various universities, catchmentauthorities (which are now part of theregional councils)are some of these.



Table 6.1 Summary of mountain erosion types and management practice

Type of erosion Factors to Consider for Examples of PracticesManagement Practice (Not inclusive)

Frost heave* Type and condition of present vegetation Retirement fencing

Sheet* Soil type (% topsoil/subsoil left & % Erosion control fenceorganic matter) Cattle-proofingGeology Rotational grazing

Wind* Rainfall, storm events Summer grazingAspect (sunny versus shady) Aerial oversowing and top dressing

Soil creep** Altitude & slopeTiming MulchingPast management (burning, grazing) Erosion control forestryPlant & animal pests Protection forestryLand Use Capability class and land use Retirement plantingTenure & ownership Feed banks(leasehold, freehold, Crown reserve) Feral animal controlAccess Destocking and spellingEconomics of practice Burning management

Streambank*** As above plus: Above as applicable, plus:

Scree * • size of catchment • water diversion

Gully*** • type of regolith • mulching

Debris avalanche** • type of vegetation required • toe stabilisation• erosion rate and ease of repair • engineering structures• spring dewatering

* = surface erosion, ** = mass movement, *** = fluvial.

The geomorphology of the types of erosion above are mostly schist, greywacke and volcanic ash.


Descriptions of the various practices for control of Mountain Land Erosion are found in chapters/sections of Part B:

2.3 Tussock Management for Surface Erosion Control

3.2 (3.3) Fencing Management for Surface Erosion Control – Hill Country and Mountain Land

4.3 Pasture Revegetation – Mountain Lands

6 Burning Management – Mountain Lands

8.4 (8.3) Managed Reversion of Retired Land – Mountain Lands

13.5 Forest Management Practices – Mountain Lands

6.5 References

Ackroyd, P., 1978: Geological history anderosion, Tussock Grasslands andMountain Lands Institute Review no37

Bishop, D.G., et al, 1984: Modernsedimentation at Falls Dam, upperManuherikia River, Central Otago,New Zealand. New Zealand Journal ofGeology and Geophysics 27, 305-312

Butterfield, G. R., 1971: The susceptibilityof high country soils to erosion bywind Proceedings NZ. Association SoilConservators Seminar on CatchmentControl, Massey University: 329-345

Colenso, W., 1884: In Memorium. Anaccount of visits to and crossingsover, the Ruahine mountain range,New Zealand, and of the naturalhistory of that region. DailyTelegraph, Napier

Cunningham, A., 1978: ErosionAssessment in New Zealandmountains, Proceedings ofConference on Erosion Assessmentand Control in New Zealand,published by the New ZealandAssociation of Soil Conservators

Floate, M., 1991: Guide to Tussockgrassland farming booklet Agresearch,Invermay, Mosgiel

Gibbs, H.S., Raeside, J.D. et al 1945: Soilerosion in the high country of theSouth Island, Dept. of Scientific &Industrial Res. Bulletin, no 92

Gradwell, M.W., 1960: Soil frost action insnow tussock grassland, NZ Journal.Science 3: 580-590

Hayward, J. A, & Barton, J. H., 1969a:Erosion by frost heaving, Soil andWater 6(1): 3-5

Hayward, J. A., 1975: The TorlesseResearch Area, Tussock Grasslandsand Mountain Lands Institute Reviewno 31

Hayward, J. A., 1969b: The use offractional acre plots to predict soilloss from a mountain Catchment.Lincoln Papers in Water Resources 7,New Zealand Agricultural EngineeringInstitute, Lincoln College 93 p

Hayward, J. A., 1980: Hydrology andstream sediments in a mountaincatchment. Tussock Grasslands andMountain Lands Institute SpecialPublication 17

Hewitt, A.E., 1996: Estimating surfaceerosion using caesium 137 at a semiarid site in Central Otago, NewZealand, Journal of the Royal Societyof New Zealand Vol 26, no 1, pp 107-118.

Hicks, D.M. et al 1996: Variation ofsuspended sediment yields aroundNew Zealand: the relative importanceof rainfall and geology. InProceedings of Symposium onErosion and Sediment Yield, ExeterIASH Publication 236.

O’Loughlin, C.L.,1984: Effectiveness ofintroduced forest vegetation forprotection against Landslides anderosion in New Zealand steeplands, inSymposium on Effects of Forest Landuse on Erosion and Slope Stability,Honolulu, Hawaii pp 275-280

Owens, I.F., 1967: Mass movement in theChilton Valley, M.A. Thesis,University Canterbury Library, 92pp

Pearce, A.J., and O`Loughlin, C.L., 1978:Hydrological and geomorphicprocesses on Forested lands – aselective overview. pp62-76 in;Proceedings of a Conference onErosion Assessment and Control inNew Zealand. New ZealandAssociation of Soil Conservators

Scheidegger, A. E. 1988: Hazards frommass movements in mountainregions. Pp 21-41 in El Sabh, M. &Murty, T. (eds), Natural and man-made hazards. Reidel Publishing Co.

Soons, J.M., Greenland, D.E., 1970:Observations on the growth of needleice, Journal Water Resources Research,6(2):579-5

Soons, J. M. and Selby, M. J. 1992:Landforms of New Zealand. 531pp,Longman Paul, Auckland.

Strange, F., 1850: The Canterbury Plains,in Canterbury papers no 3 Londonpp77-80

Whitehouse, I.E., 1984: Erosion in theeastern South Island high country- achanging perspective, TussockGrasslands and Mountain LandsInstitute Review no 42

6•7Soil Conservation A

Chapter 7 Sand Dune Erosion

7.1 Introduction

Sand dune ecosystems are very fragileecosystems, which persist underextremely difficult conditions, and theirmanagement has been a challenge toNew Zealand land managers for a verylong time. Dunes are affected by:

• wind, salt, and generally lowtemperatures from cold sea breezes

• the substrate of mobile sand, whichcombined with wind, can “sandblast”seedlings and mature plants, and canbury or expose established plants

• temperatures, whose fluctuations canbe very large, with frosts indepressions or basins during winterbut which can rise rapidly duringcalm sunny days;

• amount of moisture for plant growth,which is often small because themoisture-holding capacity of sand islimited

• summer droughts that can be quitesevere

• nitrogen, which is the most limitingelement because it is easily leachedout.

7.1.1 History of sand erosion control

On conventional sand dunes, thefollowing stages ofreclamation/stabilisation are recognised:

1 establishment and protection of astable foredune parallel with the coast(see section “Foredune”).

2 fixation of the unstable sand areabehind the foredune and furtherinland with binding plants (seesection “Main dune”)

3 establishment of a permanentvegetative cover using grasses andlegumes for pasture development, ortrees for afforestation, or nativeplants for the restoration of reservesand other high value areas (seesection “Main Dune”).

For details on the first three stages, seevan Kraayenoord and Hathaway (1986a)(pages 69-77), and more recent findings,described in the following sections ofthis chapter. Reference should also bemade to the very recent AucklandRegional Council publication, TheCoastal Erosion Management Manual.

As a primary sand stabiliser, marramgrass (Ammophila arenaria) has beengiven most prominence. Although it isan introduced plant, it has proven itselfas a sand dune stabiliser throughout NewZealand. Since the early 1920s, marramgrass has been planted, more recentlywith mechanical planters, to stabilisesand dunes because it is easy topropagate and establish.

The New Zealand silvery sand grass(Spinifex sericeus, previously hirsutus) isalso a true sand binder, found on foredunes (Whitehead, 1964), but it is noteasily propagated (Sale, 1985). Throughthe voluntary assistance provided byBeach Care and Coast Care groups,spinifex has in recent years gained moreprominence in dune revegetationprogrammes. Detailed propagationmethods may be found in regionalcouncil leaflets, such as Bay of PlentyRegional Council Information Brochureno. 6. (See also Bergin, 1999.)

The other New Zealand sand binder,pingao (Desmoschoenus spiralis), isusually found a little further inlandwhere under favourable conditions itforms rather irregular hillocks(Whitehead, 1964; van Kraayenoord andHathaway, 1986a). Hillocks can lead towind funnelling and thus to localisederosion, which is to be avoided. Pingao ismore difficult to propagate than marramgrass, but Herbert and Bergin showedthat it could be used in dunelandrehabilitation (see Bergin and Herbert,1998). Pingao is highly sought after inMaori weaving due to its yellow colour.

7.1.2 The “natural character” of sand

dunes

The preference or desirability of usingnative plants versus introduced plants forsand dune stabilisation to maintain the


CHAPTER 7

Sand Dune Erosion

natural character of dunes continues tobe the subject of much debate (Gadgiland Ede, 1998). In their comprehensivereview of sand dune stabilisationresearch and practice, they say, “Greatersuccess, especially in emergencysituations and remote areas, is likely tobe achieved by preliminary use of exoticspecies to fix nitrogen and establishcontinuous cover, followed by gradualenrichment or replacement with nativeplants. This approach would satisfyrequirements for both erosion controland a return to the natural character ofthe dunes.”

7.1.3 Preventive works

Preventive works focus on avoidance ofactivities that may disturb the fragilevegetation on sand dunes. Whereblowouts have occurred, steps must betaken immediately to remedy this, so asto avoid the problem escalating rapidly.

Human activities, such as walking, canoften be channelled onto stable areasformed by walkways and board walks.But 4-wheel drive and other motorisedactivities are less easily controlled, andbarriers may have to be put in place.Building activities in dune areas shouldbe prohibited where possible; otherwisethey should be strictly regulated inCoastal Hazard Strategy documents andsupervised.

Animal grazing and browsing should bestrictly controlled if it cannot beprohibited altogether. Domestic animalscan usually be excluded from dunelandswith the co-operation of land occupiers.Feral animals should be tightlycontrolled.

7.1.4 The Coastal Dune Vegetation

Network

The Coastal Dune Vegetation Network(CDVN) was formed in 1997. It provideslinkages between a wide range ofagencies, interest groups, iwi, nurseries,and consultants having a mutualconcern for the rehabilitation ofdegraded sand dunes, particularlyrevegetation techniques incorporatingindigenous coastal species. Financialmembers include Regional and DistrictCouncils, forest companies owning sanddune forests, and the Department ofConservation.

For more information, contact the CDVNsecretary, Forest Research, Private Bag

3020, Rotorua. Phone (07) 347 5899; fax(07) 347 5332.

7.2 Forms, processes, extent

Sand dune ecosystems are regarded asvery fragile because they persist underextremely difficult conditions. Dunes arethe most dynamic ecosystems in ourenvironment; human activities or thoseof animals very easily disturb them. Theyrequire constant vigilance andmaintenance if they are not to affectvarious assets. The erosion process insand dunes is primarily driven by windthat lifts, transports and deposits sandgrains into mobile, unvegetatedaccumulations of sand, called dunes. Thedunes may become vegetated andstabilised by highly specialisedpioneering plants that tolerate the sand-blasting effect of the wind.

When discussing sand dune stabilisation,three types of dune are recognised:foredunes, main dunes, and dunesderived from sandy cliffs. All dunes havebeen formed by wind action on sandgrains, derived from eroded rocks. At thebeginning, wind lifts, transports anddeposits sand grains into mobile,unvegetated accumulations; these maythen become stabilised by highlyspecialised pioneering plants. Thestabilised dunes can become a singleridge along the coast, or dune systemsextending many kilometres inland.

The foredune (or frontal dune)immediately faces the sea and is themost recent sand dune in a sandy orgravelly beach system. It is the ridge ofsand immediately above the high tidemark and may be covered by pioneeringplant species such as spinifex, pingao ormarram.

The sandy foredune is part of the beachitself and, along with the in-shoreseabed, this system forms a natural bufferbetween the sea and the land. The beachand its associated dune system are ableto absorb the energy of the waves, unlikea hard rocky shore, which both reflectsthe waves and is eroded by them. Thismobile beach is able to absorb waveenergy because of its gentle slope and themobile and homogeneous texture of thesand. It protects the area behind the foredune.

The main dune (or back dune) is thesandy area behind the unstable foredune. The main dune is usually well


vegetated and thus does not requirestabilisation. However, inappropriatemanagement can lead to local areas ofinstability which then do requirestabilisation, as for fore dunes. In manyparts of New Zealand, pasture orintroduced forest trees have replaced theoriginal sand dune vegetation.

Sandy cliffs that consist of induratedsand can erode to form sand dunes.Streams often dissect the cliffs so thatvalleys have been formed, and these canact as funnels for coastal winds. High-velocity winds can abrade the sandycliffs to form moving dunes that caninvade agricultural lands, local reserves,or other assets.

The total area of coastal sand dunes inNew Zealand is 305,000 ha, according toNZLRI Data. Of this, 240,000 ha are inthe North Island, especially along thewestern coasts of Northland, Waikato,Rangitikei and the Manawatu; they rangefrom 3- 20 km in width (vanKraayenoord, 1986a). Coastal sand dunesin the South Island are scatteredthroughout Nelson, Marlborough,Canterbury, Otago, and Southland.

Patrick Hesp (2000)states that sand dunesare a distinctive feature of about 1100km of the New Zealand coastline. Thesand is derived from erosion debristransported along the coast by coastalcurrents. Rivers bring down the debris,but in some coastal locations, the sand isderived from Pleistocene sandy cliffs, asat Awhitu Peninsula. Most sand ispredominantly quartz but some mayhave such a very high mineral contentthat it is mined (ironsands). Seashellsand rock fragments may be found amongsand grains, which can range from coarsesand (2.0–0.2 mm diameter) to fine sand(0.2–0.02 mm).


The control of sand dune erosion isbased on mitigating the effect of windon sand transportation. This is usuallydone by assisting common colonisingplants with supplementary planting, orby using non-vegetative materials, suchas straw/hay bales and various wind-stilling barriers. Barriers, board walks,and fencing can regulate human andanimal traffic. Domestic animals rarelyhave a place in dune managementpractices, unless very carefullycontrolled. Animal pests should be

strictly controlled in these fragileecosystems.

During periods when the dune isrelatively stable, vegetation may becomeestablished spontaneously from creepingplants such as spinifex and pingao, orthese plants may be established fromnursery grown stock. In large, unstableareas, marram may be planted, often bymeans of mechanical planters. The mosteffective planting is done in very earlyspring when moisture is available anddesiccating winds less frequent.

Regular inspections and promptmaintenance should prevent erosion indune systems, since these ecosystems arevery easily disturbed. In particular, theoccurrence of blowouts should beprevented where wind excavates theregular contour of the dune byfunnelling. This can destabilise not onlythe foredune but also the main dune andthe areas behind it, usually by coveringthem with unwanted sand. Hesp (2000)describes the process and his figure 2illustrates this.


Fig 7.1 The two blowout sand dune types and

their typical windflow patterns (P Hesp).

1. Depositional lobe

2. Deflation basin

3. Erosional walls

Because of the usually unwanted sanddeposition, it is imperative that blowoutsbe controlled as soon as practicable,irrespective of the time of the year. Ifplanting is unlikely to be effective, non-vegetative means such as straw/hay balesmay have to be used, and various windstilling barriers, such as brushwoodfences or sheltercloth may have to beerected. Frequently, both vegetative andnon-vegetative materials have to be usedat the same time. In some situations, areshaping of the contours may have toprecede the erection of barriers or theplanting of vegetation, if the funnellingeffect of the wind may destroy anyreinstatement efforts.



Descriptions of the various practices for controlling sand dune erosion found in chapters/sections of Part B:

5 Sand Dune Stabilisation Practices

15 Shelterbelts

7.5 Bibliography

The publications from the Coastal DuneVegetation Network (CDVN) are essentialreading for sand dune managers, as wellas the Auckland Regional Council’sCoastal Erosion Management Manual.Various regional councils have alreadyproduced their own locally adaptedbulletins and leaflets; a few examples aregiven.

CDVN Technical Bulletin Series

Bergin, D O & Herbert, J W 1998, Pingaoon Coastal Sand Dunes. CDVNTechnical Bulletin No. 1. Publ. NewZealand Forest Research Institute Ltd,Rotorua. 20 pages

Bergin, David 1999, Spinifex on CoastalSand Dunes. CDVN Technical BulletinNo. 2. Publ. New Zealand ForestResearch Institute Ltd, Rotorua. 32pages

Bergin, David 2000, Sand Tussock onCoastal Sand Dunes. CDVN TechnicalBulletin No. 3. Publ. New ZealandForest Research Institute Ltd, Rotorua.16 pages.

Hesp, Patrick A 2000, Coastal SandDunes – Form and Function. CDVNTechnical Bulletin No. 4. Publ. ForestResearch, Rotorua. 28 pages.

Auckland Regional Council

The “Coastal Erosion ManagementManual”, available from ARC in hardcopy format at $129.00 incl. GST, oras a CD-ROM at $49.00 in MWSformat. Phone (09) 3662000.

Other New Zealand publications

Gadgil, R L & Ede, F J 1998, Applicationof scientific principles to sand dunestabilisation in New Zealand: pastprogress and future needs. LandDegradation & Development 9, 131-142

Pearse, H C H 1958, Stabilisation of sanddrifts with blue lupins. NZ Journal ofAgriculture 97 (4), 315-324

Sale, E V 1985(?), Forest of Sand – TheStory of Aupouri State Forest. NZForest Service, Wellington.

van Kraayenoord, C W S 1986a, PlantMaterials for Sand Dune Stabilisation,pp. 69-77 in: van Kraayenoord, C WS, and Hathaway, R L (Eds.) 1986,Plant Materials Handbook for Soil

Conservation, Vol. 1: Principles andPractices. NWASCA Water & SoilMisc. Pub. No. 93.

van Kraayenoord, C W S 1986b,Management and Uses of Ammophilaarenaria (Marram Grass) and Spinifexsericeus (Silvery Sand Grass), pp. 246-9 in: van Kraayenoord, C W S, andHathaway, R L (Eds.) 1986, PlantMaterials Handbook for SoilConservation, Vol. 2: IntroducedPlants. NWASCA Water & Soil Misc.Pub. No. 94.

Whitehead, P S 1964, Sand dunereclamation in New Zealand. NZJournal of Forestry 9 (2), 146-153

Australian publications

NSW Public Works Department and SoilConservation Service 1987 [Reprint1995], Beach Dunes – their use andmanagement. Publ. NSWGovernment Printing Office, UltimoNSW, 32 pages

Soil Conservation Service of NSW 1990“Coastal Dune Management – AManual of Coastal Dune Managementand Rehabilitation Techniques”, Eds.:Conacher, P.A., Joy, D.W.B., Stanley,R.J., and Treffry, P.T. SoilConservation Service of NSW, Sydney.76 pages

“NSW Govt. Coastline ManagementManual”. This is being updated by DrRod Kidd of the NSW Dept of Landand Water Conservation. The manualmay be found at:

http://www.environment.gov.au/marine/manuals_reports/coast_manual/index.html

Regional Council and Department ofConservation publications:

Several regional councils have produceda number of leaflets/publications on sanddune rehabilitation and management.This list is not comprehensive:

Environment Waikato

• Dune Care Code

• Spinifex Seed Collection andPropagation

• Pingao Seed Collection andPropagation


• Guidelines for Spinifex Seed-HeadCollection

• How Beaches and Dunes work,Coastal Fact Sheet 13

• Why Dunes are important, CoastalFact Sheet 14

• Care Groups in Action, October 2000

• Waikato Region Beachcare Newsletter,Christmas 1999

• Websitehttp://www.ew.govt.nz/educationprogrammes/beachcare/

Northland Regional Council

• Guideline for Stabilising Sand Blows

• Guideline for Protecting Vegetationon Cliffs

• Guideline for Pasture Management onSand Country

• Guideline for Establishing PermanentTree Cover on Sand Country

[Note: these guidelines are based onthose prepared for the AwhituPeninsula Land Group, and are alsoavailable from the Group and fromEnvironment Waikato]

• Website http://www.nrc.govt.nz/

Bay of Plenty Regional Council

• Coastcare BOP Programme: CoastalPlants, Spinifex. Coast Care Info.Brochure No. 6

• Websitehttp://www.boprc.govt.nz/search/

Otago Regional Council

• St Clair to Lawyers Head: Sand DuneManagement Programme

• Ocean Grove Recreation Reserve:Sand Dune Stabilisation Programme

• Website http://www.orc.govt.nz/

Department of Conservation

• Project Crimson Supporters Kit, seealso Websitehttp://www.projectcrimson.org.nz

• NZ Native Plants in Design – CoastalForests

• Website http://www.doc.govt.nz


Chapter 8 Earthworks

8.1 Introduction

Earthworks can induce major, but usuallyshort-lived, changes in sediment yield.American information suggests that thatthe sediment yields from urbandeveloping areas can be extremely high,sometimes reaching values of 50,000t/km2/yr (Novotny et al 1981). Figures of10–12,000 t/km2/yr are quoted from areasundergoing construction in New Zealand(Herald, 1989; Williamson, 1991).

Studies in the Auckland region bear thisout, with construction sites yielding10–100 times more sediment thanuntouched land (ARC 1992a; ARA, ARWB1983a, Swales 1989). Another relevantstudy gathered sediment yield data fromfive different land uses around theAuckland.

Table 8.1 Annual soil loss in the Auckland

region Auckland Proposed Regional Plan:

Sediment Control, September 1995)

* Predicted over 20 year period

The predicted yields are a statisticalestimation of sediment yields averagedover periods longer than the flow record.A 20-year average was used, as it wasconsidered to be acceptably long-term,(Hicks 1994). The confidence limits ofthe long-term annual yield varied from afactor of two for the urban and pastureinformation up to a factor of 4-5 forearthworks. The ARC report consideredthat the larger, more infrequent stormevents yielded disproportionately moresediment from earthwork sites thansmaller events. In comparison, the veryfrequent event yielded most sedimentfrom the established urban areas (becauseof sediment exhaustion with higherstorms).

8.1.1 Impact of earthworks erosion on

the environment

Earthworks result in elevated levels ofsediment in waterbodies and addeddeposition on their beds and banks.Water clarity and aesthetic appeal isreduced. High suspended solidsconcentrations damage or kill streamplants and animals. Reduced plantgrowth flows through the food chain, toreduce insect and fish numbers.Increased channel erosion andremobilisation of sediments maymaintain high suspended solids levels forseveral years. The suitability of water foruses such as irrigation, rural watersupplies, stock watering and recreationcan be reduced (ARC, 1996).

Watercourses can be in-filled, causingloss of flood capacity. Sediment can bedeposited on floodplains, causing cropand pasture losses, property damage, andburial of fences, roads and bridges. Largevolumes can be discharged into estuariesand the ocean.


of erosion on earthworks

On earthworks, the basic erosion processis detachment, transport andsedimentation. Water is the usualeroding agent and transporting mediumthrough raindrop dislodging exposed soilparticles, and overland flow transportingthem downslope. Channelised streamrunoff also transports the eroded soilparticles to the final receivingenvironment.

On earthwork sites there are four mainerosion types:- sheet erosion, rill erosion,gully and channel erosion.

• Sheet erosion: the removal of a fairlyuniform of surface soil by waterrunoff

• Rill erosion: small channels formedby concentrated runoff flows

• Gully erosion: deep channels scouredout by concentrated runoff flows


CHAPTER 8

Earthworks

Average Annual

Measured Soil Loss*Landuse (t/km2/yr) (t/km2/yr)

Pasture 49 46 Market Gardening 49 52 Developed Urban 107 100

– Industrial Developed Urban 24 24

– Residential Earthworks 6,600 16,800

• Channel erosion: bed and channelbanks are removed by flowing water.

Choice of practices will be dictated bythe nature of soil at the construction site.Practices appropriate for stabilising sandor gravel will not work for silt or clay(and vice versa). For a discussion of howdifferent particle sizes are detached,transported and deposited, readers arereferred to Bagnold (1965).

8.3 Principles of Erosion Control

on Earthworks

Erosion control measures protect the soilsurface against rain and runoff. Theyconsist of site management, watermanagement and stabilisation. Sedimentcontrol measures capture eroded soilparticles onsite. Fine textured particlesare not easily retained once mobilised.Erosion control measures are thereforeusually far more effective.

A simple soil loss estimation model, suchas the Universal Soil Loss Equationwhich has been modified forconstruction sites, can be a very usefultool to work out what factors can reduceerosion and sediment yield.

The following principles are effective inreducing soil erosion and particletransport. They form the basis of anErosion and Sediment Control Plan

• Keep disturbed areas small and timeof exposure short. Stage construction.

• Protect disturbed areas against runofffrom above the site i.e. installperimeter controls (see section 4.1.2).

• Keep on-site runoff velocities low.

• Progressively stabilise disturbed areas(section 4.1.3).

• Retain sediment on the site (section4.2).

• Control erosion at source.

• Fit the development to the existingsite conditions. Watch steep areas.Retain watercourses.

• Retain existing vegetation if possible

• Inspect and maintain controlmeasures.

• Temporary and permanent controlmeasures are usually quite different –ensure that design for temporarymeasures is conservative.

Over the last ten years, erosion andsediment control practices have evolvedrapidly in some parts of New Zealand. Asmore has become known about theeffects of sediment on receivingenvironments, so has the demand foreffective controls become more acute.Techniques of control have continued todevelop as more is discovered about theirstrengths and weaknesses. Some havechanged significantly over the last fewyears (eg, sediment retention ponds);while others, like the use of haybales,have hardly changed at all. Theprinciples of control remain the same,but ongoing change with controlmeasures can be expected.

The techniques discussed in this sectionfollow basic principles and will, ifimplemented and maintained correctly,give good results within their designlimits. Remember however, that all ofthese measures can be expected tochange over time.

A simplified layout of erosion andsediment control practices is displayedbelow. Dust is included as it can be aproblem from earthwork sites. Thislayout includes a selection of the mostcommon measures and is far fromexhaustive. Every site is unique and acombination of these, and otherpractices, can be expected. Innovativesite thinking should be encouragedprovided new measures are based onsound principles.



Table 8.2 Control techniques for erosion

resulting from earthworks.


Erosion Control Practice

Site Management Site planning and project management that recognises and addresses

erosion and sediment control considerations. Runoff Control Systems

Water Management Diversion channels/bunds

Check dams

Contour drains

Flumes

Stabilisation Grassing

Mulching

Geotextiles

Sediment Control Practice

Sediment retention ponds

Sediment retention bunds

Silt fences/haybales

Storm water inlet protection

Pumping

Dust Control Practice

Planning Dust management plan

Implementation Water

Dust suppressants

Surface stabilisation

Other options

Descriptions of the various practices for controlling earthworks erosion found in chapters/sections of Part B:

15 Runoff Control Practices for Earthworks

16 Soil Management Techniques on Earthworks

17 Structures for Runoff and Sediment Control on Earthworks

18 Dust Control Measures for Earthworks

8.5 Bibliography

ARA, 1983a: Guideline – EarthworksErosion Management. UpperWaitemata Harbour Catchment Study,Auckland Regional Water Board

ARC 1992a: Erosion and SedimentControl Guidelines for Earthworks.Technical Publication No. 2

ARC 1994: Storm sediment yields frombasins with various landuses inAuckland area. Technical PublicationNo. 51.

ARC 1995: Proposed Regional Plan:Sediment Control. September 1995

ARC 1996: The Environmental Impactsof Accelerated Erosion andSedimentation. Technical PublicationNo. 69.

ARC 1999: Erosion and SedimentControl Guidelines for LandDisturbing Activities. TechnicalPublication No. 90. March 1999.

ARC 2000: The Effectiveness of MulchingEarthworked Surfaces. Landfacts S 04.September 2000.

ARC Environment File: No. 14/10 Br5462 – 64

Environment BOP 2000: Erosion andSediment Control Guidelines forForestry Operations, EnvironmentBOP Guideline No. 2000/01.

Goldman S J, Jackson K and BursztynskyT, 1986: Erosion and SedimentControl Handbook

Herald J R, 1989: Hydrological impacts ofurban development in the AlbanyBasin, Auckland. Ph. D Thesis,University of Auckland.

Humes Industries Limited: DrainageProducts Technical Manual

King A N, Greentree D A and Height D R1998: Construction Site Erosion andSediment Control. Soil Services, NSW.

New Zealand Forest Service 1980: CivilEngineering Bulletin 4 – Diameterrequired for small pipes August 1980.

New Zealand Hydrological Society, 1992:Waters of New Zealand.

New Zealand Institution of Engineers,Auckland Branch: A Guideline andprocedure for Hydrological Design ofUrban Stormwater Systems.

Novotny V, Chesters G 1981: Handbookof Nonpoint Pollution.

Schuler T R 1989: Controlling UrbanStormwater. WashingtonMetropolitan Water ResourcesPlanning Board.

Soil and Water Conservation Society,Empire State Chapter: New YorkGuidelines for Erosion and SedimentControl. October 1991.

State of North Carolina, USA, 1988:Erosion and Sediment ControlPlanning and Design Manual.

Swales A 1989: The effects ofurbanisation and consequentsediment generation on the UpperPakuranga Estuary, Auckland. M ScThesis, University of Auckland.

Vant W N, Williamson R B, Hume T M,Dolhin T J, 1993: Effects of FutureUrbanisation in the Catchment ofUpper Waitemata Harbour. NIWAEcosystems Consultancy Report No.220R, prepared for the Environmentand Planning Division of theAuckland Regional Council.

Williams P W, 1976: Impact ofurbanisation on the Hydrology ofWairau Creek, North Shore, Auckland.Journal of Hydrology Vol. 15, No. 2,1976.


Chapter 9 Sediment Control – Non Earthworks

9.1 Introduction

This chapter looks at sediment fromsources other than earthworks (eg, fromforestry), from various cultivationpractices and pasture activities, andfinally from channel processes and massmovement erosion. Sediment derivedfrom these sources such as these canmake up much of the sediment load ofsome of the streams and rivers of NewZealand.

Historically, any measures that mighthave been undertaken to control erodingsites were usually initiated from an on-site perspective. Reasons such as “it willlimit production”, or “that slip should beplant/grassed” were common injustifying remedial works. The initiatingreason could simply be that erosionoffended good farmers’ sense ofstewardship towards their land.Government assistance in the form ofsubsidies and grants was often availablefor these works.

Today’s perspective, however, can bequite different. Off-site water qualityconsiderations have become much moreprominent (particularly with the“effects” base of the ResourceManagement Act 1991 (the RMA)).Control measures have become muchmore selective and focused. To someextent this could be because we havemore knowledge on various practices andknow better their strengths andweaknesses. Some of our traditionalpractices were just not very effective.More importantly however, governmentmonies has declined on one hand, whileon the other there is much more pressureto ensure that adverse sediment relatedeffects do not occur from variousactivities.

Traditional soil conservation has beenalmost exclusively undertaken on ruralland. Although the terms “sustainability”and “effects” underpin the RMA, it couldbe argued that historical controls havebeen on point source discharges whoseeffects have been easier to control.Activities such as quarries with pointsource discharges have been controlledfor years. Non-point discharges, such assediment from grazing activities, have

always difficult to deal with. Forestry isone of the few rural activities wherecontrols have been imposed for aconsiderable length of time, butcropping and farming have never beensubject to specific controls in the past.Controls are now often for off-sitereasons compared to the on-site rationalethat was more usual in the past.

9.2 Sediment control from surface

erosion in plantation forests

In New Zealand approximately 6 percentof the land area is in plantation forestry.Of this area, approximately 90 percent isplanted in Pinus radiata. Most of thecomments on sedimentation fromplantation forestry concentrate on Pinusradiata forests, referred to generally as“pine forests”.

Sediment yields are generally accepted tobe less than from an equivalent pasturesite for most of a forest growing cycle(McLaren 1996). However, there is ashort period during logging whensediment levels can be elevated. This iswhen roads are prepared for loggingtrucks, landings and skid sitesconstructed, trees felled and hauled toprocessing areas. There can be a lot ofmachinery movement and large areas ofland can be bared, particularly as thecommon method of harvesting in NewZealand uses clearfell techniques. Thishappens about every 25-35 years

Forest harvesting can produce elevatedlevels of sediment (Vaughan, 1984), asdoes land preparation for planting.Harvesting is so closely followed by landpreparation that the two phasesessentially overlap.

Earthworks (such as roading) have themost impact on sediment yield.Vegetation felling by itself has a muchlesser effect on sediment generation andyield than that from earthworks (seeHicks and Harmsworth, 1989). Theimpact starts when something is done tothe vegetation – for example, when thefelled vegetation is hauled to somecollection point and then transportedaway. In some places, poor harvestingpractices can cause significant sediment


CHAPTER 9

Sediment Control – Non Earthworks

generation, particularly on steep slopesand erodible soils. These impacts areusually considered to be less than thatfrom earthworks.

Pine forests that are carefully planted,tended and harvested, can produce forestproducts for rotation after rotation withminimal generation of sedimentcompared to other productive uses onthe same land. But in many parts of NewZealand, pine forests have been plantedon steep erodible land, sometimes as apreferable alternative to pasture. Many ofthe small forests have had minimalforward planning for future harvesting.These are the forests that have a higherpotential for generation of sediment.

Increased levels of sediment arising fromforestry operations (other thanearthworks) can be a result of:

• loss of protective ground cover duringharvest

• exposure of bare ground

• soil disturbance by skidders, tractorsor cable haulers

• proximity of operations towatercourses

• concentration of runoff along vehicletracks and haul paths.

The risk of sediment generation may beincreased on:

• steep terrain

• long slopes

• loose soil or weak underlying rock

• timing of operations during wetweather

Production forestry is a commercialenterprise. Harvesting in anenvironmentally sensitive manner can bemore expensive. Where forests areplanted on steep land, there are limitedoptions for harvesting. Often, there willbe some unavoidable adverseenvironmental effects. The followingsections deal with ways of minimisingsediment generation during forestryoperations.

9.2.1 Principles of erosion and sediment

control

Plan ahead to minimise erosion. The most important aspect of forwardplanning is to tailor the plantingboundaries to match an appropriateproposed method of harvesting. Theproposed harvesting system will dictatehow much roading will be required, andthe general layout of landings, haultracks etc, is critical in minimisingsediment generation. Forward planningof forestry operations is the mosteffective method of reducing problems.This needs to be followed up with goodoperational practice, and regularmonitoring and maintenance once theworks are completed.

Minimise bare ground as far aspracticable.Ground cover helps to prevent erosion ofthe soil surface. Some land preparationfor planting (such as burning ordesiccation) can completely destroy all ofthe surface vegetation and expose thebare ground to the erosive forces of theweather. Sometimes, the operations areundertaken to prolong the exposure ofbare ground so that the new seedlingshave reduced competition from weeds.When afforesting steep erodible country,consideration should be given toalternative methods of land preparationthat minimise the exposure of bareground. This could involve oversowingwith grasses and legumes.

Minimise soil disturbance as far aspracticable.Many forestry operations involve soildisturbance even though they are notclassed as earthworks. Such operationsinclude stumping, hauling logs, ripping,V-blading, and root raking. Theseoperations can often be undertaken withminimal generation of sediment,provided they are properly planned andsupervised. Where there is a higher riskof adverse effects, such as on steepercountry or near streams, these operationsshould be avoided if possible.

Protect riparian areas, and don’tdeposit soil or debris intowatercourses.Riparian areas are critical because anydischarge may have adverse effects onthe downstream water resources. Forestryoperations carried out close to streams,or over streams, should be undertakencarefully so that any deposition of soil or


debris into the watercourse is avoided.Where there is an increased risk ofsediment entering a stream, it may beprudent to designate a protected areanext to the watercourse. Theaccumulation of waste logging debris(“bird’s nests”) at landings can also leadto problems if the landings are sitedabove steep faces, and the bird’s nestcollapses.

Avoid the concentration ofstormwater runoff.Any operation that concentratesstormwater runoff increases the risk oferosion. Some of these arecultivation/ripping operations that rundownhill rather than on the contour,hauling logs downhill to a commonpoint, or locating skid sites in a lowpoint or gully etc. Often, the effects ofconcentrating stormwater runoff are notevident until after heavy rainfall. Theseactivities can be managed to reducevolume and velocity of stormwaterrunoff.

9.2.2 Forest planning

Forward planning includes planning toestablish a forest from scratch, as well asbefore each stage of the forest rotation.The New Zealand Forest Code of Practice(LIRO 1993) is a valuable tool forassisting forest owners/contractors inidentifying any adverse effects of aparticular operation, and then inselecting the appropriate techniques toreduce potential impacts.

Inadequate forward planning oftenresults in higher cost in the long term.Professional forestry consultants have theexpertise to carry out forestry planningas well as supervise forestry operations.While it is essential to have a goodcontractor carrying out the forestryoperations, a good operator cannot makeup for poor forest planning. For example,a good ground based logging operatorwill have difficulty logging a steep sitethat should be logged by cable hauler.


erosion in croplands

Cultivation of soil is necessary to create abetter seedbed, and to minimise weedcompetition on growing crops. It usuallyexposes the soil to the erosive effects ofrain splash and concentrated runoff. Thepotential for erosion and sedimentgeneration from a cultivated site can befar greater than from the same site under

a permanent vegetative cover. Loss of soilfrom cultivated land can result inelevated levels of sediment in receivingwaters and also impact on thesustainability of the soil resource itself.

Three different types of cultivation onthree different kinds of soil are discussedhere:

• market gardening on granular loams

• cropping on loess

• orchard cultivation on free-drainingalluvial soils.

9.3.1 Market gardening and grain

cropping on granular loams

Granular soils derived from volcanic ashare some of the most intensively used inNew Zealand. Approximately 8,000 haare used for large-scale vegetableproduction in South Auckland (FranklinDistrict) (Basher, 1997), and there aresmaller pockets in Northland, NorthAuckland, and coastal Otago. Anecdotalevidence suggests that severe erosion canoccur on these soils during intenserainfalls and that this can not onlyadversely affect the sustainability of thesoil but also lower water quality.

The Auckland Regional Councilundertook a catchment study intosediment yield from vegetable growingin the early 1990s. Sediment yield fromthe 1.8 km2 trial catchment site was 49t/km2/yr over three years (Hicks, 1995).Bedload was negligible at the samplingsite. Soil loss from earlier plot studiesundertaken by Cathcart et al (unpubl) inthe early 1970s was evaluated (Basher etal, 1997). Bare soil plots averaged 5680t/km2/yr over a 2.5 year measurementperiod. The largest storms resulted inmost of the soil loss mentioned in thetwo studies.

The very large difference in yieldssuggested that large quantities of soilwere being mobilised within paddocks bystorms but little was being transported assuspended sediment load by streams.This was considered to be a function oflocal drainage characteristics (whetherfields discharge directly into watercoursesor not) together with the stronglyaggregated nature of the soil. Analysis ofthe soil indicated that the soil is resistantto dispersion into primary sand, silt andclay particles (silt and clay sized particlesusually constitute the suspended


sediment load). It was inferred that thesoil which is moved in storms istransported as aggregated material anddeposited within paddocks and drainswhen runoff velocities diminish.

When a regional plan specificallydirected towards controlling sedimentfrom the Auckland region’s landdisturbing activities was prepared andnotified in 1993, the council did notimpose any regulatory controls onoutdoor vegetable production, as thecatchment soil loss yield of 49 t/km2/yrwas similar to that from pasture fromother soil types in the region (Hicks,1994). However, opinion continued tohold that significant soil loss wasoccurring under this type of land use andthat this was affecting both soilsustainability and water quality.

A major storm in May 1996 resulted inflooding and sediment deposition inparts of Pukekohe township and focusedpublic attention on the practices ofoutdoor vegetable growers. Discussionswere then held between the PukekoheVegetable Growers Association, regionalcouncils and Agriculture New Zealand,and as a result the Franklin SustainabilityProject came into being. This was athree-year project with a emphasis onerosion management, although widerconsiderations, such as irrigation, pestcontrol and nitrate leaching, were alsoincluded. Much of the followinginformation has been obtained from thisproject.

9.3.2 Principles of control on granular

loams

Granular loams are strongly aggregated,and the aggregates are not easily broken.They do not break down to fine clay, siltand sand-sized particles but to smalleraggregates. The soil aggregates are stillrelatively large compared to clay and silt,and can quickly settle out once flowvelocities diminish. Under normalconditions, infiltration and percolationthrough these soils is very high.Measures to ensure that aggregatestability continues are important.

This characteristic markedly influencesthe type of erosion and sediment controlmeasures that can be used. Soil particlesmay be relatively easily mobilised, butthey are also equally easily retained onceflow velocities diminish. Effective erosionand sediment control does not thenmerely consist of providing settling areasfor suspended sediment to settle out. Itcomes down to minimising off siterunoff and reducing on site runoff.

For each paddock, the following pointsneed to be checked:

• Where is the water coming from?

• Where is the water going?

• How can the paddock be set up tominimise erosion and soil loss?

Six basic measures are promoted tominimise erosion and sediment loss:

• channels and interception drains tointercept above-site runoff

• benched headlands

• permanent drainage systems withinpaddocks

• contour drains

• raised accessways

• silt traps.

The first five are effectively runoffcontrol measures while the last is theonly sediment control measure. Inaddition to these measures, a number ofmanagement practices have also beensuggested. These include ripping wheeltracks, growing cover crops, use ofhedges and cultivation techniques.


Sheet erosion of topsoil following a storm

event, Pukekohe, 1999. Photo: D. Hicks.

A paddock plan will help ensure thaterosion and sediment control measuresare appropriate for the site and mesh inwith growing practices. It should beintegrated with other paddock plans toform a co-ordinated property plan, vitalfor runoff control. Each property planshould be integrated with adjacentproperty plans, where they exist.

Each measure or practice (except forpermanent drainage within paddocks,which is self-explanatory) is brieflydiscussed below in section 3.1.3.

9.3.3 Grain cropping

Cultivation of arable loess soils in theeastern South Island and southern NorthIsland can result in the loss of soil bywater runoff and by wind erosion. Thissoil loss is a consequence of most of loesssoils high susceptibility to structuraldegradation. Cultivation can readilybreak the soil down into individual siltgrains, susceptible to erosion by bothwater and wind.

In Canterbury and Otago, arable soils areconsidered to be at more serious risk ofwind erosion than water erosion (Ross etal 2000), due to strong, drying foehnwinds (nor’westers). However, a report byLandcare Research (Ross et al 2000)prepared for the Canterbury RegionalCouncil assesses the risk of erosion onarable soils; it also addresses soil loss bysurface runoff and has been usedextensively in compiling the followinginformation.

Sheet and rill erosion occur when waterruns off cultivated topsoil on slopingground. Low permeability subsoil in loesslimits downward water movement, andsideways flow through topsoilcontributes to runoff generation at lowpoints in fields. High erosion rates oncultivated downlands have been recordedsince the 1940s and found to be closelyassociated with recent cultivation (Rosset al 2000).

Rill erosion is considered to mostsignificant with up to fifty times moresoil being lost through rilling than fromother forms of erosion (ORC, 1995). Rillerosion induced by moderate intensityrainstorms on finely cultivated soils candeposit 20–93 t/ha of sediment at thefoot of cultivated slopes (Hunter et al,1990). Rill dimensions indicated locallosses on eroded slopes of up to 410 t/ha.Hunter and colleagues also reported anearlier study of soil loss from rill erosion

in the order of 50 to 101 t/ha for foursites. During a three month period in1986, 240 ha over 24 properties lost anaverage of 50 tonnes of topsoil/hectare(ORC, 1995). The depth of topsoil after60 years of intensive cultivation wasfound to be 100 mm shallower comparedto equivalent uncultivated slopes (Hunteret al, 1990).

While mean topsoil depth did not differsignificantly between areas used forpasture and areas used for cropping,topsoil depth under cropping was farmore variable and there was a higherfrequency of shallow topsoils (Basher etal, 1995). Basher’s study foundconsiderable redistribution of soil byerosion and deposition but little exportof soil. Ross et al 2000 support this andnote that erosion on downlands resultsin considerable redistribution of soil butlittle net loss. The likely reason is thatthere are few permanently flowingwatercourses close by, to transporteroded sediment off-site.

Rilling is often worse after a droughtperiod when soils are dry. Storms cantherefore mobilise and transport largequantities of soil from land undercultivation. Storms of sufficient intensityto cause significant erosion can beexpected from the larger rainfall events,such as those with a greater than a 10-year return period. Most of this soil isdeposited at the foot of slopes and cancause localised problems to occur such asblocked culverts etc.

9.3.4 Principles of control

Control of water-initiated erosion oncultivated soil includes measures thatincrease soil resistance to erosion andthose that reduce runoff volume andvelocity. The condition of the soil, therate of infiltration into the soil, thepresence of subsurface pans (both naturaland from cultivation), poor vegetativecover, and long steep slopes, are allfactors that influence erosion oncropping areas. Retaining or improvingthe structural stability of the soil is a keyelement of control.

9.3.5 Orchards and vineyards

In the 1960s, apple and pear orchardswithin the Nelson Province weresuffering from severe erosion problems.At that time, the Nelson CatchmentBoard became heavily involved withpromoting erosion control works anddeveloping practices to prevent future


problems. The Board was assisted withfunding from the Soil Conservation andRivers Control Council to developsustainable land management practicesusing six demonstration orchards. By1977, effective practices had beendeveloped and a review of theconservation works was published(Leighs, 1980). The information below isfrom that review.

The problems stemmed from a numberof factors:

• The Moutere Soils are relatively thinand non-cohesive with little or noorganic matter. The subsoil is hardclay over gravels.

• The orchards were often planted onsloping ground in a square patternusing straight rows that is best suitedto flat land.

• Rainfall in the Moutere Hills areaaverages 950 –1050 mm/year, withdry summers and mild winters.Storms of 70 mm in 24 hours, havinga return frequency of 2 years, canoccur at any season.

• In an attempt to conserve moistureand carry out weed control, theground was cultivated between therows of trees.

Cultivation resulted in the ground beingkept tilled from mid spring to midautumn, with many owners pridingthemselves on the fine tilth achieved.Over time, excessive working of the soilsresulted in a cultivation pan on thealready hard subsoil and weakened thepoorly structured topsoil. As a result,even light rains produced widespreadsheet and rill erosion. Heavy rainstormsmoved large quantities of soil from theupper slopes to lower ground and valleyfloors. It was common for the roots offruit trees on spurs and upper slopes tobe left standing on pedestals of earth,because the soil between the trees hadbeen washed downslope to deposit onthe flats and create swampy conditionsburying trees and fences.


The key principles for control of theproblems included:

• improving infiltration of rain andreducing overland flow runoff

• controlling subsurface and surfacewater, and conveying the water safelyto lower flats and natural waterways

• stabilising exposed ground with grass.

The practices that were developed toaddress the above principles were acombination of runoff control, drainagecontrol and erosion control.

9.3.7 Practices

The pattern of works included thefollowing techniques:

• Ripping of subsoil to promoteinfiltration of rain and reduce runoff;

• Installation of lateral grassedwaterways with subsurface tiledrainage to collect underground andsurface water;

• Installation of main grassedwaterways with subsurface tiledrainage to receive water from lateralsand some direct subsoil ripping;

• Use of main outlet grassedwaterways with subsurface tiledrainage to receive water from severalmain tiled grassed waterways anddeliver it to;

• Main outlets or disposal systems,usually natural streams, but couldalso be detention or retention ponds;

Grassing of all exposed ground anddrainage systems.


erosion in pasture

Surface erosion of topsoil causesreductions of 40 percent or more inpasture growth. The severity of theimpact depends on the frequency oferosion and how much land is affected.Surface erosion occurs on an annual, ormore frequent, basis. In low rainfall areaswhere vegetation is often sparse, up tohalf the land can lose some of its topsoileach time, but in high rainfall districtswhich are usually well vegetated, topsoilloss is restricted to a tenth or less of thesurface.

Farming practices that may inadvertentlycause surface erosion are those thatdeplete ground cover. Windblow,sheetwash and rilling of topsoil canoccur almost anywhere where


groundcover is depleted, even on flatland. Light soils with a high silt or sandcontent are more susceptible than loams,heavy clays or peats.

Other practices that will lead to sedimentgeneration include those that weakensoil strength, by physical destruction ofthe soil aggregates, or breakdown of thesoil structure.

Very often, erosion can be aggravated orinduced by stock management. In aliterature review on the adverse effects ofgrazing on soils, Hunter-Smith (2000)noted that overgrazing can adverselyaffect soil properties, particularlyinfiltration rate, macroporosity and bulkdensity. The extent of the damage isinfluenced by a number of factors:

• slope;

• soil type, texture, and moisturecontent

• pasture cover and species

• stocking rate, duration, and animalspecies.

A number of studies conclude thatwinter and spring appear to produce thehighest amounts of runoff and sedimenttransport. However, other studies alsoshow that when the soil is dry (generallyin summer), surface runoff and soilerosion increase during high intensityrainfall. Maintaining a dense grass swardand avoiding overgrazing, protect thesoil surface from very intense rainstorms,particularly those that occur duringsummer. Retirement from grazingsignificantly decreases surface runoff andincreases infiltration rates.


The following principles cover thecontrol of sedimentation from pastoralland use:

• Avoid land management practicesthat deplete ground cover or exposebare ground / Adopt practices thatencourage a dense or close groundcover.

• Avoid land management practicesthat weaken soil strength or breakdown soil structure.

• Encourage land managementpractices that increase infiltrationcapacity of the soils.

• Manage streamside areas carefully,and protect them whenever possible.

9.4.2 Practices

Any farm or pasture managementtechniques that minimise bare groundand help form a dense pasture sward willreduce the risk of surface erosion, limitadverse effects on soil structure, and helpprevent a decrease in the rate ofinfiltration.

ApplicationWhile it is accepted that overgrazing ofpastures cannot be completely avoided,all pastoral farmers will attempt tomaintain an adequate ground cover, andminimise the risk of overgrazing as far aspracticable. There are a range of differentfarm management practices that can beundertaken by landowners that minimisethe amount of bare ground, and helpencourage a dense pasture sward. Theseinclude the following:

• Have the farm well fenced so thatstock grazing can be closelycontrolled.

• Have a reticulated water supply thatis well planned to provide adequatewater to stock, particularly duringpeak demand periods such as midsummer. Ensure that there aresufficient troughs available, so thatgrazing animals do not have to travelfar to the nearest watering point.

• Establish a pasture improvementprogramme to provide a densepasture sward, particularly on pastoralhill country.

• Provide an adequate fertiliserprogramme to maintain a goodpasture sward.

• Avoid mob stocking or heavy setstocking during periods of drought,cold conditions or wet weather.

• Carefully site of fences, gates, troughsand farm tracks to avoid low pointsor overland flow paths wherestormwater runoff may beconcentrated.

• Keep stocking rates to a sustainablelevel for the classes of land that are


being grazed, particularly during wetperiods.

• Use grazing pressure as a pasturemanagement tool withoutovergrazing.

• Match the type and class of stock tothe land capability of the property.

In general, good pastoral farmingpractices will also help retain a densepasture sward, and will minimise bareground and reduce the risk ofsedimentation.

9.5 Surface erosion control for

other sources of sediment

Sediment can arise from sources otherthan the commonly recognised onessuch as earthworks, forestry, andcultivation. Sediment from massmovement erosion and stream channelprocesses are two.

In some areas around New Zealand, massmovement erosion is occurring on a scalethat results in large quantities ofsediment being regularly discharged towaterways. This sediment can then bereworked on an ongoing basis and thereceiving environments may have littlechance of recovery. These situations aretherefore very different one-off typesources of sediment such as earthworks.

Much of the emphasis on massmovement control has been related tothe on-site effects. Less prominent, butalso relevant, is the effect of the ongoinginjection of sediment into the receivingenvironments. Not all eroded materialenters watercourses, but what does canhave devastating effects. If sedimentcontinues to enter watercourses, orongoing processes continue to reworksediment deposited between regularstorm induced “top-ups”, then thereceiving environment may not be ableto recover to its former level. The effectsof mass movement erosion in thesesituations then not only relate to thesustainability of the on-site landuse, butalso to the off-site sustainability of thereceiving environments. A permanentdegradation of the receivingenvironment can occur.

An example of this is in the siltstonecountry north of Napier. There are aconsiderable number of studies that haveassessed the impact of storms andsubsequent erosion in this area of New

Zealand. One of these assessed the soilmobilised by a five year return periodstorm to be nearly 9000 t/km2/yr (Eyles,1971). Storms of about a 5-year returnperiod interval appear to be about thethreshold for significant mass movementerosion. Page (1992) assessed thatCyclone Bola, which occurred in March1988 and was the largest rainfall event tobe recorded in the area, generated anaverage of 420 m3/ha (42,000 m3/km2)within the same catchment. Of thissediment generated, 51 percent wasdeposited on lakebeds and 6 percent wasdischarged from the lake outlet. Thestudy considered that most of thesediment generated from this storm (89percent) was from landslide erosion, andthat sediment in secondary storageprovided only a small contribution. Thesediment deposited by larger storms wasreworked within the streams and rivers(Page et al, 1994), during rainfalls lessthan the 5-year threshold.

Not all sediment generated by massmovement erosion enters a watercourse.The proximity to watercourses, slope,surface roughness, retaining vegetation,and the characteristics of the drainagepattern will all influence how much isretained on hill slopes and how muchdischarges to watercourses. For instance,about 50 percent of sediment generatedby landslides in the Tutira area wasconsidered to enter watercourses (Page1992). The same report also assessed anumber of other studies around NewZealand and concluded that, even withdifferent terrain and triggeringconditions, about 50-55 percent of allsediment generated from massmovement sources entered streams. Thiswill obviously vary however, for thereasons mentioned above.

9.5.1 Sediment from channel erosion

The Alluvial ProcessA watercourse develops a bed shape inresponse to the volume of water that thewatercourse must carry, the size andvolume of sediment carried, the hardnessof underlying rock, the slope of thechannel and the “age” orgeomorphological stage of developmentof the catchment and its floodplain(Cathcart, 2000). A regime, whichgenerally includes a meanderingchannel, some channel erosion anddeposition of sediment, is just onevariant. If any of the catchmentcharacteristics are changed, thewatercourse will accordingly adjust its


regime to compensate. The channel mayalter –for example, to a straight orbraided form – as it accommodates thechanges, with bank erosion anddeposition also altering.

Channel erosion can contribute toundercutting and instability of adjacentland. This in turn can affect sedimentgeneration and supply to the channel.Observation by river engineers confirmsthat channel instability can sometimesbe related to a sudden introduction ofdetritus to the stream. This sets up achain reaction, by initiating bankerosion, leading to more bedload, leadingto more bank erosion. Once started, thisprocess can be difficult to control, as itrequires control over the input ofmaterial, as well as control over bedloadmovement and bank erosion.

Bankfull or near-bankfull is generallyconsidered the most effective dischargein transporting sediment and reshapingthe channel bed and banks. (Waters ofNew Zealand, 1992). In many streamsthis is about the mean annual floodevent (this is the average of the series ofannual maximum flows, whichstatistically has an average recurrenceinterval of 2.33 years – Waters of NewZealand 1992). Once eroded, depositionof clay and other fine grained materials isunlikely in streams because of a lowsettling velocity. Concentrated flows inchannels have greater depth and velocityand sediment carrying capacity thanoverland flow. As a consequence mostfine textured sediment derived fromstream bank erosion will be reworkeddownstream over time.

Channel Erosion as a Source ofSediment Stream channel erosion is a commonform of erosion and can be a majorsource of sediment. From an “effects”perspective, sediment from channelerosion is immediately available fortransport by channel flow. It is thereforedifferent to catchment derived materialmay only travel short distances and canbe re-deposited and retained in thecatchment. A good illustration of this isthe relatively small quantities ofsediment that might leave a vegetable-growing area on granular soils in SouthAuckland compared to the very muchlarger quantity that can be generatedwithin the catchment.

The contribution of sediment fromchannel erosion varies tremendously.

Gianessi (1986) reports that a study ofsediment sources in five northernMississippi watersheds found channelerosion accounted for 15 to 28 percent ofthe total sediment delivered while beingonly 1 percent of the catchment area.Channel erosion can therefore be animportant source of sediment and canhave an influence on water quality quiteout of proportion to the areal extent ofwatercourses.

The relative influence of channel erosionas a source of sediment varies similarly inNew Zealand. In one study (Page et al1992) channel erosion was considered tocontribute only 2 percent of the totalsediment yield from one catchment(Lake Tutira, north of Napier) during onestorm although there was significantchannel modification. Little sedimentwas found to accumulate between thelarger storm events and this suggestedthat sediment from channel erosion inthis catchment was significantly less thatfrom mass movement erosion.

9.6 The effect of urban

development on channel

sediment

The change of landuse from rural tourban increases the imperviousness of acatchment surface and changes itshydrological characteristics. Increases inflood-flow frequency and size result fromthe changed runoff patterns. Streamchannels will widen as they re-adjust tothe new flow regime. Herald (1989)stated that there was a three fold increasein the width of streams draining an areathat was 85 percent urbanised. Schueler(1989) supports this by claiming thatmost streams widen two to four timestheir original size if post-developmentrunoff is not effectively controlled. Thesechanges can start to occur once 10-20percent of a catchment has beendeveloped. This stream widening isprimarily accomplished by lateral cuttingof the stream bank resulting inundercutting of the zone adjacent to thechannel, treefall and slumping. Thischannel widening occurs in directresponse to changed flow regimesresulting from urbanisation. Flooddetention measures in some urbansituations throttle flood flows down tothe bank full situation and which,although controlling flood peaksexacerbates channel erosion because thebank full situation (the main erosioncausing flow) is continued for a longerperiod.


One study looking at the effects ofproposed urban development predictedan average of 9,800 tonnes/year ofsediment would result from widening ofchannels in one catchment in Aucklandas the channels adjusted to altered flowregimes over a 20-year developmentperiod (from Vant et al, 1993). Thissediment source was solely due tostreams widening and to accommodatealtered hydrology resulting from changedcatchment characteristics. This wascompared to 18,000 tonnes/yearestimated from the earthworks phase ofdevelopment (not all of which wouldreach watercourses), and 2,800tonnes/year from the remainingcatchment sources such as pastoral anddeveloped urban land uses.

Channel erosion resulting fromurbanisation of catchments can thereforebe a major contributor of sediment towaterbodies. Modification of channelswill normally last for years as streamchannels adapt. The generated sedimentis generally immediately available tostream flows and can have effects onreceiving environments out ofproportion to its relative area.

9.7 References

RC undated: Sediment yield fromchannel works on the PapakuraStream. (unpublished)

Basher LR, Mathews KM and Zhi L, 1995:Surface erosion assessment in theSouth Canterbury downlands, NewZealand using 137Cs distribution.Australian Journal Soil Research, 33, p787-803.

Basher LR, Hicks DM, Handyside B andRoss CW, 1997: Erosion and sedimenttransport from the market gardeninglands at Pukekohe, Auckland, NewZealand. Journal of Hydrology (NZ)36(1) p 73-75.

Cathcart R, 2000: Controlling streamchannel erosion in Northland. SoilConservation Fact Sheet 1, NorthlandRegional Council 2000.

Collier K, M Simons, P Simpson, J Waugh1992: Riparian zones and sustainableland management – A DOCperspective. Broadsheet Spring 1992New Zealand Association of ResourceManagement (NZARM).

Environment BOP 1998: SustainableOptions Fact Sheets SC02/98:Riparianretirement; SC07/98:Plant selectionfor retirement areas;

SC08/98:Management of retirementareas; SC09/98:Runoff managementon pastures.

Environment BOP, 2000: Erosion andSediment Control Guidelines forForestry Operations, GuidelineNumber 2000/01, July 2000.

Evans B 1983: Revegetation Manual: AGuide to revegetation using NewZealand native plants. QueenElizabeth II National Trust.

Eyles RJ, 1971: Mass Movement inTongoio Conservation Reserve,Northern Hawkes Bay. Earth ScienceJournal, vol 55, No. 2.

Franklin Sustainability Project, 2000:Doing it right. Guide to SustainableManagement.

Gianessi L P, Peskin H M, Crosson P,Puffer C, 1986: Non-point sourcepollution: are cropland controls theanswer? Journal of Soil and WaterConservation, July-August 1986.

Hicks DM 1994: Storm sediment yieldsfrom basins with various landuses inAuckland Area. ARC EnvironmentDivision, Technical Publication No51, Auckland Regional Council,Auckland 34 p.

Hicks DM 1995: Sediment yields from amarket gardening basin nearPukekohe. ARC EnvironmentDivision, Technical Publication No61, Auckland Regional Council,Auckland 24 p.

Hicks DL 1995: Control of soil erosionon farmland. MAF Policy TechnicalPublication 95/4, Ministry ofAgriculture and Fisheries. 45 p.

Herald J R, 1989: Hydrological impacts ofurban development in the AlbanyBasin, Auckland. Ph. D Thesis,University of Auckland.

Hunter GG, Lynn IH 1990: Storminduced soil losses from SouthCanterbury and North Otagodownlands. D.S.I.R. Land resourcesTechnical record 22. 7 p.

Hunter-Smith J 2000: Draft Report – TheAdverse Effects of Grazing on Soils –A Review of Literature. EnvironmentBOP, Environmental Report 2000/14,July 2000.

Leighs MJ 1980: A Review of Water andSoil Conservation Works in NelsonOrchards. Nelson Catchment Boardand Regional Water Board.


Maclaren JP 1996: Environmental Effectsof Planted Forests. FRI Bulletin No.198, New Zealand Forest ResearchInstitute.

Murphy G 1992: Riparian ZoneManagement – A Review of theLiterature. Prepared for the Bay ofPlenty Regional Council.

New Zealand Forest Research Institute1992: Proceedings of Riparian ZoneWorkshop – Implications for ExoticForestry, compiled by JA Fenton.

NIWA 1994: Managing riparian zonesalong agricultural watercourses. I.Background concepts, and II. Interimguidelines. National Institute ofWater and Atmospheric ResearchLimited, Hamilton.

Otago Regional Council 1995: NorthOtago Sustainable Land ManagementGuidelines.

Page MJ, Trustram, NA and Dymond JR1992: Sediment budget to assess thegeomorphic effect of a cyclonicstorm, New Zealand. Geomorphology9 (1994) p 169-188.

Page MJ, Trustram, NA and DeRose RC1994: A high resolution record ofstorm induced erosion from lakesediments, New Zealand. Journal ofPaleolimnology 11, p 333-348.

Quinn JM 1994: Framing the Issue: WhyAre Riparian Areas so Important?Paper presented to NZARM AnnualConference 1994.

Ross CW, Basher LR & Baker CJ 2000:Management of erosion risk on arablesoils: a review. Landcare ResearchReport LC9900/090 prepared forEnvironment Canterbury regionalCouncil. 119 p.

Schuler T R 1989: Controlling UrbanStormwater. WashingtonMetropolitan Water ResourcesPlanning Board.

Shepherd TG, Ross CW, Basher LR andSagger S 2000: Soil managementguidelines for sustainable cropping.Landcare Research.

Spiers JJK 1985: Loggers Language A NewZealand Terminology. New ZealandLogging Industry ResearchAssociation Inc. February 1985.

Spiers JJK 1986: Logging OperationsGuidelines. New Zealand LoggingIndustry Research Association Inc.and National Water and SoilConservation Authority.

Stringer OA 1978: The river scene: Thedownstream viewpoint. Paperpresented to conference on “ErosionAssessment and Control in NewZealand”, New Zealand Association ofSoil Conservators 25th AnniversaryConference, University of Ilam,Christchurch August 1978.

United States Department of Agriculture1954: A manual on conservation ofsoil and water – Handbook forprofessional agricultural workers. USDepartment of Agriculture, SoilConservation Service AgricultureHandbook No.61.

Vant W N, Williamson R B, Hume T M,Dolhin T J, 1993: Effects of FutureUrbanisation in the Catchment ofUpper Waitemata Harbour. NIWAEcosystems Consultancy Report No.220R, prepared for the Environmentand Planning Division of theAuckland Regional Council.

Vaughan L 1984: Logging and theEnvironment. NZ Logging ResearchAssociation

Vaughan L 1990; Visser R and MSmith1993 (eds.): New Zealand ForestCode of Practice, 2nd Edition. NewZealand Logging Industry ResearchOrganisation.

Wendell G, L Schipper, P Beets, MMcConchie 1992: Riparian Buffers inNew Zealand Forestry. New ZealandForest Research Institute.

Wilkie DR: The curving furrow – Soilconservation by contour methods.Bulletin No. 10. Soil Conservationand Rivers Control Council, NewZealand.

Williamson RB, CM Smith, ABCooper1996: Watershed RiparianManagement and its Benefits to aEutrophic Lake. Journal of WaterResources Planning and ManagementJanuary February 1996.


Part B

Practices

iSoil Conservation B

1 Cropland Management for Surface Erosion Control 1•11.1 Introduction 1•1

1.2 Conventional tillage 1•1

1.2.1 Guidelines 1•1

1.3 Minimum tillage 1•2

1.3.1 Guidelines for minimum tillage 1•2

1.4 No-tillage cultivation 1•2

1.4.1 Tips for successful direct drilling 1•3

1.4.2 Results of direct drilling 1•3

1.4.3 Pest monitoring and management 1•3

1.5 Stubble retention 1•4

1.6 Strip or Contour Cultivation 1•5

1.7 Ripping of wheel tracks 1•5

1.8 Cover Crops 1•5

1.9 Fallow 1•5

1.10 Soil structure maintenance 1•5

1.10.1 Introduction 1•5

1.10.2 General guidelines (from Shepherd et al 2000) 1•6

2 Pasture Management for Surface Erosion Control 2•12.1 Lowland pasture management for surface erosion control 2•1

2.1.1 Climate 2•1

2.1.2 Pasture type and species 2•1

2.1.3 Soil fertility 2•2

2.1.4 Grazing management 2•2

2.1.5 Other factors 2•2

2.2 Hill pasture management for surface erosion control 2•3


2.2.2 Pastoral hill country 2•3

2.2.3 Utilisation of feed 2•3

2.2.4 Fertility transfer by stock 2•3

2.2.5 Natural reseeding 2•3

2.2.6 Hieracium management 2•3

Contents – Part B

ii Soil Conservation B

2.3 Tussock management for surface erosion control 2•4


2.3.2 Guidelines 2•5

2.3.3 Retirement management practice 2•5

3 Fencing Management for Surface Erosion Control 3•13.1 Lowlands 3•1

3.2 Hill country & Mountain land 3•1

3.2.1 Size of flock and stocking rate 3•1

3.2.2 Grazing Pressure 3•1

3.2.3 Paddock shape and orientation 3•2

3.2.4 Soil Conservation Factors 3•2

3.2.5 Siting factors 3•2

3.3 Specifications 3•2

3.3.1 Cattleproof fence 3•2

3.3.2 Recuperative spelling fence 3•2

3.3.3 Erosion Control Subdivision or Conservation fence 3•3

3.3.4 Rabbit Proof fence 3•3

3.3.5 Retirement fence 3•3

4 Pasture Revegetation Practices 4•14.1 Lowlands 4•1

4.1.1 Guidelines for lowland pasture revegetation 4•1

4.2 Hill Country 4•2

4.2.1 Guidelines for hill country pasture revegetation 4•3

4.2.2 References 4•3

4.3 Mountain Lands 4•3


4.3.2 Guidelines for High Country Revegetation 4•4

5 Sand Dune Stabilisation Practices 5•15.1 Marram Planting 5•1

5.1.1 Practice 5•1

5.1.2 Application 5•2

5.2 Spinifex planting 5•2

5.2.1 Description 5•2



iiiSoil Conservation B

5.3 Pingao planting 5•3




5.4 Coastal forest and pasture planting 5•5




6 Burning Management – Mountain Lands 6•16.1 Introduction 6•1

6.2 Guidelines For Good Burning 6•1

7 Fodder Bank Establishment 7•17.1 Introduction 7•1

7.2 Guidelines for Fodder Bank Establishment 7•1

8 Managed Reversion of Retired Land 8•18.1 Lowlands 8•1

8.2 Hill country 8•1


8.2.2 Fencing 8•1

8.2.3 Weed control 8•2

8.2.4 Pest control 8•2

8.3 Revegetation 8•3

8.3.1 Choice of species 8•3

8.3.2 Planting 8•3

8.3.3 Post-planting maintenance 8•5

8.3.4 Nurse crops 8•6

8.3.5 Concluding Remarks 8•6

8.4 Mountain lands 8•6

9 Runoff Control Practices – Lowland 9•19.1 Introduction 9•1

9.1.1 Contour furrows 9•1

9.1.2 Factors to Consider in Design 9•1

9.1.3 Practical Points and General Recommendations for

Constructing Furrows 9•2

9.2 Broad Base Terraces and Absorption Terraces 9•3

iv Soil Conservation B

9.3 Graded Banks 9•3

9.3.1 Description/Purpose 9•3

9.4 Interception drains 9•4

9.5 Headlands 9•4

9.6 Contour Drains 9•5

9.7 Raised Accessways 9•5

9.8 Silt Traps 9•6

9.9 Subsoiling – amongst trees 9•7

9.10 Tiled drainage beneath waterways 9•7

9.10.1 Grass waterways beneath trees and along waterways 9•8

9.11 Grade line (contour) planting 9•8

9.12 Hedges 9•8

9.13 Contour banks 9•8

9.14 Broadbased terraces 9•9

10 Dewatering Techniques for

Deep-seated Mass Movements 10•110.1 Introduction 10•1

10.1.1 Geotechnical Principles of Slope Stabilisation 10•1

10.1.2 De-watering 10•2

10.1.3 Overview of the Site 10•2

10.1.4 Surface drainage 10•2

10.1.5 Sub-surface Drainage 10•3

10.1.6 Surface Re-contouring 10•4

10.1.7 Graded Banks 10•5

11 Gully Control Structures 11•111.1 Flumes & Chutes 11•1



11.1.3 Types of Flumes and Chutes 11•1

11.2 Pipe Drop Structures 11•3


11.2.2 Types of Pipe Drop Structures 11•4

11.3 Sink Holes 11•5


11.4 Detention bunds/dams 11•5


11.5 Diversion Banks = diversion bunds on earthworks 11•6

vSoil Conservation B

11.6 Graded Waterways=grassed waterways in pasture & cropland 11•6


11.7 Drop structures 11•7


11.7.2 Types of Drop Structure 11•7

11.8 Debris Dams 11•9


11.8.2 Types of Debris Dams 11•10

11.9 Surface Protection in Gullies 11•12

11.9.1 General 11•12

11.10Mechanical Infilling of Gullies 11•12

12 Pole Planting 12•112.1 Description/Purpose 12•1

12.2 Application 12•1

12.3 Practice 12•1

12.1.1 Pole dimensions 12•1

12.3.2 Planting poles 12•2

12.3.3 Pole placement and spacing 12•2

12.3.4 Pole protection 12•3

12.3.5 Other benefits 12•3


13 Erosion Control Forestry 13•113.1 Earthflow stabilisation with erosion control pole planting 13•1

13.1.1 Shallow Earthflows 13•1

13.1.2 Deep Earthflow 13•1

13.2 Erosion control forestry for gully stabilisation 13•2

13.3 Erosion Control Forestry on Earthflows and

Other Deep-seated Movements 13•2

13.4 Erosion Control Forestry of large Mudstone and

Argillite Gullies (Farm Wood lots) 13•3

13.4.1 Gully perimeter planting 13•3

13.4.2 In-gully planting 13•4

13.4.3 Post Harvesting Management 13•4

13.4.4 References 13•4


13.5 Forest Management Practices – Mountain Lands 13•6


vi Soil Conservation B

13.6 Protection Forestry 13•6

13.6.1 Species 13•6

13.6.2 Planting regimes 13•6

14 Shelterbelts 14•114.1 Siting 14•1

14.2 Design 14•2

14.3 Establishment 14•2

14.3.1 Site preparation 14•2

14.3.2 Time of planting 14•3

14.3.3 Planting stock quality 14•3

14.3.4 Weed control 14•3

14.3.5 Damage from rabbits, hares, and possums 14•3

14.3.6 Windbreaks 14•3

14.3.7 Irrigation 14•3

14.3.8 Management & Maintenance 14•3

14.4 Windbreak design and plant species for different sites 14•4

14.4.1 Shelterbelt, medium-tall hardwood, one row 14•4

14.4.2 Shelterbelt, medium-tall hardwood, one row, underplanted 14•5

14.4.3 Shelterbelt, medium-tall hardwood, deciduous, two row 14•5

14.4.4 Shelterbelt, medium-tall hardwood, evergreen, two row 14•6

14.4.5 Shelterbelt, conifer, one row 14•7

14.4.6 Shelterbelt, conifer, two row, timber 14•8

14.4.7 Shelterbelt, conifer, multiple row, timber 14•8

14.4.8 Shelterbelt, low-medium coastal, two row 14•9

14.4.9 Shelterbelt, tall coastal, two row 14•9

15 Runoff Control Practices for Earthworks 15•115.1 Diversion Channels and Bunds 15•1


15.1.2 Installation 15•1

15.1.3 Maintenance 15•1

15.2 Check Dams 15•1



15.2.3 Limitations 15•2


15.3 Contour Drains 15•2


viiSoil Conservation B




15.4 Flumes 15•3





16 Soil Management Techniques on Earthworks 16•116.1 Grassing 16•1


16.2 Mulching 16•1


16.3 Geotextiles 16•2


17 Structures for Runoff and Sediment Control

on Earthworks 17•117.1 Sediment Retention Ponds 17•1


17.1.2 Location 17•1

17.2 Sediment Retention Bunds 17•3


17.3 Silt Fences 17•3

17.4 Haybale Barriers 17•4

17.5 Stormwater Inlet Protection 17•5

18 Dust Control Measures for Earthworks 18•118.1 Introduction 18•1

18.2 Guidelines 18•1

18.3 Watering 18•1

18.4 Dust Suppressants 18•2

18.5 Surface Stabilisation 18•2

18.6 Windbreak Fencing 18•3

Tables

Table 4.1 Seed mixtures 4•3

Table 11.1 Permissible Velocities for Grass Lined Channels 11•6

Table 12.1 Classes of pole (giving typical dimensions) 12•1

Table 16.1 Mix of grass seed and fertiliser for Auckland 16•1

viii Soil Conservation B

Surface Erosion control

Chapter

1. Cropland Management

2. Pasture Management

3. Fencing Management

1.1 Introduction

These management practices encompassconventional tillage, minimum tillage,no tillage, stubble mulching and contourcultivation. Runoff managementpractices for arable land are describedseparately (see 10).

It is important to understand theterminology used with respect tocultivation, minimum tillage and directdrilling in NZ.

• Conventional Tillage – Is the practiceof establishing a crop or pasture inthe field which has been tilled orcultivated. (Includes the fullcultivation process to seed bed stage).

• No Tillage – To establish a crop orpasture where no cultivation iscarried out at all. This term issynonymous with Direct Drilling.

• Minimum Tillage – is where the soilsurface may have only one or twopasses over it by using say harrows orlight discing then sowing the seeds(rather than full cultivation process).

It is important to note that mostcultivation equipment will mechanicallymove soil downhill leaving upper slopesand knobs bare of soil. Erosion greatlyspeeds up this movement, which can beclearly seen on paddocks that have beenploughed one way for many years.

1.2 Conventional tillage

1.2.1 Guidelines

Many farmers prefer conventional tillagewith a plough, prior to seedbedpreparation with harrows or discs.Downslope soil loss can be reduced bythe following practices:

• Cultivating on the contour wherepossible

• Maintaining slow tractor speedreduces the distance soil is movingdownhill

• Travel slower across steeper faces andif going downhill on gentle slopesusing rotary hoe, maxitill or grubber(as this equipment can move a waveof soil downwards if travelling atspeed)

• Cultivating at the correct time i.e.when soil is moist and friable tominimise the number of passesrequired

• Use a reversible plough to throw thefurrow uphill

• Use other equipment that will havethe least impact i.e. paraplow orMaxitill in preference to rotary hoe(i.e. over-use of a rotary hoe willdamage the soil structure)

• Where possible use single passcultivation with multiple implementsattached, or a one pass cultivator;may need higher HP tractor oralternatively use a contractor with thespecialised equipment

• Judicious use of suitable herbicidesmay reduce the number of passesrequired

• Where possible avoid cultivatingslopes over 13 degrees especially ifthe slopes are longer than 30 metres

• Minimise the bare ground by timingthe tillage operation (considerrotation type and cover crops)

• Avoid leaving a fine seedbed (surfaceroughness is vital with a range ofaggregate sizes which will promotebetter infiltration thus reducing thesurface runoff and erosion).

• Take care to minimise compactionfrom machinery by not cultivating inwinter months, or when the soil iswet in spring and autumn

• Leave dry, ephemeral watercoursesuncultivated i.e. in permanentpasture – Refer to Grassed Waterways.

1•1Soil Conservation B

CHAPTER 1

Cropland Management for

Surface Erosion Control

• Plant steeper vulnerable slopes intrees and fodder banks.

Finally if cultivation is deemed to be anecessary practice on the farm then it isvital that ongoing monitoring of the soilaggregate stability is carried out e.g. byvisual soil assessments, Shephard (2000).

1.3 Minimum tillage

This tillage practice has been used inNew Zealand for many decades, more sobefore direct drilling technology wasmastered. There has been a significantmove into minimum tillage in the areasof New Zealand commonly affected bydroughts i.e. the eastern side of the SouthIsland (especially after the drought of theeighties and late nineties), also Wairarapaand Hawkes Bay in the North Island.

Minimum tillage may involve usingsimple cultivation equipment andpassing over a paddock only once ortwice e.g. with harrows thenbroadcasting or drilling the seeds. Themore common approach today is to usean approved herbicide to reduce thevegetative top cover thus reducing theenergy (i.e. fuel used) and machinerypasses to prepare a seedbed.

There are advantages over direct drilling,such as minor levelling of a paddock andgenerally lower cash costs and machinerylasting longer.

1.3.1 Guidelines for minimum tillage

Aim to only carry out sufficientcultivation to prepare the paddock fordrilling (use suitable equipment as amaxitill or one way pass cultivator)

• Leaving a rough cloddy seedbed andcultivate to a shallow depth.

• Use approved herbicide to killexisting vegetation (may need twosprays depending on weeds present)this also helps the ground break up,conserves soil structure and moisture.Note that if the soil structure is finewith poor aggregates then it ispreferable to direct drill

• On slopes of 13-20 degrees, minimumtill only when coming out of longterm pasture. If sowing Brassicaecrops then include suitable grassspecies in the mix for good groundcover

• Take care not to get wheelcompaction on clay soils

1.4 No-tillage cultivation

Modern technology developed in NewZealand has provided sophisticatedcoulter designs such as the “cross slotdrill” which have eliminated many ofthe earlier problems and variable resultswith traditional drills when sowingpasture and crops.

Some of the benefits of direct drillscompared to cultivation can besummarised as follows:

• Minimal surface erosion of topsoil

• Soil improvements as the soilstructure is preserved and improvedwhilst leaving pasture and cropresidues including stubble on thesurface

• Enhances earthworm numbers andother soil fauna

• Fewer new weeds introduced

• Time savings in the number of passes

• Fuel conservation up to 80% less fuelused and lower machinery costs

• Reduced nitrogen mineralisation andcarbon oxidation to the atmosphere

Such cultivation systems are already wellestablished in many countries, such asthe USA and Canada, and parts ofEurope. The benefits and several of therisks have been described in books suchas “No-Tillage – Science and Practice” byC J Baker, K E Saxton and W R Ritchie(1996), and “Successful No-Tillage –Practice and Management” by W RRitchie, C J Baker and M H Hamilton-

1•2 Soil Conservation B

Steeper area in poplars – no fencing.

Cultivation on more gentle sloping land. Photo: M Harris.

Manns (1998. In addition, the NorthOtago Sustainable Land ManagementGroup has a very useful overview of“Direct Drilling and Pesticide Use” atwww.noslam.co.nz/info_sheets/directdrilling.shtml See also “North OtagoSustainable Land ManagementGuidelines, produced by Otago RegionalCouncil, (1995)”.

Choice of drillIn general, the more recent the design ofdirect drills, the better the results. Newtriple disk designs with small presswheels give better results than olderdesigns. Triple disc drills perform bestwhen moisture conditions are good, orin sandy or stony soils.

1.4.1 Tips for successful direct drilling

The following steps are recommended forsuccessful direct drilling:

• Test soil well in advance. If lime isrequired, put it on 6-12 months priorto drilling. Identify run out paddocksin May/June.

• Spray old pasture while soil moisturelevels are reasonable and when thepasture and weeds are activelygrowing, usually between Septemberand November. An excellent plant killis essential – seek good advice oncorrect chemicals.

• Where moisture retention is aproblem, use the double spray –fallow technique to improve moistureavailability. Trials have shown that upto 12 times more moisture is retainedto 20 cm depth compared tounsprayed sites. Old pasture killedwith herbicide [or other measures],followed by fallow for at least 6weeks, does not transpire and thusretains soil moisture. Plant roots alsobreak down.

• Inspect newly drilled paddock every2-3 days while seed is germinatingand seedlings are becomingestablished; take action if insect, slugor weed problems occur.

• Correct timing. If you can wriggle apocketknife into the soil up to theend of the blade, soil conditionsshould be suitable for direct drilling.If not, it is either too dry, or the rootshave not broken down sufficiently.

• Do not direct drill when conditionsare too wet. Smearing of soils canresult if conditions are not suitable;this will drastically affectgermination.

• Best sowing depth. Sow no deeperthan 13 mm for most seeds. If sowndeeper, the establishment rate formost pasture plants will drop sharply.

• Harrowing, if the drill used did nothave its own press wheels, harrowinghelps to produce loose soil to coverthe seed in the slots, but be verycareful to avoid soil compaction atthe seed slots. That would also reducethe establishment rate of the pastureplants.

1.4.2 Results of direct drilling

Ross (2000) found that soil organicmatter levels in the top 20 cm underlong-term (21 years) no-tillage in theManawatu were maintained at 85-94% ofthose under permanent pasture. Thiscompared with a decline of 40% in soilcarbon after 17 years of cropping bytillage. Total weight of earthworms underlong-term no-tillage cropping was 77% ofthat under pasture, which comparedwith a 99.8% decline under 29 years oftilled cropping. The aggregate stability ofthe no-tilled soil was 85-92% of thatunder pasture, and five times better thanthat after 23 years of continuous cerealcropping by tillage. Over the years no-tillage soil became progressively easier todrill (requiring decreasing levels oftractor power) which has been explainedby the soil’s increasing crumb structureand aggregate stability. Such anobservation has since also beenconfirmed in Canterbury (D, M and ARedmond; R and M Scott; D Ward, pers.comms (1999), in Baker (2000) b).

1.4.3 Pest monitoring and management

During conventional cultivation, plantpests may be mechanically damaged ormore easily found by various birds, suchas gulls and starlings. With a directdrilling programme, this does not occurand the risk of pests damaging cropsincreases. Control is possible, but it doesincrease the use of chemicals. But thereare a number of practices that can beused to minimise the use of chemicals inthe pest control programme.

Recommendations for pest monitoringand management are:


Grass grubDig 10-15 inspection holes across apaddock. If there are more than twogrubs per spade square, insecticideshould be used.

SpringtailsSpringtails damage brassica crops soonafter sowing, often before seedlingsemerge. To establish if springtails arelikely to be present at damaging levels,place a white handkerchief on the soil inthe afternoon, seven days after sowing. Ifmore than five springtails have jumpedonto the handkerchief after 2-3 minutesat ten sampling points across thepaddock, then damaging levels areprobably present. But spray only ifspringtail levels are high enough.

Argentine Stem WeevilArgentine Stem Weevil [ASW] affects allimproved pasture and damages by larvalfeeding and stem mining. ASW resistantryegrass “high-endophyte” varieties havebeen developed but the endophyte, aliving fungus inside the grass, producestoxins which can lead to animal healthproblems. Biological control is availablein the form of the parasitoid waspMicroctonus hyperodae. But TaranakiRegional Council expects that it will take5-10 years for this parasitoid to build upand disperse to become an effectivecontrol agent of ASW. Assuming that50% parasitism of ASW is then achieved,a 6% increase in pasture DM productioncould be expected (TRC LandManagement Information Sheet No. 9).Contact the Regional Council orAgResearch for assistance.

SlugsSlugs are a problem in wet conditions orwhere there is lots of trash. They are ofparticular concern to seeds and emergingseedlings of direct drilled crops becausethe drill slots provide favourableconditions for slugs. Problems with slugscan occur when numbers exceed 15 perm2.

Steps to take before sowing: Place 5wet jute sacks diagonally across thepaddock, three weeks before sowing.Cover the sacks with pegged downplastic to prevent them from drying out.If there is an average of more than nineslugs under each sack after two nights,slugs are present at levels that maydamage the seedlings. Mob stocking at500 ewes/ha for three consecutive nightsprior to herbicide application can

achieve good control of such slugpopulations.

Steps to take after direct drilling:Inspect drill slits two days after sowing. Ifthere are more than four slugs per metreof row, then immediate use of slug baitsis recommended, with the followingoptions:

• Thiodicarb pellets @ 7.5 kg/ha if rainis imminent

• Methiocarb pellets @ 7 kg/haimmediately after rain

• Methaldehyde pellets @ 20 kg/haimmediately after rain.

The establishment of “beetlebanks”The establishment of “beetle banks” tomaintain habitat for insect predators andto reduce the use of chemicals isrecommended. When cultivatingpaddocks, create a ridge 0.4 m high and1.5-2 m wide within the paddock. Leavea gap about 25 m at each end to allowfarm machinery to move around it.These areas will accumulate vegetativematerial and provide habitat for insectpredators, which will assist in keepinginsect numbers down. As a guideline, asquare 20 ha paddock (450 x 450 m) willonly need one ridge in its centre toensure adequate coverage by insectpredators. The beetle banks will take twoto three years to become effective formsof insect control.

1.5 Stubble retention

Prudent farmers try to retain the stubbleof the previous crop as a form of windbarrier, rather than burning the stubble,which is still practised by many. Aresearch team at Crop & Food ResearchLtd [Team leader Dr Prue Williams] isdeveloping sustainable managementsystems for arable and horticultural landsand is studying amongst others whathappens to the carbon in crop residuesunder various cultivation treatments[Glyn Francis et al.]. Crop & Food carriedout a review on crop residues for theFoundation for Arable Research, Lincolnin 1996 (Dr Patricia Fraser, pers. comm.)but a copy is not yet to hand.

Evidence is mounting that all forms ofcultivation oxidise soil carbon and recentUS data suggest that the entire carboncontent of a preceding wheat crop can beoxidised in 19 days following planting


(Baker, 2000 b). Under regularcultivation, soil structure deteriorates,and thus the soil becomes moresusceptible to surface and wind erosion.But with stubble retention and lesscultivation, humus build-up occur at arate depending on local soil and climatefactors, and soil moisture is retained,especially in the top 300 mm of theprofile, which leads to improvedinfiltration and crop yields (Baker 2000b). He quotes Anon. (1994) who reportedthat when soil organic matter content isincreased from 2% to 6%, a 65% increasein available soil water results. And thatTriplett et al (1968) had found that no-tilled corn with 70% residue groundcover significantly out-yieldedconventional corn, but with 45% residuecover, there was no difference betweentreatments.

1.6 Strip or Contour Cultivation

In the USA, cropping lands prone towind erosion are generally strip-cultivated, i.e. strips of vegetation, “grasshedges” are kept between areas that arecultivated and cropped. Those hedges actas major catchers of wind eroded soil,and of nutrients in surface runoff (seee.g. Eghball, Gilley, Kramer andMoorman 2000). Such “minishelterbelts” have been evaluated in theHawke’s Bay by its Regional Councilsince the mid 90’s (D Bloomer, pers.comm.), especially in asparagus growingregions. The HBRC brochure “SustainableLand – Asparagus Strip Cropping” (May1996) recommends strips of oats sown inmid June-July at 120 kg/ha over everysecond asparagus bed. The crop issprayed off in end August at least 3weeks before asparagus harvesting starts.In a season that lacked the typical strongNorwesters, the cost saving through lackof windblast to the asparagus spikes wasvalued at $750/ha. The total cost ofestablishing and spraying the oats hedgeswas $229. Asparagus pickers commentedthat harvesting conditions had improvedas a result of increased shelter andminimal dust problems.

1.7 Ripping of wheel tracks

Wheel tracks are a key zone for initiationof surface runoff and erosion withinpaddocks. They are highly compacted, sohave infiltration rates. They also collectrunoff from higher ground and channelit. Various trials have demonstrated thatcultivation or ripping of wheel tracks

markedly increases infiltration andsignificantly reduces soil erosion.

Wheel track ripping, using a shallowtyned implement towed behind a tractor,should be carried out as soon as possibleafter planting.

1.8 Cover Crops

A cover crop is grown to be ploughedinto the soil rather than harvested attimes of year when a paddock is not usedto grow vegetables. It stabilises the soilagainst erosion, adds organic matter,improves soil structure, and uses residualnitrogen from previous crops. Byimproving soil structure, cover cropshelp retain high infiltration rates.Commonly used cover crops on granularloams include greenfeed or Massif oats,ryegrass, Phacelia and mustard.

1.9 Fallow

At some stage it will be necessary toleave the soil fallow. The most effectivemeasure to minimise erosion risk in thissituation is to plough a cover crop backinto the ground. The plant residues adddry matter to the soil, which helps bindit together. Other effective measures areto leave the soil in an open and loosecondition, or form ridges using a tynedimplement with only a few tynes.

1.10 Soil structure maintenance

1.10.1 Introduction

Retaining or improving soil aggregatestructure and stability helps reducesurface erosion by sheetwash andwindblow. Aggregate size and stabilityare important factors because a coarsestructured, stable aggregate is relativelyhard to mobilise but is quickly depositedwhen runoff/wind velocities decrease. Afine textured soil that easily breaks down


Ripping compacted wheel tracks. Photo: Franklin Sustainability Project.

to even finer sized particles is more easilymobilised by runoff or wind, and is moredifficult to retain or settle out. Aggregatesize influences soil transportability and isin turn influenced by aggregate stability.Both are affected by:

• Natural physics and chemistry of thesoil type,

• Amount of organic matter in the soil,

• Type and frequency of cultivation.

The structure of most soils declinesunder cultivation and recovers underpasture. Soil structural degradationadversely affects cropping. Good soilstructure leads to lower tillage costs,more days when the soil is suitable forcultivation, better seed germination,emergence, plant growth and vigour, andhigher crop yields.

The type of cultivation equipment usedcan have a significant effect on soilstructure. Machinery passage itself willphysically alter soil structure. Rotarycultivators generally produce finerseedbeds than do ploughes, discs ortyned implements. The rate of change instructure stability is dependent on thesoil type and on the cropping system.Long term, continuously cropped soilsrequire more cultivation passes toproduce a fine seedbed and are thereforemore susceptible to erosion, than arepaddocks with short-term cropping, no-tillage cropping, or a pasture cover.

There is a strong relationship betweenaggregate stability and soil organicmatter. Cultivation exposes more soilsurfaces to aeration and this increases thedecomposition of organic matter by soilorganisms. This loss of organic mattercontributes to loss of soil structure andreduces the ability of the soil to recover.Organic matter is conserved withminimum soil disturbance andmaximum residue input, such as occursunder a no-till system.

1.10.2 General guidelines (from

Shepherd et al 2000)

Minimum or no-tillage cultivationsystems should be utilised. These avoidor minimise cultivation and minimisestructural damage. Surface residueincreases organic matter and thereforesoil structure. Aggregate stability issignificantly higher under these tillagesystems than from cultivated cropping.

Cultivation should be undertaken at thecorrect moisture levels. Cultivation whensoils are too wet causes smearing andthin plough pans that reduce infiltration.

Cultivation implements that minimisestructural damage should be selected.These avoid, or minimise, cultivationand therefore minimise structuraldamage.

Practices that avoid compaction of thesoil should be used. These include usinglow ground-pressure machinery (widetyres, tracked equipment etc);minimising traffic (by reducingcultivation runs; planning the paddocklayout etc); and cultivating with a tynebehind the wheels to at least 10-15 cmdepth.

• Ripping of subsoil pans should becarried out.

• Stock should be removed when thesoil is too wet.

• Crop rotations that suit the soilshould be used. Soils that aresusceptible to compaction andstructural degradation will requirelonger pasture and shorter cropcycles.

For further details readers are referred tothe Visual Soil Assessment publications(Shephard et al, 2000), Volumes 1 and 2,Field Guide and Soil ManagementGuidelines for Cropping and PastoralGrazing on Flat to Rolling Country.


2.1 Lowland pasture management

for surface erosion control

Overgrazing of pasture can be completelyavoided, though there are times in anyyear when feed is short and grazingpressure may be high. To minimisewindblow, sheetwash or rilling, thecritical thing is to maintain just enoughground cover, so that if a gale or acloudburst strikes, a high proportion oferoded soil particles are trapped in thesame paddock before they have a chanceto move further.

Maintaining adequate ground cover fitsin with things which most farmers doanyway in the course of grazingmanagement: rotation of stock betweenpaddocks, and avoidance of mob-stocking or heavy set-stocking duringdrought, cold conditions or wet weather.

Residual ground cover can be assessedduring a paddock walk to determine ifground cover is adequate to protect soilfrom surface erosion;

• If one or two paces out of every tenstrike bare soil or mud, ground coveris still enough to protect most of thebare patches against wind and rain.

• If between two and seven, cover isinsufficient to prevent wind andrainsplash from detaching soilparticles, but residual grass and litterstill trap most of them before theymove out of a paddock.

• If more than seven, detached soil islikely to leave in dustclouds or runoffduring adverse weather.

The remedy is to spell the paddock if atall possible before it reaches this stage,and temporarily destock if it does. Thereis an obvious cost in terms of short-termfeed utilisation. On the other hand, thereis also faster recovery of pasture, andgreater long-term production, ifpaddocks can be lightly grazed whenweather conditions are adverse for plantgrowth. Measurements on farms showthat maintaining sward density:

• Traps 90% or more of windblown andwater-washed soil before it leaves thepaddock;

• Helps reduce pugging andcompaction of wet soil

• Prevents pasture growth losses ofanything from 8 to 91% thefollowing season.

The upper levels of pasture loss mayseem high but have been recorded atdiverse locations (Hawkes Bay,Manawatu, Canterbury, Otago). Thehighest losses occur where topsoil isstripped by water or wind; and whengrass is trampled into mud onwaterlogged, pugged soil. In either casegrowth cannot recover for a year orlonger, even though climatic conditionshave improved meanwhile. Light set-stocking or rotational grazing withadequate spells reduce the risk.

To ensure a healthy vegetative cover thatwill ensure erosion is minimised thefollowing factors, that affect pasture/farmmanagement, need to be recognised:

2.1.1 Climate

Temperature and day length are the twomain environmental factors determiningrate of plant production. Generally thehigher the temperature, the greater thegrowth growth rates and higher thepasture production (as long as soilmoisture and fertility are not limiting).Examples of this can be seen in NZ withwinter growth rates varying from 20-25kb dry matter/ha/day in temperateNorthland to 5 kb DM/ha/day in Gore.The key is to select pasture species thatare suited and adapted to the localclimate and environment.

2.1.2 Pasture type and species

As a general rule where farmed animalsare to be grazed the pasture ratio shouldbe made up of approximately one thirdin legumes (ie red and white cloverspecies) while the remaining two thirdsin grasses (ie ryegrass, cocksfoot, timothyetc) and herbs (eg plaintain andchickory). The species will varythroughout NZ but should be selected as


CHAPTER 2

Pasture Management for


suitable for that particular environment.Where droughts are more frequent thenone should include dryland type suitablegrass/clover species which will withstandheavy grazing and periods without water.Some drought prone pasture mixes couldinclude wheatgrass, cocksfoot, and fescuespecies as well as specialised lucernepasture where suitable.

2.1.3 Soil fertility

New Zealand soils are naturally low innutrient status, with phosphorus,nitrogen and sulphur being the mostlimiting macronutrients on unimprovedsoils. On newly developed pasturessuperphosphate and if necessary limeand molybdenum need to be applied toensure the clovers are healthy and fixatmospheric nitrogen. As the soilnitrogen levels increase the grasses canestablish and become more dominantthus ensuring a good effective groundcover for 12 months of the year.

The other important factor to consider isthe replacement of nutrients removed byanimals and lost from the soil. The soiltype, stocking rate and level of nutrientreserves built up in the soil are veryimportant factors in working outaccurately the maintenance fertiliserrates. It is essential that an ongoing soilfertility monitoring programme isundertaken at regular intervals to ensuremaximum pasture production (includingpersistence of plant species) andoptimum animal performance isachieved.

2.1.4 Grazing management

From a soil conservation point of viewthe aim of grazing management is toensure high pasture production as well asmaintaining pasture quality whichresults is a complete protective coverover the soil mantel. Pasture regrowthfollows the classic “S” shaped curvewhich has three phases where phase oneis at the start after grazing with slowregrowth. If the paddock has beenovergrazed (ie where less than 500kgDM/ha) then regrowth will be slow. Withphase two we find the most rapid growthand phase three is a slowing down asmore pasture length creates shading.Therefore if maximum pastureproduction was the only objective asystem that kept the pasture in phasetwo would be best. Under rotationalgrazing this means grazing down to 3-5cm at each grazing and allowing themto grow back to 15-20cm before

regrazing. But under a set stockingsystem this corresponds to setting astocking rate so that pasture length isnever below the height range of 3-6cm.However, maximum pasture productionis not the sole aim in grazingmanagement especially where we areprotecting the soil from erosion. Pastureheight may not be important comparedto the spread of grasses and clovers (andother weeds and herbs) over the exposedsurface.

Consideration of the types of grazinganimals that are farmed is importantbecause that will also determine theamount of pasture grazed and residualthat should be left.

In summary the need is to vary grazingcontrol and a stable vegetative cover is tohave a sufficient number of paddockswhich can only be achieved bytemporary or permanent fences. Thiswould necessitate having at least 30-35paddocks on the farm and in the winterperiod mob rotational grazing (usingtemporary electric fences) can be used forthe daily or tow day shift within apaddock.

2.1.5 Other factors

Pasture renovationIt is paramount that annual pasturerenovation be carried out worked in withthe farm rotation programme. Asdiscussed elsewhere cultivationtechniques may be desirable on somesites ie where there has developed a soilpan but from a soil conservationviewpoint where soil erosion is aproblem and drought conditions prevailthen it is strongly recommended that notillage or direct drilling technologies beused.

Plant and animal pest controlIt is imperative that ongoingmaintenance programmes are carried outto control prevent or minimise anyexisting or new plant or animal pest.

Irrigation managementUsing additional water from an irrigationsystem will ensure pasture species persistlonger and help maintain the protectivesoil cover.


2.2 Hill pasture management for

surface erosion control

2.2.1 Introduction

This is the practice of continuouslyimproving hill country pasture so thatthe ground is protected from sheet,wind, and shallow slip erosion by a closeand vigorous mixture of clovers andryegrasses. Subdivision fencing allows thecontrol of grazing, thus improving swarddensity, and recent research hasdeveloped pasture species suitable fordifferent climatic conditions and soiltypes.

2.2.2 Pastoral hill country

On hill country also, depletion of groundcover cannot be completely avoidedduring droughts or cold, wet winterswhen there is not much fresh growth.But it can be reduced by the samemodifications to grazing management asfor lowland pasture, i.e., maintaininggreater residual feed in paddocks throughrotational grazing, avoidance of mob-stocking or heavy set-stocking, anddestocking the worst-affected paddocks(if feasible). There is also an additionaltechnique which makes a big differenceto density of ground cover on hillcountry – pasture improvement.

Since the 1940s, farmers have convertedsome 4 million hectares of hill countryfrom low-producing to improved pasture,by oversowing and regular fertilisation.While carried out primarily to increaseproduction, this practice has also had thebeneficial side-effect of establishing adense sward. Field trials have shown thatestablishment of improved pasture onhill country:

• Reduces surface erosion by 50 to 80%,relative to levels measured inunimproved pasture:

• Reverses pasture growth losses of 40%or more, (combined effects of over-grazing, sheet erosion and fertilitydepletion). In combination withcloser subdivision and better grazingmanagement, this reversal hasenabled increases in stock carryingcapacity from 2-7 stock units ahectare up to 8-12.

2.2.3 Utilisation of feed

It is important to utilise any extra grassfeed to ensure pastures do not becomerank (due to low stocking rates) and

dominated by Cocksfoot, Yorkshire fogand lower fertility demanding or shadetolerant grasses and pests such as porinaand grass grub. This utilisation can beachieved by more strategic fencing andincreasing stock numbers over theperiods when surplus dry matter isproduced. Furthermore, aerialoversowing and topdressing will providea better ground cover and more growthin the winter months. Clover responsecan be significant early on afteroversowing until the available soilnitrogen levels are raised especially byspecific topdressing and or rotationalgrazing.

2.2.4 Fertility transfer by stock

This can be one of the limiting factors inmanaging hill country pastures as stocktend to concentrate their grazing on thewarmer crests of hills which can beovergrazed (bared off) and subject tofrost heave, sheet and wind erosion. Thisimportant transfer of fertility results invarying nutrient levels and pastureproduction. The key is strategic fencingof shady blocks from the sunny so theycan be grazed at the appropriate timeswith suitable stock types.

2.2.5 Natural reseeding

Where clover establishment has beenpatchy seed can be spread by stockfollowing close grazing and summerspelling to encourage good seed set. Theimpermeable coat of clover seed enablesit to survive through the animal andgerminate in the dung. Some of the driersunny blocks can be grazed in the springand early summer then spelled to allowseed set and natural spread which willassist in improving the pasture groundcover.

2.2.6 Hieracium management

There are two opposing views on the roleof hieracium species (hawkweed) in thehill country particularly unimprovedlands which may support tussockgrassland associations. One view sees theplant as beneficial because they arecolonising land that can no longersustain original grassland vegetation thusreducing the amount of bare groundwhich may be vulnerable to acceleratederosion. The other view sees these plantsas aggressive invaders that have takenover from other species, so that theircontrol would enable more desirableplants to be re-established.


There are four main options forcontrolling Hieracium namely,agricultural development, usingherbicides, biological methods andgrazing management. Grazingmanagement appears to be one of themost feasible current strategies forlimiting the spread of this plant ontolow input land. Exclosure studies inCanterbury and Otago show grazing canreduce Hieracium flower density 40-foldlimiting expansion by seed.

Low intensity spring-summer grazingsignificantly reduced plant number andground cover of the upright Hieraciumspecies ie praealtum (king devil) but notthe prostate species ie pilosella (mouseear). Conversely, high intensity grazingmay assist establishment.

2.3 Tussock management for

surface erosion control

Note that the comments and principlesmade above for hill pasture managementalso apply for tussock countryparticularly where the land lies below900m a.s.l.

2.3.1 Introduction

Grazing management in the highcountry has changed dramatically overthe past thirty years from traditional setstocking on large blocks of land torotational grazing of small blocks atdifferent times of the year. Furthermore,the period of grazing and stock grazingpressures are now recommended for eachsite.

There is no common set ofrecommendations for maintenance ofimproved tussock grassland. Each block,or group of blocks, will be managed for aspecific purpose e.g. a sunny face toprovide winter grazing.

There is now a greater tendency tomanage each block individually. Thebasic objectives for successful pasturemanagement and optimal dry matter aswell as minimising the action of, sheetand rill erosion occur are to:

• Sustain a satisfactory level ofproduction.

• Maintain the productive pasturespecies.

• Ensure good ground cover ismaintained all year round.

• Ensure continuity of feed supplyespecially at critical stages of year.

• Utilise feed effectively so overallinputs are justified.

Level of productionApart from soil moisture which can bemanipulated, nutrient supply is themajor factor affecting pastureproduction. It is well known that, inintensively managed pastures, maximumregrowth occurs when the sward isneither grazed too short (no less than 3-5cm), nor left to grow too long (no morethan 12-18cm) before regrazing. This canonly be achieved by mob stocking androtational grazing. Intensivemanagement is more difficult on tussockgrasslands due to the terrain,unpredictable climate and most growthoccurs during a short season. In such anenvironment it is more important toprovide feedbanks where and when thestock need them than to achieve overallmaximum pasture growth.

Maintaining productive pasturespeciesFor sustainability and long term successfrom oversowing tussock land, it is vitalto ensure pasture species persist andencourage spread of the introducedspecies. Because of the low soil nitrogenstatus, clovers dominate newly oversownpastures but as soil fertility levelsimprove grasses become moreproductive. There grasses have not beenintroduced annuals such as brome grass,browntop and sweet vernal will competewith the clovers. The practice of spellingfor natural reseeding is more rewardingfor grasses than for clovers.

Ensure good ground coverIt is essential to have an effective coverover the surface to minimise any erosionimpacts. The most difficult time of theyear will be in the winter and latesummer when soil moisture deficitsoccur. Selection of suitable species whichmay include herbs for that environmentis important.

Pasture quality when requiredA high legume proportion in the sward,and a high ratio of green to deadmaterial, will ensure good pasture qualityespecially when stock have high feedrequirements such as ewes pre and postlambing and during flushing or mating.


Effective pasture utilisationThis can only be achieved by sufficientstock numbers and subdivision fencing.The aim is for 60-70% pasture utilisationon improved areas.

As a rule of thumb a stocking rate of atleast one stock unit for every tonne ofdry matter that is grown is required. Forexample, land in the semiarid areaswhich is moisture limited and producingapproximately 2 tonne DM/ha/yr shouldcarry at least 2 SU/ha/yr whereas ayellow brown earth soil in the midtussock area producing 5 tonnes wouldcarry approximately 5 SU/ha/yr. At leastthirty hill blocks are needed to operatean effective management system thatincludes breeding replacement stock, andwintering on saved pasture. Because ofthe great variations in the mountainlands in New Zealand many differentgrazing practices occur. The approachtaken here is to provide guidelines basedon altitudinal, climate and generalvegetation zones.

2.3.2 Guidelines

Alpine ZonesThis zone varies around New Zealand,but basically occurs higher than 1500 mabove sea level. In some localities it mayinclude reasonable vegetative cover(alpine grasses and shrubs), butcommonly has bare ground, rock andscrees with snow cover for considerableperiods. Frost heave is very active.Includes alpine vegetation.

Management Practices Avoid unnecessary impacts on this zone.Where grazing is acceptable it should beat very low stock rates (eg 1 stock unitper 7-10ha or more for short summerperiod ie2-3 months). Care needs to betaken when setting stock rates as theytype of season and environmentalinfluences can impact on this. It isrecommended in good growth seasonswhen there is a surplus of feed on theflats and lower hills of these propertiesthat the summer alpine lands be spelledcompletely from grazing. Furthermore,this zone is suite to active and passiverecreation so minimal tracking and soildisturbances should take place. Resourceconsents and approval from the Crownsagency will be required on any pastorallease land for any developmentproposals.

• Recognise the long term impact ofgrazing on visual landscape. Most ofthese areas have important vistas andany overgrazing can be clearly seen ata distance. This applies particularly ifany burning is carried out or escapedtussock fires occur. It is recommendedthat no burning be carried out in thiszone.

• Where possible destock all grazinganimals and remove feral pests. Thiswill depend on the type of countryand balance of the remainder of thefarm. For it to be successful requireswell sited retirement fences to keepfarmed animals out of. It is importantto note that in some of the morerugged terrain of the Southern Alpsfencing may not be possible so thebest alternative may be to graze solelycattle in the valley floor complexes.

• Where practical construct retirementfences. If Crown leasehold land,consider future land use options.

2.3.3 Retirement management practice

PracticeThis is the practice of fencing out an areaof severely eroded high country (usuallyLUC VII and VIII), which is often at thetop of a mountain range. In somecircumstances the topography of the areamay enable stock to be completelyremoved without the need for largelengths of retirement fences.

Furthermore, to assist in the revegetationprocess of the retired land all stock thatwere removed will require alternativegrazing elsewhere i.e. offsite feed. This isusually provided from the lower altitude,more productive land i.e. which may beAOS & TD or irrigated Lucerne to carrythe same or equivalent displace stock on.

HistoryRetirement schemes were not reallyadvanced at all until the SoilConservation and Rivers Control Councilprovided grant incentives to farmers tocarry out the practice from about 1955onwards. This ceased in 1992 when grantmonies for soil conservation purposeswere terminated by government.

ApplicationToday the works still continues onprivate land but are at a slower rateunless it is part of the Crown LandTenure Review process on Pastoral


Leasehold land where the high altitudeor severely eroded lands are retired andtransferred to the Crown’s estate tobecome the responsibility of DoC.

The same land use change can occur onFreehold land where severely erodingland exists and needs to be destocked toassist in the natural or man assistedrevegetation programme. There may beopportunities to involve the QueenElizabeth II Trust if there are uniqueareas of tussock grassland and bushwithin the same block.

The most important factor in anyretirement block is to ensure thatongoing animals and plant pest controlis carried out as well as regular fencemaintenance. It is vital that proper firemanagement control such as firebreaktracks and watering facilities are in placein case any escaped or wild fires occurwhich will damage the vegetationrecovery programme.

Tall Tussock Grassland ZoneThis zone commonly in the 800-1500 mabove sea level range is dominated bynarrow leaved snow tussock(Chionochloa rigida) with sub-dominantblue tussock (Poa colensoi) and otherinter-tussock species including herbs andweeds

There may be hawkweed ingression atlower altitude. Sunny faces are moredepleted due to concentrated stockgrazing. They are normally utilised forsummer grazing in the December to earlyApril, by dry sheep with stocking levelsless than 0.2 stock units per hectarewhere unimproved. May include patchesof native scrub and forest, particularly onthe gullies and wetter sites.

• Use conservative stocking rates forthe enterprise. This will depend onwhether the land has been improvedor not and how much sunny landcompared to shady land exists. Ifunimproved land then stocking ratesof 1 su/1.5ha-4.0ha may beacceptable (check local guidelines)whereas on improved land the ratecan vary from 2 su to say 5su per haper year on the best moist/warmcountry.

• Use conservative grazing approach.

• Aim for the best protective coverpossible. From a soil and waterconservation viewpoint this will

require regular monitoring beforestock are put out on a block andwhen to remove a mob from theblock particularly in a dry summerperiod. Stock may need to bemustered from a block earlier thanplanned in a drought season toensure an adequate ground cover ismaintained into the winter. Therecovery following a drought can beenhanced by applying seed andfertiliser in the autumn if requiredwhen sufficient moisture wasavailable.

• Consider species diversity anddevelop only sustainable areas. Thismay mean very low grazing pressureson important biodiverse areas oralternatively fence them out and nograzing at all. Furthermore, areasshould be developed according to theLand Use Capability classification ofthe land and consideration of thebiodiversity values.

• Practice conservative burning policies(Refer Burning).

• Spell native blocks during goodgrowth years. This is a valuablemanagement tool particularly onblocks that have bare ground andmay be unsuited to oversowing orfurther subdivision. This will help inseed set and may build up a smallfeed reserve.

• Use cautious approach tomanagement of hieracium land.

• Continually monitor and checkgrazing loads. The fundamentalprinciple from a soil conservationviewpoint is to ensure optimumground cover at all times of the year.This means the blocks should beregularly monitored by eye or usingvarious sampling/measuringtechniques which will provide gooddata and trends on speciescomposition, species change, amountof dry matter produced and bareground recovery. This monitoring willprovide information which will allowchanges to be made at an early phaseon the time and period of grazing,grazing intensities and type of stockused.

Short Tussock GrasslandThis varies in altitude between 350 m –800 m above sea level. In some locationsit includes scabweed country, but the


typical cover is depleted short tussock(Festuca spp) interspersed by exoticgrasses and weeds. It is more typical ofthe South Island lower foothills andbasin lands in the semi arid areas.

Some of this grassland was severelyaffected by rabbits and burning, but hasrecovered well with better fencing andstock control. Hieracium flatweedingression is a major concern.

The supply of feed in this zone is criticalto the viability of many run properties(i.e. early spring flush feed) and is oftenused for younger stock. The carryingcapacity varies greatly depending on theseasonal rainfall and on farmdevelopment. Shady faces are vital forstock grazing in the late summer periodswith average carrying capacity of 0.3-0.4su/ha/yr.

Management Practices • Constant management of rabbit and

other animal pests as well as anymajor plant pest problems (includingHieracium spread invasion). Theimportant factor here is for regularongoing monitoring of the mainanimal and plant pest populations toensure the threshold levels are notpassed. Remedial plans need to be inplace and ongoing control to ensurethat a good ground cover is alwaysmaintained.

• Have drought strategies in place.Although droughts may be a regularoccurrence on some of this country itis vital to have strategies in place tominimise the impact that lack ofwater and grazing animals can haveon depleting the surface groundcover. Some of these matters havenbeen discussed previously such asdeferring grazing or grazing only inthe early spring with the hogget flockand spelling to enable early seed setand natural spread of seed. The use ofdrought tolerant species on suitablepaddocks at the lower altitude isadvisable but grazing managementand soil fertility status needs to beregularly monitored. Furthermore,using the most suitable lucernepasture species for the site isrecommended but specialmanagement is required (eg grazingand weed control) to ensure thesevaluable feed reserves persist for sometime. Spelling lucerne blocks ordeferred grazing may be necessary torestore them after a prolonged

drought or heavy grazing. The otheroption is to have specific fodderfeedbanks established at strategic sitesaround the farm which may or maynot be used annually.

• Develop only sustainable areas thatcan withstand grazing. Otherwiseconsider managing them as forestryblock or retire from grazing stock.

• Adopt conservative stocking policies.This is important on those landswhich have been eroded and lost partor all of their topsoil as well as thethreat of Hieracium plant invasion. Italso depends whether they have beenimproved and the animal pestpopulation threat.

• Aim for maximum protective cover.Similar management considerationsto the other hill country zones.

• Exclude cattle from this land duringwet periods. This is to minimisepasture species damage in improvedareas but also will reduce the problemof cattle pulling out whole tussockplants (esp hard and silver tussockplants) in a wet season.

• Allow annual grasses to seed at leastonce per three years. This has beendiscussed elsewhere and is importantto ensure improved and unimprovedpastures persist. The provision ofalternative feed sources as lucernepaddocks or special fodder banks andusing irrigation on the lower flats andterraces will help facilitate removingstock off tender sunny country.

• Destock this land between November-February for seeding of sunny facesand spell blocks for a full seasonduring good growth years. (Refer tocomments above).

One of the most important managementtools is to continually monitor and checkgrazing all the grazing loads.


3.1 Lowlands

Fencing can also assist surface erosioncontrol, if the fencelines follow the landtypes based on the principles of theNZLRI Land Use Capability Classificationsystem. The key objective is to separatelower- producing, erosion prone units,from better arable or pastoral land.

Firstly, fence out Class VI and VII landand Class III & IV Land i.e. non-arablefrom arable. This practice can be used tominimise sheet and rill erosion on thesteeper land by establishing morepersistent pasture species. Trees can alsobe established on this steeper land, basedon an open planting regime i.e.poplars/willows, or on more vulnerablesites as a close planting regime i.e. pineor gum woodlots. Another alternative inthe drier environments and sunnydepleted faces, is fencing out these areasand establishing suitable “stock fodderbanks” which may only be grazed onceor twice a year. Suitable species willdepend on locality but could include thefollowing:

Shrub willows; Saltbush (Atriplexhalimus), Dorycnium spp, Tagasaste-treelucerne (Chamaecytisus palmensis), Treemedick (Medicago arborea), Shrubbywattles (Acacia spp), Perennial lupin(Lupinus spp) as well as nut trees i.e.Black locust (Robinia pseudoacacia),Honey locust (Gleditsia triacanthos),Carob (Ceratonia siliqua) and otherssuch as Chestnut spp and various Oakspp.

Secondly, fence out “High Class soils” onthe farm i.e. Class I & II fromsurrounding more erosion prone arableland. Better productive sites, withoutlimitations of fragipan, soil compaction,poor drainage or stoniness can beintensively managed on a sustainablemanagement programme.

Thirdly, fence out fertile alluvialfloodplains and riparian zones (unitswith a w sub-class) from the adjoininglandform. These flood-prone areas needspecial management such as nocultivation and preferably direct drilling

for crop and pasture establishment, so asto avert sheet wash by floodwater.

3.2 Hill country & Mountain land

Fencelines for hill country can also bemade on the basis on LUC classifications.Separating classes I – V from the lowerversatility units should be the firstpriority, where these classes are presenton hill country properties principles forfencing these classes are similar tolowland areas.

Ideally class VI land should be fencedseparately from class VII land as this willfacilitate good grazing management,which will in turn reduce surfaceerosion. Class VIII should always befenced off where possible for protection.

In the high country of New Zealand,fencing is still an important managementtool for sustaining the soil and farmproduction. Although fences differ indesign they have a similar objective,namely to facilitate stock grazing so asnot to deplete the sensitive high countryvegetative sward.

Fencing for the control of grazinganimals has been practiced since the firstsheep and cattle were introduced intoNew Zealand from the 1840s onwards. In the high country the basic design offences has not changed other than morespecialised wires i.e. 12.5 gauge, andusing tanalised posts instead of flatstandards where the site allows.

3.2.1 Size of flock and stocking rate

Size of the flock and block size isimportant if grazing sheep. Stocking rateswill determine the paddock size ie 2000ewes need 44 ha paddocks for grazing at45 stock units per hectare if a properrotational grazing system is to bepractised.

3.2.2 Grazing Pressure

In general, there is more risk than benefitfrom periodic high grazing pressure onboth unimproved and improvedgrasslands, especially in semiarid lands.On improved grasslands in sub-humid


CHAPTER 3

Fencing Management for


and particularly in humid zones, periodichigh intensity stocking for herbagecontrol and active nutrient cycling isessential as long as grazing is not toground level.

3.2.3 Paddock shape and orientation

The key factors here are the area andlength of fence and animal behaviour.The need to ensure stock water isavailable is paramount so watercoursesshould not be fenced out from stock inwater short areas especially if areticulation scheme is not available.

3.2.4 Soil Conservation Factors

Fencing will be to either act as anexclosure to stock (i.e. a debrisavalanche) or as an enclosure to stock(i.e. to allow different grazing pressureson different land classes) e.g. sunny fromshady. The latter may also be to spellplants for seeding or to permit seedlingsto establish. The enclosure fence wherepossible, should follow the land-capability class boundaries wherevertopography and climatic constraintspermit i.e. snow drifts.

3.2.5 Siting factors

• Fences should be up and down aslope where possible to minimisedamage from the snow

• Where slopes must be crossed lookfor stable sites, glacial ridges, oldterraces and changes in slope

• For mustering purposes site fences onridge tops so they can be seen (butnot on exposed ridges as snowdrifting may occur, preferable in thissituation to be on the lee of theslope).

• Where acceptable from a soil andwater and landscape perspective,dozing the line followed withregrassing may save costs in future.

• Cross streams at gorges or narrowconfined points.

• Siting of gates is important for ease ofaccess and movement of stock i.e.rotational grazing

3.3 Specifications

It is not the intention of this manual tolist all the types and specifications as therange is almost unlimited with various

combinations of HT (high tensile) andMS (mild steel) can be used. Boundaryfences must be stockproof and built tolast a long time.

The standard fence has seven wires,heavy strainers, posts and battens (orwarratahs/metal standards) with posts125mm or more about 4m apart. In thehigh country on areas subject to snow,steel standards, T irons and other sturdymaterial should be used. On the roughterrain standards/posts should be closertogether with the wire strains being a lotshorter eg 200-240 meters. If electricfencing is to be used it should be of astandard that if the power is off thefence is still stockproof (Note electricfencing is not recommended for highaltitude sites particularly retirementfences).

An alternative where snow drifting is aproblem, is to construct speciallydesigned fences that collapse in total (egthe “Hurricane chain fence”) or wherethe wires loosen off. Nevertheless,maintenance of these snowline fencesand other high country fences must becarried out on an annual basis to ensurethey exclude stock from erodible areas.

Descriptions and illustrations follow forfences that are specifically to controlgrazing for the purpose of soilconservation.

3.3.1 Cattleproof fence

This is an existing fence (such as asummer snowline fence) upgraded to thestandard of a new fence that will allowcattle to be grazed. It may involvewooden posts, metal standards andbarbed wire or electric top wire. Thisenables cattle to be held within the blockthus facilitating more even grazing of thetussock cover and reducing the need toburn.

3.3.2 Recuperative spelling fence

This is a fence erected on lower altitudecountry where pasture has been depletedby sheet, wind and frost heave. It is oftendry sunny land that has a short growingseason ie early spring flush with grassseeding later when the vegetativematerial dies out and exposes bareground. Once a fence is constructedoffsite feed provisions need to beplanned so the stock can be completelyremoved from the block for a year or inthe critical summer seed set time,enabling pasture recovery. Offsite feed


may be from a fodder bank or irrigatedpasture.

3.3.3 Erosion Control Subdivision or

Conservation fence

These terms are synonymous with oneanother. The fences are used to separateeroded from non eroded areas, or toseparate shady from sunny faces. Theyfacilitate better grazing control, enablingeroded or sunny faces to be grazed forshort periods, and stock to be moved ofwhile pasture recovers.

3.3.4 Rabbit Proof fence

This type of fence can be made byplacing rabbit netting (usually 40mmdiagonal measure netting) over anexisting fence, or constructing a newfence (on the property boundary orwhere rabbit numbers are high). It isessential to bury the bottom of thenetting approx. 150mm into the ground,or if the surface is rocky then turn up onthe ground and pack well down withstones to prevent rabbit burrowing. Gatesshould be swung close to a concrete orboard sill buried in the gateway so thatrabbits cannot wriggle underneath. Onceerected, follow up poisoning, or usingthe RHD virus or other techniques suchas shooting can be more effectivelycarried out to reduce the rabbit numbersand allow revegetation to take place.

3.3.5 Retirement fence

To specifically exclude all stock grazingfrom severely eroding land. This can taketwo forms. One is at high altitude, whereseverely eroded land is retired and left tonaturally revegetate. This usually takesmany years ie 15-20 before naturalseeding and ground cover starts to show.Ongoing fence maintenance and feralanimal control is essential forrevegetation to succeed. The other formis on low altitude country that has beeneroded. Here protection forestry species(for slope stabilisation and control ofsedimentation) are established after allstock have been removed.

It is essential that all matters areconsidered when planning a retirementfenceline. Walk all lines before handwith an accurate measuring wheel notingall dips, angles, corners, where snow maylie and drift, gateway sites and creekcrossings (floodgates, netting). Draw aprofile of the line to assist in costingsand type of material required. From thefield notes a schedule of all the materials

can be made and this is useful in pricingand tendering for materials and labour.

On the operational side, fencing gearmay be best bundled and dropped by ahelicopter. This will negate the need toconstruct unsightly access tracks, whichmay initiate further erosion. Each dropsite should be well flagged and accessiblefor the fencing contractors. Foradditional information on specificationsand design contact the local RegionalCouncil. Any retirement fencing shouldbe designed and constructed to last aslong as possible, therefore the costs willbe higher than other fences due toadditional tiedowns, strainers and thelike, being required.


Rabbit-proof fence. Photo: ORC.

Retirement fence, showing weed control as

well as bush remnant. Photo: John Douglas, Environment BOP.

Revegetation & Reversion

Chapter

4. Pasture Revegetation Practices

5. Sand Dune Stabilisation

6. Burning Management – Mountain Lands

7. Fodder Bank Establishment

8. Managed Reversion of Retired Land

4.1 Lowlands

Lowland pasture revegetation may takeplace as a pasture renewal programme,following a cropping season at a wholepaddock scale, or smaller filling in‘patches’ from localised erosion. Lowlandpastures are generally dominated byperennial ryegrass and white clover,other species can also be expecteddepending on the demands placed onthe pasture by management andenvironmental factors.

4.1.1 Guidelines for lowland pasture

revegetation

(Adapted from Langer, (1990), Pasturestheir ecology and management)

TimingWhen water and nutrients are non-limiting, temperature plays a dominantrole in the germination of herbageplants.

Traditionally new pastures are sown inthe autumn. Autumn sowing allows theseed to go into a warm but sometimesdry seedbed, but with decliningtemperatures. These conditions arenormally satisfactory for ryegrass, butspecies more sensitive to cool conditionsmay be disadvantaged.

Spring sowing will ensure adequatemoisture and rising temperature, but theonset of early drought may cause someplant mortality where irrigation is notpossible.

FertiliserSoil tests should be undertaken beforedetermining fertiliser rates.

Phosphorus is essential for successfulpasture establishment, particularly tostimulate the growth of seedlinglegumes. Generally applied at 20-35 kgP/ha when sowing, for some soils such asyellow-brown loams applications as highas 45-60 kg P/ha may be necessary.Superphosphate is often used, wheresowing inoculated legumes, a revertedform should be considered as the

strongly acid superphosphate maydamage rhizobia.

Lime should be used to bring the pH ofthe soil to 6.0 or above to assist in theestablishment of well-nodulated legumes,it is less important for grasses as they cantolerate lower pH levels.

Sulphur deficiency is widespread in NZsoils, but is usually corrected by 22-34 kgS/ha. Generally most deficiencies areovercome when superphosphate isapplied at sowing. On soils such asbrown-grey earths, where sulphurdeficiency is much greater than that ofphosphorus, sulphur-fortified super maybe used. Elemental sulphur can be usedwhere no phosphorus is required.

Potassium is often deficient on volcanicsoils, although soils from greywacke aremore commonly adequate in potassium.Legumes are less competitive than grassesin extracting available potassium fromsoil, and a deficiency can therefore resultin poor legume establishment unless it iscorrected by applying KCl or potassicsuperphosphate.

It is uncommon to apply nitrogenfertiliser when sowing new grass. Thestimulus the grass component receivesfrom the nitrogen may causeconsiderable competition with theaccompanying clovers.

Seedbed preparationWhen undertaking conventionalcultivation the aim is to achieve a fineand firm seed-bed, so that the small grassand legume seeds make good contactwith moist soil particles. Seeds shouldnot be sown more deeply than 50mm,and frequently 25mm is moreappropriate for small seeds.

Other guidelines for direct drilling,minimum tillage and conventionalcultivation are covered in Chapter 1.

SpeciesA brief description of the mostpredominant pasture species is givenhere, seeding rates and mixes are notdiscussed. For more details on the


CHAPTER 4

Pasture Revegetation Practices

suitability of different species, readers arereferred to Burgess and Brock (1985), TheProceedings of the NZ GrasslandAssociation.

Perennial ryegrass, generally dominateslowland pasture mixes. Generally rapidlyestablishing and requires high soilfertility, can struggle when faced withdrought and insect damage. Severalcultivars can withstand heavy treadingand continuous grazing.

Italian ryegrass, although an annual,many types do persist for several years.Noted for their rapid establishment,which can jeopardise the establishmentof other species. Requires high soilfertility and shows good growth in theautumn and winter.

Cocksfoot is well adapted to moderatefertility and low soil moisture. Somecultivars show tolerance to grass grub,and once gully established to argentinestem weevil.

Tall fescue, while slow to establish, doesshow a wide range of adaptability. Mosttall fescue show reasonably good droughtresistance and little frost damage duringwinter. Tolerant of acid and alkalinesoils, withstand poor drainage, and somecultivars show resistance to grass gruband Argentine Stem Weevil attacks.

Phalaris is slow to establish, but onceestablished withstands hard grazing andwinter pugging and may persist for manyyears. Does well over cool seasons, but indry summer conditions may becomesemi-dormant.

White clover, a wide range of cultivarsare available with varying tolerance tosevere grazing and treading. Generallyrequires soils of medium to high fertility,with evenly distributed rainfall. About200 kg N/ha/yr can be expected to befixed from the atmosphere.

Red clover, is easily recognised by itstaproot, which aids in its tolerance to dryconditions. Requires high fertility andshows resistance to pests and diseases.

Grazing managementThe first grazing of a new pasture isimportant and should be done beforerapidly establishing species are able toshade out the less vigorous seedlings, thefirst grazing would normally take placesix weeks following sowing whenryegrass is about 10cm high. Grazing

should be done with a highconcentration of light stock for a briefperiod. Frequent defoliation for the first6-9 months will help the slowerestablishing species, especially legumes.

4.2 Hill Country

Slip oversowing involves the applicationof seed and fertiliser to bare ground leftafter the soil and sub soil has beenremoved by erosion. It is a temporaryrepair, pending spaced planting of treesor retirement from grazing to ensurepermanent stabilisation.

The treatment of slip scars by oversowingdates back to the 1940s with work bySuckling and others at Te Awa in thePohangina Valley in the Manawatu.(Suckling 1962) It was found that anumber of factors lead to the success orfailure. Available moisture was critical,followed by seed mix, fertiliserapplication and grazing control.

The advent of aerial sowing by fix wingaircraft, (McCaskill 1973) and laterhelicopters, led to larger operations inHawkes Bay, Manawatu and Wairaraparegions after major storms. The costs ofdoing this work were high but in manycases subsidies reduced costs to farmers.Trails in the Whangaehu Valley in theManawatu resulted in the restoration ofslip scars to half of the production fromuneroded areas, 7 to 8 years after a stormevent (Garrett, 1981). Similar results wereobtained in post Cyclone Bolaoversowing trials in Hawkes Bay. (Quilter1993) and (Lambert 1993) Althoughpasture growth will take decades torecover, and possible never reach preerosion growth, research has shown thatrecovery time can be shortened by 20years with the right management. (HBRC1999)

It was common practice during the 70sand 80s for subsidies to be madeavailable through Catchment Boards forslip oversowing after an extreme storms.Oversowing was carried out from EastCape to the Wairarapa during thisperiod.

The establishment of pasture on slip sitesis usually very difficult because little soilremains and skeletal soils on eroded sitesare subject to extremes of wetting anddrying. Root development is restricted bythin or non existent soils and the plant’sability to obtain nutrients is very limited.


4.2.1 Guidelines for hill country pasture

revegetation

Factors to be considered are:

MethodsSlip oversowing can be done by handapplication, or for larger areas byaeroplane. A helicopter is the mostefficient as many small areas can betreated at one time, especially ifconcentrated fertilisers are used, and theseed-fertiliser mix can be better targeted.Pelletised seed and fertiliser will aidgermination and initial growth. Somefarmers have placed a large mob of sheepon the area seeded for a short period toassist in bedding in the seed.

Moisture LevelsTreatment is recommended only whenthere is adequate soil moisture, or whenthere is a reasonable chance of rainfalloccurring soon after the operation.

Seed MixturesSeed selections should include a mixtureof legumes and grasses, which aretolerant of dry conditions and lowfertility. Lambrechtsen (1987)recommends the mixture in Table 4.1.The clovers must be inoculated and allseed preferably pelleted.

Table 4.1 Seed mixtures

Kg per ha Species

6-10 Ranui perennial ryegrass

4-6 Huia white clover

1-3 Mt Barker or Tallarook subclover

Fertiliser Mixes and RatesFertiliser must be applied with the seedalthough there is some debate as towhether nitrogen needs to be applied ifinoculated clovers are included in theseed mixture. Initial rates of 150 to200kg/ha are suggested, usuallysuperphosphate with added mineralsdepending on the soil requirements.Diammonial phosphate at 100 to 150kg/ha has been used with a repeatapplication 6 months later.

Grazing ControlThe control of grazing by farm and feralanimals is considered essential for thesuccess of oversowing of slips. This canbe difficult if large areas are treated atonce, or there are large paddocks. Sheep

will overgraze these sites, although theymay also transfer some fertility.

A 12 months grazing restriction is theminimum suggested by most trials andlarger scale operations.

The oversowing of slip scars on Class VIIhill country has been carried out in smalltrials and larger scale programmes forover 50 years. Some of this work hasbeen highly successful and has hastenedthe recovery of pasture production, whileother examples have had minimal effecton recovery due to other factors such asweather, overgrazing and site exposure.At the best a 50% recovery after 5 to 10years can be expected.

4.2.2 References

SUCKLING, F.E.T. (1962)., Te Awa SoilConservation Experimental Area,Manawatu Catchment Board and SoilConservation and Rivers ControlBoard, Wellington

HAWKES BAY REGIONAL COUNCIL,Environment Topics, Repairing SlipDamage 1999

Lambrechtsen, N.C. 1987 Grasses andLegumes for soil conservation, PlantMaterial Handbook.

4.3 Mountain Lands

4.3.1 Introduction

Revegetation has been one of the mostimportant erosion control methods usedin the high country since the 1950swhen topdressing and oversowing ofbare depleted faces first started. Theearlier techniques were basic but astechnology improved e.g. helicopterapplication, using coated seeds withinoculum, and fertiliser, more suitableplant species, better ground cover hasbeen established for longer periods.

When revegetation is carried out,alternative feed is provided to stock oncethey have been completely ortemporarily removed from an erodedblock. Offsite feed can be anywhere onthe property but is normally on thelower altitude soils. Offsite feed can bepasture improvements, or supplementaryfeed, or an alternative fodder bank.

• The following practices can be usedto revegetate eroded faces:

• Cultivation, direct drilling, minimumtillage


• Aerial Oversowing and Topdressing(AOS &TD)-pasture species

• Native plant establishment (mayinclude hand planting)

• Forestry protection & productionplanting

• Dryland pasture & alternative cropproduction.

Historically there was a smaller range ofplants to use in the 1940s and 1950s, butadvanced research in plant selection,seed pelleting (with inoculum) andmethod of application has enabled betteradapted plants for a variety of sites.

There is a multitude of research andpublications highlighting various speciesfor a particular site. The followingguidelines identify key legumes, grasses,herbs and shrubs that can be safely used.

4.3.2 Guidelines for High Country

Revegetation

• Follow the illustrated table forcommon species

• Legumes before grasses (select byspecies, and don’t worry aboutcultivars)

• Most reasonable success is up to800m above sea level (above thisresults are variable – depends on frostheave/shelter)

• Expect poor survival in harsh winters,be prepared to re-sow

• Well established species

Legumes

alsike clover

white clover

red clover

lucerne

Maku lotus

Grasses

cocksfoot

browntop/sweet vernal

chewings

timothy

perennial ryegrass

Yorkshire fog

• Species showing potential

Legumes

caucasian clover

perennial lupin

birdsfoot trefoil

canary

Grasses

tall fescue

tall oat

smooth brome

wheat grasses

Herbs & shrubs

salt bush

sheeps burnet

• Others which may have a specialised role

Legumes

zig-zag

crown vetch

sub clover

sweet clover

new trifoliums

Grasses

red fescue

phalaris

brome


Figure 4.1 The most suitable pasture species

in relation to temperature, soil moisture and

levels of soil fertility.(After D. Scott and Keith

Widdup, AgResearch, Lincoln)

• Seeding rate varies considerably,depending on the size of seeds andenvironment and can range from 6 –10kg/ha.

Fertiliser RecommendationsYellow- brown earth (brown soils): 250kg/ha moly super in year one and 250kg/ha Superphosphate every 2-3 yrs thenlime as practical

Yellow-grey earths(Pallic soils): 200 kg/hamoly sulphur super (28% S) for 2-3 yrsOr 150 kg/ha moly sulphur super (33%),lime as reqd.

At high altitudes more fertiliser will berequired due to the greater leaching ratesbut the economics will need to beconsidered. Soil tests are vital to ensuresunny and shady faces receive their ownfertiliser maintenance recipe mix.Inaccuracy of fertiliser application is notgood where bare ground is beingrevegetated, as stock can congregate onthe fertilised strips and overgraze thenew plants. Use precision equipment iehelicopter with high analysis materials.Avoid waste application on non-productive sites.


5.1 Marram Planting

HistoryAs a primary sand stabiliser, marramgrass (Ammophila arenaria) has beengiven most prominence in the past 80years. Although it is a plant introducedfrom Europe, it has proven itself as asand dune stabiliser throughout NZ. It isalso found in some inland areas: on theVolcanic Plateau and at Erewhon Park inthe Rangitata Valley. Since the early1920s, it has been planted, more recentlywith mechanical planters, to stabilisesand dunes because it is easy topropagate and establish. With theincreasing public concern about the‘natural character’ of sand dunes, otherplants are gaining more prominence.They will be discussed separately. TheNorth American Ammophilabreviligulata has been tried in NZ butwas found to be less effective than theEuropean A. arenaria.

DescriptionMarram grass (or European beach grass)is a tall, erect, spring and summergrowing perennial grass, formingcompact clumps with 60-80 cm highculms. It spreads rapidly through loosesand by strong rhizomes, several metreslong, which send up new shoots throughaccumulated sand and with new rootsforming at the nodes. The leaves arebluish green, up to 6 mm wide, ribbedon the upper side and sharply pointed.They are usually tightly rolled up tominimise transpiration. The flowerheadsare compact, narrow, cylindrical spike-like panicles that develop fromDecember to January. The seedheadswhen ripe are green yellow but marramgrass does not set much seed under NewZealand conditions. It is vegetativelypropagated by splitting two-year oldplants into tufts.

5.1.1 Practice

Marram grass is always propagatedvegetatively by transplanting bundles ofculms gathered from vigorous 2- or 3-year old clumps.

Planting material can be collected fromwell-established existing stands, or from

special selected sheltered “nursery” areaswhere the marram grass production isboosted by extra fertiliser applications ofnitrolime (calcium ammonium nitrate)[or equivalent] at 125 kg/ha in springand autumn. Or it can be obtained fromsome commercial nurseries, often byprior arrangement.

The planting stock is dug with a sharpspade 5-10 cm below the surface. Thetillers should have at least 2 nodes andare trimmed to about 50-60 cm. Some 5to 8 culms per bundle are planted in a20-25 cm deep V-shaped hole and wellheeled in. Spacing varies from 60 x 60cm to 100 x 100 cm depending on thedegree of wind exposure. Planting is inrows at right angles to the prevailingwind (preferably on the contour) whilethe bundles should be staggered in therows to prevent wind funnels. Plantingtime is June, July and August. Small areasand steep dunes are planted by hand,while more extensive areas are machine-planted, using specially developedmarram planters pulled by a crawlertractor, or a 4-wheel drive tractor withextra wide tyres.

Nitrolime (calcium ammonium nitrate)[or equivalent] applied at 45 kg/ha inspring and autumn will increase growthand early establishment.

Marram grass is more suitable forplanting on top of the foredune, and onthe main dunes and blowouts furtheraway from the coast. It has also beensuccessfully used to stabilise inland sanddunes near Cromwell and to controlblowing pumice sand along the DesertRoad, north of Waiouru.

Marram grass is unpalatable to stock.

Except for topdressing, the exclusion ofstock, and the speedy repair of any blow-out that may occur, no specialmanagement practices are required.Snails can be troublesome in newlyplanted areas and may need to becontrolled.


CHAPTER 5

Sand Dune Stabilisation Practices

5.1.2 Application

Where sand requires stabilisation withmarram planting, the following stepsshould be taken first:

1 ContouringWhere possible, shape the sand dune tocontours that blend in with thelandscape and make the area less proneto the erosive forces of the wind. Anywind funnelling should be avoided andprevented. Where slopes are less than15°, contouring can be done with abulldozer, or possibly even with a tractorand blade.

Experience with repairing breaches in theforedune of major beach systems in NewSouth Wales, Australia, following stormdamage suggests that it is often morecost-effective to use heavy earthmovingmachinery to reshape the beach andrebuild the foredune.

2 Temporary RetirementThe area has to be fenced beforehand tokeep stock out. A standard 2-wire electricfence will keep out most cattle, so longas it is maintained and the power is kepton all the time. Since possums, hares andrabbits can still get into the area to bestabilised, they will need controllingalso, unless there is good control in theadjoining areas.

3 Binding The SandOn actively moving dunes in windsweptareas, only a dense cover of marram grasswill bind sand sufficiently for otherplants to establish.

5.2 Spinifex planting

Spinifex sericeus (formerly known as S.hirsutus Labill.) is a native of the NewZealand and Australian coasts where itoccurs naturally on the seaward side ofthe foredune. It diminishes towards thesouth of the South Island and is notfound much below Dunedin. With theincreasing public concern about the‘natural character’ of sand dunes, thisgrass is gaining more prominence and itis often used in preference to marramgrass. The use of Spinifex requires long-term planning and commitment.

Spinifex has been described verycomprehensively in the CDVN TechnicalBulletin No. 2 “Spinifex on coastal dunes– Guidelines for seed collection,propagation and establishment” by

David Bergin. See especially pages 21-25“Recommendations”.

5.2.1 Description

Spinifex is a 30 cm high stoloniferousperennial grass with stout overgroundcreeping runners (stolons), several metreslong which develop roots and leafyshoots at the nodes. The leaves aresilvery hairy, 7-8 cm wide, with incurvedmargins and a fine tapered point.

It is one of the few dioecious grasses, themale and female flowers developing ondifferent plants in the spring. The maleflowers are arranged in loose terminal oraxillary clusters. The female flowersdevelop into large 15 cm round, spinyterminal seedheads which when ripe inmid summer break off and are blownalong the beach by the wind. Volunteerseedlings are often found amongstdriftwood above high water mark.

Sometimes large colonies of plants ofonly one sex are found; these havegrown from one single seedling.

5.2.2 Practice

Spinifex can be established from seed orfrom cuttings made from runners. Seedgermination and strike rate of cuttings isoften disappointing.

Seed is best sown in January-Februarywhen the seedheads can be collectedfrom the beach, either as completeseedheads or as individual seeds removedfrom the seedheads. Seed should beburied 2.5-4 cm deep. Seed sown inspring gave much better survival andgrowth. Soaking the seed for 24 hours inseawater will improve germination. Seedcan also be collected in mid to latesummer and then sown in a nursery forplanting out later, which may take 9-15months. All seed should be free of smutfungus (Ustilago spinificis), whichdestroys the embryo. Or seed can becollected and stored in jute or papersacks for sowing in the next spring.

Cuttings, particularly tip cuttings, 50-60cm long, prepared from vigorousgrowing stolons are planted in earlyspring, some 20-30 cm deep and at aspacing of 1 x 1 m. For increasedeffectiveness, it is better to grow thecuttings first in a nursery with bottomheat and misting, and treatment withrooting hormones and a high phosphatestarter fertiliser. Then a strike rate of 80%


can be achieved, and plants will be readyfor field planting in 18 months.

Spinifex can also be established bytransplanting volunteer seedlings,carefully dug up without disturbing theroot system, but the above nurserypropagation techniques are moreeffective. They require long termplanning and commitment.

Planting is at similar densities as thosefor marram: 50-60 cm in windy sites; upto 1 metre in more sheltered sites. Atleast 5 cm of sand should be above thepotting mixture at planting. Theaddition of slow release fertiliser (e.g.Magamp) at about 30 g per planting holeand mixed in with the sand, assists plantestablishment and vigour. Urea and other‘strong’ fertilisers should be avoided.

Time of planting: spring or autumnplanting gave similar results in the FRItrials, but summer planting should beavoided. Early spring planting [June toearly August], well before the equinoctialgales, may be the best time because ofmore favourable temperature andmoisture conditions.

Spinifex is less palatable than pingao, butmay still be browsed by rabbits. Plantedareas should be protected fromdisturbance by beach users through theerection of barriers and walkways;grazing animals should be excluded byfencing.

Maintenance should be by regularinspection of fences, walkways, andreplanting where plants have died.Without a uniform vegetative cover,wind may dislodge sand again. Fertiliserapplications on established stands at 200kgN/ha increased cover by up to 52%.One application remained effective for 3years.

5.2.3 Application

Spinifex is mainly used for foreduneplanting, especially to repair gaps in thevegetative cover on the seaward side. Forexample, near walkways and other highuse areas, where closer planting may bedesirable. Without disturbance, theextensive stolons grow down slopes andin hollows collecting sand andsmoothing out the duneface.

Because of the need for nurserypropagation of Spinifex for its effectiveestablishment, the plant lends itself to

community involvement, such asBeachcare or Coastcare groups. It is lessreadily applied to large-scale revegetationprojects, unlike marram grass. Thepreparatory steps outlined for marram,apply here also, with possibly greateremphasis on local communityinvolvement.

1 ContouringSince Spinifex is more commonly usedon the foredunes, it is even moreimportant to shape the sand dune tocontours that blend in with thelandscape and make the area less proneto the erosive forces of the wind. Anywind funnelling should be avoided andprevented. Where slopes are less than15°, contouring can be done with abulldozer, or possibly even with a tractorand blade.

Experience with repairing breaches in theforedune of major beach systems in NewSouth Wales, Australia, following stormdamage suggests that it is often morecost-effective to use heavy earthmovingmachinery to reshape the beach andrebuild the foredune than using manuallabour.

2 Temporary RetirementThe area has to be fenced beforehand tokeep people and stock out. A standard 2-wire electric fence will keep out mostcattle, so long as it is maintained and thepower is kept on all the time. Timberrailings and timbered walkways will benecessary to control pedestrian traffic.Since possums, hares and rabbits can stillget into the area to be stabilised, theywill need controlling also, unless there isgood control in the adjoining areas.

3 Binding The SandOn actively moving dunes in seawardareas, only a dense cover of Spinifex,usually with pingao, will bind sandsufficiently for other plants to establish.

5.3 Pingao planting

HistoryDesmoschoenus spiralis (pingao) is asedge that occurs only in New Zealandwhere it can be an effective sand binderon foredunes, but it is much moredifficult to propagate, so that it has notconsidered as a major stabilisationspecies. It has always been valued inMaori weaving for its yellow colour.With the increasing public concern


about the ‘natural character’ of sanddunes, this sedge is gaining moreprominence and it is often used incombination with Spinifex and inpreference to marram grass to stabiliseforedunes and other seaward-facingsituations. The use of pingao requireslong-term planning and commitment.

Recent research by FRI has shown thatpingao can be used as a sand stabiliser.See CDVN Technical Bulletin No. 1“Pingao on coastal sand dunes” by D OBergin & J W Herbert for detailedinformation; especially pages 18-19“Recommendations”.

5.3.1 Description

Pingao, also called golden sand sedge, isendemic in NZ, the only representativeof its genus. It forms rope-like, ratherwoody rhizomes on foredunes; its tuftsof coarse, grass-like curved leaves are 2-5mm wide, 30-90 cm long, smooth andvarying in colour from green to orange;its culms are triangular in cross-sectionand 30-90 cm long. The seedheads arepanicles, which carry nut-like seeds thatare smooth and dark brown. The seedsare arranged in a spiral pattern.

Pingao was common throughout NZ butalways rare in Southland. Nowadays,there are only scattered populations ofpingao, due to grazing and browsinganimals, and competition with marram.It is often associated with Spinifex,which grows in similar situations.

Vigorous stands of pingao are observedwhere there is sand movement as on theseaward faces of foredunes. Young plantscan withstand 10-20 cm of sandaccumulation, whereas well-establishedstands can tolerate up to 70 cm of sandannually. But excessive accumulation orerosion causes dieback. Esler found onthe Manawatu coast that pingao was lessresistant to sand removal than othersand binders [Spinifex and marram],although its woody rhizomes maypersist.

FRI studies have shown that there aredistinct provenances of pingao, somewith a sprawling rhizomatous habit, andothers with a more tussock-like form.Others differed markedly in frost-tolerance and important differences inweaving properties were also found. It istherefore important that local materialbe used in revegetation plantings.

5.3.2 Practice

Pingao is best established from seedlings.Planting from rhizome cuttings is muchless reliable. Seed can be collected inlarge quantities during December to earlyJanuary, depending on summertemperatures. The seed should be shinybrown-black when collected. It should bestored in jute or paper bags, not plasticbags. The seed germinates readily withhusks still attached. Seedlings should be9-15 months old by the time they areplanted in autumn or very early spring.

Time of planting: some people preferautumn planting; others winter or earlyspring planting. In one trial, springplanting resulted in a much highersurvival rate of pingao. At planting, theplants should be 25-30 cm tall and haveat least 5 leaves. As for Spinifex, theyshould be planted with at least 5 cm ofsand above the potting mixture; 30 g ofslow-release fertiliser (e.g. Magamp)should be mixed in with the sand in theplanting hole. Planting densities as forSpinifex and marram: 50-60 cm in windysites; up to 1 m apart in more shelteredsites, and when using the rhizomatousstrains.

Maintenance should be by regularinspection of fences, walkways, andreplanting where plants have died.Without a uniform vegetative cover,wind may dislodge sand again. Fertiliserapplications on established stands at 200kgN/ha increased cover markedly. Sincebrowsing by rabbits and other animals isa major cause of pingao failure, controlof rabbits and hares is essential, as is theexclusion of grazing animals, such ascattle, sheep, goats and deer. Theplanting sites should also be protectedfrom disturbance by beach users. Suitablebarriers and walkways should be erected.

5.3.3 Application

Just like Spinifex, pingao is mainly usedfor foredune planting, especially inclusters to simulate its natural growthpatterns. Since excessive sandaccumulation or erosion will retardgrowth or cause dieback, survival andgrowth of pingao can never beguaranteed. Where strong on-shorewinds cause high rates of sandmovement, pingao should not be used.Marram would be more suitable. Whilemarram could be used as a “nurse plant”for pingao, there is evidence that it couldout-compete pingao, particularly onexposed sites. The use of pingao should


therefore be more carefully consideredbeforehand than that of Spinifex andmarram.

Because of the need for nurserypropagation of pingao for its effectiveestablishment, the plant lends itself tocommunity involvement, such asBeachcare or Coastcare groups. It is lessreadily applied to large-scale revegetationprojects, unlike marram grass. Thepreparatory steps outlined for marram,apply here also, with possibly greateremphasis on local community, includingiwi involvement, if the pingao is to beused for weaving once well-established.

1 ContouringSince pingao is more commonly used onthe foredunes, it is even more importantto shape the sand dune to contours thatblend in with the landscape and makethe area less prone to the erosive forcesof the wind. Any wind funnelling shouldbe avoided and prevented. Reshapingwith machinery should be considered ifmanual methods are likely to beineffective or too costly. Temporaryshelter in the form of straw/hay balesand shelter cloth may be appropriate, ifthe likelihood of successful pingaoestablishment is high.

2 Temporary RetirementThe area has to be fenced beforehand tokeep people and stock out. A standard 2-wire electric fence will keep out mostcattle, so long as it is maintained and thepower is kept on all the time. Timberrailings and timbered walkways will benecessary to control pedestrian traffic.Since possums, hares and rabbits can stillget into the area to be stabilised, theywill need controlling also, unless there isalready good control in the adjoiningareas.

3 Binding The SandOn actively moving dunes in seawardareas, only a dense cover of pingao,usually with Spinifex, will bind sandsufficiently for other plants to establish.

Note:The CDVN Technical Bulletin No. 3“Sand tussock on coastal dunes –Guidelines for seed collection,propagation and establishment” byDavid Bergin, describes the use of SandTussock in great detail. But since thisgrass “plays a minor role in sandstabilisation and foredune development”,

it has been decided not to include it inthese “Soil Conservation Practices”.Readers who wish to use this grass areadvised to consult the Technical Bulletin.

5.4 Coastal forest and pasture

planting

Stable sand dunes were covered withnative shrubs and trees before the arrivalof man. Few areas are left with relativelyundisturbed coastal forest; many areashave been converted to a planted forestof Pinus radiata, usually with a belt ofmacrocarpa facing the seawinds.Increasingly, local residents want toconvert coastal areas to the vegetationthat once was there. To determine thecomposition of that vegetation requireslocal expertise or knowledge that shouldguide planting options and strategies.Long-term planning and commitment isessential for those locals who wish toembark on this path with assistance fromlocal/regional councils and scientificadvisers.

5.4.1 Description

This section will deal with a selection ofnative and introduced plants that areknown to be tolerant to coastalconditions, and with a planting strategythat may bring success. A prerequisite isthat the area to be planted has alreadybeen stabilised with the grasses/sedgedescribed in the previous sections.

5.4.2 Practice

Once the dune has been stabilised,planting a permanent ground cover ofshrubby or even tree species can beattempted. These species need to behardy and persistent, and preferablynitrogen fixing. Yellow tree lupin(Lupinus arboreus) was commonly usedbut it has been affected by the fungusColletotrichum gloeosporioides in recentyears, so that it is less common today.But it appears to be coping with theblight, and its use should bereconsidered. Alternative shrubbylegumes, such as tree lucerne(Chamaecytisus palmensis) and someshrubby wattles (Acacia sophorae and A.cyanophylla) are known to survive incoastal areas. The latter are currently notrecommended because they spread bysuckering and seeding, and can suppressother vegetation. And tree lucernerequires reasonably sheltered locations.


Whether one wishes to use theseintroduced legumes to enrich the sandyenvironment with nitrogen prior toplanting shrubs or trees, is a localdecision, dependent on local regulations,if any. The alternative is to use slow-release NPK fertilisers at planting, asdescribed in the previous sections.

1 Native SpeciesNative species may be planted forpreference, or for conservation areas, orto complement areas set aside as reserves.The following native species are windand salt tolerant, and will eventuallyform a dense stand. They can form thenucleus of a coastal forest and have beenranked in order of preference.

Phormium tenax Flax

Coprosma repens Taupata

Pittosporum crassifolium Karo

Pittosporum tenuifolium Kohuhu

Griselinia littoralis Papauma or Kapuka

Myoporum laetum Ngaio

Metrosideros excelsa Pohutukawa

Muehlenbeckia complexa Pohuehue

Cortaderia fulvida / toetoe Toetoe

Other native species are harder toestablish on sand dunes, or may not befound in the particular botanical district.Advice should then be sought from theDept of Conservation, and/or landmanagement officers of regionalcouncils.

2 Permanent TreesAs an alternative to the use of nativespecies, a permanent tree cover can beplanted to provide shelter, or a tree crop,or a mixture in the form of agroforestry.In general, a mixture of native andintroduced species is recommended withthe choice of species rather dependenton what the land manager can afford,and desires as end use of the trees.

3 Permanent PastureTo establish and maintain permanentpasture on sandy soils can be verydifficult because such soils usually havethese features:

• Low natural fertility

• Inability to hold moisture

• Lack of organic matter

• Proneness to erosion

These features, together with exposure tostrong, often very cold salt-laden winds,dry summer conditions, and oftensignificant areas of steep contour, meanthat special management techniques areneeded to establish and maintain apasture cover that will providesustainable stock grazing and avert therisk of wind erosion. When pasture hasbeen depleted, sand will start to blowagain.

5.4.3 Application

1 Native PlantsThe native plants, preferably obtainedfrom local seed sources, should behardened off one month prior toplanting between May and August at 2-3metre spacing among the grass/sedgevegetation which has effectively stilledsand movement. The earlier they areplanted, the better. The addition offertilisers such as nitrolime and magampwill help establishment and survival.Controlling weeds around the plants intheir first year will also help to ensuresurvival until they are big enough tosuppress weeds. Kikuyu grass may haveto sprayed carefully with a herbicidesuch as “Gallant” or equivalent; it tendsto smother plants otherwise.

Since native plants are quite palatable tostock, an effective permanent fence is amust.

2 Permanent Trees

Shelterbelts

For successful plantings, a permanentfence is a pre-requisite. The area shouldbe about 7 metres wide for best resultswith an appropriate mixture of plants, atright angles to the prevailing wind. Thenarrower the area, the less effective theshelter, and the more likely that stock,especially cattle and horses, will reachover the fence and damage the plants.

During May to August, plant a sequenceof flax (Phormium tenax), pohutukawa(Metrosideros excelsa), taupata(Coprosma repens) or ngaio (Myoporumlaetum), and macrocarpa (Cupressusmacrocarpa), with flax facing the seawinds.

Woodlots

The hardier timber species, used for farmforestry, can also be considered in coastalareas when they are salt wind tolerant:


Pinus radiata Radiata pine

Pinus pinaster Maritime pine

Pinus muricata Bishop pine

Pinus nigra Corsican pine

Cupressus macrocarpa Macrocarpa or Monterey cypress

Cupressus leylandii Leyland cypress

Eucalyptus botryoides Southern mahogany gum

Cypresses and pines usually achievebetter growth rates and forms in a coastalenvironment. The growth of gums andwattles can be highly variable, dependingon site conditions. Silver poplar andsome shrubby willows can be used toprovide shelter for more productive treecrops. Other poplar and willow speciesare not recommended because of poorgrowth form and susceptibility to saltspray. From 100-500 metres landwardfrom the foredune or cliffs, spray burnand wind shear will stunt timber species.But beyond 500 meters, normal growthrates and forms can usually be obtainedbehind the shelter provided by the treesto windward.

In the autumn prior to planting,establish suitable planting sites in theexisting vegetation. Do not clear thatvegetation if a new phase of winderosion is to be avoided. Plant treeseedlings at 1000 stems per hectare (3-metre spacings) between June andAugust.

For management, do not thin or prunetrees in the zone 100-200 meterslandwards from fore dune or cliff;maintain as a protective shelterbelt.Between 200 and 500 metres, a standardpruning and thinning regime can beapplied to pines and cypresses. Prune upto 6 metres in successive lifts, and thinto a final stand density at 200-400 stemsper hectare.

Agroforestry

At sites more than 500 metres from foredune or cliff, agroforestry can beconsidered, preferably if a protectiveplanting, or at least an adequateshelterbelt has been established in thezone from 100-500 metres. Any of thetimber species recommended forwoodlots can be used for agroforestry.Plant tree seedlings at 200-400 stems perhectare in June to August in sites pre-prepared during the autumn.

Sheep have to be excluded for 1-2 years;cattle for 3-4 years until the seedlings are

sufficiently high to escape browsing. Ifthis is impossible, the seedlings will haveto be individually protected to establishthe agroforestry block that will providehighly desirable shelter for stock.

While standard pruning and thinningprocedures can be followed, the risk ofcreating ‘top heavy’ trees should bewatched carefully in this windy, coastalenvironment. Top heavy trees have beenmuch more prone to breakage whenthinned to 50-100 stems per hectare.

On sandy soils, the loss of pastureproduction as the trees get heaviercrowns, is likely to be greater than thatrecorded on heavier soils. But thebeneficial shelter effect to stock is alsolikely to be greater in this coastalenvironment with its strong, often coldsea winds.

3 Permanent PastureTo establish pasture on sandy soils, theland should firstly be of an even contour.Shaping with bulldozer or tractor withblade to prevent wind funnelling may berequired beforehand. Then a cereal, suchas barley or oats, or annual lupin, whichestablishes quickly, should be sown as acovercrop in autumn, with the pasturespecies germinating more slowly underthe cereals. Careful grazing will berequired to maintain the covercrop,while allowing the perennial pasturespecies to come through.

To revegetate small areas such asblowouts, see sections on Marram,Spinifex or Pingao.

To maintain pasture on sandy soils, thekey components are:

• Soil fertility

• Pasture species

• Pest control

• Grazing management

• Choice of stock

Soil fertility

Sand country is naturally low in organicmatter and the major plant nutrients,NPK. The free draining nature of thesesandy soils causes rapid leaching offertiliser. While improvements in soilfertility and organic matter can beachieved over time, careful management


is required to maintain them. To improvesoil fertility, an appropriate fertiliserprogramme needs to be determined by

• Getting the soil and stock healthtested to determine their currentstatus

• Assessing nutrient losses from thefarm by determining current stockingrate, stock type and management,pasture yield and management, andsoil type

• Based on the above, working outfertiliser application rates for annualmaintenance and improvement ofsoil fertility status.

Because leaching of nutrients on sandcountry can be high so that it is difficultto build up the nutrient status of thesoil, the following practices should beconsidered:

• Slow release fertilisers e.g. RPR,especially on acid soils

• Organic fertilisers e.g. poultry manureto build up organic matter andmoisture and nutrient retention

• Potassium applications in the spring

• Nitrogen applications to coincidewith pasture growth e.g. spring andautumn

• Fertiliser applications little and often.

Pasture species

Kikuyu grass (Pennisetum clandestinum) isextremely common in dune regions fromWaikato and Bay of Plenty northwards,but may be found in dunes furthersouth. While it is frost-tender, it didpersist for 2 winters at Lincoln. As apasture grass, it needs heavy grazing inlate spring-early summer to maintainpalatability. On sand country, it can dieoff in severe summer droughts andbecome a fire hazard, or it can lead togaps in the sward that can then lead toblowouts. If not grazed hard, it can bevery competitive with existingvegetation, and seedlings of nativeshrubs and trees have very little chanceof establishment among kikuyu (Edgar &Connor 2000, p. 575). On the otherhand, its mat-forming growth habitmakes it a suitable species for erosioncontrol. Where kikuyu dominatespastures, efforts can be made tointroduce and maintain temperate

grasses to obtain better autumn andwinter production. The following pastureplants can complement kikuyu:

Temperate grasses

Perennial ryegrass Lolium perennevarieties

Annual ryegrass Lolium multiflorumvarieties

Cocksfoot Dactylis glomerata

Prairie grass Bromus willdenowii (B. unioloides)

Yorkshire fog Holcus lanatus

Tall fescue Festuca arundinacea

Phalaris Phalaris tuberosa

Legumes

White clover Trifolium repens

Red clover Trifolium pratense

Lucerne Medicago sativa

Subterranean clover Trifolium subterraneum

Birdsfoot trefoil Lotus corniculatus

Herbs

Plantain Plantago major or P. lanceolata

Chicory Cichorium intybus

These species are best introduced byundersowing into a kikuyu sward thathas been suppressed by heavy grazing,harrowing or spraying. In the absence ofa kikuyu sward, these species can beundersown into a covercrop such asbarley or oats, or annual lupins.

Pest control

Not only do pests such as hares, rabbitsand possums have to be controlled indune country if pasture is to establishand persist, but also slugs and snails, andvarious insect pests, such as grass grub,black beetle, black field cricket, andclover flea have to be controlled. Plantpests, such as thistles and ragwort canestablish in pastures, opened up by insectdamage.

Grazing management

Because of the difficult and changeableconditions prevailing in coastal sandcountry, grazing management has to becareful and flexible to cope with theseconditions that can lead to surpluses andsevere deficiencies in food availability. Arotational grazing system should bedesigned around:

• Adequate subdivision

• Adequate stock water and shelter


• Deferred grazing and spelling

Any incipient sand blows should befenced off and treated immediately withgrass/sedge planting (see sections onMarram, Spinifex or Pingao).

Choice of grazing stock

The choice of stock for sand country canhave a major impact on pasturepersistence, or the recurrence ofblowouts, and should be carefullyconsidered. In general, cattle damagecoastal pastures far more quickly thansheep, but light-weight cattle breeds withcarefully controlled grazing regimes maykeep such damage to acceptable levelsthat prevent recurrence of erosion.


6.1 Introduction

In the high country of New Zealandburning has been carried out for manycenturies. From the 1840s it was used toopen up rank tussock grassland, whichwas unable to be readily grazed by sheep.Without any controls, escaped firesfrequently burned several hundredhectares of erosion prone land. Stockwere commonly grazed back on theburnt areas, overgrazing any regrowthand camping on the sunny faces, thusaccentuating frost heave, sheet, and winderosion.

It is now recognised that burning ofvegetation should only be carried out inspecial circumstances and not used as aregular development tool each year.Alternative options may have minimalimpacts on the soil and water. Burningon large blocks is difficult to control andfraught with many risks if the windchanges.

Before deciding to burn, consider:

• The objectives for burning tussockand intertussock grasslands (landclearance, rejuvenation of nativepasture or removal of unwantedshrubs or weeds);

• What conservative burning practicesare appropriate given the various landtypes and vegetation being burnt;

• Time since the previous burn.Burnings impact on vegetationdepends on the history ofmanagement (burning of snowtussock should be only carried outonce every 12-15yrs. to avoiddepletion of biomass)

• Post fire management. It is essentialto burn when the ground is damp inthe winter months and spell the siteafterwards. Follow up AOS &TDwhere feasible on the lower altitudesites will be beneficial. Lenientgrazing in the first two yearsfollowing the burn, will assistregrowth. Ensure rabbits and haresare controlled on the sunny faces.

• Whether post-fire spread of ingressiveweeds e.g. hieracium spp., can becontrolled

• In the wetter Lakes regions wherefern and scrub are a problem burningmay need to be carried out in latewinter followed up with AOS & TDand erection of erosion controlfence(s) for ongoing grazing control.

• Have a Burning Management Plan inplace for a five year period(incorporate with the Farm Planprogramme).

6.2 Guidelines For Good Burning

Source: Otago Regional Council DraftCode of Practice for Vegetation Burningin Otago 2000

Safety and control (RiskManagement)• Ensure that the necessary consents or

permits are obtained from therelevant agencies and be familiar withyour insurance policy provisions.

• Seek advice from a meteorologicalservice prior to burning on theexpected weather conditions.

• Avoid lighting fires while a strongwind is blowing, when a strong windis predicted, or when conditions aresuch that a fire is likely to spreadbeyond the limits of the area that isintended to be burned.

• Use experienced people to light andcontrol a fire.

• Do not leave fires unattended.

• Be familiar with the contact phonenumber of the local rural fire officerfor the rural fire authority.

• Ensure that a helicopter is available ifit is needed.

• Where available, use a snow cap tocontrol the area to be burned andform an effective barrier for fires.


CHAPTER 6

Burning Management – Mountain Lands

• Make use of fire breaks where a snowcap is not present by pre-burningprior to the main burn, to preventthe spread of fire into unwantedareas.

Controlling the Nuisance Effects ofSmoke • Endeavour to burn when weather

conditions are such that smoke islikely to dissipate by the followingmorning.

• Avoid conditions that could result inash deposition on skifields during theski season and other recreational uses.

Firebreaks and access tracksFirebreak tracks which can be dualpurpose for vehicle access and stockmovement have been used for manyyears. The main objective is to act as adefined structure where a fire can be litfrom or alternatively be used as a backburning barrier. The other role is to forma normal control barrier for any naturalor escaped fire thus allowing men andvehicles onto the site to help control theburns.

Since about 1950 a large number offirebreak tracks were constructed usinggrant assistance. In the South Islandemphasis was on comprehensive rangeand mountain land “Strategic FirebreakAccess Schemes” where all propertieswere linked into an overall communityfire control management plan. Thisworked well where one had thecooperation of landowners. Withinproperties was another matter.

Summary of Guidelines:• Consult with the appropriate rural

fire authority

• Plan the over all route proposed usinglocal knowledge and expertise e.g.geologist if potentially unstable slopesare present. Survey the whole trackand mark all strategic corners, whereculverts or cutoffs should be withpegs or coloured cloth so theoperators can see from a distance

• Site so there is minimal landscapedisruption

• If feasible, use a hydraulic excavatorin preference to a bulldozer, tominimise site disturbance

• Use only experienced operators withthe appropriate horsepowermachinery

• In some instances an environmentalimpact assessment may be advisable(check on Resource consentrequirements)

• The average width of the carriagewayshould be 3 metres

• The grade of the track should notexceed 1:8 (i.e. 7 degrees) exceptwhere ground conditions demand eghard rock, bluffs. Here for shortdistances, grade can be increased to1:5 (i.e. 11 degrees)

• On hill formation, frequent culvertsor cutoffs need to be constructed withrockwork at both the inlet and outletto prevent erosion of water tables,downhill batters and waterways.

• Stream crossings should beconstructed to a hard bottom.Alternatively they can be armouredwith rockwork, rock filled or adequatesized culverts placed (i.e. able to takestorm flows)

• Uphill batters should be graded backto a slope sufficient to minimiseslumping and regressing.

• Revegetation of the disturbed landshould be carried out as soon aspractical with suitable speciesaccording to the site and altitude. Ageneral mix could include 3kg Whiteclover, 2kg Montgomery Red or Alsikeclover, 3kg Yorkshire fog, 3kgBrowntop/Fescue and 2 kg Cocksfootplus suitable fertiliser as 100kgSulphate of Ammonia and 300kgMolybdic superphosphate (orsulphur). Apply the dressing with atractor mounted spinner or blower orhelicopter bucket.


7.1 Introduction

This practice has been around for sometime but it is only recently that specialspecies have been planted as alternativefodder. The technique is applicable todrier environments in the high country,but may be useful anywhere in NewZealand. It involves establishing speciallyadapted plants that will provide a bankof additional supplementary feed on oneor two small areas, once or twice a year.Fodder banks may be planted on sitessubject to frost heave, sheet and winderosion but in general are established for“offsite feed at low altitude” to enablehigher altitude eroded sites to bedestocked or permanently retired.

Fodder banks have become moreprominent due to many successful farmtrials and research plots in particular inthe 1980s in Marlborough, theHakataramea Valley and Central Otago.These provided useful data on growthform, dry matter production and the bestmanagement practices. Traditionallyfodder banks have been used as adrought management strategy, but todaythey are considered a wise option inerosion control.

7.2 Guidelines for Fodder Bank

Establishment

Because of the large number of plantsavailable to use in fodder banks generalguidelines only will be provided. Morespecific details should be soughtAgresearch, Alexandra. Theestablishment of any feedbanks shouldbe undertaken as an integral part of themanagement of the whole property.

The key to these new species is that theyare suited to harsh sites, often dry and oflow fertility. Many of the earlier specieswere grasses and legumes, now the trendis to use shrubs and other herbs as well.

The success of fodder banks depends onplanning, in advance so all the details onthe soils, fertility, moisture regime, area,type of fencing, type of grazing stocksuitable, planting equipment and whereto purchase large numbers of qualityplants, are known.

Common species to use (list is notinclusive, many suited to drier sites).

Shrub speciesMediterranean saltbush Atriplex halimus

Doryncium Doryncium species

Bluebush Kochia prostrata

Tagasaste Chamaecytisuspalmensis

Tree medick Medicago arborea

Ceanothus Ceanothus species

Mountain Cerocarpusmahogany montanus

Shrubby germander Teucrium fruticans

Shrubby wattles Acacia species

Perennial lupin Lupinus spp

Tree speciesPoplars Populus species(various)

Willows Salix species (esp. Kinuyanagi)

Other tree crops Black locust and various nut trees – site specific

Pasture species grassesWheatgrasses (Agropyron Species), Tall oatgrass (Arrhenatherum elatius),Cocksfoot (Dactylis glomerata), alsofescue species and native grasses.

Pasture species legumesMilkvetches Astragalus species

Crownvetch Coronilla varia

Birdsfoot trefoil Lotus corniculatus

Sainfoin Onobrychis viciifolia

Serradella Ornithopus sativus

Lucerne Medicago sativa

Sweet clover Melilotus species

Clovers Trifolium species

HerbsChicory Cichorium intybus

Sheeps burnet Sangisorba minor

On other sites where there is moremoisture and possibly shading some ofthe more traditional grass-clover mixescan be used but they may require higherinputs of fertiliser to ensure sustainedproduction.


CHAPTER 7

Fodder Bank Establishment

8.1 Lowlands

Generally, the managed reversion oflowland areas take place alongstreambanks or wetland areas. Fencingoff areas will be one of the mostimportant factors in retirement of theseareas, this is not only vital to protectplants from stock but also removes thepressure from streambanks reducinganother source of erosion.

A great deal of information has alreadybeen produced about riparianmanagement. For further details onrestoration of vegetation in these areasreaders are referred to the MfEpublication Managing Waterways onFarms, A guide to sustainable water andriparian management in rural New Zealand,June 2001.

8.2 Hill country

8.2.1 Introduction

In the early years of soil conservation,indigenous species were not favoured bycatchment boards. The prevailing viewwas that they should not be used onfarmland, either because they were tooshallow-rooted to stabilise slopes, or tooslow-growing to stabilise them fast, ortoo unprofitable a use of productiveland. If a farm conservation planprovided for fencing-off standing bushremnants or reverting scrub, it wasusually on a steep face or gully, viewed ashaving no productive value.

On the other hand, many millions ofhectares of bush, scrub and tussock,reserved in the ranges and mountainlands between 1890 and 1940, weremanaged by the former Department ofLands or New Zealand Forest Service forthe purpose of watershed protection. Theview was that retaining indigenous coverin river headwaters controlled soilerosion, reduced flooding, andmaintained water supply.

With growing public interest inconservation since the 1970s, thesituation has changed. Many farmersnow fence off remnant bush, scrub andwetland, because they perceive them as

having ecological value, and asenhancing the farm landscape. It is notuncommon for landowners to fence offsmall areas of pasture that are difficult tofarm, such as on streambanks or ingullies, and plant them up in nativespecies.

Many farms already have patches of treeor shrub cover on land which has neverbeen cleared, or which has reverted. Insome instances such land is stable,though more often than not, it has beenleft uncleared – or has reverted – becauseof past instability. Retention andrestoration of such areas is a low-cost soilconservation option. The key elementsare:

• Construct a stock-proof perimeterfence

• Undertake regular control of animalpests

• Eradicate invasive weeds

8.2.2 Fencing

If stock are still grazed on adjacent land,a good fence is essential to success ofrestoration. Canopy trees may remain formany decades in the presence of


CHAPTER 8

Managed Reversion of Retired Land

Streambank planting of native shrubs in hill

country. This site was pasture 6 years before.Photo: D Hicks.

browsing stock, but if seedlings arepersistently browsed they do not survive;ultimately the stand will open up andcollapse if regeneration is insufficient toreplace the old trees.

What kind of fence to build, will dependon what a farmer can afford. Atraditional retirement fence, with timberposts at 3 to 4 metre spacing and 6 to 8wires with attached battens, is clearly thebest king to ensure permanent stockexclusion. At a cost of some $10 a metre,it is also the least affordable.

Go for one of these where the landownercan afford it, or where financialassistance is available from a regionalcouncil or QEII Trust. Where not, go fora low-cost fence design – it may not beas good, but it will achieve satisfactorystock exclusion most of the time. Somelow-cost options are:

• Electric fence with permanent postsat 3-4 metre spacings, and either twohot wires (for cattle) or four (forsheep). These cost about $3 a metre

• Light fence with timber posts or ironfencing standards at 5 to 10 metrespacings, with four wires (twoelectrified) for sheep or two wires(one electrified) for cattle; andadjustable tensioners on each wire.These cost about $1 to $1.50 a metre.

It is a good idea to have a gate in thefence; alternatively removable woodenrails between tow posts; so that if stockaccidentally get inside, they can be easilyremoved.

Remember that a retirement fence canoften be made more attractive to afarmer, by placing it in a position whichfacilitates grazing management. It maysubdivide an existing paddock in two,giving a better rotation; or create a racethat concentrates stock along a ridge or aslope-foot when mustering.

8.2.3 Weed control

Once stock are excluded from a patch ofreverting scrub or bush, there is a riskthat regrowth may be things that farmersdon’t want – blackberry, gorse orsubtropical shrubby weeds – as well asnatural regeneration. It is best to nipthese in the bud, while infestations arestill small. If they get away on a retiredhill country face or in a gully, theinfestations will become near-impossible

to control without damaging the naturalregeneration.

Knockdown herbicides such asglyphosate are effective on grasses andmany woody weeds, but extreme care isneeded when applying them close toregenerating natives. Where infestationsare small, spot-spray using a funnel,spray wand or similar to direct sprayaway from seedling foliage. Residualherbicides such as oxydiazon arepreferable as they give longer controlbefore re-spraying. While more toxic,they are usually herb-specific i.e. causeminimal damage to the seedlings ofwoody species.

Alternatively clear rank grass and weedsby hand or slasher, to free seedling thatare small and slow-growing, and leave asa mulch around their stems to helpsuppress regrowth.

Another option is to cut 0.5 metrediameter mats of permeable erosioncontrol fabrics and place these aroundeach seedling. Heavy cardboard or oldcarpet can serve the same purposeequally well.

A third way is to use plastic tree-guards,or off-cuts from dynex tree protectors. Aswell as keeping seedlings free fromchoking by grass, these give some shelterfrom wind. One dynex sleeve shouldprovide 5 to 10 seedling-sized off-cuts. Ifmore substantial artificial shelter isneeded on windy sites, peg shelterclothto windward on wooden stakes or ironfencing standards, but use heavy-dutymaterial that will last several years.

8.2.4 Pest control

Protection of seedling from goats, deer,rabbits, hares and possums is essentialfor good natural regeneration. Shootingor poisoning will generally be neededonce a year for effective pest control.Additional protection may be obtainedby applying repellent chemicals e.g. egg-paste to seedling. Doing this is low-costbut laborious, so is an option only insmall retirements.

Fenced-off retirements at least have theadvantage that pests in them can beeasily targeted. Depending on the type ofvegetation and when different animalsfeed on it, it may be possible to hit themhard once a year, instead of having tospread effort. Some regional councilshave charts showing what times of year


possums feed on different plants, andsimilar information may be available forthe other animal pests.

8.3 Revegetation

Where gullies or steep faces are toounstable to contemplate spaced plantingof poles in pasture or close afforestationwith commercial timber species, theoptions are either to fence, and awaitnatural reversion; or to assist it byplanting native seedlings in the rankgrass. The first option is clearly preferablebecause it is low-cost. In wet regions likeNorthland, Auckland, the Waikato,Taranaki, Nelson, Westland andSouthland closed-canopy scrub will coverthe site within 10 to 20 years. However itwill take between 20 and 30 years for aclosed scrub canopy to form inseasonally dry regions (Gisborne, HawkesBay, Wairarapa, eastern coastline of theSouth Island); and longer than 30 in dryinland regions of the South Island.Where faster revegetation is desired, thesecond option becomes necessary.

Key elements of successful indigenousrevegetation are:

• Fencing

• Choice of species

• Planting

• Post-planting maintenance

• Pest control

• Weed control

For three of these – fencing, pest controland weed control – details are the sameas for restoration of bush remnants andreverting scrub. Readers are referred tothe preceding discussion of these topics.The remaining three are discussed below.

8.3.1 Choice of species

What species to plant, may be decided asmuch by a landowner’s personalpreference or by ecologicalconsiderations i.e. what the originalvegetation was, as by species’effectiveness for soil conservation. TheQEII Trust’s Revegetation Manual (Evans1983) and DOC’s Trees for Tomorrow(Simpson 1995) are standard referencesfor ecological restoration. The PlantMaterials Handbook, Volume 3 (VanKraayenoord and Hathaway 1986) is still

a standard reference about use of nativespecies for soil conservation. However inthe years since its publication, more hasbecome known about their suitability forthis purposed in different environments.The following list gives species thatvarious regional councils recommend forplanting on hill country slopes(including gullies).

Note that it is a nationwide list. Some ofthe species on it are suited to a mildclimate so will establish well in northernparts of the country but may fail in cold,frosty conditions farther south. Equally,some of the hardy southern species maynot thrive in hot, dry northern summers.Use local knowledge – observation ofwhat grows well naturally in a region –when selecting plants from the list. Beopen to other species as well – there maybe ones that, though restricted in theirnatural distribution, are suited to soilconservation at particular localities.

Refer to Table 8.1 on the following page.

8.3.2 Planting

Obtain seedlings from nursery stock,preferably propagated from local seedsources. These have a better chance ofsurvival, as they have been selected fromtrees that are adapted to local soilsclimate.

Bare-rooted seedling are easiest tohandle, but have to be planted soon aftersupply. Potted seedlings can be held forlonger, but are bulky. Root-trainerseedlings can be held for a long time,and are light to shift, but do not havemuch foliage which lessens their chancesof survival (though good results canoften be obtained).

Seedlings should preferably be notshorter than 20cm and no taller than50cm, as smaller or larger seedlings can


Tree planting of gully system and retirement

from grazing. Photo: Wellington RC.


Table 8.1 Species for Revegetating Hill Country Slopes.

Tolerance to: Value for:Salt Wind Drought Frost Damp Slope stabilisation

Agathis australis kauri m m m l m *Alectryon excelsus titoki m m m m m *Aristotelia serrata makomako l m l h m ?Beilschmiedia taraire taraire l m m l m *Beilschmiedia tawa tawa l m m m m *Brachyglottis repanda rangiora m m m m l ?Carmichaelia spp broom l l-h m-h h l @Carpodetus serratus putaputaweta l m m m l ?Cassinia spp tauhinu m m m-h m-h l-h @Coprosma lucida karamu m m m m h ?Coprosma parvifolia leafy coprosma l m m h l ?Coprosma propinqua mingimingi l m m h m ?Coprosma repens taupata h h m m l ?Coprosma robusta karamu m m m m m ?Cordyline australis cabbage tree m h m h h @Coriaria spp tutu l m l-m m l @Corokia cotoneaster korokio m h m h l ?Cortaderia spp. toetoe m h h h m @Corynocarpus lavig. karaka h h m l l *Cyathea spp. tree fern l l l h h @Dacrydium cupressinum rimu m m l m m *Dacrycarpus dacryoid. kahikatea l m l m h *Dodonea viscosa akeake h h h m l @Dicksonia spp. tree fern l l l h h @Entelea arborescens whau m m m l m @Fuschia excorticata tree fuchsia l m l m m @Griselinia littoralis broadleaf h h m m m *Hebe spp hebe l-m m-h l-m m-h l-m ?Hoheria spp lacebark l m l-m m m @Knightia excelsa rewarewa l m m m l *Kunzia ericoides kanuka l m h h m @Leptospermum scopar. manuka m m h h h @Macropiper excelsa kawakawa l l m l m ?Melicytus ramiflorus mahoe l m m m m @Metrosideros spp rata l-h m-h m-l m l *Myoporum laetum ngaio h h m m l @Nothofagus fusca red beech l m m h m *Nothofagus menziesi silver beech l m m h m *Nothofagus solandri black beech l h h h m *Nothofagus cliffort. mountain beech l m h h m *Olearia spp leatherwood h m m h l @Paratrophis banksii towai l m l l m *Phormium tenax flax h h m m h @Phyllocladus glaucus tanekaha m m m l m *Pittosporum colensoi black mapou l m m h m @Pittosporum eugenioid. lemonwood l m l m m @Pittosporum crassifol. karo h h m m l @Pittosporum ralphii karo h h m m l @Pittosporum tenuifol. kohuhu l m m h l @Plagianthus spp ribbonwood l-h m-h m m m-h ?Podocarpus totara totara l m m h m *Podocarpus halli mountain totara l m h h m *Prumnopitys ferruginea miro l m m h m *Prumnopitys taxifolia matai l m h h m *Pseudopanax arboreus five-finger l m m m l @Pseudopanax lessoni five-finger l m m m l @Pseudowintera axill. horopito m h m m m @Schefflera digitata pate l l l m m ?Solanum aviculare poroporo l m h l m ?Sophora spp kowhai l m m h m @Weinmannia racemosa kamahi l m m m m @Vitex lucens puriri m m m l m *

Abbreviations:l = low tolerance m = moderate tolerance h = high tolerance @ = fast growth and root spread; good value for slope stabilisation* = slower growth and/or root spread; lower value for slope stabilisation ? = limited growth and/or root spread; some value for slope stabilisation

be difficult to establish. All plants shouldbe hardened off in the open for aboutone month prior to planting.

In northern regions, seedlings are bestplanted in May or June, so that theyestablish over the winter and springmonths before summer drought sets in.In central and southern regions,seedlings are best planted in mid-winteror early spring, when the plants aredormant and after they have beenhardened off by frost. Any speciesvulnerable to out-of-season frosts shouldbe planted as late as possible, inSeptember or October.

Grass and weeds should be sprayed orhand-weeded 2 to 4 weeks prior toplanting. A 0.5 to 1m diameter spot ateach planting site is usually sufficient.This gives seedlings a chance to put on aspurt of growth before the grass andweeds grow back. Where there is concernabout spray side-effects or residues, useamine-based salt preparations such asglyphosate – they have low toxicity andbreak down fast.

Dig a planting hole that’s large enoughto accommodate the seedling’s roots withplenty of loose earth around andunderneath once back-filled. Take carethat the seedling isn’t too deeply buried.Seedlings should be planted with pottingmix, but ensure that the potting mix isnot exposed. Keep it about 2cm belowground level and cover with soil ormulch.

Direct seeding may be worth trying, asan alternative to plantation of seedlings.Australian landcare groups report goodresults with this technique. In NewZealand there has been little experiencewith species other than manuka andkanuka, which may be established byscattering seed-pods or laying brush withripe seed-pods still attached.

Establishment can be helped byfertilising with one or two 25g magamppellets in the planting hole. Alternativelyuse 25-50 grams of granulated magamp.Magamp is preferable to urea ornitrolime which can easily burn plantroots. It is better to under-fertilise thanover-do it; too much fertiliser around itsroots will kill a plant.

For indigenous seedling, an initialspacing of 2.5 to 3.5 metres isrecommended, so that growing treesestablish reasonably dense ground cover.

Most native species are naturally self-thinning, but if timber production iscontemplated, hand-thinning to 5-7metres at 20-plus years may helpimprove growth rates. Fast canopyclosure, with suppression of competingweeds or grass within 2 to 3 years, can beachieved by planting at 1 metre spacings.However this entails 10,000 seedlings ahectare, so is feasible only on smallrestoration sites (or projects with bigbudgets e.g. road earthworks).

8.3.3 Post-planting maintenance

Post-planting maintenance can be crucialfor seedling survival, but may not bepractical due to other demands on time.The options are:

• Only plant what can be cared for, or

• Plant at a higher density to allow forpartial planting failure, and let naturetake its course

It is unrealistic to expect 100% successwith any tree planting, but post-plantingcare and attention can greatly improvetheir chances. When good-qualityseedlings are planted and protected, 70-80% usually survive. Where animal pestsare present, they are the single largestcauses of tree mortality, and muchhigher percentages may need to bereplanted.

In inland regions, blank (re-plant) gapsin August or September, because May toJuly may have been too early for frost-tender seedlings; or too dry on free-draining soils. In coastal districts, blankin May or June, so that seedlings have achance to establish before drought sets inthe following summer.

If planting indigenous timber trees witha view to harvest, it is a good idea toretain some documentary evidence thatthey are planted and not naturalregrowth, so that harvest isn’t precludedby existing or future forestry legislation.

If planting indigenous trees in the hopeof a commercial return, pruning isadvisable to improve growth form, andavoid knotty sawlogs. Most New Zealandpublications on silviculture dealspecifically with radiata pine, but thetechniques they recommend could beapplied equally well to many timberspecies, including natives. For high-quality clearwood, some farm forestersmake a practice of annually pruning


small side-growth branches on theirnative timber stands.

Where trees are planted close to a valley-bottom stream in hill country, they needto be clearly separated from the stream,so they they can be felled without anyrisk of the channel being disturbed bylogging machinery, or blocked by slash.Even where there is no intent to harvest,this is a good idea, because the channelstill needs to be managed so as tomaintain its flood capacity. Considerplanting patterns that restore the naturalflow path of floodwater. Trees that fall inmay block the flow path, and need to beremoved. Dense plantings, or weedregrowth enclosed by plantings e.g. self-sown willows, blackberry or gorse, mayalso block or deflect the flow path. Anydebris which has lodged in the bed mayneed to be removed, if it is likely toimpede passage of floodwater or direct itagainst banks which have been planted.

8.3.4 Nurse crops

Given that many indigenous plants areslow-growing, a nurse crop of exoticshrubs may be worth considering, toprovide quick ground cover and shelter.The plants on this list form an opencanopy, which admits enough light fornative seedlings to grow underneath, andeventually dies out when it is overtoppedby the emerging natives.

Albizzia spp. Brush wattle

Banksia spp. Banksias

Callistemon spp. Bottlebrush

Chamaecytisus palmensis tree Lucerne

Lupinus arboreus tree lupin

Tamarix chinensis tamarisk

Tree Lucerne in particular is good fodderfor native birds, and can enable naturalrevegetation from seeds they bring in.Shrubby acacias are excluded from thelist because many species are self-seedingand can from dense thickets. Casuarinasare excluded because there is someconcern about their weed potential,though one species, river she-oak(Casuarina cunninghamii) may besuitable.

8.3.5 Concluding Remarks

That indigenous plant materials can beeffective slope stabilisers in hill country,has been demonstrated by numerousstorm damage surveys carried out from

the 1960s onwards. Closed-canopy scruband bush – whatever its speciescomposition – reduces mass movementand gully erosion by 90% or more,compared with adjacent land in pasture(Clough and Hicks 1993, Hicks 1995).Reverting scrub, with an as-yet opencanopy and discontinuous root mass,achieves reductions of up to 50%.

It is true that there is limited scope forspaced planting of indigenous species ingrazed hill country pasture, unless theseedlings are protected or the pasturelightly grazed. Close planting ofindigenous timber trees on unstable hillcountry, while occasionally practices,remains a less attractive commercialoption than fast-growing exotics. It is thethird category of hill country – unstablesites with low potential for grazing orforestry – where indigenous revegetationwill be increasingly used as a soilconservation measure.

8.4 Mountain lands

This is the practice of fencing out an areaof severely eroded high country (usuallyLUC VII and VIII), which is often at thetop of a mountain range. In somecircumstances the topography of the areamay enable stock to be completelyremoved without the need for largelengths of retirement fences

Furthermore, to assist in the revegetationprocess of the retired land all stock thatwere removed will require alternativegrazing elsewhere i.e. offsite feed. This isusually provided from the lower altitude,more productive land i.e. which may beAOS & TD or irrigated lucerne to carrythe same or equivalent displaced stockon.

Retirement schemes were not reallyadvanced at all until the SoilConservation & Rivers Control Councilprovided grant incentives to farmers tocarry out the practice from about 1955onwards. This ceased in 1992 when grantmonies for soil conservation purposeswere terminated by government.

Today the works still continues onprivate land but are at a slower rateunless it is part of the Crown LandTenure Review process on (PastoralLeasehold land) where the high altitudeor severely eroded lands are retired andtransferred to the Crowns estate tobecome the responsibility of theDepartment of Conservation.


The same land use change can occur onFreehold land where severely erodingland exists and needs to be destocked toassist in the natural or man assistedrevegetation programme. There may beopportunities to involve the QueenElizabeth II Trust if there are uniqueareas of tussock grassland and bushwithin the same block.

The most important factor in anyretirement block is to ensure thatongoing animal and plant pest control iscarried out as well as regular fencemaintenance. It is vital that proper firemanagement control such as firebreaktracks and watering facilities are in placein case any escaped or wild fires occurwhich will damage the vegetationrecovery programme.


Runoff Control & Dewatering

Chapter

9. Runoff Control Practices – Lowland

10. Dewatering Techniques for Deep-Seated Mass Movement

9.1 Introduction

The practices described apply to the ruralenvironment and are particularlyapplicable on arable lands. Somepractices can also be used to controlsiltation and erosion in urbandevelopment.

Some of these practices have not beenused much in New Zealand (e.g.terracing), but are suited to intensiveland use on slopes of 12° and less. Theearthwork control practices should beconstructed in the early phases oferosion control, e.g. graded furrows,banks as they become permanentfeatures. The other soil conservationpractices can be integrated with themafterwards, i.e. conservation tillage,cultivation, soil/pasture aeration andothers.

Many of the runoff control practiceshave been used in New Zealand from the1950s. They were first tried on soilconservation reserves and trial sites asThe Pisa Flats (Central Otago), the AdairReserve (South Canterbury), GlenmarkCatchment (North Canterbury) and theEarnscleugh Reserve site (nearAlexandra). The techniques weregenerally successful on low terraces anddry downland sites. From the 1970sonwards these practices seemed to loosefavour as farm development and grantmonies were directed into pastureimprovement.

9.1.1 Contour furrows

These can be simple systems constructedon a paddock either on the contour orwith a slight gradient (may also be calleda diversion or drainage furrow).

Contour furrows are used to detainrunoff on hill slopes. They break slopesup into a number of short lengths,preventing runoff reaching velocitiesthat could cause rilling or sheetwash.Dung, fertiliser and soil are also retainedin the furrows, creating fertile, moistconditions. More useful in drierenvironments where mass movementerosion is not a problem.

Graded or diversion furrows are used todivert water away from problem areas,e.g., slips, gully heads, gateways, orslopes where sheet/rill erosion occurstaking the water to safe disposal sites.They are useful to irrigate sunny knobsor ridges. Such sites are also idealoutfalls, since they tend to spread waterout, rather than concentrating it in oneplace as occurs in a gully. Graded furrowscan be used to increase the effectivecatchment area of a pond or dam.

Graded furrows for Drainage can be aneffective and inexpensive method ofdraining slopes over 10°, where othermethods are impractical. Put in at 10 m

intervals, they may lead to a safe disposalsite or grassed waterway.

9.1.2 Factors to Consider in Design

SlopeMore water will be detained on flatterland than steeper land. Furrows onsteeper slopes must be installed closertogether because they have less water-holding capacity. Furrows can beinstalled effectively on slopes of up to25%.


CHAPTER 9

Runoff Control Practices – Lowland

Graded Furrow, North Otago.

Photo: Otago Regional Council.

ClimateIn areas where high intensity rainfallfrequently occurs, furrows would bespread closer together.

Soil TypeFewer contour furrows are required onfree-draining soils where infiltration israpid. In clayey soils or wet conditions,furrows should be graded to a safedisposal site.

Size of furrowA furrow made by a grader may detain 2-3 times the amount of water detained bya furrow constructed with a single-furrowplough.

VegetationCrops and lucerne which may have ahigh percentage of bare ground andrapid runoff, require furrows to be closertogether than will grassed pastures.

Access and CultivationContour spacing should allow forvehicular access routes if these have beenthe custom – a compromise maysometimes be necessary betweendesirable contour spacing and actualpractice. Contours should also be spacedand positioned for future cultivation.

MachineryFurrows can be formed by a number ofdifferent machines, and their capacity isstrongly influenced by the resultingcross-sectional area. When a two furrowplough is used to form the pasturefurrow, they can be ploughed so that thesecond turf is placed on top of the first.Construction by grader type machines isthe preferred method on slopes less than15% and plough furrows on slopes over15%. For steeper slopes, bulldozers havebeen used, but it is more difficult toachieve the critical grades required.

9.1.3 Practical Points and General

Recommendations for Constructing

Furrows

Contour furrows• A system of contour furrows can be

expected to detain 25mm of runoff.

• Pasture furrows are useful on slopes of7° to 18°. On flatter arable areas,broad-based terraces that are noimpediment to machinery are moreuseful.

• Generally, vertical interval betweenfurrows is 2 – 4m, the steepest slopepoint. A 2m vertical interval isequivalent to 7-l0m horizontalinterval on a 10°-12° slope. On 25%slopes, the interval would be 8m

• Furrows should be compacted byrunning the rear tractor wheel alongthem when returning empty to thestart of the next contour line.

• Furrows should be seeded andfertilised once constructed so theyquickly get a protective cover ofvegetation. In lucerne, furrows arebest sown in sub. clover.

• Cattle should be kept off newlyfurrowed paddocks furrows. May notbe suitable on deer farms as deer liketo wallow and damage areas of somepaddocks.

• Furrows should be broken every 50mto l00m in a zig-zag fashion down thecontoured slope. This preventsovertopping and scouring occurringat any one place.

• Do not construct contour furrows onland prone to slips or tunnel gullyerosion, as increased infiltration canworsen the problem. Graded furrowscan be constructed only above orbelow these areas.

• Maintenance with a shovel over thefirst year is required.

• Furrows become filled up with soil,dung and vegetation after 3-4 years.The decision to reconstruct these orcut new furrows depends on whetherthe mitigation of soil loss and runoffhas been achieved.

Graded Furrows• Normally a grade of 1:80 should not

be exceeded or scouring of furrowscould result.

• A quickset level or some otheraccurate surveying equipment shouldbe used for graded banks and similarstructures since fall may be only1:200 or 1:400.

• Graded furrows can be spilled out ondry knobs or ridges. These are idealdisposal sites since water is spreadrather than concentrated as it is in agully. Soil moisture levels are alsoimproved on these normally dry sites.Regular maintenance is vital.


9.2 Broad Base Terraces and

Absorption Terraces

These structures are used on flattercropping land (2° – 7°). These arecommonly 9 to 12 metres wide and300mm high and is no impediment tothe passage of machinery.

On gentle slopes (2° – 5°), a small moundwill detain quite a large amount of water.However, with increasing slope, theamount detained falls off quickly.

Like pasture furrows these structuresbreak long slopes into short lengths,encouraging absorption and preventingrilling.

On slopes steeper than 5°, terraces overl00m should be graded at 1:300. Excesswater is slowly led off the land withoutscouring to a grassed waterway. Storagebehind terraces on these slopes isinsufficient to cope with heavy rain. Ifthey are not graded overtopping mayoccur. (Since grades are slight, it isessential to use a Quickset level, Abneylevel or other surveying equipment).

Machinery for Broad Base TerracesTerraces are built up with a grader and/ora whirlwind terracer, which is a tractordrawn implement which cuts a furrowand feeds it into an auger revolving atvariable speed. By changing the auger’sspeed, soil can be thrown a distance ofup to 8m.

LimitationsTerraces are not used extensively in NewZealand, in contrast with denselypopulated places overseas where flat landis scarce. Orchards and vineyards havebeen successfully established on terracesconstructed along steep slopes (20°) inthe Heathcote Valley, eliminating sheeterosion under the trees, also for kiwifruitorchards in the Tauranga district.

Terracing is very expensive and requirescareful stockpiling of topsoil. Surveyingis vital to ensure correct gradients andsafe outfalls are established. Do not usethis technique where slip erosion canoccur. On slopes that may be susceptibleto water logging from ponded water,graded terraces are more appropriate.

9.3 Graded Banks

9.3.1 Description/Purpose

Graded banks are a series of earthbanks/channels formed on long slopes atvery slight grades, to control the surfacestormwater flow and divert the runoff tostable grassed waterways. The purpose ofthe graded banks is to dispose of surplusoverland flow so that the risk of rill orgully initiation is lessened. They alsoimprove drainage on clay soils.

ApplicationGraded banks are generally smaller thandiversion banks and broad-based terraces.The banks are normally constructedusing a grader on slopes of 12-15°.Graded banks can be constructed onsteeper slopes using a bulldozer, but thegrades are more difficult to accuratelycontrol. Over the years, a range ofmachinery including ploughes andwhirlwind terracers have been used toinstall graded banks.

Ideally, the grades on the banks shouldbe as low as possible, depending on howfar the water needs to be diverted.Typically, grades are between 1:80 to1:100. However, for short distances,grades can be as steep as 1:60 as long asscour velocities are not reached. Gradedbanks are installed at spacings as close as20 m apart. They need to be accuratelysurveyed and grassed as soon as possibleafter construction. The outlet for thegraded banks needs to be a waterwayspecifically planted (grass/trees) andmanaged so that it can handle thevolumes of runoff without eroding.Controlled waterways should ideallyspread water so that it is notconcentrated.

Graded banks have been successfullyused in Northland to control gullying on


Graded banks on moderately steep

Bentonitic mudstone country. Photo: Gisborne

DC.

gumland soils, and on in the Wither hills(Marlborough) for tunnel gully controlby diverting surface runoff back to ridgesto spread the water flow. They have alsobeen used in the Gisborne District inconjunction with pole planting anddewatering to control earthflow andgully erosion. In this case they wereconstructed using bulldozers, and poleswere planted on the outside of thegraded bank to help stabilise the worksin the medium to long term.

9.4 Interception drains

These perform the same function asdiversion channels/bunds (see RunoffControl on Earthworks Section 16) forgreater design detail if required.

Description/PurposeInterception drains are site specificmeasures to intercept and convey runoffat non-erosive velocities away fromcropping soil to a stable outlet. They arepermanent drains, and should be part ofan integrated system, as opposed tocontour drains which are temporary.

InstallationGrade should be less than 2 % otherwisethe channel will erode and need to bestabilised with rock or erosion controlfabric. The outfall may need to beprotected against erosion with rock, or aflume. Because of their permanent natureand the small margin for error, thesechannels must be surveyed to obtain aneven grade. An abney level, inclinometeror similar should be appropriate for thispurpose.

Earth bunds can be used to control anddirect runoff. They need to be wellcompacted. Any excavated channel orbund wall should be grassed to preventscouring.

No criteria are presently advocated for adiversion channel in a cropped field. Ifone is required, then it is suggested thatdesign be for the 1 in 20 year (5% AEP)storm.

LimitationsBecause of the interception drain’s lowgrade and the well-aggregated soil type,sediment will often deposit.Sedimentation can quickly compromisethe capacity of the drain to transportflow and cause localised damage to cropsat break out points.

Care needs to be taken in theirplacement so that the drains can beaccessed and any sediment removed.Constructing the channel on a steepergrade so that sediment is not depositedwill result in the channel itself erodingand requiring erosion control. Checkdams made from rock (see EarthworksChapter 16), sand bags etc are oftennecessary to control channel velocity.Sandbags filled with a mixture of sand,gravel and concrete are very effective insteep sided drains. The concrete will setand hold the sandbags together (use UVresistant bags if possible). Check damsconstructed in this situation will also actas sediment retention measures becausevelocity reduction results in sedimentdeposition on.

MaintenanceInterception drains need to be inspectedregularly, particularly during andimmediately after heavy rain, to ensurethat the drain is working correctly and isnot blocked. Deposited sediment needsto be removed and any necessary repairsto the drain should be undertaken. Theoutfall should be checked to make surethat is free from erosion. Check thatmachinery movement through the sitehasn’t damaged the channel.

9.5 Headlands

These can be significant sources of runoffdue to severe soil compaction.

Description/PurposeHeadlands are used for access andmachinery turning at the ends of


Sandbags used to control drain velocity.

Photo: Franklin Sustainability Project.

cropped fields. They can be designed tointercept and divert runoff to permanentinterception drains. They are still used inthe normal manner for machineryaccess.

InstallationThey should be constructed at no morethan a 2% grade (to avoid scour). Theycan be sloped back to form a benchedheadland. Alternatively a wide shallow“V” channel can be formed between theheadland and end of the rows (this lattertype requires more shaping). They canalso be formed within paddocks, to breakup the length of long paddock runs. Ifconstructed to a broad shallow design,machinery can be easily driven across.Ideally the headland should be grassed(and it can then be on a slightly steeperslope as a grassed surface is moreresistant to erosion).

LimitationsCommon problems are insufficientcapacity, erosion prone outfalls, too steep(so they erode), or too shallow a grade(so they don’t flow or depositioncompromises their capacity). They haveoften been constructed so they dischargestraight to an accessway and then to aroad or road side water table.

They may not have been installed whereneeded through paddocks, because theresultant uneven ground can affectmachinery operations.

MaintenanceRemove any accumulated sediment. Re-grass as required. Repair as necessary.

9.6 Contour Drains

These perform the same function asthose described in Section 15.3, RunoffControl on Earthworks.

Description/PurposeThey are temporary drains used tointercept runoff across slope in apaddock and channel it to permanentinterception drains.

InstallationContour drains should be as short inlength as possible and constructed at nomore than a 2% grade (to avoid scour ofthe channel). As a rule of thumb theyshould be about 300 mm deep and from20 to 80 metres apart. Drain spacingshould be 20 metres apart for slopes of

10% or more, and can increase to 30 andthen 50 metre spacing as slope gradientchanges to 3-10 % and to less than 3%respectively. As a general rule, contourdrains should never be more than 80metres apart. The actual positioningshould be determined by the presence(or otherwise) of an erosion proof outfall.They should be put in immediately aftersowing a crop.

LimitationsCare is needed with contour drainsbecause they concentrate site runoff.Insufficient capacity, erosion proneoutfalls, and construction on too steep agrade (so they erode) or too shallow agrade (so they don’t flow) are commonproblems. They are often too far apart, ordo not discharge into a permanent drain.They are often not installed by growerswhere needed, because the unevenground hinders machinery movementand spraying operations.

MaintenanceContour drains should be inspectedduring periods of prolonged rainfall andas soon as possible after heavy rainfall.Any necessary repairs should be made.

9.7 Raised Accessways

Description/PurposeRaised accessways are constructed fromroads into fields, to facilitate machineryand truck entry. A raised accessway is nota runoff conveyance system, it is ameasure by which runoff is divertedaway from this heavily trafficked andvulnerable spot.

InstallationAccessways should be placed away fromthe lowest point in the field where waternaturally flows. They should be raisedabove the surrounding area so runoff isdiverted sideways into the nearest


Contour drains. Photo: Franklin Sustainability Project.

interception drain. This can be done byplacing aggregate on the accessway tomound it up, this gives all weather accessand reduces the potential for futurelowering of the accessway. Any accesswaythat goes directly onto a road will needto be piped to allow roadside drainage.The size of culvert should be checkedwith the local council or drainageengineer.

Maintenance:Maintain the raised nature of theaccessway to ensure runoff cannot flowdirectly to the road. Keep the culvertunder the accessway open and clear.

9.8 Silt Traps

Description/PurposeSilt traps are pits dug at intervals alonginterception drains. Their purpose is toretain sediment on-site and minimise off-site sedimentation.

InstallationThey should be used in combinationwith measures that reduce runoff. Thesmaller the quantity of runoff directedthrough these measures, the lesssediment they need to trap. Silt traps canbe constructed either by excavating pitsor constructing bunds, and passingrunoff through them prior to dischargeoff site. Excavations can consist merelyof a hole, preferably longer than it iswide and with the outlet at the oppositeend to the inlet. Earth bunds can be usedas well, or in conjunction withexcavations, to form a silt trap.

There are currently no guidelines on trapsizing. On strongly aggregated soil types,it is probably appropriate to excavate aseries of scoops along a drain rather thanconstruct a more formal silt trap at thelower end of a paddock. However, for lesswell aggregated soil types, and assumingthat there is an offsite need for sedimentto be retained, then sediment retentionponds should be used. A drainage pipe,such as those detailed in earth bunds (seeEarthworks Chapter) can be used to drainthe pond or depression, or the pondshould be kept shallow enough so that itis not a safety hazard. Silt traps should belocated in side drains as well aspermanent drains. They need to belocated away from turning areas forsafety reasons.

In permanent drains, small check damsconstructed out of rock, timber orerosion control fabric can be used toretain sediment as well as to reduce thevelocity of water in the drain. Scoops(slightly deeper excavations) at 15 – 20metre intervals along a drain can also beused.

LimitationsSilt traps quickly fill with sediment andwill need regular digging out.Minimising the quantity of runoff willreduce the frequency at which this needsto be done.

At present there is no specific designrequirement for silt traps – they aremerely “holes on the ground”. Room is


Figure 2(10) raised accessway Photo –

Photo: Environment BOP.

Silt traps. Photo: Franklin Sustainability Project.

always at a premium on grower’sproperties and these measures arenormally “squeezed in”. They may,therefore, quickly fill with sediment andlose their effectiveness during a largestorm. Some recognition of catchmentsize, inflow velocity and silt trapcapacity, undertaken at initial design,would improve their effectiveness.

The reason that they work as well as theydo is because of the very stronglyaggregated nature of the granular loams.This allows the particles to settle easilywhen runoff velocity diminishes. Silttraps will not be as effective on other soiltypes that readily break down to clay andsilt sized particles, because their typicallysmall capacity will not allow sufficientsettling time to retain the fine particles.Sediment retention ponds (seeEarthworks Chapter) would be moreeffective in situations where the soil iseasily broken down to clay and silt.

MaintenanceSilt traps should be regularly cleaned ofsediment so their effectiveness isretained. Any drainage pipe should beinspected to ensure that it is not blockedand is functional.

9.9 Subsoiling – amongst trees

Subsoiling involves the ripping of soil toa depth of about 460 to 600mm toshatter the compacted pan, particularlyunder tractor wheel marks, as thecompacted soil obstructs waterinfiltration.

ApplicationSubsoiling can be undertaken using a D4bulldozer with trailing twin rippershydraulically mounted, or mounted on ahydraulic toolbar. Soil conditions need tobe dry enough to shatter, but not so drythat the machine cannot rip to therequired depth. Work is normallyundertaken in spring/early summer(October/November) or early winter(May/June). Subsoiling runs could be upto 100 m long on slopes of less than 8°,but limited to 40 m on steeper slopes.Two rips per row are necessary to breakthe compacted layer under tractor tyremarks.

Subsoiling was not wholly accepted bylandowners as being necessary, and theirconcerns related to the questionabledrainage benefits as well as thepossibility of killing roots of producing

trees. Although the Nelson CatchmentBoard recommended that moreinvestigation be undertaken to quantifythe effects of subsoiling, staff had littledoubt that the practice was absolutelynecessary to improve the infiltration andprovide a basis to initiate undergrounddrainage patterns.

9.10 Tiled drainage beneath

waterways

These include a 100 mm field tile(drainage pipe) in a trench (170 mmwide and at least 900 mm deep), andbackfilled with coarse drainage metal,then grassed. The tiled grassed waterwayscollect water from the subsoiling anddrain water via the drainage metal andfield tiles safely down to the outlets. Thesystem also provides drainage for thewetter soils of the flats and valley floors.

Main and outlet waterways usuallyfollow natural flow paths and need littleshaping to function effectively. Thedepth of drainage metal provides agreater capacity for drainage, and createsa break in the hard pan, allowing betterinfiltration to occur across the orchard.Water is able to rise as the volumes


Installation of tile drains through orchard.

Photo: Nelson Catchment Board.

increase, and some surface flow mayresult during peak flow conditions. Thissurface water infiltrates into the drainagemetal and tile drain as the flowdecreases. Approximately 3.5 to 4 cubicmetres of drainage metal is required forevery 20 metres of trench.

9.10.1 Grass waterways beneath trees

and along waterways

This practice includes the grassing ofbare areas, particularly followingsubsoiling and tile draining, to stabilisethe ground surface and reduce the risk ofrilling and gullying.

Grassing down is normally carried out inthe winter following subsoiling. Thegrassed waterways need to be maintainedthrough to the outlets. This may create

management problems if there iscultivation carried out on flatter landprior to the grassed waterway ending. Alldrainage systems need to be graded andvegetated so that they are able to conveystormwater runoff flows withoutscouring.

9.11 Grade line (contour) planting

The grade line system involves plantingtrees along the contour on sloping landin parallel and on a similar grade to thesubsoiling and tile drains. The drainagesystem feeds into tiled grassed waterwaysand then to natural channels.

Early attempts to plant orchards on thecontour failed because the widefluctuations in spacing preventedefficient spraying. Subsoil drainageallows the spacing between trees to bemore uniform.

9.12 Hedges

Hedges can act as natural barriers toretain sediment. They can be effective atthe bottom of a sloping paddock,between a paddock and drain, and whencombined with a headland, can be usedto intercept and divert runoff from up-slope. They are a permanent measureand are cheap to establish and maintain.

Hedges should be trimmed annually toencourage thick growth, particularly atground level. Any gaps should beblocked or filled with young plants.

9.13 Contour banks

Description/Purpose Contour mounds or banks are similar tocontour furrows except they are spacedfurther apart and are twice the size.

ApplicationContour mounds can be used insituations where the close pattern ofcontour furrows creates managementproblems such as if the area is to becultivated frequently. Contour moundscan be constructed on similar slopes topasture furrows, and while not asefficient as the pasture furrows, they stillprovide for good moisture retention, andrunoff control.


Completed grassed waterway.


Grade line system for new orchard on

rolling land.


9.14 Broadbased terraces

Description/PurposeThese are structures designed to detainsurface runoff on flatter sites (3-12%slopes), and safely convey the runoff to asafe disposal point, normally a grassedwaterway.

ApplicationBroadbased terraces have a large storagecapacity, and usually have such a shallowbatter slope that machinery andimplements are able to traverse themwith ease. They are normally used onrelatively flat country that is intensivelyfarmed. Broadbased terraces can beconstructed with a range of earthmovingmachinery. In New Zealand, a special-built machine, the “whirlwind terracer”,was trialled in the South Island atInvermay Research Station, andsuccessfully used in the Otago regionduring the 1950s and 1960s.


10.1 Introduction

The feasibility of stabilising massmovements varies according to the formof mass movement involved. In general,Earthflows occur in the weatheredregolith and so are usually between 1and 3 metres in depth. However, indeeply weathered crush zones they canbe much deeper. The weatheredmudstone in which they form frequentlycontains a high proportion of theswelling clay, montmorillonite(bentonite).

Slumps, with their semi-circular failureplane, form in deep relatively uniformcohesive materials, includingunconsolidated deposits such as claysand weaker tertiary sediments. They maybe ten’s of metres deep at their greatestdepth.

The depth of a Slide will frequently bedetermined by the depth of a particularsedimentary layer that may have lowershear strength or different hydraulicproperties.

Various forms of mass movement mayoccur in the same failure. For examplethe lower part of a slide may bedisrupted during failure to form anearthflow or debris flow, while the uppersection may contain the rotationalfeatures of a slump.

From these definitions it can be seen thata deep earthflow is unlikely to be as largea feature as a deep slump or slide andthere is therefore a greater possibility ofimproving the stability of an earthflow.

Deep slumps and slides are majorfeatures that are challenging even withthe unrestricted use of civil engineeringtechniques. This is illustrated by theexpense of the stabilisation works in theCromwell Gorge. While this approachlies outside the scope of this handbook,the geotechnical principles involvedapply equally well to smaller massmovements.

10.1.1 Geotechnical Principles of Slope

Stabilisation

The stability of any slope is a balancebetween restraining forces (the shearstrength of the soil) and destabilisingforces (gravity). Both of these forces areaffected by the angle of the slope so thatthe opposing forces in a gentle slope areless than those in a steep slope. Wheretectonic uplift is still occurring, andstreams are actively down-cutting, theslope angle will be determined by thestrength of the material it is composedof. Weak material will only support agentle slope (after its toe has beenremoved by stream erosion) while verystrong rock will form vertical bluffs.

In effect this means that many slopeswhere there are high rates of uplift,including North Island East Coasttertiary sediments, are quite close tofailure in their natural state. Only minorchanges in the strength of the soil or toshape of the slope (e.g. the removal ofthe toe of the slope by stream erosion orroad cutting construction) may causefailure to occur.

It is convenient to regard soil shearstrength as having two components:

Cohesion is not affected by the weightof the soil overlying it and also remainsrelatively constant over short periods oftime. It can be increased by the presenceof the roots of plants and can also bereduced over time by weatheringprocesses. In the case of weaklycemented mudstones containing swellingclays the most rapid weathering processis excessive drying. Shrinkage forcesbreak the existing bonds and re-wettingallows swelling to occur more freely thanbefore. In one example near Gisborne afreshly exposed hard mudstone slopebecame a moving earthflow in only 11years. Vegetative cover and shade istherefore very important to preservecohesion.

In another well documented situation,railway cuttings in Britain failed up to 80years after their construction due toprogressive failure of the slowlyweakening clay material.


CHAPTER 10

Dewatering Techniques for

Deep-seated Mass Movements

The frictional component of shearstrength is dependent on the normalforce (at right angles to the shearingforce) and so greater frictional strength ismobilised deeper in the slope. Opposingthis normal stress is the pressure of waterin the pores of the soil (the pore waterpressure). Consequently an increase inthe pore water pressure reduces thenormal force which in turn reduces theshear strength of the soil.

10.1.2 De-watering

The aim of de-watering is to manipulatepore water pressure as an effective way toincrease the shear strength of the soil.The essential principle is to minimise thevolume of water getting into the slopeand maximise the volume leaving theslope. The means of doing this will varywith the type of mass movement beingstabilised but surface drainage, sub-surface drainage, surface reshaping, andafforestation are basic tools.

Where agricultural or forestry land is atrisk and some land movement can betolerated the lower cost approachesdescribed here can be adopted. Whereassets such as buildings and roads areaffected by the movement a morecomprehensive engineering approachwould be warranted for the higher levelof protection it might provide.

10.1.3 Overview of the Site

• Obtain all available informationabout the movement.

• Study stereo aerial photographs todetermine the boundaries of themovement as it may be greater inextent than is obvious.

• Look for similar features on the otherside of the ridge and lineations acrossmore distant slopes that may indicatea fault or crush zone. (In crush zonesthe underlying material has beenweakened by fault action andmovements, usually earthflows, aredeeper and harder to control.Groundwater flow patterns may beless predictable in crush zones andfault lines and in places natural gasmay be a complication.)

• Search for any available geologicalinformation to determine lithology.The dip slope and strike of strata willbe significant in the case of slides.

• How deep is the movement? Theheight of the head scarp will givesome indication but in the case ofearthflows may exaggerate the actualdepth.

• Locate possible infiltration zones andponding areas which are oftenproduced by previous movement inthe upper parts of slumps andearthflows. (While ponds raise waterlevels in the surrounding area andpresumably increase pore waterpressure beneath them, it is dryhollows with substantial catchmentsthat have the greatest impact oninfiltration. Particularly in deepslumps in argillite, these hollowsconcentrate run-off and allowed it toflow to deeper layers through highlypermeable crushed rock. As the failureplane is almost certainly of lowerpermeability, high pore waterpressures can develop there,triggering movement.)

• Examine the toe of the slope to see iflack of support there has triggered themovement.

• How active is the movement?(Slumps and slides that have reachedequilibrium may be reasonably stable,as long as their toe is not eroded orcut away. Many older farm buildingsin hill country are located on theback slope of old slumps, as they mayhave been the only level sites notprone to flooding. Earthflows cannotbe regarded as having long termstability unless extensive stabilisationwork has been carried out, and theyare almost certainly not suitable asbuilding sites.)

10.1.4 Surface drainage

The rotational movement of slumpfailures can form a back-tilted zone,below the head scarp, which has poorsurface drainage. Tension cracks andgeneral deformation in this zone allowsinfiltrating water ready access to theshear plane. The careful use of surfaceditches can drain these ponding areasand reduce infiltration. The number,location and depth of the ditches mustbe determined for individual sites.

Attention to detail is important toprevent scour erosion of the ditches,particularly in unconsolidated sedimentswhere a surface cover of grass may bemore important than a tree cover whichleaves the soil bare. Drop structures and


flumes may be needed to safely removedrainage water from the movement(those structures have been coveredelsewhere in this handbook). As analternative, willow poles can be plantedwhere scour is a problem. The ditchesneed to be maintained and cleared ofdebris regularly to remain operative.

Where dry hollows with significantcatchment areas exist, run-off should bediverted from them and out-fall ditchesprovided. Smoothing of the land surfaceto shed water may reduce infiltration anda covering of low permeability materialsuch as a clay soil could be considered.

Any part of the slump that has verybroken ground may require re-contouring by bulldozer to encouragerun-off. Tension cracks should be filledand compacted to reduce infiltration.

Graded banks, very similar in appearanceto small farm tracks, may be used tocarry run-off to stable land off the slumpor slide. The grade must be appropriatefor the soil type to prevent scour, butsteeper grades remove water moreeffectively.

Slides and earthflows may have differentland forms in the upper regions but thesame principles regarding infiltrationreduction apply. The shear plane may bemore directly exposed in the upper slideand the prevention of infiltration here iscritical.

10.1.5 Sub-surface Drainage

This is intended to remove water thathas already infiltrated into the soil andso also the lower pore water pressure atthe shear plane. Two variations havebeen commonly used on farm scaleworks, particularly in the Gisborne-EastCoast Region:

Horizontal boring, in which holes ofabout 50 to 60mm diameter are drilledon a gradient up into the unstable massfor distances up to 30 metres. The holesare lined with perforated pvc pipe ofabout 25 to 35mm diameter and thewater removed is collected by a system ofsurface polythene pipes and disposed ofsafely off the movement.

Portable drilling equipment based on asmall motorised posthole borer has beenconstructed for this purpose (seephotograph) and the unit can be carriedby two people in rough terrain.

The location of the boreholes is more ofan art than a science as the method relieson intercepting naturally occurring highpermeability zones in the movement.Experience in interpreting landscapes forsigns of old tension cracks, shear planesand even fault lines is important. Someremarkably high flows have beenrecorded such as at a site near TokomaruBay in which 3000 litres per hour wasremoved after heavy rain by 18 bores(Hall J. 1973). Volumes of this size couldnot be expected from material ofuniformly low permeability.

Spring taps, in which obvious springson unstable hill slopes are excavateddown to parent material to locate thesource of the flow. An envelope ofpolythene, filter fabric and a permeablematerial (gravel or polystyrene beads)surrounding a length of perforated pipeis buried so as to intercept the flow andtake it to the surface. From here it can bepiped to safe disposal or used as a stockwater supply. It is critical that the openend of the perforated pipe extends to theground surface on the up-slope side toallow the entry of air to the system. Ifnot, air locks will stop all flow in thepipe.

As the lower sections of an earthflow aregenerally too mobile for horizontal boresthe most effective subsurface drainagewill be in the upper sections wheregroundwater from higher land may beentering the earthflow. Monitoring of anumber of earthflows revealed evidenceof this.

The best location for spring tap andhorizontal bore installations tends to bewhere ground water seeps to the surfacein the upper section of flow. The locationof these areas is best done in late


Portable drilling equipment used to install

horizontal bore draws. This bore produced an

initial flow of approx 3,500L per hour and

prevented further slope instability. Photo: Don Miller.

summer when vegetation changes aremost distinct.

Cut-off drains can be used to interceptgroundwater flow, usually, but notnecessarily above the unstable part of theslope. They can be particularly usefulabove unstable sections of farm track asthey will improve both slope stabilityand surface wetness. Location willdetermined by observation of wet zoneson the slope and by the presence ofinfiltration areas further up-slope.

In general the greater the depth, thegreater the possibility of interceptinggroundwater, although depth is limitedby available equipment and by safetyconsiderations. The most effective cut-offdrains will penetrate to a zone ofpermeable material or fractured rock inwhich water is flowing. Unstable slopesbelow ash covered ridge tops could alsobenefit from extensive use of cut-offdrains although expensive. Althoughuntested, this could also reduce theincidence of shallow slips which arecommon in those circumstances.

Drains consist of a perforated pipesurrounded by a bed of permeable filtermaterial in a back-filled trench. Thechoice of permeable filter surroundingthe pipe is important. While not aproblem in fractured hard rock, the forceof the escaping water can erode thesurface of softer materials and lead toclogged, ineffective drains. For examplethe use of gravel as a filter in non-cohesive sediments may allow migrationof silts to occur. Filter fabrics should beused around the outside of the filtermaterial, rather than directly around thepipe, to maximise flow. Details of filter

design can be found in engineering textbooks (e.g. Cedergren, 1968).

Commercially available filter materialsinclude vertical multi-layered compositefilters to collect and remove seepage overseveral metres depth.

Back-fill material should prevent inflowof surface water unless a second shallowperforated pipe is included for thispurpose, as the flow of surface waterdown to greater depths could causeinstability if pipe outlets becomeblocked. Tree roots, particularly ofwillows, could block drains if trees areplanted too close. If cut-off drains arereasonably straight it may be possible toclear and flush them using a highpressure pump and suitable nozzle, aswith horizontal bores.

10.1.6 Surface Re-contouring

Smoothing and re-contouring is aimed atspeeding run-off and eliminating hollowsthat could pond. Earthflows have a moredisrupted and broken ground surfacethan slumps or slides and are generallymore shallow. They move in a moreregular pattern, being triggered byconsistent heavy rain over a number ofdays. (In a Gisborne trial an average of40mm rain per day over 10 days createdapproximately as much earthflowactivity as Cyclone Bola did with about400mm in 3 days)

Smoothing and re-contouring the brokensurface to reduce infiltration is the mostimportant component of de-watering anearthflow, but this may not be possibleuntil initial surface drainage work haslowered the ground water table. Inextreme cases it may take one or twoyears after ponding areas have beendrained by ditches before the land is dryenough for bulldozer operation.

Re-contouring involves the movement ofhillock material to larger hollows in theground surface by bulldozer, to facilitaterun-off of surface water. Care should betaken to preserve topsoil for laterrespreading. Smoothing removes the“hump and hollow” surfacecharacteristics of earthflows. Cultivatingand re-sowing pasture follows, and trialshave shown that substantial growth ofsummer feed results, due in part to theshallow ground water table.


Surface recontouring of an earthflow north

of Gisborne. The areas of recontoured,

smoothed and cultivated land contrasts

strongly with the areas of untreated

earthflow. A graded bank removes water from

an area of springs to stable land for safe

disposal. Photo: Don Miller.

10.1.7 Graded Banks

Graded banks, serve to shorten thelengths of run-off overland flow paths, sodecreasing the possibility of infiltration.They can also remove the outflow fromspring taps and horizontal bores, wherethese have been installed. These banksare the most vulnerable part of thedrained earthflow as water isconcentrated there.

Successful earthflow de-watering projectshave generally had substantial plantingsof willows or poplars along the lowersides of the graded banks where theirroots strengthen the soil and compensatefor the increased pore water pressureunder those banks. A major failureduring Cyclone Bola occurred in a de-watered earthflow where the willowswere less than 2 years old.


Gully Structures

& Pole Planting

Chapter

11. Gully Control Structure

12. Pole Planting

11.1 Flumes & Chutes


Flumes and chutes are structures thatconvey stormwater over a gully head orscarp slope to discharge at a pointsufficiently downstream so that theplunge pool does not migrate upstreamto undermine the gully head. In manysituations, flumes are used as a short tomedium term measure to control activeheadward erosion, until associatedprotection planting has stabilised thegully head.

11.1.2 Application

The design of the flumes and chutes iscritical to ensure that they are able toconvey peak flows. Flumes were normallydesigned to convey (at least) a 10 yearstorm event where the storm duration isequivalent to the time of concentration.Many of the runoff control systemsdescribed below are covered in detail byEyles (1993), using design practicesdeveloped by Mr Ian Cairns and othersoil conservators in the Central VolcanicPlateau Region. When siting the runoffcontrol structures, alternative flow pathswere often incorporated into the designso that when design flows wereexceeded, the stormwater flowsdischarged to alternative outlet pointswhere damage would be minimised as faras practicable. Most failures relating toflumes were due to incorrect sizing forthe design storm, or failure at the inlet ofthe flume by undermining or scouringaround sides of the wingwalls.

The design of flumes and chutes haschanged over the years. While all flumesand chutes in the past were designed andconstructed for specific sites, differentproducts to convey small to mediumflows are now available commercially,and can be bought ‘off the shelf’. Asthese products have become available,designs have adapted to incorporatesome detention of stormwater above thegully heads with the discharge into thenewer types of flumes, chutes or pipes.

When constructing any gully controlworks where storm water is going to bedirected to a pick up point and then

safely conveyed past the critical gullyhead, water should not be directed intothe flume or chute until downstreamworks have been completed. Considerthe consequences of a storm half waythrough the job. It is often a good ideato divert stormwater away from theworks site until the project has beencompleted. Alternatively, if the job isgoing to take two or three days, wait fora spell of good weather, and then try andcomplete the downstream part of the jobbefore building the bunds and stopbanksto divert water into the runoff controlstructure.

11.1.3 Types of Flumes and Chutes

Lip FlumesThese are wooden structures thatconcentrate the surface flow anddischarge it safely over a gully head orscarp slope.


CHAPTER 11

Gully Control Structures

Plan of lip flume, 0.56m3/s capacity.

All measurements are in millimetres.

Source: Environment Waikato.

They need to be constructed using fromeither ground treated tongue and groovetimber or H4 marine plywood. It shouldbe dry, tightly jointed and double nailedwith galvanised treaded flathead nails atall bearers and joints. The structureneeds to be well footed, and sealedcarefully at the inlet using .005mm

polythene or equivalent. Wing walls atthe inlet need to be well founded andsealed where they join with the sidewallsof the flume. Bunds (stopbanks)controlling the flow of water into theinlet also need to be well founded andcompacted.

Cantilever & Escarpment FlumesCantilever and escarpment chutes arewooden flume structures designed to bebuilt upslope of a gully head andcantilevered into place.

These structures have the same functionas the lip flumes. They are used only insituations where the gully heads aremore than 3 metres deep, and often 10metres or more, or where all work has tobe undertaken from above the gully headand cantilevered into place.

The same principles and constructionmethods apply as for lip flumes.However, construction needs to belighter to allow for the difficulty oflevering the structure into place. Neithercantilever nor escarpment flumes arecurrently used because they areexpensive and difficult to construct.They have been superseded by the use ofsmall detention bunds or ring bankdischarging into drainage coil piping.

Suspended Flumes or pipesSuspended flumes are supported by wiresout over gully heads. They are used onlyin locations where it is not possible to


Plan of a typical cantilever flume.

All measurements are in millimetres.


Plan of a suspended flume designed to drop surface flows safely over a 30m head in a difficult

hill environment. This was one of a series of measures that included diversion banking to

concentrate flows into detention bunds, retirement and planting of the gully head and outflow

areas. This flume, constructed in the early 1960s, has disappeared but the gully system

remains stable.

build conventional drop structuresbecause of access, difficulty.

The suspended flumes are used to conveythe stormwater flows far enough awayfrom the gully head so that the dischargedrops and forms a plunge pool thatdissipates the energy without migratingback up slope to undermine thestructure.

The design of suspended flumes variesdepending on site conditions. Thewooden constructions are expensive tobuild and difficult to maintain. Theavailability of plastic pipes in a range oflengths and diameter has meant that theprinciple of suspended flumes can beused with pipes as an alternative.However, their use is restricted to a fewsites only, and often in conjunction withother practices such as detention bunds.

Wooden ChutesWooden chutes are used to conveystormwater flows down steep slopes, orescarpments. They sit on the groundsurface and need to be anchored atregular intervals. They discharge at thetoe of the slope or escarpment into adissipating chamber.

Wooden chutes are constructed out ofeither ground treated timber or H4marine plywood. The actual designs varydepending on how critical the site maybe. They are often used to take thedischarge from a culvert pipe safelydown a steep slope. While Armcofluming, plastic piping or other productshave largely superseded them, they arestill used in some locations as they canbe constructed on site, and can be sizedto accommodate the design flow.

Butyl Rubber Flumes:Butyl rubber flumes are used to conveystormwater down slopes or escarpments,in locations where the slopes are not toosteep (less than 20°) and the flume is dugpartially into the ground for support.The discharge point needs to beprotected to avoid scour problems.

Butyl rubber flumes were used insituations where water control wasrequired for the medium term, and thevolumes of water or length of dischargewas too great for wooden chutes. Theywere often manufactured to suit specificjobs and were considered to be a costlyoption to employ. Butyl rubber flumingis not used often now as there are a wide

range of alternative cheaper productsthat can be used with equal success.

Corrugated Armco or Steel Flumes:These flumes act as chutes to conveyflows safely down steep slopes orescarpments. They may be partially duginto the slope so that they are supported,or can sit on the ground surface and beanchored at regular intervals. Thedischarge point at the toe of the slopeneeds to be adequately protected toavoid scouring problems.

These flumes are used very widely,particularly in association withconveyance of runoff from roads andtracks taking discharge from culvert pipessafely down steep slopes below the road.

Alternatively, they can be used in asimilar fashion to lip flumes and extendover the edge of a gully head.

11.2 Pipe Drop Structures


Pipe drop structures transport surfacestormwater from above a gully head tothe gully floor, bypassing the gully head.

ApplicationPipe drop structures have largely replacedwooden flumes and chutes, especiallywhere design flows are relatively low.They have a number of advantages overwooden structures:


Butyl rubber flume. Photo: Gisborne DC.

• They have a lower cost compared towooden structures;

• They are more versatile and havelower maintenance requirements;

• Inlet systems for pipes are oftenprefabricated, which reduces the riskof failure;

• More pipes can be added if required;

• They have a degree of flexibility andcan move if the gully changes shape.

Pipe drop structures are often used inconjunction with bunds or diversionbanks, as well as detention banks ordams. They vary markedly in design andconstruction, but essentially there aretwo main types:

• Rigid pipes carrying surfacestormwater over or away from thegully head to a safe disposal point;

• Flexible pipes following the gullyhead or side wall contours down to asafe disposal point. The flexible pipescan be of collapsible form, whichexpand to take design flows.

All pipe drop structures need to be firmlyanchored throughout their length and atthe discharge point. When they arerunning full, they have to cope with theweight of water as well as the energy ofwater. Pipe structures that drop water toconsiderable depths will need to copewith high water velocities, and need tobe sufficiently robust to withstand thepressure changes. Specialised engineeringdesign may be required to ensure thatthe structure is sound and well founded.

11.2.2 Types of Pipe Drop Structures

Drop Man Hole InletThe drop man hole inlet structure has aninlet into a man hole that allows thesurface stormwater to descend safely tothe required depth, before dischargingout of an outlet pipe to a safe disposalpoint. The drop man hole inlet requirescareful design to ensure that thestructure is able to convey the designflow. The design also needs to ensurethat the structure is able to cope with theforces exerted by the energy of the waterdropping down the man hole, and thehigh pressure from the head of water.

The structure is commonly used wherethe gully head is not excessive (less than

3 metres depth) as a substantial amountof earthwork may be required forinstallation. These structures are used toprovide long term protection withrelatively low maintenance requirements.

Rigid Pipe Drop StructureThese structures involve the use of rigidpipe either discharging out over the gullyhead into a plunge pool, or conveyingsurface stormwater around the gullyhead and down the side of the gully to asafe disposal point. Where the pipes arerun down the slope, they need to be wellanchored at regular intervals.

In the past, costly steel or concrete pipeswere used for protection of valuableassets. More recently, plastic or PVCpipes have become available, and aremore cost effective.

Flexible Pipe Drop StructuresDrainage coil is a non-collapsible flexiblepipe e.g. Nova-flo. When used with abund or ring bank around the top of thegully head, it has proven very successfulfor treating small gullies (less than 5metres high). The drainage coil is used toconvey surface stormwater around thegully head to a safe disposal point on thegully floor.

The drainage coil pipe used shouldalways be the non-perforated pipe thatdoes not have drainage holes in the pipewall. The pipe should be either anchoredor dug in to ensure that it is stable. Inletsystems vary in design, but oftenincorporate a small drop man holeburied so that the inlet is at ground level.Alternatively, a simple culvert inlet witha small head wall can be used to drainstormwater from diversion bunds ordetention banks. For remote sites,sandbags filled with a dry cement/soilmix can be used to construct small inletheadwalls. Sandbags should be filled tono more than one third full. The endsare folded back to seal the sandbag, andthe headwall is constructed using thesandbags in a manner similar to layingbricks, but allowing for a slight batterslope on the headwall. Following rain,the headwall hardens and becomes apermanent structure.

Collapsible tubing such as Pyvac,Structure-flex, or UV stable lay-flat pipingcan be laid to remove surface stormwaterfrom gully heads. The collapsible tubingcan be factory prepared to cope withspecified volume of runoff, and is


sufficiently flexible to follow the contourof the scarp face. Operates in a similarfashion to drainage coil pipe, in that it isoften used in conjunction with a ringbank or diversion bund that protects thetop of the gully head, and directs waterinto the pipe inlet.

The collapsible tubing needs to be wellanchored at regular intervals to preventthe pipe from twisting and blocking.Also, the collapsible pipe needs to befirmly attached to the inlet pipe as theweight of water over the gully drop offcan exert enough force to split or breakthe material. Pyvac tubing is produced inrolls 30 metres by 1500 millimetres witha full width fitting over a 457 mmdiameter pipe. Sheets can be welded tofit larger pipes.

Collapsible tubing has been largelysuperseded by drainage coil pipe becausedrainage coil is cheaper, more versatileand requires less maintenance.

11.3 Sink Holes


A sink hole is a drill hole usually with adiameter of 1-1.2 m, and a depth inexcess of 6 m, filled with rock rubble. Itis located immediately upslope from abund on low angle terraces in anephemeral drainage line. The practice issimilar to normal soak holes used for thedischarge of stormwater from houses,except the soak hole is extremely deep.

ApplicationThe downstream sink hole and bundshould be located no closer than 150metres from the gully head or terraceedge to ensure that groundwater rechargevia the sink hole does not weaken thescarp face and cause collapse. The holesare normally used in series to provide forcontinuous drainage directly into theground. Their use has been confined todeep gravel/sands river terraces in theWaikato region. The inlet system shouldbe fenced to prevent stock andmachinery from going too close to thestructure. Careful attention should begiven to ensuring an efficient filtersystem is put in place to prevent sealingand failure of the system. The practice isdescribed in detail by Eyles (1993).

11.4 Detention bunds/dams


Detention bunds are up to 1 m high witha 150mm or similar-sized discharge pipethrough the base. They are usually usedin series along a valley floor or upslopefrom a gully head and are often usedwhen limited storage is required.

Detention dams are larger structures upto 2.5 m to 3m high and provide agreater storage volume than detentionbunds. They are often located closeenough to the gully head so that thedrainage coil discharge pipe is able toconvey water over the gully head to asafe disposal point on the gully floor.Sometimes two or three detention damsmay be constructed in series down anephemeral valley, with the downstreamstructure conveying stormwater over thescarp face/gully head.

There is provision made for anemergency spillway around the side ofthe structure (at 2 m height), with theoverflow directed to a safe alternativeoutlet as far as practicable.

ApplicationBoth detention bunds and dams detainstormwater so that flood peaks areattenuated. They are normally used inconjunction with other works involvingfencing, retirement from grazing andplanting of the gully head and associatedsteep escarpment area. The detentionstructures are normally designed toprovide for a minimum of 100 cubicmetres of storage for each hectare ofcontributing catchment. The method isused primarily for small catchments ofup to 40 hectares only.

Care needs to be taken with both thedetention bunds and dams, so that


Sink hole cover for heavily stocked locations.


discharge from the outlet pipe does notcause scour.

The detention bunds and dams can onlybe used if the site is appropriate, andthey provide protection for small tomedium storms (5 to 10 year returnperiod) if used in conjunction with otherworks as described above. The practicesare described in detail by Eyles (1993)and Environment BOP Fact sheet 10/98.

11.5 Diversion Banks = diversion

bunds on earthworks

Description/PurposeDiversion banks are earth banks thatguide surface flows across a slope to asafe or controlled outlet. They are usedeither to protect a gully head, bydiverting surface stormwater flows to asafe outlet, or to collect surfacestormwater and move it to a particularspot for safe disposal or detention.

ApplicationDiversion banks are often used inconjunction with a controlled outlet pipeor flume system. They should bedesigned for the 1 in 20 year storm eventand constructed on a grade that does notexceed the scour velocity of theparticular soils on site. This may be aslow as 1.25%, so a survey grade line willbe required. Diversion banks need to bewell compacted and grassed as soon aspossible following completion. If thebatters are not too steep, the diversionbank will blend in with a paddock andbecome part of the landscape.

This practice is described in more detailby Eyles (1993) and is similar to thediversion channels/bunds described inthe chapter on earthworks.

11.6 Graded Waterways=grassed

waterways in pasture & cropland


A graded waterway is a mechanicallyconstructed grassed waterway thatintercepts and transports surface runoffto a safe outlet. The function of a gradedwaterway is to regain the naturaldrainage grade in a gullying drainagesystem, enabling flows to be managedwithout scour or further gully formation.

ApplicationAs with diversion banks, it is essentialthat the waterway grade keeps runoff toless than the scour velocity. All gradedwaterways should be well compacted,and grassed as soon as possible afterconstruction. Alternatively, natural stableground can be used.

Maximum erosion proof grades ofwaterways depends on factors such asthe design flow, the shape of thewaterway, frequency of peak flow etc.Instead of slope angles, maximum flowvelocities are generally recommendedbeyond which erosion of channel willstart to occur. For grassed channels, aconservative estimate of this velocity isabout 2m/s (ref: Hydraulic Designmanual, Humes Industries Ltd). Thisvalue will vary depending on the typesand state of the vegetation, e.g. type ofvegetation, height, density, stiffness etc.

The following table indicates acceptablevelocities of flow within various grasslined channels on a range of slopes.

Table 11.1 Permissible Velocities for Grass

Lined Channels

Channel slope Lining Velocity* (m/s)

0-5% Grass/legume 1.25

Kikuyu 1.85

5-10% Grass 0.91

Kikuyu 1.5

>10% Grass n/a

Kikuyu 1.25

*For highly erodible soils decrease permissiblevelocities by 25%

(Modified from Soil and Water ConservationEngineering; Schwab et al).

Simple engineering formulas can be usedto work out the design flow and channelsize and shape. Channels that will havevelocities greater than about 2 m/s mayneed to be reinforced or armoured (usingfabric, rock etc).

The graded waterways can be installed insituations where the eroding gully headis not excessively high. The site needs tobe excavated to provide a firmfoundation prior to filling to correctgrade. The fill material should be formedin layers and firmly compacted as fillingproceeds. Different geotextiles are nowavailable with which to reinforce andgrass the graded waterways. Reinforcedwaterways can withstand significantly


higher velocities i.e. can be installed atsteeper grades without erosion occurring.

Graded waterways are suitable for grassedspillways at detention dam sites. Careshould be taken when grazing stock ongrassed waterways, as any exposure ofthe ground surface can initiate furthererosion.

11.7 Drop structures


A range of drop structures are used toconvey surface water safely over a gullyhead. Most of the structures are capableof controlling a gully head drop of up to3 metres in height. However, many areonly used on short gully head drops of 1to 1.5 metres.

ApplicationWhile many of the drop structures areexpensive to construct, they are alsorelatively permanent. Generally there isless reliance on planting for long termstabilisation. Some of the drop structuresdescribed below can also be used inpermanent streams.

All drop structures should be carefullydesigned to ensure that they are able toconvey the design flow. Provision needsto be made for storm flows in excess ofthe design storm, when the structure isovertopped. Drop structures are designedto be site specific. Sometimes, the idealstructure may be a combination of twoor more of the generic structuresoutlined below.

Drop structures generally have twocomponents:

• The ‘drop’ section where the water isconveyed from one level down to alower level; and

• The ‘energy dissipator’ which may bea plunge pool or a flexible structuresuch as a tyre mattress.

The wide range of construction materialsand geotextiles now available means thatmany different types and combinationsof drop structure can be built.

11.7.2 Types of Drop Structure

Concrete Drop StructuresThese are constructed entirely ofconcrete, concentrate the surface

stormwater at the top of the gully head,and discharge the water over aconstructed chute to dissipate safely atthe toe.

The inlet needs to carefully designed andconstructed so that the structure is notoutflanked. These types of drop structureare expensive, but will last for decades.They have been used to protect valuableassets and are often used as spillways fordams. The structure requires carefulengineering design to ensure that thedesign storm can be conveyed, and thatthe energy is dissipated at the toe of thestructure.

Geotextile/Interlocking ConcreteBlock Structures:These structures are similar in principleto the concrete drop structures exceptthey are built using geotextile cloth withinterlocking concrete blocks laid on thesurface of the cloth to hold it in place.


Concrete drop structure – spillway over dam.

Photo: horizons.mw.

Geotextile interlocking concrete blocks.

Photo: Wellington RC.

The structure is designed to be sitespecific and is built completely on site.

This type of structure is expensive, buteffective and permanent. The site needsto be carefully prepared with experiencedstaff carrying out the construction.

Rock Rip-rap Drop StructuresThese drop structures use rock rip-rap toconvey surface stormwater from the topof a gully head safely down to thebottom of the drop. The mediandiameter size of the rock rip-rap shouldbe calculated to ensure that it is able tolock together and withstand the forcesgenerated by the stormwater.

These structures have proven to besuccessful, as the rip-rap provides astructure which is flexible, and canadjust to any changes in the shape of the

gully head over time. The inlet to thedrop structure is often formed fromgabion mattress or concrete to ensure itis not outflanked. The rip-rap may needto be topped up after a few years, as thestructure settles. Geotextile cloth shouldalways be laid underneath the rip-rap.During construction, the channel slope isshaped first. Geotextile cloth is then laiddown, the inlet constructed, and rockrip-rap placed. Depending on the site,the inlet may be constructed last.

Sheet Piling Drop StructuresSheet piling is sometimes used to controlsmall gully heads (up to 1.5 metres inheight) particularly in difficult situationssuch as permanently flowing streams.Sheet piles are lengths of channelledsteel that are driven in as piles butinterlock so that the completed structureforms a continuous wall. The sheet pilingis driven (or vibrated) into position withheavy machinery and provides apermanent gradient control.

The sheet piling needs to be long enoughso that the plunge pool formed at the toeof the drop structure does notundermine the sheet pile walls. Thismethod is an option that can be used forriver training works.

Rock Gabion Drop StructuresRock gabion drop structures are normallyused to control small gully heads up to 1metre in height. They can be used inboth ephemeral gullies and permanentlyflowing watercourses. The structurecomprises a rock mattress (rock inside awire mesh) that forms the downstreamenergy dissipator, with a rock basket onthe upstream end. The stormwater flowsover the rock basket to land on the rockmattress. The rock mattress needs to belong enough, that any plunge poolforming at its toe does not underminethe structure. Engineering design will benecessary to ensure that the structureworks.

These structures will require machinework to construct. They should becarefully designed so that the size andweight of rock used in the structure isable to stay in place. The mattress isflexible and will drop if a plunge poolstarts to form at the downstream end.Also, it is important that the rock basketand sides of the rock mattress are carriedup the sides of the channel and fittedinto the natural ground contours in ahydraulically smooth fashion. Geotextile


Sheet piling drop structures.

Photo: Gisborne DC.

Rock gabion drop structure.

Photo: Wellington RC.

cloth should be laid underneath thestructure to ensure that fine material isnot scoured out. When these structuresare used in rocky streams, tyres aresometimes wired onto the top of themattress to protect the wire mesh frombeing broken by the moving rockbedload during major floods.

Normally these structures have a life of15 to 25 years, and are supported byplanting.

Boulder/Tyre Drop StructuresThese structures are similar in principleto the rock gabion/mattress dropstructures except they are constructedout of boulders and tyres, with the tyreswired together and laid over thedownstream part of the structure. Theyare normally used in permanentlyflowing streams where a substantialstructure is required to control channelgullying.

These structures have been used in theWairarapa by the Wellington RegionalCouncil and require experienced peopleto ensure they are designed andconstructed properly.

Geotextile Reinforced Grass DropStructures:These structures are used in ephemeralgullies to safely convey the surfacestormwater over a gully head or steepslope. They comprise a shaped grasschute that has been reinforced bygeotextile so that the roots of the grassare bound together and are able to resistnormal scour velocities during floodflows. The site is shaped and thegeotextile is laid down. Topsoil is thenspread over the geotextile and grassed.The grass should be firmly establishedbefore the drop structure is used. Someform of energy dissipater such as rock orgabion basket is required at thedownstream end of the structure.

These structures have only becomepossible with the advent of a range ofgeotextiles in recent years. Theappropriate geotextiles should be chosenfollowing careful design to ensure thatthe velocities will not scour out thestructure.

11.8 Debris Dams


Debris dams are structural controls thatare built in the floors of eroding gullies.Their purpose is to stabilise the gullyfloor so that trees can be established tostabilise the gully sides. While trees arethe main long term tool for V-shapedgully control, they can be difficult toestablish if water channels continuallyundermine the toes of the hill slopes.

ApplicationDebris dams are largely confined to thecontrol of V shaped gullies. They arenormally built in series over time, withthe base of the upstream debris damlevel with the top of the debris dambelow. However, locating a suitable siteto commence debris dam construction isan important part of ensuring theirsuccess. It is important that the site isable to give sufficient support to thesides of the dam.

Debris dams can be used for a number ofpurposes (Gair 1973):

• To provide a stable area in an erodinggully, thus facilitating theestablishment of trees or othervegetative control measures;

• To effect grade control, byeliminating bed level fluctuations;

• To reduce channel slope angle;

• To raise bed height, therebysupporting the base of gully walls;

• To increase bed width, and with itreduce water velocity;

• To centralise water flow in thechannel;

• To trap and hold debris. This not onlygives associated tree planting a bettergrowing medium, but also helpsreduce deposition downstream.

The height of debris dams (where thewater flows over the centre of thestructure) should be approximately600mm on completion of construction.Following the formation of a plungepool, this height may increase to 750-800mm. It is important that debris damsare not built higher than this, as theywill become prone to failure. Over theyears, many different types of debris dam


design have been built. A common factorin all types of construction is the needfor the structure to be stable when thedownstream scour hole has formed.

In the Gisborne – East Coast region ofthe North Island, early dam constructionmade use of manuka brush, which wasoften in plentiful supply. The brush wasused for fascines, acting as an energydissipator below the debris dams. Themost successful design was the TimberDebris Dam. The Timber Debris Dam wasbased on an earlier Pole and NettingDam designed by J Gair, but used groundtreated timber as suggested by H Pearce.OM Borlase of the Poverty BayCatchment Board, and WR Howie, Waterand Soil Division, Ministry of Worksundertook early design work on debrisdams. During the 1960s and 1970s,thousands of debris dams wereconstructed, particularly in the Gisborne/ East Coast area. However, very few havebeen built since loss of Governmentsubsidies for soil conservation work, in1988.

11.8.2 Types of Debris Dams

Timber Debris DamThe timber debris dam is built out ofground treated timber and comprises adam wall across the gully floor 0.9 mhigh with a lower centre section 0.6 mhigh. The dam has wing walls thatextend up the side slopes of the gully sothat the top of the wing wall is aboveflood level (at least 1 m above the top ofthe dam face). The dam is anchored with1.8 m long fence posts, as well astiebacks to warratah standards. The damwall and wing walls are constructed fromground treated 150mm x 25mm railsspaced approximately 25mm apart. Thelowest board on the dam face is dug intothe gully floor at least 75 mm deep, withgeotextile cloth anchored and laid partway up the inside face of the dam wall.

The following have been noted asimportant tips for construction:

• If posts are not driven in, make surethey slope up-stream.

• Where posts cannot be used (becauseof rocky ground) standards can beused. They must, however, be tiedback.

• The bottom board must be levelotherwise it will be difficult to line upthe other boards. It is essential thatthe weir rail is level, or else the waterwill be channelled to one side.


Timber debris dams. Photo: Gisborne DC.

Timber dam construction guidelines.

Material specifications for an average sized dam.• 50m of 150mm x 25mm Merch Grade Pinus Radiata

(ground treated).• 6 posts 1.8m long (No 2; 1/2 rounds or No 3 rounds)

for dam and wings.• No 2, 1/2 round post with at minimum a 100mm face

for weir rail or double thickness 150mm x 25mm.

• 4 standards for tie backs and wings.• 2kg galvanised 100mm nails.• 8m sarlon mesh.• 20m No 8 galvanised wire.

• All bottom boards must be hardagainst the ground to preventundercutting.

• Wings must extend above flood level.

• Geotextile should be spread outevenly before backfilling.

• Backfilling is essential; otherwise thedam will blow out.

• Do not place successive dams closerthan 3m apart. This is because a scourpool forms at the base of the dam,and could undermine another damdownstream.

• Where possible, place the debris damupstream of an existing strong point,such as a tree, or rock face.

Pole and Netting Debris DamsThe pole and netting debris dam wasdesigned in the 1960s, by trying differentdesigns and modifying them over time.The pole and netting dam is similar tothe timber debris dam except it isconstructed using netting, warratahstandards, polythene and manuka poles.The manuka poles were used for the railson the wing walls and weir rail. Ifmanuka was not available, polar orwillow poles were used instead.

The pole and netting dam was largelysuperseded by the timber debris dam inthe 1970s.

Brush Debris DamsBrush debris dams were comprised ofmanuka brush laid 0.5-0.6m thick acrossthe gully floor after rough shaping of thechannel. The brush is laid so that theends overlap, in alternate layers withbutts upstream, then butts downstream.Two rows of standards are then driventhrough the brush in a line across thechannel. No.8 wire is then threadedthrough the top of the warratahs andtightened to pull the brush down firmly.The weir is formed from the brushmaterial with two wings up the sides ofthe gully. At least 12 poplar or willowpoles were planted on the edge of thestructure in a pair pole planting pattern.

By the 1970s brush dams were seldomused. However, they did prove usefulwhere there was some degradation in thegully floor, but the sides of the gully

were still stable. The brush acted in asimilar manner to mulch and helpedprevent the mudstone gully floor fromdrying out and flaking. The dams werecheap, trapped moving bedload (rubble)efficiently, and the manuka brush lastedwell. However, the dams did not copewith any erosion of the valley sides, andwere not used if brush was not readilyavailable to the site.

‘Bag’ Debris DamsThe ‘bag’ debris dam was developed tohelp collect debris as it moved down thegully, and the collected debris was usedas an energy dissipater. The dam wascomprised of angle iron eitherbolted/wired together or driven in a lineacross the gully floor. Wire netting meshwas then attached to the structure andfolded back so that it formed a bagdownstream of the angle iron thatcollected debris. As the bag filled, ittended to settle and act as a mattressdownstream of the supporting structure.

In the Manawatu and Rangitikei-Wanganui areas, this technique wasdeveloped and used successfully. In otherareas, where there was more risk oflateral movement, they required a lot ofmaintenance and were not usedextensively.

Log and Slab DamsSome of the first dams used boulders,logs or other debris already trapped inthe gully floor. These later evolved intousing split logs (slabs) or poles placedvertically across the gully floor orhorizontally across the gully with lateralsupport.

‘Spider’ DamsSome dams used pre-fabricated angleirons that were then placed as a debrisdam structure across the gully floor.Ranges of different materials were usedto form a mattress below the dam.

Various DesignsVarious other designs were usedthroughout New Zealand, incorporatingaspects of the designs outlined above.Often, the designs were “one-off”. Attimes, they were constructed off-site,then brought in and installed in thegully.


11.9 Surface Protection in Gullies

Surface protection is occasionally usedfor gully control. The appropriatetechniques are described in Section 16.2(Mulches, coatings and tackifiers). Thispage merely gives a few additionalobservations specific to their use ingullies.

11.9.1 General

Surface protection in gullies can involvegrassing (the most common measure),mulching, fabric and all manner ofcoating substances such as aggregate,chemicals. Surface protection helps toprotect the soil against raindrop, sheetand rill erosion, which can aggravateexisting gullies. (Also see Earthworks –stabilisation)

Surface protection will not controlerosion from concentrated flows, soappropriate runoff control measuresshould be applied first. Vegetative surfaceprotection such as grassing orhydroseeding will establish moresuccessfully if there is adequate moisture,suitable soil medium, and fertiliser. Nonvegetative surface protection such asmulches, or geotextile fabrics will giveinstant cover, but may need follow upmaintenance or establishment ofvegetation in the longer term. Somesurface protection measures (such as,geotextile fabrics, aggregate etc) can beapplied to the flow paths of surfacestormwater runoff.

Types of Surface Protection used ingullies:

• Grassing

• Hydroseeding

• Mulching

• Geotextile Cloth

• Erosion Control Blankets

• Aggregate

11.10 Mechanical Infilling of

Gullies

Description/PurposeThis involves the contouring of theground surface with earthmovingmachinery to infill small U shapedgullies or tunnel gullies. It is carried outin association with surface stormwaterrunoff control and follow-up surfacerevegetation. The gully infilling isnormally part of the surface contouringto change drainage patterns, andprovides for a more productive land useon eroded ground without anyproductive potential.

ApplicationMechanical infilling of gullies is anexpensive option that can be undertakento address small U shaped gullies, andtunnel gullying problems. Mechanicalinfilling is usually carried out inconjunction with reconstructingdrainage patterns, and establishingstabilising ground vegetation. Wheresmall U shaped gullies occur on shortterrace faces, gully infilling, contouringof the face to an easier grade, controllingstormwater runoff to reduce velocities,and follow up topsoil and grassing cancontrol the active gully erosion andsubstantially reduce the risk of furthergullying.

Infilling of severe tunnel gullies wascarried out at the Wither Hills,Marlborough with diversion banksinstalled to direct water back to ridges,and follow-up planting and surfacerevegetation.

Mechanical infilling of gullies is notcarried out to any great degree in NewZealand because of the large costsinvolved. However, following infillingoperations at Wither Hills, studies haveshown that although the costs are high,increased returns from production alonefollowing treatment can pay for the workover a period of time. Furthermore,increased production costs do not takeinto account other benefits such as easiermanagement, more versatile land use etc.

Mechanical infilling of gullies shouldonly be considered on small scale gullyerosion problems. The costs for infillinglarge gullies will generally beprohibitively expensive, and potentiallyprone to failure



12.1 Description/Purpose

Tree planting is the traditional long-termcontrol measure for gully erosion inmany parts of New Zealand. Inparticular, tree planting is used (inconjunction with other measures) for thecontrol of V shaped gullies. The mainpurpose of planting is to stabilise thesides of gullies. However, planting alsostabilises the gully floor in the long termas tree roots extend beneath the channel.Well planned planting stabilises the soilthrough the binding of roots, as well asdropping soil pore water pressure byevapotranspiration. Planting of trees in agully’s catchment will also help tomodify and attenuate peak runoff forsmall to medium sized storms.

12.2 Application

Normally specific planting for gullycontrol is confined to willows and poplarspecies only. Specialised planting may beused in particular regions e.g. Erythrinain Northland or native planting adjacentto ecologically important areas. Willowsand poplars are normally planted asstakes or poles for specific gully controlworks where there is a potential gullyerosion problem. This is carried out bypair planting or staggered planting alongeither side of the gully floor or erodingchannel. Alternatively, poles can be spaceplanted on the sides of gullies to helpcontrol associated erosion which canaggravate the gully erosion. Willow polesare also used for control of tunnel gullyerosion. Extra long poles (up to 5 mlong) may be necessary in some placesand these are planted directly in theholes formed by the tunnel gullyerosion, or by pair planting if the tunnelhas collapsed. Polar poles may also beused, although willow poles performbetter on wetter sites. The poles shouldhave protectors fitted. Poplars andwillows are ideally suited to gully controlas they have a strong fibrous root systemthat helps to stabilise gully floors.,Because they are deciduous, they do notshade out grasses and other groundcover, providing they are not planted tooclose to each other. Initial pair poleplantings of poplar or willows at 5 or 8m spacing, has proven to be too closeafter two or three decades. Ideally pair

pole planting should have final spacingsof 10 to 15 m between pairs. On verysmall gullies, staggered planting ispreferable to pair pole planting.

If willows or poplars are planted out asstakes, the plantings will need to beprotected from stock.

12.3 Practice

This involves the planting of unrootedpoplar and willow poles into land whichis susceptible to slips. The aim is to havea surface spread of roots through theground which holds the soil togetherwhile also drawing water from theground. Pole length can range from 1 –305metres, depending on whether stockare excluded while they establish.

Poles, like all living plants require carefulhandling and care must be taken toprevent them drying out prior toplanting. Studies show that standing thepoles in fresh water for 48 hoursimmediately after cutting will improvetheir establishment in the field. Initialroot growth draws on moisture reservesin the pole. In some areas it is thepractice to expose fresh wood at the baseof the pole shortly before planting. Thisis done by pointing the pole with a sharpcutting tool prior to planting.

12.1.1 Pole dimensions

Pole length and small end diameter(SED) will vary according to siteconditions, where they are to be planted,and the class of animal that will begrazing the paddock.

All animals will rub against poles as wellas chewing the bark where it is notprotected, so a bigger stronger pole, wellrammed into the ground, will resistmovement better than lighter ones.

Table 12.1 Classes of pole (giving typical

dimensions)

Class of Pole Length SED

Cattle 2.5–3.5, 25mm

Sheep 2.0–2.5m 25mm

Retirement pole(stake) 1.0-2.0m 15 to 20mm

CHAPTER 12

Pole Planting

12.3.2 Planting poles

Poles should be in the ground beforeshoots start to appear, generally beforethe end of August, although some specieshave a later bud break.

There are many ways to plant poles, butthe most effective ways are driving ordigging. For driving, the pole big endneeds to sharpened using an axe orslasher. In some areas poles are slice cutin the nursery which eliminates the needto point the pole on site. If the ground isdry and hard, a pilot hole is made with acrowbar, the sharpened end is placed inthe hole and then driven into theground not less than 60cm, with a poledriver. This is a length of pipe, larger in

diameter than the pole, about 800mmlong and fitted with two handles. A spikemay be fixed to the driver in order tomake a pilot hole. If the ground is moistand soft, poles can be driven without theneed for a pilot hole.

When digging a hole, use a spade to diga hole 60cm deep. Place the pole in thecentre of the hole and ram the earthwith a rammer from the bottom to thetop, taking care to avoid damaging thebark.

It is necessary for the pole to be firm inthe ground while roots are developing.Any movement caused by animalsrubbing or by wind will cause the rootsto break. If a space develops between thebark and the earth the pole will dry out.In some soils, poles may work loose asthe soil dries and shrinks. Re rammingmay be necessary and farmers should bemade aware of this requirement.

12.3.3 Pole placement and spacing

Poles will struggle to grow in thin or drysoil. So sites where there is moisture, anda sufficient depth of soil to allow thepole to penetrate 60cm are essential.Poles will eventually grow into largetrees, so spacing should be carefullyconsidered. Trials have shown that a 10mspacing will develop a full root matbetween trees at five to ten years age.Between ten and twenty years age, rootswill spread more than five metres fromthe trunk, so the stand may be thinnedby removing every second tree i.e. a finalspacing of 20m. However this should notbe done on particularly unstable sites,where root density at five to ten metresradius may be insufficient to bind thesoil.

Poles need only be planted on theunstable parts of a slope, identifiable bymicro-relief which indicates past failure(regrasses slip scars and debrishummocks). So actual planting densityin a paddock will vary, and average outat less than the theoretical 100 poles perhectare. Establishing a pole in a harshenvironment will result in losses, socloser spacings may be required initially,with thinning carried out later.Alternatively, plant replacement poles inthe gaps.

Willow poles form a fibrous root mat anddo better in wet conditions, such asalong stream beds and gully floors.Poplar poles are better on drier sites,although large leafed varieties should not


Widespread gully (above) and spaced

(below) planting of poplars to control

erosion on mudstone soils, Kanakanaia

Valley. Photo: Don Miller.

be planted in windy sites. Poles areplanted to prevent erosion, so it ispreferable to site them where erosioncould occur sometime in the future.Planting them in eroded sites (which iscommon) is worth doing, as it willstabilise remaining subsoil. However, it ismuch harder to establish poles once thedamage has been done.

12.3.4 Pole protection

All poles that are planted in paddockswhere animals will graze must haveprotectors fitted to them. Ideally cattleshould be kept out of the paddockswhere the poles have been planted for 18months to 2 years to allow the poles toestablish. Other animals such as goatsand deer should also be kept out for thesame length of time. In practice thisrarely happens, so protectors are fittedinstead.

Pole protectors, or plastic sleeves, arecommonly used where grazing animalsare presents. There are two types,‘Netlon’ and ‘Dynex’ and both are slidover the poles at the time of planting.They are 1.7m long for use with cattlepoles and slightly shorter for sheep poles.Netlon sleeves need to be stapled at thebottom to the pole.

If stock can be excluded for two years, 1-2m stakes may be planted, a substantialcost saving, compared with the largerpoles.

12.3.5 Other benefits

Willows and poplars are deciduous, andthis is a great benefit to pasture andanimal production. Firstly the trees willprovide shade for animals from spring toautumn. Then, in winter the trees haveshed their leaves, so pasture growth isnot suppressed. Even in summer, enoughlight filters through the trees for pastureto keep growing. Field trials havedemonstrated that annual dry matterproduction is depressed by about 15-25%beneath growing trees (Gilchrist et al1993, McElwee 1998, Parminter andDodds pers. Comms.). The canopyshading loss is roughly counterbalancedby pasture growth between the trees, onground that otherwise would have beenlost to erosion (Gilchrist et al 1993, Hickset al 1995, Hicks pers. Comm.). Theleaves are high in nutrient and areexcellent food value in times of droughtor other feed shortages, dropping about500kg of dry matter a hectare in autumn.

Poplars are recognised in other countriesas having a high value for timber, fibreand match production. In New Zealandthis value has not really been appreciatedby growers or industry. Unpruned andinaccessible trees are of no value toanyone, including the farmer. Occasionalsilviculture is recommended to:

• prevent trees from splitting andfalling (which can damage fences andblock tracks)

• maintain good growth form (whichminimises pasture suppression by thecanopy)

• ensure a defect-free trunk, if harvestfor timber is envisaged.

Specific recommendations for matchingpoplar and willow cultivar to sites,planting techniques, and silviculture, aregiven in a series of pamphlets recentlypublished by HBRC and TRC.

12.4 References

Hathaway, R. & Van Kraayenoord, C,(eds) 1987. Plant Materials Handbook,Water and Soil Division MWD

Wilkinson, A. G. FRI Bulletin No. 124, 17The poplars, Forest Research Institute2000

Hawkes Bay Regional Council,Environment Topics – PlantingPoplars and Willows 1995


Forestry & Shelterbelts

Chapter

13. Erosion Control Forestry

14. Shelterbelts

13.1 Earthflow stabilisation with

erosion control pole planting

13.1.1 Shallow Earthflows

The trigger for renewed movement of anestablished earthflow is an excessiveamount of infiltrating water causingswelling of the clay and raising porewater pressure. Control is obtained byeither reinforcing the soil with tree rootsor by reducing the pore water pressure(either by de-watering orevapotranspiration by trees).

Tree species that will establish from polesare almost without exception deciduousand so soil moisture removal during thewinter months will be virtually zero.There will certainly be moisture removedwhen they are in leaf but the value ofthe moisture deficit that will be carriedinto winter has not as yet beenadequately established. The stabilisingimpact of deciduous trees is mainlythrough the reinforcing effect of theirroots and it is necessary that they beplanted at close spacings.

The depth of penetration of tree rootswill have a significant impact on theireffectiveness but may be limited by soilconditions. The lower surface of theshear plane may have fine grainedmaterial and water may be perchedthere, both conditions that will restrictroot growth.

Planting patterns: Five approacheshave been used successfully:

• Very close planting of Salixmatsudana on a regular 3 metre by 3metre spacing with no surfacedrainage. With 7 year old trees thissuccessfully held a small, previouslyactive, earthflow near Gisborneduring cyclone Bola. The only otherearthflow at that site that did not failwas afforested with P radiata.

• A wider spacing (approx. 6 metre by 6metre) of poplars and willows plantedafter a long period of de-watering. Avery active earthflow affected area ofbentonite clay, planted in this fashionsurvived 600mm of rain in 3 days

during Cyclone Bola (photos) withminimal damage, but the works werevery expensive to install.

• Willow poles closely planted in rowsadjacent to graded drainage banks inassociation with a comprehensive de-watering program. Poles were alsoplanted wherever scour could havebeen a problem in other surfacedrains. The works, on a number ofproperties, were spread over severalyears to allow gradual drying out ofthe saturated soil. Both slope stabilityand pasture production were greatlyimproved.

• Planting of poles in pasture on a widespacing (greater than 10 metres) withno de-watering. These generallyreduce erosion by 50% or morecompared with unplanted ground. Akey factor may be the absence ofexceptionally wet winters in thefollowing 5 to 7 years, which couldgive the trees a chance to establish,and to reinforce the soil mass. Thislower cost method may be mosteffective where earthflow movementis sporadic.

• The planting of small gullies withinthe earthflow, may prevent worseningearthflow movement as gullyenlargement is a major destabilisingfactor.

13.1.2 Deep Earthflow

The forces involved in deep seatedmovements are such that even if theroots of trees could penetrate to theshear plane the relative proportion ofshear strength they could provide wouldbe minimal. Stabilisation of deep-seatedmovements relies on de-watering and themost effective trees to lower the watertable will be fast growing evergreenspecies.

If poles of deciduous species are closeplanted (within a 10 metre spacing) thegeneral increase in shear strength of thesoil mass may have a slight benefit, but acombination of de-watering andafforestation will give the most effectivelow cost treatment.


CHAPTER 13

Erosion Control Forestry

Full stabilisation of a deep-seatedmovement, even if it is possible, willusually involve a full civil engineeringinvestigation and subsequent works.

13.2 Erosion control forestry for

gully stabilisation

Large scale planting or afforestation withPinus radiata, Eucalyptus or other speciesis often used for catchment planting.However, pines should not be used tooclose to eroding gully channels as theydo not have the secondary fine rootsystem necessary for gully control, andthey will shade out the willows andpoplars. Normal distances back from theeroding gully is at least the height of amature pine tree. If production treessuch as pines are planted, considerationshould be given to proposed harvestingoptions in the future, and whetherplanting boundaries are appropriate toallow for sustainable production forestry.Trees are normally planted out as openrooted seedlings, and stock are excludedfrom the planted area. Seedling trees canbe planted out with protectors, butcareful grazing management is required,and the cost is higher.

Planting for long term control on Ushaped gullies is only carried out if thegully head is not too deep (less than 2m). Normally, planting is undertaken onU shaped gullies to provide a vegetativeregime that is easy to maintain, or toprovide an income when trees areharvested. Normally, willows and nativespecies are planted within the purelyprotection areas, while other species suchas pines, or special purpose productionspecies are planted in areas that can besafely harvested without compromisingthe gully control works.

Most U shaped gullies are fenced andpermanently retired from grazing,because the gullies are a hazard for stock.It is advisable to provide a gate or rails(preferably wired shut) in a retirementfence to make it easy to get stock out ofthe retirement area if they do manage tofind their way in.

Gully control planting (using willows orpoplars) is sometimes carried out atstrong points such as a confluenceupstream above an eroding gully head.In these situations, the areas should befenced off to ensure that the plantingsbecome well established. Once theplantings are established, careful grazing

management can be carried out, takingcare that the trees are not damaged.

13.3 Erosion Control Forestry on

Earthflows and Other Deep-

seated Movements

Deep seated mass movements have beensuccessfully stabilised by erosion controlforestry on both large scale eg MangatuForest, and small scale, as with farmconservation woodlots. Research carriedout at Mangatu Forest in Pinus radiataforests on argillite earthflowsdemonstrated the dramatic effect thatthe forest had on ground water levels. Inaddition there are significant soilreinforcing effects produced by the deepvertical sinker roots that develop about 7years after planting.

At Mangatu the entire ground surfacewas covered, but where only discreteearthflows are being planted furtheraction may be needed to ensuremaximum effect on groundwater. Even atMangatu there were areas where acombination of high groundwater(possibly due to subsurface flow) andpoor soil permeability created trees withvery shallow roots. These trees weresubject to severe windthrow once thearea was opened up by adjacent logging.

The chances of success may be improvedby additional measures:

• De-watering of the earthflow beforeplanting may increase stability in theearly years after planting, before thetrees are fully effective. It may alsoreduce windthrow problems in smallblocks with a greater proportion ofexposed edge trees.

• Ridge tops and areas of disturbedground above the actual earthflowmay be infiltration zones that feedgroundwater into the earthflow.These may need minor earthworks toincrease run-off and reduceinfiltration, and should possibly alsobe planted to increase moisture loss.Soil water control will increase asgreater proportions of a catchmentare planted.

• In argillite country groundwater isknown to flow between adjacentcatchments, and this increases theimportance of groundwater controlwhere only discrete catchments are tobe planted. If obvious infiltrationzones exist in neighbouring


catchments they may needtreatments as detailed above.

• Areas that are obviously wet and areasin which continuing root strengthafter felling is required, could beplanted in water tolerant coppicingspecies such as redwood or suitablevarieties of poplar or eucalyptus. Thechoice of tree may be influenced byerosion control forestry subsidyrefulations.

• Roading should be carried outcarefully to ensure that run-off is notconcentrated or directed into criticalareas, such as zones of permeablerock or gully heads, where it mayincrease gully instability.

• Planting densities for P radiata andDouglas Fir are recommended at 1500stems per hectare in order to obtainroot zone canopy closure formaximum early stabilityimprovement (1250 sph under theECFP 2001 regulations). Of equalsignificance is the subsequentsilviculture regime as excessivelyzealous thinning too early can openthe canopy and reduce the erosioncontrol properties of the forest.

• The Gisborne District Councilrecommend thinning to 600 sph afterthe low prune and a final stocking of350 to 400 sph. An alternative usedby forestry companies producing un-pruned farming grade timber is tothin to 350 sph after 7 to 9 years,which is satisfactory form a soilerosion control perspective.

• Poplar may be planted at 500 stemsper hectare under the ECFPregulations. An initial density of 1000sph, using stakes rather than poles,would obtain more rapid rootreinforcement.

13.4 Erosion Control Forestry of

large Mudstone and Argillite

Gullies (Farm Wood lots)

Standard erosion control forestry practiceapplies where entire catchments arebeing forested. Where only the gullyitself, and some surrounding land, isbeing treated as a farm wood-lot, thelimited understanding of the effects offorest on gully activity needs to beappreciated.

The reduction of groundwater flow tothe gully walls, through afforestation ofthe gully’s catchment, is almost certainlya major stabilising factor. Seepage forcesand pore water pressure in a gully wallincrease the incidence of the shallowslump failures that lead to gullyenlargement. A reduction in the volumeof groundwater that seeps from furtherup-slope will be beneficial and thegreater the proportion of catchmentplanted the greater the benefit will be.

Evapotranspiration and interception bythe forest will probably be of greaterimportance than the root strength oftrees, except for those trees immediatelyadjacent to or on the gully walls. Areduction in surface run-off through thegully bed, is a further process by whichafforestation aids gully stabilisation.

A less recognised role of the trees aroundthe rim of a gully is their effect on themicroclimate in the gully itself. Shadingand wind protection of exposed faces bylarger trees can greatly improve thepossibility of them being revegetated.

13.4.1 Gully perimeter planting

The larger the proportion of catchmentplanted, the greater the reduction in run-off. A compromise on the location of thefence around the gully will occur if bothgrazing and stability are desired. Onmore unstable gullies a wide margin (atleast 30 metres, but preferably muchmore) is required, while more stablegullies in mudstones may be fencedcloser. Economies of scale will applywhen harvesting takes places and a largerarea may give a greater return in terms ofboth stability and dollars.

Fencing costs may be minimised if areliable, well constructed, electric fence isused and if cattle-only grazing is carriedout on the remainder of the paddockduring critical periods of treeestablishment.

Appropriate tree varieties can be bestjudged from other local experience orthe NZ Farm Forestry Association mayprovide information. While P. radiata isalmost universally successful for plantingaround gully perimeters, some varietiesof Eucalyptus and Acacia are better suitedto specific regions and conditions.Cypress species, in particular Cupressusmacrocarpa and C. lusitanica may havelimited uses but only where soils are welldrained, as they cannot tolerate highwater tables. Their foliage may cause


abortion in cattle. Details on eucalyptus,cypresses, and other species, for specificsites are given in the Plant Materialshandbook for Soil Conservation, Volume2.

Tree planting densities will be as fornormal production forestry practice(1500 stems/ha for P.radiata is typical)unless under an agro-forestry regime.One can expect a less rapid erosioncontrol benefit from wider spaced agro-forestry trees. The silviculture regimepractised on farm scale gully wood-lotsmay be affected by the large proportionof edge trees, the degree of exposure towinds and the need to maintain arelatively dense tree stocking for erosioncontrol benefits.

13.4.2 In-gully planting

Planting in an active gully will oftenonly be successful when the surroundingforest is several years old and has alreadyreduced groundwater flow. There seemsto be little point in attempting torevegetate the gully floor when freshsediment is still being deposited andsigns of improved stability, such as theestablishment of pampas grass seedling,may be useful as a guide as to when treeplanting should commence. Rather thanhaving two age classes of timber trees,the use of suckering trees that willsurvive logging damage may have meriton gully walls. Robinia pseudo-acacia hasbeen observed growing well in such sitesand Acacia melano-xylon has also beenproved useful in tough conditions,although initial establishment can bedifficult. A. dealbata is an exceptionallydrought tolerant tree, particularly duringits establishment phase, but is has still tobe proven in the acid soils found inmany argillite gullies. Other Acaciavarieties were tested in Mangatu Forestby FRI, who will still have informationavailable. In acid sulphate affected gulliesin Northland the Coral Tree (Erythrina xsykesii) has been found to be very easy toestablish, although it is best suited tofrost free areas.

Pines appear to be hardier than poplarsin the tough conditions found in gullywalls, but if suitable suckering poplarscan be established, such as Bolleana(Populus alba “Pyramidalis”) or the P.albax glandulosa crosses; Yeogi 1 and Yeogi 2(which require well drained sites), theymay provide long term stability despitelogging damage.

The walls of argillite gullies will almostcertainly be more difficult to revegetatethan mudstone. In either case repeatedattempts may be required untilbenevolent weather conditions occur andthe plants are able to establish. Thepresence of seed sources of native plantssuch as Tutu, will often allow naturalregeneration to occur once the majorgully movements have been controlledby the surrounding forest.

A technique for establishing poplars andwillows on steep slopes in some soilconditions is brush layering, in whichcutting sized material is laid in a crossedpattern on contour terraces across theslope, which are then back filled withmaterial from the next terrace up-slope.Most commonly used on road cuttings,the technique would only be warrantedwhere very good soil moisture waspresent and the soil had adequatefertility. Details can be found inSchiechtl, 1980.

13.4.3 Post Harvesting Management

Where suckering trees and shrubs havebecome established in the gully floor andwalls, provision should be made toexclude stock immediately afterharvesting to allow these plants torecover from damage sustained duringlogging, prior to forest re-planting.

13.4.4 References

Schiechtl Hugo Meinhard. 1980.Bioengineering for Land Reclamationand Conservation. (English-languagetranslation co-ordinated by N.K.Horstmann). University of Alberta Press.

Forestry company manuals, where theseare able to be accessed, may providestandard methods of forestestablishment.

After 1989 when Regional Councils wereformed, and Government subsidies wereno longer available for soil conservationactivities, financial assistance witherosion control forestry became theresponsibility of each Council. Someresponded with ratepayer funded grantsand others set aside capital funds fromthe sale of assets from which dividendswere invested in conservation works.Others entered into joint ventures withfarmers, where eventual proceeds fromthe sale of logs would be shared betweenthe Council and the landowner.


13.4.5 Application

Erosion control forestry began on theeast coast of the North Island with theplantings on Mangatu in 1961. The6,500ha area was subject to slumps andearthflows, with severe gullying andaggradation in stream channels. The landwas waterlogged in winter. The onlyvegetation consisted of poor pasture anda scattering on titoki and manuka (Willis 1973)

Early plantings were made with radiata at2200 stems per hectare and by 5 yearscanopy closure had occurred and therewas a marked improvement in slopestability. Observations showed that thetop 10 to 20cm was much drier and therewas a noticeable reduction in sub surfaceflows into stream channels. Aggradationhad slowed and many of the streams andgullies had degraded to a stable base.

Current practice is to plant seedlings at adensity of 1250 spha in 4m rows at 2mcentres, with a thinning to about 400spha while maintaining early canopyclosure. (Marden 1995) With the ECFP itis possible to plant other species of trees,including exotic species such as Douglasfir and poplar, as well as indigenousforest plant. Much more is known aboutthe effects of radiata on slump andearthflow erosion control due to the 40years plus experience with this particulartree. With other species it is necessary toplant to similar tree densities in order toobtain similar erosion control results.

Seedling planting requires carefulhandling of the seedling tree fromnursery to placing in the ground.Handling, transport to the site, preparingthe ground, and eliminating plantcompetition is essential. Factors thatshould be considered are:

Handling and transportAfter undercutting and wrenching in thenursery, the seedlings are lifted andplaced in either plastic bags or cardboardboxes. There are advantages anddisadvantages with each method ofpackaging. In boxes the seedlings areprotected from crushing and sweatingthat might occur in plastic bags.However, with care, the bags can bemore economical to transport. In theGisborne area helicopters are sometimesused to transport men and seedlings intoremote and difficult access sites. Aplywood pod has been designed that cancarry up to 3000 seedlings, and be slung

under the helicopter. The pod is a 1.2metre cube which is reusable and itprovides good protection for seedlings inplastic bags.

PlantingThe key to planting success is placing awell rooted plant in the ground wherethe roots are undistorted, facingdownwards and contained by a wellworked soil. Planting into a single spadecut with exposed roots and poorcompaction raises the odds against treesurvival. In drier areas a technique hasbeen developed with considerable successwhere a double cut is made and the soilbetween the cuts is thoroughly loosenedbefore placing the tree in the ground.

Reducing weed competitionIt is normal forestry practice to use aherbicide to eliminate plant competition.A sprayed circle of 75 to 100cm is usual,the larger size is used where there islikely to be growth of vigorous pasturespecies. If the new tree is heavilyovergrown by other plants it can lead todeformation and shortage of soilmoisture, both factors essentially killingoff the chance of a useful treeeventuating.

Other speciesErosion control forestry can includespecies other than radiata. Siteconditions and accessibility may suggestthe use of alternative species. Forexample, willows could be used wherethere is active gully erosion, macrocarpasfor sheltered and accessible sites, and inalong stream margins indigenous plantscould be encouraged, and even planted.

Where poplars are selected the treedensity needs to be much closer thannormal open spaced planting.

Post planting careTrees which are planted at 1200 to 1400spha will need to be thinned to around400 spha. The aim is to get early canopyclosure, and thin out unwanted ordistorted trees, while maintaining aneven spread throughout the planted area.Pruning is a choice of the owner and aregime should be selected at or about thetime of planting. Many factors willinfluence this decision, including, accessto the site, proximity to processing,severity of erosion on the site andavailability of time and resources to dothe work


13.5 Forest Management Practices

– Mountain Lands

13.5.1 Introduction

The practice involves planting a range ofseedling trees on mountain faces orgullies subject to erosion. The trees arenormally planted as seedlings but inspecial cases may be planted poles orstakes

Forestry plantings have beentraditionally established to control slips,gullies, debris avalanches and to a lesserextent for frost heave, sheet wash, windand creep erosion. There was very littleplanting in the mountain lands prior tothe 1950s partly due to emphasis onother forms of revegetation and farmdevelopment but also there wasinsufficient information about theperformance tree species at high altitude.

However, the work of Forest ResearchInstitute at Craigieburn and Kaweka hasprovided valuable information onappropriate tree species, plantingtechniques, growth rates and othermanagement details.

13.6 Protection Forestry

This applies where forestry plantings arecarried out with the objective ofcontrolling soil erosion. The trees may intime have other benefits such as timberproduction, weed suppression,recreational uses, water qualityenhancement and carbon storage.

13.6.1 Species

Many species have been tried in the 130years of European settlement. The mostsuccessful are the conifers. Other speciese.g. Poplars, Willows, Silver birch (Betulaverrucosa) and sycamore (Acer

pseudoplatanus) are less common in thehigh country.

The most common species recommendedtoday are as follows:

Mountain pine: (Pinus mugo) very hardy,multiple leaders, shrubby. Mostsuitable for the severe high altitudesites up to say 1600m depending onthe site.

Lodgepole pine: (Pinus contorta) veryhardy and seeds well but notrecommended today because ofwilding spread.

Scots pine: (Pinus sylvestris) not used somuch today, may be prone to possumdamage.

Corsican pine: (Pinus nigra) a hardyspecies (up to 800m) best on driersites.

Ponderosa pine: (Pinus ponderosa) oftengrouped with P.nigra as a species forhard sites but is more prone to pestsand diseases when stressed.

Radiata pine: (Pinus radiata) the mostversatile quick growing species but islimited to the lower altitude orwarmer sites below 540 m above sealevel.

Douglas fir: (Pseudotsuga menziesii)species well suited to the more moistareas of the high country.

European larch: (Larix decidua) was usedin the past but site tolerance and slowgrowth make it not as popular,deciduous, also potential wildingspread.

Alders: (Alnus spp) still useful on moistsites esp on scree slopes andstreambanks. Deciduous but can growat altitudes up to say 1600m.

Other species are available such as thenatives (normally slow and difficult toestablish unless self seeded) and somehardy eucalypts (subject to frosting andpest problems).

13.6.2 Planting regimes

Generally for erosion control purposesseedling trees need to be planted at1200-1600 stems/ha i.e. 2.5m x 2.5m to3 x 3 m spacing. Follow up grass controlis essential once the seedlings are planted


Erosion control forestry on Class VIIe land.

Photo: I H Cairns.

but on many of the high country sitescompetitive grass and weeds may not bean issue.

On heavy rainfall areas where soilfertility status is low the addition offertiliser tablets for each tree isrecommended (trace element deficienciesoccur in this environment). On the easierslopes that are readily accessible, followup weed and pest control is essential.Silvicultural management needs to beconsidered, depending on the objectiveof erosion control.

Where frost heave is active severedifficulties may arise with seedling treestock, so an alternative is to aerialoversow Pinus species seed with fertiliserand a small quantity of suitable grass-clover mix to reduce the frost lift.Otherwise, use Maku lotus, which is aspecies that adapts well to low fertilitysites. Time sowing when moisture ispresent and stock have been removedfrom the block.

Where the trees listed above have beenplanted for erosion control purposes onareas with slight to moderate sheet, windand creep erosion, they can be managedto enable harvest for income. Harvestingmust be carried out using cabletechniques or special logging equipment(e.g. Wyssen Hauler) for sensitive areas.The need for a detailed harvesting plan isparamount. Refer to chapter on slips formore information on harvest of treesfrom erosion-prone land.


CHAPTER 14

Shelterbelts


Field shelter by planting windbreaks iswidely practised on both agricultural andhorticultural croplands and pastoralfarmland. Many windbreaks were plantedand fenced for grazing control and wereoften planted as part of regional winderosion control schemes, which couldattract a subsidy of up to 70% (Stringer1978; Wethey 1984). By 1984, winderosion control schemes protected 1.9million hectares of land susceptible towind erosion (Salter 1984). Windbreakshave been multipurpose, providinglivestock shelter and enhanced cropgrowth (Sturrock 1981), in addition tomitigating wind erosion. Considerableresearch has been directed at windbreakdesign and performance, suitable species,the advantages of shelter, and its costsand benefits (Gilchrist 1984, Sturrock1984).

In the “Plant Materials Handbook for SoilConservation” 1986, Volume 1, p. 125-138, Allan Wilkinson gives a brief butcomprehensive summary of windbreaksiting, design, establishment andmanagement, and examples ofwindbreak designs, both for shelter only,and as “timberbelts” for woodproduction. He emphasises the need foran integrated shelter system for a farmproperty. His format is followed here.Section 14 of the “Radiata Pine Growers’Manual” (McLaren 1994) containsadditional information specific to pinetimber belts.

14.1 Siting

The following are the major factors to betaken into account:

• Orientation should be as near aspossible to 90º from the harmfulwind direction.

• Windbreaks should be as long aspossible, connect with existing shelterto obtain continuity, and gaps shouldbe avoided where possible, becausewind velocity is increased by up to25% around ends and through gaps.

• The interval between windbreaksdepends on the expected maximumheight and degree of shelter required.

• Physical restrictions need to be takeninto consideration, includingproperty boundaries and roads, powerand telephone lines, microwave andcellphone towers, water races,irrigation systems (especially borderdyking), filed drainage systems suchas tile drains and perforated pipes(novaflo), and existing fence lines.

• Existing natural shelter. On hillcountry, windbreaks should becomplementary to natural shelter.

• Aesthetic considerations. Wherepossible, windbreaks should follownatural boundaries, and shouldinclude designs and species thatenhance the existing local character;they should form part of an overallshelter system for the farm whichincludes hedges, woodlots and wide-spaced tree plantings.

• The farmer’s objectives. These may bemany, including minimising width ofwindbreaks, provision of shelter at aspecified time of year (e.g. lambing,spring flowering, autumn crop andfruit harvest), minimising wintershading, minimising competitionwith adjacent crops or pasture,maximising the return from timbersales, increasing the pollen and nectarsupply for honey bees, and providingfood and shelter for desirable birdspecies [and insect predators].

Patterns of wind abatement in the vicinity of shelterbelts of

different density (after Caborn 1965).

14.2 Design

Some weighting of objectives is necessaryto produce designs that meet the needsof the farmer and the rural community.In the past, lack of perception andinsufficient effort devoted to design hasresulted in a limited number of speciesbeing planted throughout New Zealandwithout due regard to the localconditions and the visual landscape.Unreasoned antagonism to shelter hasarisen in many areas. Single purposedesigns such as radiata pine hedges haveevolved and the contribution whichshelter can make, both in increasedproduction and the quality of rural life,has not been fully recognised.

The requirements for each windbreak sitewithin a designed shelter system shouldbe carefully examined. The resultingwindbreak design should incorporateexperience resulting from previousdesigns, the best local knowledge ofspecies performance, and considerationof the following factors:

Height of windbreak

A significant reduction in wind velocityis obtained for a distance of 10-15 timesthe height of the windbreak. Themaximum reduction occurs at a distanceof 1-4 times the height, depending onthe permeability.

Permeability (density)

Very dense impenetrable windbreaks canresult in severe turbulence in the lee ofthe shelterbelt and a resumption ofunhindered wind velocity at a relativelyshort distance form the shelterbelt.Permeable shelterbelts provide a greaterreduction in wind velocity further outfrom the shelterbelt but less reductionvery close to the shelterbelt. Theoptimum density for a permeablewindbreak is between 40% and 50%.This may be obtained by:

• Planting trees of the appropriatenatural density, e.g. pines have adense crown, gums and poplars havelight crowns;

• Varying the spacing of trees withinthe row;

• Varying the number of rows of trees[multiple rows];

• Tending, including side trimming,pruning, and thinning.

Continuity of shelter

All species have a limited life and mustbe replaced before they become over-mature. Continuity can be obtained byplanting at least two rows of trees, eachrow having a different rotation length, orby replacing alternate shelterbelts atdifferent time, as part of an overallmanagement plan for the property.

Stability of windbreaks

Windthrow occurs in areas with shallowsoils, which experience high velocitywinds. The risk of windthrow can bedecreased by:

• Good design, e.g. two rows of specieswhich have different growth rates arecombined to give additional stabilityto the windbreak: the slower growingspecies is established on thewindward side to reduce movementof the root plate of the fast growingspecies;

• Good establishment techniques, e.g.the use of high quality tree stockswith some form of deep cultivationand careful planting;

• Good management: the milling andreplacement of trees nearing the endof a rotation is the most significantstep that can be taken to reducefuture windthrow.

14.3 Establishment

For the establishment of shelterbelts, it isextremely desirable that 100% survivalbe obtained. Thus the greatest possibleattention must be paid to site preparation,planting techniques, tree stock quality,fertiliser requirements, release spraying,protection form animal damage, andirrigation. The following are key pointsto note. (See Chapter 2 of the PlantMaterials Handbook for more detail.)

14.3.1 Site preparation

Some form of cultivation isrecommended prior to planting. The sitecan be cultivated well in advance(including deep ripping) and sown downin a covercrop such as oats or ryegrass toprovide initial shelter for trees planted insprayed spots or lines, and to suppressweeds. Shallow line ripping can becarried out in winter with at least two orthree rips per line to avoid anypossibility of root runs down singleripped lines. The tractor wheels can berun back over the rips to consolidate thesoil and improve the surface for pre-plant herbicide application.


14.3.2 Time of planting

On sites where frost and waterlogging arenot a problem, autumn planting is oftenvery successful, especially if summerdroughts are common. Wet or frosty sitesare best planted in September-October ifa period of stagnation in growth is to beavoided. Many North Island sites can beplanted any time from May until lateAugust. Many sites in the South Islandand some in the central North Islandcannot be planted until after the dangerof severe frost damage is past.

14.3.3 Planting stock quality

Planting stock quality has a major effecton survival, early growth rate, rootsystem configuration and resistance towindthrow. Container grown stockgenerally shows a higher survival andfaster early growth rate than bare-rootedstock. However, where adequate survivaland growth of bare-rooted stock can beobtained, this is preferred.

14.3.4 Weed control

Although good weed control can beobtained short-term by cultivation priorto plantings, more encouraging long-term control can be obtained by the useof herbicides. Only a limited range ofherbicides is suitable for use inwindbreak establishment because of thesusceptibility of many of the species used(particularly hardwoods) and the needfor 100% survival of good healthy trees.Accurate calibration of sprayingequipment is essential to obtainsuccessful weed control without damageto the trees. Residual herbicidescommonly used include hexazinone,simazine, and terbumeton/terbuthylazine(see Appendix 2 of the Plant MaterialsHandbook, Vol 1).

Those farmers who do not wish to useherbicides, should consider variousmulches, including black plasticmulches, or spraying with hot water oreven silage leachate, or cultivation withthe old push hoe.

14.3.5 Damage from rabbits, hares, and

possums

The susceptibility of young trees toanimal damage necessitates the use ofindividual protection devices, rabbitnetting, or electric fencing (see Section2.10.2 of the Plant Materials Handbook).Damage can also be reduced by:

• A programme of eradication of

animal pests in conjunction with theregional council;

• Good establishment to enable trees toattain a size out of reach of animalpests as quickly as possible;

• Spot spraying well in advance ofplanting to allow the growth of tallgrass around the planting spots beforeplanting. (All three pests are lesslikely to run through rank grass thanalong clear-sprayed lines).

• Several Regional Councils also suggestpest repellent made by mixingtogether 5 fresh eggs, 150 ml of whiteacrylic paint, and 600 ml of water.Apply to seedlings with a paintbrush.

14.3.6 Windbreaks

Windbreaks should always be protectedfrom grazing animals. The distance fromfence to trees for areas carrying onlysheep should not be less than 1 metre.Where cattle are present, the minimumrecommended distance from the fence tothe trees is 2.5 m. Where this distance tothe windbreak is not acceptable,electrified top wires are recommended,but the farmer should ensure that this isa fail-safe electric fence. Animals have anuncanny knack of getting into ashelterbelt as soon as the power is offwhere they can do a lot of damage

14.3.7 Irrigation

Trickle irrigation is recommended on drysites to ensure 100% survival andoptimum growth rates. The amount ofwater to be applied depends to a largeextent on the plant species, weather andsoil type. Average application rates varyfrom 10-25 litres per tree every 7-10days; light sandy soils require morefrequent application than loam or claysoils. Additional advice may be soughtfrom regional councils’ irrigation officersor companies supplying the equipment.

14.3.8 Management & Maintenance

While some species for shelterbeltsrequire little maintenance once wellestablished, shelterbelts do require somemaintenance to prevent them from:

• Spreading out beyond the boundaryfence;

• Becoming too tall and castingexcessive shade;

• Becoming too dense, causingexcessive turbulence behind theshelterbelt.


Thinning may be applied, but grazing ofthe vegetation inside the fence is notrecommended. This is to prevent theformation of a browse line under whichwindspeed could increase, or loss ofbeneficial insect predators

14.4 Windbreak design and plant

species for different sites

Wilkinson l.c. describes in considerabledetail, a range of shelter designs fromone row to multiple rows hardwoodswhich can be deciduous or evergreens,with or without underplanting; also rowsof conifers, again with a range ofvariations. The conifers may or may notbe used for timber. He also describesshelter for coastal situations. The shelterdesigns are described in terms of shelterheight, spacing, management, siteselection and applications. The sitetolerances of the suggested shelterspecies are indicated in Appendix 1 ofthe Plant Materials Handbook for SoilConservation, Volume 1. Additionalinformation may be found in the FactSheets of the New Zealand PlantMaterials Research Collective. Adviceabout locally suitable species can besought from Regional Councils, LandcareGroups and members of the FarmForestry Association.

In all following figures, the winddirection is always from the left in theprofile diagrams.

14.4.1 Shelterbelt, medium-tall

hardwood, one row

* P. nigra ‘Italica’ is rust-susceptible; use only in inlandCanterbury, Marlborough and Otago.

** Salix matsudana and its hybrids may be defoliated bylarvae of the willow sayfly.

Initial Spacing1-2 m between trees for horticulturalshelter

2-3 m between trees for farm shelter

ManagementLow maintenance. Optional sidetrimming to reduce width of belts andretain live green branches on the lowerportion of the stem.


Medium-tall hardwood, one row.

Profile Windward view

Medium height species

Alnus cordata

glutinosa

incana

rubra

Phebalium squameum (syn. P. billardieri)

Salix discolor

Tall species

Populus alba‘Pyramidalis’ (suckers)

P. X euramericana‘Tasman’ or ‘Veronese’ or‘Crow’s Nest’

P. nigra ‘Italica’*

Salix matsudana**

S. matsudana X albahybrids**

S. matsudana Xpentandra hybrids**

Site selection/applicationsMoist, reasonably fertile soils andirrigated cropland. Branch trimmings canprovide supplementary stock fodderduring mid-summer.

Use deciduous shelter for east-westwindbreaks where winter shading is aproblem.

Preference is now for perimeter belts tobe of poplars and willows; internal beltsof alder; in milder climates, the semi-evergreen Alnus acuminata.


hardwood, one row, underplanted

Initial planting2-3 m between tall-growing species

1 m between low-growing species

ManagementLow maintenance. Occasionally side trimto reduce fence overhang.

Site selection/applicationsDeciduous hardwoods should be used ineast-west belts on naturally moist orirrigated soil types. Most of the abovelow-growing species can be used with thedeciduous hardwoods but care should betaken to select Acacia spp. tolerant ofmoist sites, e.g., Acacia floribunda, A.melanoxylon, A. retinodes.

On drier sites use eucalypts combinedwith drought-tolerant, low-growinghardwoods or slow-growing conifers.

Hardwood shelterbelts can providefirewood, visual amenity, nectar andpollen for honey bees, and branchtrimmings for supplementary fodder(deciduous species and Chamaecytisus).In milder climates fruit or nut-bearingshrubs such as feijoas, guavas andhazelnuts can be used for underplanting.


hardwood, deciduous, two row

*‘Tasman’ and ‘Veronese’ can be pruned for veneer logs.If this is done, prune annually to restrict stem diameterover stubs to 100 mm.

**P. nigra ‘Italica’ is rust-susceptible; use only in inlandCanterbury, Marlborough and Otago

*** Salix matsudana and its hybrids may be defoliatedby larvae of the willow sawfly


Medium-tall hardwood, one row,

underplanted.


Low- or slow-growingevergreen species

Acacia spp.

Bambusa oldhamii

Callistemon spp.

Chamaecytisuspalmensis

Corokia spp.

Cortaderia spp.

Cryptomeria japonica

X Cupressocyparisleylandii

Olearia spp.

Phormium cookianum

P. tenax

Pittosporum spp.

Thuja plicata

Tall, fast-growingspecies

Tall species listed in14.4.4 or Eucalyptus spp.Listed in 14.4.4

Low- or slow-growingspecies

Abelia grandiflora

Acacia floribunda

A. melanoxylon

A. retinodes

Callistemon spp.

Chamaecytisuspalmensis

Cortaderia spp.


Cupressus lusitanica


Melaleuca spp.

Olearia spp.

Phormium cookianum

P. tenax

Pittosporum spp.

Thuja plicata

Medium-tall growingspecies

Alnus cordata

glutinosa

incana

rubra

Populus alba‘Pyramidalis’ (suckers)

P. X euramericana‘Tasman’* or ‘Veronese’*or Crows Nest

P. nigra ‘Italica’**

Salix matsudana***

S. matsudana X albahybrids***

S. matsudana Xpentandra hybrids***

Medium-tall hardwood, evergreen, two row.

ProfileWindward view

Initial spacing2-3 m between tall-growing specieswithin the leeward row

1-1.5 m between low-growing species inthe windward row

1-1.5 m between rows

ManagementWider-crowned species may requireoccasional side trimming.

Site selection/applicationsMoist, reasonably fertile sites andirrigated cropland. Provides low wintershelter for stock; high permeable shelterfor pasture and crops during the growingseason. Use as east-west windbreakswhere winter shading is a problem. Low-growing species and Salix spp. providefood sources for honey bees. Branches ofChamaecytisus palmensis and poplar andwillow tree species can be used assupplementary fodder.


hardwood, evergreen, two row

Species marked * are timber production species.


Medium-tall hardwood, deciduous, two row.


Low- or slow-growingspecies

Moderate to high rainfall areas.

Low- or slow-growingspecies listed in 14.4.3

Low rainfall areas

Abies pinsapo

Cupressus arizonica

C. torulosa


Pinus ponderosa

P. nigra subsp. Laricio

Sequoiadendrongiganteum

Fast-growing species

Moderate to high rainfall areas.

Eucalyptusbotryoides*

E. delegatensis*

E. fastigata*

E. fraxinoides*

E. globoidea *

E. muellerana*

E. nitens*

E. obliqua*

E. ovata

E. regnans*

E. saligna*

Low rainfall areas

Eucalyptusamygdalina

E. cordata

E. gunnii

E. nicholii

E. pulchella

E. viminalis

Initial spacing4 m between trees in the leeward row

1.5-2 m between trees in the windwardrow

2 m between rows

If fast initial shelter is required, plant theleeward row at 2 m between trees; thinto 4 m apart at age 4-6.

ManagementLow maintenance. Side trim occasionallyto restrict width of belt. In shelterbeltsthe timber species require annualpruning to restrict stem diameter overstubs to 100 mm and to remove multipleleaders.

ApplicationsTimber, firewood, visual amenity, andnectar and pollen for honey bees andbirds. Low-growing species can providesupplementary stock fodder from branchtrimmings. It is essential to select thespecies and seed sources suited toparticular sites.

14.4.5 Shelterbelt, conifer, one row

Initial spacing2-3 m between trees

ManagementHigh maintenance: Clear prunealternate trees (annually); pruning liftsshould concentrate on restricting stemdiameter over pruned branch stubs to100 mm. Fan prune and/or side trimremaining trees. Fan pruning entailsremoving branches that grow at 30-90’ tothe windbreak. Prune logs to at least 6 mabove ground.

Low maintenance: Optional sidetrimming to reduce the width of thewindbreak.

Site selection/applicationsOne-row pruned belts are not suitable forprimary shelter in very exposed coastalor inland South Island locations.


A. Pruned for Timber, Windward view

Conifer, one row.

B. Unpruned, Windward view


Pinus radiata

P. muricata (blue strain)

Medium-growing species


Cupressus lusitanica

C. macrocarpa

X Cupressocyparz s. leylandii

Pinus nigra subsp. laricio

Pseudotsuga menziesii

Slow-growing species

Abies pinsapo

Cedrus atlantica

C. deodara

Cupressus arizonica

C. torulosa

Sequoiadendron giganteum

Thuja plicata

14.4.6 Shelterbelt, conifer, two row,

timber

Initial spacing2-3 m between trees and rows

ManagementClear prune all trees in the leeward rowto restrict stem diameter over prunedbranch stubs to 100 mm. This mayrequire annual pruning until the desiredlog length is attained (6 m is thecurrently recommended height forpruning).

The windward row may be trimmedmechanically to confine branch growthbetween the windbreak fences.

Site selection/applicationsThe windward row provides addedstability (resistance to windthrow) thusthis design is recommended for shallowsoils. If desired, evergreen hardwoodscapable of growing up to 6 m tall can besubstituted for conifers in the windwardrow.

14.4.7 Shelterbelt, conifer, multiple row,

timber

Initial spacing2-3 m between trees and rows

Rows should be staggered to overlap gapsin preceding rows. Delay establishmentof slow-growing leeward rows untilsufficient shelter is provided by the twoprimary rows.

ManagementClear prune second (and subsequentrows if desired) to restrict stem diameterover stubs to 100 mm. Multiple rowtimber plantings should be pruned andthinned as woodlots. Amenity rows canbe pruned to 3-4 m to allowunderplanting with shade tolerant shrubs


Tall conifer, two row, timber.


Tall conifer, multiple row, timber, cold inland areas of the South Island.

Profile – pine / douglas fir / larch Profile – 2 rows of pines, 1 of larch Windward view

Slow-growing species

Low rainfall areas

Abies pinsapo

Cedrus deodara

Cupressus arizonica

C. torulosa

(2) Medium-high rainfallareas


Pseudotsuga menziesii

Thuja plicata


Hardwood specieslisted in 14.4.3


Pinus nigra subsp.Laricio

P. ponderosa

P. radiata

for fodder, pollen, nectar and fruitproduction.

ApplicationCold areas of the inland South Islandwhere drifting snow is a major problem.Homestead shelter.

14.4.8 Shelterbelt, low-medium coastal,

two row

*In many regions, this species is an invasive weed.Check with Regional Council staff before planting.

Initial spacing/site selection1-2 m between rows and between plantswithin rows

Spacings need not be rigid. Species canbe grouped together in clumps to varythe width of the windbreak. Use the flax(Phormium) or toetoe/pampas(Cortaderia) with cabbage trees(Cordyline) on poorly drained heavy

soils. In frost-fee areas Norfolk Islandpine (Araucaria heterophylla) can beplanted at 5-8 m intervals in the leewardrow. Species for the South Island shouldbe selected for greater cold tolerance.

Establishment and managementIt may be advantageous to provide initialshelter for the plants by erecting a brushfence or commercial windbreak clothbarrier along the windward boundary ofthe shelterbelts. Maintain stockprooffences.

ApplicationsLow shelter for stock, visual amenity,shelter and food sources for birds andbees.

14.4.9 Shelterbelt, tall coastal, two row


Low-medium coastal shelter, two row.

Profile

Windward species

Acacia sophorae*

Coprosma repens

Cortaderia spp.

Phormium cookianum

P. tenax

Senecio reinoldii

Tamarix chinensis

Teucrium fruticans

Leeward species

Albizzia lophantha

Araucaria heterophylla

Banksia integrifolia

Cordyline australis

Corynocarpus laevigatus

Dodonaea viscosa

Erythrina sykesii

Lagunaria patersonii

Metrosideros excelsa

Myoporum laetum

M. insulare

Olearia paniculata

O. traversii

Pittosporum crassifolium

P. ralphii

Pomaderris apetala

Quercus ilex

Tall coastal shelter, two row.


*In many regions, this species is an invasive weed.Check with Regional Council staff before planting.

Initial spacing2.5-3 m between trees

2 m between rows

ManagementLow maintenance. Occasionally side trimto restrict overhang on leeward side ofwindbreak.

ApplicationsCoastal areas of both islands. For primaryshelterbelts, it may be necessary toprecede the tall tree belt with low tomedium height shelter species as in14.4.8 and increase the number ofleeward rows to two or more.


Windward species

Acacia sophorae*

Coprosma repens

Cortaderia spp.

Phormium tenax

Senecio reinoldii

Tamarix chinensis

Leeward species

Cupressus macrocarpa

X Cupressocyparis leylandii

Eucalyptus botryoides

Pinus muricata

P. radiata

Earthworks

Chapter

15. Runoff Control Practices

16. Soil Management

17. Structures for Runoff & Sediment Control

18. Dust Control

15.1 Diversion Channels and

Bunds


Diversion channels are shallow opendrains which intercept and conveyrunoff to stable outlets at non-erosivevelocities. They are used to protect workareas from upslope runoff and to divertsediment laden runoff to appropriatesediment retention systems. Bunds arecompacted ridges of soil which interceptrunoff and convey it to diversionchannels.

15.1.2 Installation

Diversion channels and bunds need to bedesigned to ensure that they havesufficient capacity to convey the runofffrom the required storm. They must havea positive grade to a stable outlet. Gradeshould be less than 2 % otherwise thechannel itself will erode and will thenneed to be stabilised (with rock, timberor fabric). The outfall may need to beprotected against erosion rock, a flume.Because of the small room for error, thesechannels should be surveyed to obtainan even grade.

They should be constructed with atrapezoidal cross section with internalside slopes no steeper than 3 horizontal:1 vertical. Earth bunds should beconstructed in layers no greater than 200mm in thickness and be well compactedas construction proceeds. As a generalprinciple, any excavated channel orbund wall should be sown with grassseed or hydroseeded to prevent scouring.

Design requirements will vary fromregion to region and can be found inplanning documents, erosion andsediment control guidelines etc. If thereis no guidance, it is suggested that designbe for the 1 in 20 year storm event.

Because of their low grade, deposition ofsediment will often occur in diversionchannels quickly compromising theircapacity. Channels will need to be sitedso deposited sediment can be removed.Care is therefore needed in theirplacement. If the channel/bund is

constructed on a steeper grade to avoiddeposition, the channel itself willgenerally erode (if constructed in earth)and will require specific measures tocontrol this i.e. rock armour or fabriclining. A common problem is inadequatecompaction of bund walls, orchannels/bunds put in by eye, whichbreach or overtop at weak areas or lowspots.

15.1.3 Maintenance

Diversion channels need to be inspectedduring and immediately after heavy rain,to ensure the diversion is workingcorrectly and is not blocked or itsfunction otherwise impaired. Anynecessary repairs need to be undertaken.The outfall should be checked to ensurethat it is free from erosion. Check thatmachinery movement through the sitehasn’t damaged the channel/bund.

15.2 Check Dams


Check dams are small structuresconstructed across a drainage way orchannel. Their purpose is to reduceerosion of the channel by reducing thevelocity of flow. Although these measuresalso trap sediment, this is generally asecondary function.

15.2.2 Installation

The catchment of individual check damsshould be limited to four hectares. Theycan be constructed from rock, logs,fabric, or sandbags and must havesufficient mass to withstand the energyof a concentrated flow. A check damshould be no more than 1000mm inheight and have sides that are at least300mm higher to ensure that any flowpasses over the centre of the structureand not around the sides (which willusually wash it out). The toe and sides ofthe structure should be well protectedagainst undercutting or outflanking.They should be constructed so that thecrest of one structure is level with the toeof the next upstream (this can be hard toachieve in practice).


CHAPTER 15

Runoff Control Practices

for Earthworks

Fabric structures should be not morethan 0.6 metres high, anchored well atthe toe and well tied back. They are lessrobust and their catchment should beless than 2 hectares.

15.2.3 Limitations

Check dams are often undercut oroutflanked and the failure of one canaffect the integrity of other structures.

These are not commonly found ontemporary earthwork sites because theytake time to implement and can bedifficult to construct correctly. They canbe costly.

15.2.4 Maintenance

Check dams need to be inspectedregularly after every storm. Theinspection should check that theintegrity of the structure is not impairedby erosion or piping. Sediment must beremoved sufficiently often to ensuredrainage channels behind each damretain their flow capacity.

15.3 Contour Drains


These are temporary drains constructedat the end of work or when rain isimminent to reduce slope length andconvey runoff safely off an earthworksite. They are removed/obliterated oncework recommences.

15.3.2 Installation

They should be as short in length aspossible and constructed at no morethan a 2% grade (to avoid scour of thechannel). As a rule of thumb they shouldbe about 500 mm deep and constructedat about 30 metre intervals. They shouldbe closer together on steep sites whereasspacing can be increased on flatter sites.They should be as short as possible andconstructed at no more than 2%longitudinal grade (to avoid scour of thechannel). The position of these measuresshould be determined by the presence(or otherwise) of an erosion proof outfall.They must never discharge over areas offill (install a flume).

15.3.3 Limitations

Care is needed with cut-off drainsbecause they concentrate site runoff.Each drain must discharge to an outfallthat is stable against erosion. Commonproblems include insufficient capacity,erosion prone outfalls, constructed ontoo steep a grade (so they erode) or tooshallow a grade (so they don’t flow).

15.3.4 Maintenance

They should be inspected during periodsof prolonged rainfall to ensure they arefunctioning. Immediately after eachheavy rainfall, any necessary repairsshould be undertaken.


Contour drains.

Diversion band.

15.4 Flumes


Flumes convey runoff down a bank orover an erodible site without causingerosion. Pipes, polythene, geotextiles,fabric flumes, sheets of iron, concretelined chutes, rock etc can all be used forthis purpose. Fabric flumes can bebought “off the shelf” (made to orderfrom 200 – 600 mm diameter).

15.4.2 Installation

Flumes should be sized according toindividual site requirements. The flumeshould be capable of carrying aminimum discharge equivalent to the10% AEP (Annual ExceedenceProbability) strom where the stormduration is equal to the time ofconcentration. Some flume dimensionsfor specific discharges are given in thereference “Gully erosion controltechniques for pumice lands” Eyles(1993). On critical sites protectingvaluable assets, the design strom can beas large as 2% AEP.

The catchment of temporary flumesshould be a maximum of 2 hectares. Allcatchment flow needs to be directed tothe flume. This is often achieved througha pipe installed through a compactedearth bund. Its invert should be laid atground level and have a positive slope ofat least 3 %. All flow needs to be directedto the flume and the flume should besecurely attached to the inlet pipe. Theflume needs to be water tight, and theoutfall should be protected againsterosion. The flume should be laiddirectly down the slope if possible and bewell supported and anchored. It shoulddischarge to a stable outfall (rocksarmour or erosion control fabric.).

A secondary flow path may be requiredshould the inlet become blocked or ifflows exceed its capacity. Fish passagemay need to be provided if a flume is toconvey stream flows around temporaryworks in their channels.

15.4.3 Limitations

The inlet to the flume needs to be welldug in or protected (e.g. with concrete)to guard against undercutting oroutflanking of flow. The wing wall, inletand flume need to be well secured toeach other. Wing walls may not direct allrunoff to the flume. The flume may notbe waterproof or runoff may splash out

of the flume and erode adjacent areas.Subsidence may occur on fills and thismay affect the functioning and integrityof the structure.

15.4.4 Maintenance

Flumes should be inspected daily duringperiods of prolonged rainfall,immediately at the finish of each rainfall,and weekly during periods of no rain.Any necessary repairs should be madeimmediately. The flume should beretained until the area has beenpermanently stabilised or the flow hasbeen removed.


Plastic flume.

Stabilisation of an earthworks sitecommonly involves grassing, mulching,or fabric. Less common are all manner ofstructural measures, aggregate, variouschemicals etc. Only the first three arediscussed here.

16.1 Grassing


Established grass protects the soil againstraindrop, sheet and rill erosion.

ApplicationTopsoil should be spread over the site toa minimum depth of 100mm to create asuitable medium for the establishment ofgrass seed. Grass seed should be appliedin either spring or autumn when there isenough moisture for germination andestablishment but is not too cold toinhibit growth. Mulch can increase thesuccess of grass establishment into thedrier summer period. Annual grasses willgenerally give quicker establishment andmore vigorous winter growth than willperennial grasses. Fertiliser should beadded. A grass seed and fertiliser mixsuggested in one region is as follows

Table 16.1 Mix of grass seed and fertiliser for

Auckland

Ref ARC TP 90

Different seed mixes will be required fordifferent regions. Discuss this with alocal seed merchant. Hydroseed (seed,fertiliser and paper or wood pulp sprayedon as a slurry) can be used. Hydroseedcan also be used in the cooler months(provided mulch is also applied), ondifficult slopes, and as a temporary coverdirectly onto subsoil surfaces.

LimitationsHeavy rain soon after sowing can washseed away (this particularly applies tohydroseed which is not incorporatedinto the soil). Grass can take a while toestablish and additional measures, suchas mulch, may also be required to protectthe site against erosion while the seedestablishes, particularly in the driersummer months. Poor grassestablishment can often be attributed toinsufficient fertiliser application.

MaintenanceSeed may need to be resown after heavyrain, or dry weather.

16.2 Mulching


This is the application of a protectivelayer of straw, hay, wood fibre, bark orsimilar to the soil surface to protect itagainst raindrop impact and shallowsheet flow. It will give instant protectiononce applied and so can be used forerosion control when grass growth isslow (winter or summer) or whereimmediate protection is required. Hayhas its own grass seed source but cancarry weed seeds. Straw lasts longer.Mulch can “nurse” the establishment ofgrass seed. Bare subsoil can dischargeover six times as much sediment as frommulched subsoil areas (ARC 2000) andmulch on topsoil is much more effectiveagain.

Figure 2(38) mulching Photo

InstallationMulch can be temporary or permanent,applied with grass seed and fertiliser orwithout. Mulch should be applied at 30mm loose thickness (measured at time ofapplication) and be anchored by


CHAPTER 16

Soil Management Techniques

on Earthworks

Seed

Fertiliser

Mix

Temporary

Annual ryegrass(e.g. Tama) and clover

Permanent

Perennial rye,brown-top andred/whiteclover

D.A.P.

N.P.K.S.(18:20:0:2)

Rate (kg/ha)

300

perennial – 120

browntop – 45

clover – 45

240

spraying a tackifier or crimping into thesoil with discs. It can be applied by handon small areas but mechanical “blowers”will give a more even spread over largerareas. Runoff control measures need tobe installed before mulching.

LimitationsMulching will not control erosionresulting from concentrated flows soappropriate control measures should beapplied first. Hay and straw will breakdown over time and another applicationmay be required. Insufficient mulch maybe applied. Some tackifiers can beenvironmentally harmful. Wind canblow mulch away, from exposed sites,unless it is anchored with wire netting.

Maintenance:Mulched areas need to be inspected afterhigh winds or heavy rain and mulchreapplied as necessary.

16.3 Geotextiles


Geotextiles are fabrics used to stabiliserunoff conveyance systems. They can beused to line temporary channels,spillways, outlets to culverts, underriprap etc. An enormous variety ofgeotextiles is sold by manufacturers’agents. Which to choose, is dictated bycost as much as site.

InstallationThe fabric must be dug in at the inlet.An anchor trench 500-mm wide by 500mm deep needs to be excavated acrossthe top of the channel and up the sides.The fabric should be laid in the trenchleaving the top 1.5 metre extending outof the trench. The fabric should bepinned at 0.3 metre centres along thetrench, and soil then backfilled andcompacted over the fabric. The top 1.5metre is then folded back over thecompacted soil and pinned at 0.3 metrespacing. The fabric is rolled out downthe channel and pinned on a 0.5 metregrid pattern down the sides and lengthof the channel. The fabric needs to becontinued to an erosion proof outfall.

LimitationsThe big drawback to most fabrics is thatthey allow some water through. Unlessthe channel is very smooth, some watercan permeate and scour under the fabric.Polythene is waterproof but difficult tofasten (it rips if it is pinned). Somealternative fabrics address this problembut they are more expensive.

Pinning/fastening of fabrics is oftenpoorly done, particularly at the inlet end.All fabrics must be securely dug in at thetop of the work and well fastened. Thefunction of the fabric liner dependsentirely on the fastening. Where shearstresses are high more expensive fabricsand better fastening will be required.

MaintenanceThe fabric should be inspected regularlye.g. weekly and during and after eachheavy rainfall to check it is securelyfastened at the inlet and down its length.


Mulching.


Geotextiles. Source: Auckland Regional Council,

Technical Publication No 90.

1.5m minimum

Anchor type is to be to the manufacturersrecommendations. Anchor spacing is to be

to the smaller of the manufacturersrecommendation or the dimensions shown.

Where the slope terminates in an area of concentratedflow the geofabric must be laid through the flowpathand pinned down on a 500mm grid.

750mm minimum

1m minimum overlap at the end of a roll

1.5m maximum

1m minimum

150mm minimum

overlap

750mmminimum

Earth is to becompacted priorto laying top lap over

17.1 Sediment Retention Ponds


A sediment retention pond is designed toretain sediment on site from temporaryconstruction and minimise off-sitesedimentation. It is the most commonsediment control measure utilised whenthe site is more than 0.3 hectares.

InstallationSediment trapping efficiency is primarilya function of sediment particle size andflow velocity. To maximise the timeavailable for sediment to settle out ofsuspension, a pond should retain asmuch storm runoff as possible, have lowenergy inlet flow, and low decant rate atthe outlet.

The following design principles arecurrently promoted for clay soils. Silt or

sand soils should require less pondcapacity.

17.1.2 Location

These are generally constructed at thebottom of slopes, or at the lower end of asite, and in a position that allows accessfor sediment removal. It is usually betterto locate them immediately below theworks site (to optimise sedimentretention), and, if possible, away fromthe actual works area in order not tocompromise site earthworks. Theyshould not be located in streams. Pondcatchments should be kept to less than 5hectares if possible.

• Pond capacity. The capacity of pondsshould be 200 m3 for each hectare ofcontributing catchment for slopegradients less than 10% and less than200 m in length. Otherwise 300 m3


CHAPTER 17

Structures for Runoff and

Sediment Control on Earthworks

Sediment retention pond. Source: Auckland Regional Council, Technical Publication No 90.

Extra crest width may be required to providefor machinery access for cleaning out.

All bare surfaces to bestabilised with vegetation.

Manhole to act asPrimary Spillway.

Emergency spillway is to be wide, shallow andlevel. Where possible over the existing groundretaining the existing grass cover. Bare areas tobe stabilised with concrete or similar.

Runoff Diversion Channel/Bund to ensureall flow enters at the inlet end.

Level Spreader full width of inlet end,stabilised from the beginning of the inlet tothe pond invert and appropriate soft mattinggeotextile. Level Spreader to be 100-150mmabove the invert level of the spillway.

3:1 inlet batter tobe smoothed and

free of voids.

Pinned geotextile overlaid withlarge rock to break up flow.

per hectare of catchment of pondcapacity is required.

• Alternatively: use Pond surface area(m2) = 1.5 peak inflow rate (l/s).

• The inflow rate is calculated using the5% AEP rainfall event.

• Pond depth. This should be between1 – 2 metres in depth. The pondshould be a minimum 1.0m deep.

• Length to width. A pond should be atleast three times longer than it iswide (baffles may be necessary toachieve this).

• Inlet. Inlets should be level, have aninlet batter of no more than 3horizontal to 1 vertical slope, and bethe same width as the pond.

• Primary Spillway. A piped outlet isrequired to act as the primaryspillway. For ponds less than 3hectares, the outlet pipe can be 150mm diameter. Concrete manholesand pipes are required for largercatchments. At least two antiseepcollars should be installed along thepipe through the pond embankmentand the soil needs to be wellcompacted to guard against possiblefailure along the pipe.

• Emergency spillway. This should caterfor the 1 % AEP rainfall event and beconstructed in solid ground (not fill).Lining to protect it against erosionwill be necessary. It should be at least6 metres wide or the width of thepond (whichever is the greater).

• Permanent water storage. This is thecomponent that remains in the pondto dissipate inflow energy and shouldbe 30 % of the pond capacity.

• Operating volume. This is the volumebetween the lowest decant level andthe primary spillway invert andshould be 70 % of the pond capacity.

• Outlet decants. These should allowthe removal of the relatively cleanwater only. The currentrecommended rate is 3 litres/sec/ha ofcontributing catchment and isachieved by a perforated floatingdecant pipe. This pipe is generally100 mm in diameter and has a seriesof 10-mm holes drilled at 60 mmspacing along its length. This isattached to the primary spillway. Onedecant is required for each 1.5 hectareof catchment. A single decant shouldextend through the operating volumeof a pond. If more than one decant isrequired, then the additional decantsshould be positioned so they “kickin” proportionally through theoperating volume of the pond.

• Pond Cleaning. Ponds should becleaned out when they are 20 % fullof sediment.

• Pond efficiency. Ponds constructed inaccordance with the above criteria areabout 70-80 % efficient in retainingsediment (from trials on clay basedsoils in the Auckland region).Increasing the pond size does notsignificantly increase the efficiency ofsediment retention.

• Chemical treatment. Some trials withpoly aluminium phosphate (PAC),alum and polyacrylamide have beenundertaken to promote flocculationof suspended sediment particles andaid sediment retention. While resultslook promising, further investigationis being undertaken.

LimitationsThe embankment and spillway can beweak areas. Careful attention should bepaid to good compaction and erosionprotection here. Settlement of fill mayaffect embankment heights. Pipesthrough embankments need antiseepcollars and good compaction to protectagainst piping. Ponds are hazards andshould be fenced.

MaintenanceThe most common problem is decantholes blocked with floating material suchas straw mulch. These should be checkeddaily. Ponds should be inspected on a


Sediment retention pond.

regular basis and after every storm.Checks should be made for outflanking,scour, spillway protection structuralsoundness of the embankment and theoperation of the decants. Repairs shouldbe undertaken to ensure the pondremains in good working order. Sedimentshould be removed from the pond whenit is 20 % full. The pond should beretained until the upslope area hasstabilised.

17.2 Sediment Retention Bunds


A sediment retention bund is acompacted ridge of soil used to retainsediment from small areas of sheetflowonly.

InstallationThe catchment of a sediment retentionbund should be less than 0.3 hectares.The bund should be well compacted forstrength. Its outlet should have aperforated pipe upstand, about 100 mmbelow the spillway level, supported by arigid stake/waratah and connected to anon-perforated pipe that passes throughthe earth bund. The spillway should belined with an impermeable or non-woven fabric so excess flows can besafely discharged. The spillway should bethe lowest point on the bund and afreeboard of 250 mm should be allowedfor. The spillway should be stabilised e.g.lined with impermeable or non-wovenfabric and this should be well pinned.

LimitationsThey should never be used in waterwaysor where runoff has concentrated. Lackof compaction is often a problem,particularly if the discharge pipe is

trenched through the embankment.Spillways on earth bunds are oftenpoorly constructed.

MaintenanceThey should be inspect regularly (e.g.weekly, during and after each heavyrainfall). They should be repaired orreinstated as necessary. Sediment shouldbe cleaned out when the bund is 20 %full. The perforated upstand should beunblocked when it becomes clogged.

17.3 Silt Fences

Description/PurposeA silt fence is a geotextile barrierattached to posts and used to retainsediment from small areas of sheet flowonly.

InstallationThey are constructed from wovengeotextile fabric. The supportingpost/waratah should be at two metremaximum spacing and 400 mm intoground. The fabric should be trenched200 mm deep into the ground andupslope of the supports. Gravel filledsandbags laid end to end over the toe ofthe fence can be used where trenching isnot practical e.g. where there is surfacerock. The top of the fabric should be 400mm above the ground and have atensioned wire (2.5 mm HT) along thetop of the silt fence. The fence should bealigned along the contour as much aspossible. Ensure that the fence cannot beoutflanked by flow by turning the endsup equivalent to the effective height ofthe fence. Returns should be constructedevery 30 metres to confine sedimentload. The fence should be tied back toanother waratah for additional supportin low points.


Sediment retention bund. Source: Auckland Regional Council, Technical Publication No 90.

160mm non-perforated pipe through bund

1m approx

250mmSpillway

Spillway stabilisedwith geotextile

Stabilisedoutlet

Perforated pipefixed to waratahstake

Cross Section

A silt fence can be reinforced with sheepnetting, chain mesh netting or similarbehind it. The netting must also be duginto the ground.

LimitationsSilt fences should only be used tointercept sheet flow. They should not beused across flowing watercourses orsimilar areas of concentrated flow, asthey do not have the strength to stand

the energy of concentrated flows. Thefabric quickly becomes blocked withfines and the fence then acts more like asediment retention measure. They oftenfail through being undercut.

MaintenanceThey should be inspected after heavyrainfall and repaired as necessary.Sediment should be removed when itcreates bulges in the fence. Tiebacksshould be installed at low spots. Fencesgenerally fail by being undercut. This canbe addressed by extending the fabric upslope and then digging the toe inapproximately 1 metre away from thefence.

17.4 Haybale Barriers

Description/PurposeA haybale barrier is a temporarysediment retention measure. They canalso be used to intercept and divertrunoff from small catchments.


Silt fence.

Haybale barriers. Source: Auckland Regional Council, Technical Publication No 90.

Trench cut into ground

2 Re-bars or 50mm x 50mm stakesdriven 300mm minimum into theground. Stakes are to be driven flushwith the top of the bales.

Bound balesplaced alongcontour.

Entrench bales a minimum of100mm into the ground.

Angle first stake toward thepreviously placed bale.

100mm Vertical Face

Bedding Detail

Flow

String Binder

Undisturbed Ground

InstallationThe catchments should be less than 0.2hectare for sediment retention purposesand 0.5 hectares for runoff diversions.The haybales should be placed on thecontour and the ends of the barrierturned up to stop runoff flowing aroundthem. Each bale should be butted up tothe previous bale and each bale stakedwith 2 stakes driven 300 mm into theground. The first stake in each baleshould be driven towards the previouslylaid bale to force the bales together.Holes under/between the bales will needto be securely plugged with clay.

For runoff diversion purposes, start fromthe discharge point and then install thehay bales on a maximum 2 % grade. Theline needs to be surveyed (an abney levelor inclinometer is sufficient – do notsurvey by eye), and the hay bales willnormally weave around a slope as theterrain varies. Make sure that thedischarge point is secure against erosion,by installing erosion control fabric orrock armour.

LimitationsHay bales are impermeable and do notfilter. They should never be used inpermanently flowing waterways or whererunoff has concentrated. They have ashort life of only a few months. Oncethey get wet, they become very heavyand difficult to move around on site.

MaintenanceFor “tight”(sites, they can be removedafter rain provided they are replacedbefore the next rainfall. Otherwise, leavethem in place and inspect them regularly(weekly, during and after heavy rainfall).Sediment should be removed when it ishalf way up the bale. The barrier shouldbe reinstated or repaired after each heavyrain event (or if it is damaged by sitemachinery).

Figure 2(45) Hay bales Photo

17.5 Stormwater Inlet Protection

Description/PurposeThese are sediment retention measuresinstalled around roadside stormwaterinlets to capture eroded material,preventing its entry to stormwatersystems and eventual discharge toreceiving environments.

They can be constructed from sandbags,silt fences, rock/gravel barriers, or

haybales. An excavated depressionupslope of the measure helps retention.

InstallationThe catchment of each protectivemeasure should be less than 0.3 hectares.Individual measure should be spaced upslope of the stormwater inlet in theroadside water table. The impoundmentdepth should be slightly lower than thekerb to avoid redirecting the flow.Treated runoff should then flow aroundthe outside of the sandbags and into thestormwater inlet. The stormwater inletshould not be covered with, silt fencefabric (because this will quickly block)but left open so flows can enter into thestormwater system. Consider whetherponded water will encroach on aroad/private dwelling.

Rock should be 100 mm in diameter andfaced with 25 mm gravel on the sideaway from the inlet. Side slopes shouldbe a maximum of 3 horizontal: 1vertical. Concrete blocks and wire meshcan be used as a support for the stone.

• Sandbags can be used as barriers –they should be half full only.

• Silt fences should be no more than500 mm in height.

• Coarse filter fabric can be laid againstwire mesh and both nailed onto awooden frame secured by sandbags.

LimitationsBecause flows have to be retarded toretain sediment, control measures canimpede the entry of stormwater intostormwater inlet systems, and causesurface flooding. They should beregarded as a backup system only.

MaintenanceControl measures need to be repaired ifdislodged or broken. They should beinspected, and sediment removed aftereach heavy rain.


18.1 Introduction

Dust is generated when soil is repeatedlydisturbed and broken down into fineparticles. On susceptible soils (such asvolcanic ash when dry, or loess) dustassociated with earthworks can be severe,and difficult to control. Repeatedtracking of soils with machinery notonly breaks down the soil particles butalso aerates them so that they becomequite “fluffy” and suspended asparticulate material in the air. This issimilar in principle to sediment controlwhere finer particles are more difficult tosettle out. On high-risk sites, fine soil onthe ground can become very dry andaerated, and roll in waves as machinespass even when there is no wind. Oncethe site is subject to any wind, the dustgets very difficult to control. Dust fromproblem sites can travel for kilometresand cause a range of problems to healthand property.

18.2 Guidelines

Dust management should be consideredearly in the planning stages of anyearthworks project. Planning andmanaging to minimise dust problems isthe best option. If dust management isonly addressed after it has become aproblem, it is almost impossible to bringunder control.

A Dust Management Plan should beprepared prior to any works beingundertaken on a susceptible site. In theBay of Plenty, where resource consentsare required for some earthworksoperations, a Dust Management Plan is anecessary part of the consentapplication.

The main practice used to control duston earthworks is the application of waterto keep soil moisture high enough toprevent dust generation. A DustManagement Plan should include thefollowing elements:

• The potential effects of dust if itcauses a nuisance off site.

• The soil characteristics of the site andwhether the timing of operations willhelp or hinder dust control.

• Any methods that can reduce thedust e.g. restricting the amount ofbare ground exposed, staging ofworks.

• If water is used, the plan shoulddetail the water source, sourcecapacity and availability. If the sourceis marginal, then on-site storage maybe necessary. If the water is sourcedfrom a municipal reticulated watersupply, then written confirmationfrom the territorial authority will berequired.

• Other types of control that may beused.

• Contingency plans (e.g. for severewind problems). Contingency plansshould outline other options if theprimary method of control turns outto be ineffectual.

A Dust Management Plan normally alsoprovides for signage at an earthworks sitegiving a 24 hour contact number fordealing with dust complaints that mayarise from the operations. The provisionof this 24 hour contact number ensuresthat the contractor has a managementplan in operation to deal with dustcontrol.

The timing of works can be crucial fordust management. If the earthworks canbe carried out during wetter seasons,then dust control will be less of aproblem.

18.3 Watering

Description/PurposeThe application of water to maintain soilmoisture so that the soil does not dryout sufficiently to generate dust.

ApplicationWater is normally applied for dustsuppression in one of two ways; by watercart or by sprinkler. Either systemrequires a minimum amount of water to


CHAPTER 18

Dust Control Measures

for Earthworks

achieve effective dust control over anopen earthworks site. In the Bay ofPlenty, the minimum amount of waterrequired to control potential dustproblems is 5 mm/day.

The use of water carts is the mostcommon system of dust control. Watercarts can carry from 3,000 to 10,000litres. The use of water carts is limited bythe ability of the vehicle to access theareas that require wetting down. Asprinkler system is often used onearthworks sites where there are largeareas open, or where the terrain may betoo steep for water carts. Sprinklersystems are also commonly used on siteswhere some irrigation may be useful toestablish vegetation followingcompletion of earthworks.

Water should be applied as a dustsuppressant at a rate of 5mm/day beforesoil moisture levels start to drop. Thiscan vary with individual sites, andprevailing weather conditions, butconsideration should be given toapplying water after 7 days without rainduring spring/summer periods onsusceptible sites, This period can bereduced markedly in windy conditions,where on-site conditions should beassessed to commence treatment. Watershould continue to be applied until thenext rainfall event that results in surfacerunoff from the site.

LimitationsThe main limitation is availability ofsufficient water during mid-summer.When the water cannot be taken frommunicipal water supplies, alternativeoptions may be required. Sometimes, areservoir can be used.

The use of a groundwater borespecifically for the operation is acommon practice.

18.4 Dust Suppressants

Description/PurposeDust suppressants are polymers orchemicals applied to the exposed surfaceto protect it from the wind, and soreduce the dust generating capacity ofthe treated area.

ApplicationDust suppressants are a recent alternativethat has been used on some sites in NewZealand over the past two to three years.There is a range of dust suppressants

available. They are normally aproprietary blend of naturally derivedsurfactants and acrylic polymers,provided in a aqueous emulsion form sothat they dissolve readily in water foreasy application. Upon application, theyprovide a protective surface that reducesdust emissions.

Alternatively, some dust suppressants actas a binder. These include salts (CaCl2,and MgCl2), and ligin sulfonates.Chemical suppressants include salts,lignin sulfonate, wetting agents, latexes,plastics, vegetable oils and petroleumderivatives. The use of petroleumderivatives may require a resourceconsent.

LimitationsDust suppressants generally cannotwithstand machinery being driven overthe treated area. The cost means thatthey are only used in places where thetreated area is not be worked for a periodof time. There are questions regardingwhether they have a contaminationeffect on the soil. The potentialcontamination effects are unknown atthis time.

18.5 Surface Stabilisation

Description/PurposeProtection of exposed ground surfaceusing a range of different materials orproducts to reduce the potential for dustgeneration.

ApplicationSurface stabilisation is a dust controloption that is normally only used as alast resort if other dust control measuresare not fully effective. Hydroseeding mix,aggregate or geotextiles may be placedon to the exposed soil to reduce dust.Sometimes, aggregate just need to beapplied on areas where there are a lot ofvehicles travelling. Alternatively,aggregate may be applied in conjunctionwith a water cart as a back up.

LimitationsSurface stabilisation is normally onlyused as a last resort because of the itscost, and also because the surfacestabilisation may then need to beremoved to finish the job.


18.6 Windbreak Fencing

Description/PurposeWindbreak fencing comprises low fencesconstructed from fabric to reduce windvelocities on site, and reduce dustgeneration.

ApplicationWind break fencing using geotextile orwindbreak fabric is sometimes used onsmall sites to help control particularareas that may be difficult to access usingother methods, or to assist with theestablishment of vegetation as anadditional dust control measure.Windbreaks or silt fences can also beuseful in keeping machinery off criticalparts of the earthworks site.

LimitationsWindbreak fencing only controls dust onsmall areas because the fences arerelatively low; no more than 1 metrehigh, and protect ground for no morethan 10 metres to leeward. Even here,they can be ineffective due to eddiesround either end of a fence. Unless well-staked and constructed out of heavy dutyfabric, they will be demolished by strongwinds.


Appendices

A •1Soil Conservation B

New Zealand Land Use Capability

The classification has three components – a class, a subclass and a unit. The diagram belowillustrates the relationship between the various components of the land use capability classification.

New Zealand Land Use Capability Units(From Our Land Resources a bulletin to accompany NewZealand Land Resource Inventory Worksheets, 1979)

The Land Use Capability (LUC) is an ordered arrangement of the land according to those propertiesthat determines its capacity to sustain production permanently. The LUC takes into accountphysical limitations, management requirements and soil conservation needs.

As a basis for the land use capability assessment, an inventory is made of the facts about the land.The facts recorded are: rock type, soil, slope, erosion degree and type, and vegetation. Additionalinformation on climate is also factored in. This information is then displayed on the New ZealandLand Resource Inventory Worksheets as land inventory units.

These units are areas of land which, in terms of the five physical factors mapped, have uniformcharacteristics. They are therefore ‘homogeneous’ for every factor.

The classification has three components – a class, subclass and a unit. The diagram below illustratesthe relationship between the various components of the land use capability classification:

I II III IV V VI VII VIII

erosion (e) wetness (w) soil (s) climate (c)

1 2 3 etc.

dominant kind of limitation

similar management and conservation requirements

Class

Subclass

Unit

degree of limitation

The capability class is the broadest grouping of the capability classification. It is an assessment ofhow versatile the land is for sustained production taking into account its physical limitations. Itgive the general degree of limitation to use.

There are eight classes represented by roman numerals. Classes I-IV are suitable for cropping,pasture or forestry while Classes V-VII are limited to pastoral or forestry use. The limitations reach amaximum with Class VIII land which is not suitable for grazing or production forestry; it best servesa protection function.

The capability subclass is identified by a lower case letter in the land use capability code. It dividesthe land within each class according to the major kind of limitation to use. There are four kindsidentified but only the dominant one for each land unit is recorded.

Appendix INew Zealand Land Use Capability Classification

A •2 Soil Conservation B

The four subclasses are:

e erodibility where susceptibility to erosion is the dominant limitation to use

w wetness where a high water table, slow internal drainage, and/or flooding constitutesthe major limitation to use

s Soil limitation Where the major restriction to use is a limitation within the rooting zone.This can be due to a shallow soil profile, stoniness, rock outcrops, low soilmoisture holding capacity, low fertility (where this is difficult to correct),salinity or toxicity

c climate Where the climate is the major limitation to use

In the land use capability code, the capability unit is represented by an Arabic number. It groupstogether land inventory units which require the same kind of management and the same kind andintensity of conservation treatment. Units of land having the same land use capability unit numberare capable of growing the same kind of crops, pasture or forest species and have about the samepotential yield.

The capability units are arranged in order of decreasing versatility of use and increasing degree oflimitation to use.

Capability Classification

* land use capability classes IV – VII which have wetness as the major limitation and those units in very lowrainfall areas of those occurring on shallow soils are normally not suited to production forestry.

ClassCropping

Sustainability* General Pastoral & Production

Forestry Suitability* General Suitability

IHigh

Multiple land use

Pastoral orForestry land

Catchment protection land

HighMedium

Medium

Low

Unsuitable

Low

Unsuitable

II

III

IV

V

VI

VII

VIII

incr

easi

ng li

mita

tions

to u

se

decr

easi

ng v

ersa

tility


Debris Avalanche

These maps only indicate the presence of erosion

within large LRI units. As such, they exaggerate

how much area is affected by a given erosion type.

Appendix II

Erosion Distribution Maps (from the NZLRI)


Soil slip and Earth slip





Earthflow Erosion








Gully Erosion


Scree Erosion





Slump Erosion





Tunnel Gully Erosion





Wind Erosion




Indexes

Index to Part A

burning, A2•3

channel erosion, A9•9

cultivated soil, A9•3, A9•5

debris avalanches, A6•3

deep-seated mass movement, A4•1

deflation surfaces, A1•3

earth flows, A3•3

earthworks, A8•1

falls, A3•1

flows, A3•1, A4•1

forestry, A9•1

frost heave, A6•2

grain cropping, A9•3

granular loams, A9•4

gullies, mountain, A6•4

tunnel, A5•3, A5•5

U-shaped, A5•2, A5•5

V-shaped, A5•2, A5•6

gully erosion, A5•1

ice and erosion, A6•2

marine sediments, in gully erosion, A5•2

market gardening, A9•3, A9•4

mass movement, shallow, A3•1

deep, A4•1

mountain gullies, A5•3

mountain lands, A6•1

needle ice, A6•2

orchards and vineyards, A9•5

overgrazing, A2•3

pasture, erosion from, A9•6

periglacial erosion, A6•2

plantation forests, A9•1

rill erosion, and grain cropping, A9•5

rills, A2•2

sand dunes, A7•1

sediment, deposition, A9•9

sediment control, earthworks, A8•1

forests, A9•2

non-earthworks, A9•1

shallow mass movement, A3•3, A3•4

sheet and rill erosion, A1•2, A2•1, A6•2

slides, A3•1, A4•1

slip erosion, A3•1

slips, definition, A3•2

slumps, A4•1

surface erosion, plantation forests, A9•1

surface runoff, A2•2

torrents, A6•4

tunnel gullies, control, A5•3, A5•5

urban development, A9•9

U-shaped gullies, A5•2, A5•5

vineyards, A9•5

V-shaped gullies, A5•2, A5•6

wind blow, A1•3

wind erosion, A1•1, A6•2

Index to Part B

alpine zones, B2•5

banks, graded, B9•3

boring and drilling, B10•3

broadbased terraces, B9•9

bunds, B15•1, B17•3

burning management, mountains, B6•1

check dams, B15•1

chutes, B11•1

coastal planting, B5•5

contour banks, B9•8

cultivation, B1•5

drains, B15•2, B9•5

furrows, B9•1

planting, B9•8

contouring and recontouring, B10•4

cover crops, B1•5

cropland management, B1•1

cultivation regimes, B1•5

debris dams, B11•9

dewatering, B10•1

diversion banks, B11•6

channels, B15•1

drainage, subsurface, B10•2

tiled, B9•7, B10•2

drains, B9•4, B15•2

drilling, B1•3, B10•3

drop structures, B11•7

dust control, B10•2, B18•1

earthflows, B10•1, B13•1, B13•2

earthworks, dust control, B18•1

runoff control, B15•1, B17•1

sediment control, B17•1

soil management, B16•1

erosion-control forestry, B13•5

fallow, B1•5

fences, types of B3•2, B3•3

fencing B3•1

fire management, B6•1

firebreaks and tracks, B6•2

flumes, B11•1, B15•3

fodder banks, B7•1

forest management, mountains, B13•6

forest, coastal, B5•5

forestry as erosion control, B13•1

furrows, B9•2

graded waterways, B11•6

grasses, coastal pasture species, B5•8

grassing earthworks, B16•1

gullies, B11•1, B13•2

haybale barriers, B17•4

headlands, runoff from, B9•4

hieracium, B2•3

hill country, fencing, B3•1

pasture management, B2•3

revegetation, B4•2, B8•4

interception drains, B9•4

irrigation, B2•2

lowlands, pasture management, B2•1

runoff control, B9•1

fencing on, B3•1

revegetation of pasture, B4•1

marram, B5•1

mass movements, stabilising, B10•1

mountain lands, forest management, B13•6

retirement, B8•6

revegetation of pasture, B4•3

mulching earthworks, B16•1

native plants in coastal plantings, B5•6

no-tillage cultivation, B1•2

nurse crops, B8•6


pasture quality, B2•4

pasture, coastal, B5•5

revegetation, B4•1

species, B5•8, B7•1

types, B2•1

pest control, B1•3, B1•4,B2•2, B5•8, B8•2

pingao, B5•3

planting, shelterbelt, B14•3

pole planting, B12•1

poles, B13•1

ponds, sediment retention, B17•1

raised accessways, B9•5

retired land, B2•5, B8•1

revegetation, of pasture, B4•1

of retired land, B8•3

runoff control, B9•1, B15•1, B17•1

sand dune, stabilisation, B5•1

shelterbelt planting, examples, B14•4

shelterbelts, B14•1

silt traps, B9•6

sink holes, B11•5

slope stabilisation, B10•1

slumps, B10•1

soil fertility and pasture mgmt, B2•2

soil management for earthworks, B16•1

spinifex, B5•2

stormwater inlets, B17•5

strip cultivation, B1•5

stubble retention, B1•4

subsoiling amongst trees, B9•7

surface erosion, croplands B1•1

fencing and, B3•1


terraces, B9•3

tillage practices, B1•1

tussock, B2•4, B2•6

water transport structures, B11•7

watering dust, B18•1

weed control, B8•2

wheel tracks, B1•4

willows and poplars, B12•3

windbreaks, B14•1, B14•4

woodlot erosion control, B13•3

soil conservation technical handbook

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