soil conser b ch11 18 jun01

7/31/2019 Soil Conser b Ch11 18 Jun01

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Gully Structures& Pole Planting

Chapter

11. Gully Control Structure

12. Pole Planting


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11.1 Flumes & Chutes

11.1.1 Description/Purpose

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 active

headward 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 by

Eyles (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 scouring

around 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 job

before building the bunds and stopbanksto divert water into the runoff controlstructure.

11.1.3 Types of Flumes and Chutes

Lip Flumes

These are wooden structures thatconcentrate the surface flow anddischarge it safely over a gully head orscarp slope.

111Soil Conservation B

CHAPTER 11

Gully Control Structures

Plan of lip flume, 0.56m3/s capacity.

All measurements are in millimetres.Source: Environment Waikato.


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They need to be constructed using fromeither ground treated tongue and groove

timber 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 Flumes

Cantilever 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 to

be 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 bank

discharging into drainage coil piping.

Suspended Flumes or pipes

Suspended flumes are supported by wiresout over gully heads. They are used onlyin locations where it is not possible to

112 Soil Conservation B

Plan of a typical cantilever flume.All measurements are in millimetres.Source: Environment Waikato.

Plan of a suspended flume designed to drop surface flows safely over a 30m head in a difficulthill environment. This was one of a series of measures that included diversion banking toconcentrate flows into detention bunds, retirement and planting of the gully head and outflowareas. This flume, constructed in the early 1960s, has disappeared but the gully systemremains stable.


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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 Chutes

Wooden 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. They

were 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 a

similar 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.

Application

Pipe drop structures have largely replaced

wooden flumes and chutes, especiallywhere design flows are relatively low.They have a number of advantages overwooden structures:


Butyl rubber flume. Photo: Gisborne DC.


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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 and

construction, 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 firmly

anchored 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 Inlet

The 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 the

high 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 Structure

These 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 PVC

pipes have become available, and aremore cost effective.

Flexible Pipe Drop Structures

Drainage 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/soil

mix 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 piping

can be laid to remove surface stormwaterfrom gully heads. The collapsible tubingcan be factory prepared to cope withspecified volume of runoff, and is



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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 to

fit 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. It

is 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.

Application

The 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 the

scarp 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 of

up to 40 hectares only.

Care needs to be taken with both thedetention bunds and dams, so that


Sink hole cover for heavily stocked locations.Source: Environment Waikato.


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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 = diversionbunds on earthworks

Description/Purpose

Diversion banks are earth banks thatguide surface flows across a slope to a

safe 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.

Application

Diversion 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=grassedwaterways 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.

Application

As 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 Design

manual, 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 GrassLined Channels

Channel slope Lining Velocity* (m/s)

0-5% Grass/legume 1.25Kikuyu 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 filling

proceeds. Different geotextiles are nowavailable with which to reinforce andgrass the graded waterways. Reinforcedwaterways can withstand significantly



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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 are

only used on short gully head drops of 1to 1.5 metres.

Application

While 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 carefully

designed 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 is

conveyed 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 Structures

These 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.


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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 Structures

These 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 forces

generated 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 Structures

Sheet 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 channelled

steel 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 Structures

Rock 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 pool

forming 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 basket

and 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.


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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 Structures

These 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 the

downstream 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 Drop

Structures: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. Some

form 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.

Application

Debris dams are largely confined to thecontrol of V shaped gullies. They arenormally built in series over time, withthe base of the upstream debris dam

level 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 other

vegetative 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



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design have been built. A common factorin all types of construction is the needfor the structure to be stable when the

downstream 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 have

been built since loss of Governmentsubsidies for soil conservation work, in1988.

11.8.2 Types of Debris Dams

Timber Debris Dam

The 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 as

important 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 water

will 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 postw ith at minimum a 100mm facefor 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.


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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 dam

upstream of an existing strong point,such as a tree, or rock face.

Pole and Netting Debris Dams

The 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. If

manuka 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 Dams

Brush 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 useful

where 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 Dams

The 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 mesh

was 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 Dams

Some 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 Dams

Some dams used pre-fabricated angle

irons 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 Designs

Various 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.



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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). This

page 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 up

maintenance 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 ofGullies

Description/Purpose

This involves the contouring of the

ground 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.

Application

Mechanical 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 and

substantially 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 be

prohibitively expensive, and potentiallyprone to failure



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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 agullys 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. Erythrina

in 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 m

long) 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 unrooted

poplar 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 hours

immediately 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 resist

movement better than lighter ones.

Table 12.1 Classes of pole (giving typicaldimensions)

Class of Pole Length SED

Cattle 2.53.5, 25mm

Sheep 2.02.5m 25mm

Retirement pole(stake) 1.0-2.0m 15 to 20mm

CHAPTER 12

Pole Planting


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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 the

bark 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 carefully

considered. 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 the

unstable 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 controlerosion on mudstone soils, KanakanaiaValley. Photo: Don Miller.


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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 goats

and 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 is

not 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 are

excellent 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, 17

The poplars, Forest Research Institute2000

Hawkes Bay Regional Council,Environment Topics PlantingPoplars and Willows 1995



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Forestry & Shelterbelts

Chapter

13. Erosion Control Forestry

14. Shelterbelts


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13.1 Earthflow stabilisation witherosion 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 their

roots 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 planted

after 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 several

years 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 seated

movements 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


19/49

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 forgully 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 trees

such 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 be

fenced 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 onEarthflows 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 the

area 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 disturbed

ground 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



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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 than

having 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 Forest

by 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 to

recover 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 forest

establishment.

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.



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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 and

earthflows, 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 about

the 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 transport

After 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 sometimes

used 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 competition

It 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 species

Erosion 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 than

normal open spaced planting.

Post planting care

Trees 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, access

to the site, proximity to processing,severity of erosion on the site andavailability of time and resources to dothe work



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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, wind

and 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 130

years 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 possum

damage.

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 or

warmer 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 purposes

seedling 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.


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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 fertiliser

and 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 cable

techniques 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.



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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). Windbreaks

have 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 windbreak

siting, 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 GrowersManual (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 90 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 farmers 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 ofdifferent density (after Caborn 1965).


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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 and

insufficient 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. The

optimum 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, each

row 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 stability

to 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 single

ripped lines. The tractor wheels can berun back over the rips to consolidate thesoil and improve the surface for pre-plant herbicide application.



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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 sites

are 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 to

windthrow. 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 of

herbicides 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, andpossums

The susceptibility of young trees toanimal damage necessitates the use ofindividual protection devices, rabbitnetting, or electric fencing (see Section

2.10.2 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 off

where 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 sought

from 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.



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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 plantspecies 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 shelter

designs 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-tallhardwood, 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 Spacing

1-2 m between trees for horticulturalshelter

2-3 m between trees for farm shelter

Management

Low maintenance. Optional side

trimming 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 albaPyramidalis (suckers)

P. X euramericanaTasman or Veronese orCrows Nest

P. nigra Italica*

Salix matsudana**

S. matsudana X albahybrids**

S. matsudana Xpentandra hybrids**


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Site selection/applications

Moist, 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.

14.4.2 Shelterbelt, medium-tallhardwood, one row, underplanted

Initial planting

2-3 m between tall-growing species

1 m between low-growing species

Management

Low maintenance. Occasionally side trimto reduce fence overhang.


Deciduous 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 provide

firewood, 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.

14.4.3 Shelterbelt, medium-tallhardwood, 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.

Profile Windward view

Low- or slow-growingevergreen species

Acacia spp.

Bambusa oldhamii

Callistemon spp.

Chamaecytisuspalmensis

Corokia spp.

Cortaderia spp.

Cryptomeria japonica

X Cupressocyparis

leylandii

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.

Cryptomeria japonica

Cupressus lusitanica

X Cupressocyparisleylandii

Melaleuca spp.

Olearia spp.

Phormium cookianum

P. tenax

Pittosporum spp.

Thuja plicata

Medium-tall growingspecies

Alnus cordata

glutinosa

incana

rubra

Populus albaPyramidalis (suckers)

P. X euramericanaTasman* or Veronese*or Crows Nest

P. nigra Italica**

Salix matsudana***

S. matsudana X albahybrids***

S. matsudana Xpentandra hybrids***


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Medium-tall hardwood, evergreen, two row.

ProfileWindward view

Initial spacing

2-3 m between tall-growing specieswithin the leeward row

1-1.5 m between low-growing species in

the windward row

1-1.5 m between rows

Management

Wider-crowned species may requireoccasional side trimming.


Moist, reasonably fertile sites andirrig

soil conser b ch11 18 jun01

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