sh18 greenhithe deviation - nz transport agency · harbour bridge in hobsonville. starting at the...

NEW ZEALAND TRANSPORT AGENCY

SH18 GREENHITHE DEVIATION

Erosion and Sediment Control

Comparison with NZTA Draft Erosion & Sediment

Control Standard Requirements

NEW ZEALAND TRANSPORT AGENCY

SH18 Greenhithe Deviation


Comparison with NZTA Draft Erosion & Sediment Control

Standard Requirements

Prepared By Opus International Consultants Limited

Glenn Jarvie Senior Environmental Engineer

Environmental

Level 3, The Westhaven

100 Beaumont Street, PO Box 5848

Auckland, New Zealand

Reviewed By

Warren Bird

Team Leader Water Resources

Telephone: +64 9 355 9500

Facsimile: +64 9 355 9584

Approved By Date: 11 February 2011

Matt Thompson Reference: 1-T0063.00/01AE

Business Manager Environmental Status: FINAL

© Opus International Consultants Limited 2008 o:\env\transport\nzta\proj\1-t0063.00 - nzta erosion and sediment control standard road test\600 deliverables\610 reports\sh18\sh18

greenhithe deviation - nzta escs road test_final_20110211.docx



Comparison with NZTA Draft Erosion & Sediment Control Standard Requirements

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February 2011 FINAL i

Executive Summary

The purpose of this study is to provide a comparison between the Erosion and Sediment Control

measures employed on the SH18 Greenhithe Deviation project and the Erosion and Sediment

Control requirements under the “Draft Erosion and Sediment Control Standard for State Highway

Infrastructure (August 2010)”. The study was commissioned to help NZTA understand the

ramifications of adopting this standard for highway projects nationwide.

The SH18 Greenhithe Deviation project itself consisted of the construction of a 5.5km length of 4-

lane motorway, three interchanges, and 4 bridges including duplication of the Upper harbour

Bridge. The majority of the project site drains to the Lucas Creek Estuary in the upper reaches of

the Waitemata Harbour. Due to the sensitivity of the Lucas Creek Estuary to sediment discharges,

rain activated chemical treatment systems for sediment retention ponds on the project were

required as a condition of the Auckland Regional Council Land Use (earthworks) Consents.

The adoption of the draft NZTA standard would not have fundamentally altered the consenting

requirements or design objectives, but it would have impacted on the size of the devices and the

capital cost of the project relating to Erosion and Sediment Control. The primary reason for this

discrepancy is that the NZTA guideline considers the site specific environment risk and increases

the design criteria for projects that have sensitive receiving environments.

The regional requirement applies blanket criteria for the Auckland region irrespective of the

receiving environment. Adoption of the NZTA erosion and sediment control guidelines for the

Greenhithe Deviation would have increased the required storage of the sediment retention ponds

and decanting earth bunds by about 80%. In some locations the extra storage could have been

achieved by increasing the size of the sediment retention ponds but in many areas designation and

environmental constraints would have necessitated construction of additional devices.

Cost estimates indicate that adopting the NZTA standards for the Greenhithe Deviation would

increased the cost of erosion and sediment control works by about 30%. Construction of the

additional sediment retention ponds would have increased the contract period but the extra work

required to build the additional devices is considered insignificant compared to the overall project

timeframe.

The inclusion of the Erosion and Sediment Control Field Guide for Contractors provides extensive

guidance (over and above that provided in ARC TP90) for the construction, maintenance, and

decommissioning of Erosion and Sediment Control practices in a concise and user-friendly format.

It is expected that this will greatly improve contractors’ understanding of erosion and sediment

control concepts and potentially led to improved maintenance regimes and reduced cost/time for

preparation of contractors’ environmental management plans.




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February 2011 FINAL ii

Contents

1 Project Description .............................................................................................................. 1

1.1 Project Location ............................................................................................................ 1

1.2 Delivery Method ............................................................................................................ 1

1.3 Project Scope ............................................................................................................... 2

1.4 Site Constraints ............................................................................................................ 2

1.5 Natural Site Features .................................................................................................... 2

2 Environmental Factors ........................................................................................................ 3

3 Designed Solutions ............................................................................................................. 6

3.1 Design philosophy ........................................................................................................ 6

3.2 Project Requirements ................................................................................................. 10

3.3 Erosion and Sediment Control Practices ..................................................................... 14

3.4 Cost ............................................................................................................................ 16

3.5 Time ........................................................................................................................... 20

4 General Comments and Discussion ................................................................................. 22

4.1 Design/Construction Issues ........................................................................................ 22

4.2 Project Completion ..................................................................................................... 22

4.3 General Comments ..................................................................................................... 22

Appendices

Appendix A: Design Erosion and Sediment Control Plans

Appendix B: Design Information




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February 2011 FINAL 1

1 Project Description

1.1 Project Location

The project on which this comparison is based is a portion of SH18. It is located in

Auckland between the Constellation Drive interchange on SH1 in Albany and the Upper

Harbour Bridge in Hobsonville. Starting at the Albany Highway, the motorway route passes

through lifestyle type properties in the east and then through bush and reserve areas to

sub-division development areas east of Greenhithe Road. West of Greenhithe Road the

route passes through more bush areas to denser residential areas and finally joins the

existing Upper Harbour Drive near the Upper Harbour Bridge (see Figure 1).

This portion of SH18 is named the Greenhithe Deviation and will form part of Auckland’s

Western Ring route. This Highway is in the process of being extended from the Upper

Harbour Bridge to the Westgate Interchange on the North Western Motorway in Massey.

Once completed this motorway network will connect the North Shore to Waitakere City and

will provide motorists with an alternative motorway route between the North and North

Western Motorway. This study focuses on the Erosion and Sediment Control aspects of the

Greenhithe section of works only.

Figure 1: SH18 Greenhithe Deviation Location Plan

1.2 Delivery Method

The project was procured and delivered using a traditional measure and value contract.

source: maps.google.com

Tauhinu

Interchange

Greenhithe Rd

Interchange

Albany Highway

Interchange

Upper Harbour

Bridge




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1.3 Project Scope

The Greenhithe Deviation involved the construction of;

• 5.5km length of 4-lane motorway,

• Three motorway interchanges at Albany Highway, Greenhithe Road, and Tauhinu Road

• 4 road bridges including duplication of the Upper Harbour Bridge

• Several large fill embankments including a 25m high embankment at the eastern end of

the highway beside Ashby Place. This fill embankment required approximately 750,000

m3 of earthworks, up to 12m of undercut of soft foundation soils, the installation of wick

drains, and geogrid reinforcing within the embankment to maintain stability.

• Modifications to the local road network to allow construction of the highway and to

maintain access to the affected residential properties. Alterations were made to William

Pickering Drive, Unsworth Drive, Ashby Place, and Chester Ave.

• Noise walls along noise sensitive section of the route

• Retaining walls to support the highway and to prevent/limit encroachment past the

designation boundary. A variety of wall types were constructed including mechanically

stabilised earth (MSE), soil nail, and a combination of timber and concrete pole walls.

The motorway was completed at a cost of $107million dollars and opened in 2006.

1.4 Site Constraints

The designation of the route is generally quite narrow and crosses a number of steeply

incised stream valleys. The motorway route passes through several areas that were/are

considered to be ecologically sensitive. To minimise damage to these area retaining walls

were constructed to limit encroachment into these areas.

The combination of steep topography and tight designation posed practical problems for the

installation of temporary silt control devices.

1.5 Natural Site Features

The Greenhithe section of the Upper Harbour motorway drains to the Lucus Creek /

Paremoremo Estuary. This estuary discharges into the Upper Waitemata Harbour and

eventually the Hauraki Gulf.

The New Zealand Geological Society (NZGS) record indicates that the geology of the upper

catchment comprises interbedded sandstones and siltstones of the East Coast Bays

Formation (i.e. subset of the Waitemata Group soils and rock that is common in Auckland).

The upper layer of this material has typically weathered to form silty clay soils. Recent

alluvial soils have been deposited in stream valleys. A significant depth of these soft alluvial

soils were encountered along the route, particularly near Ashby Place where up to 12m of

alluvial soils were undercut to allow construction of the Ashby Place fill embankment.

Alluvium and Waitemata Group soils types generally comprise soils of high plasticity that

are characterised by high shrinkage, high winter groundwater levels, and low permeability.




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2 Environmental Factors

The following table provides a brief outline of the various environmental factors and data

relating to the design, construction and implementation of the SH18 Greenhithe Deviation

project.

Environmental Factor Discussion

Terrain The catchment area generally consists of bush/pasture land

and medium density residential development. The main road

alignment starts in an industrial/commercial area at the eastern

end and terminates at the Upper Harbour Bridge at the western

end, passing through alternating areas of low to medium

density residential development, semi-rural and reserve/bush

areas

Site Area Approximately 55 ha

Size of earthworks Approximately 45 ha

(based on measured Site Clearance area )

Volume of material

moved

~1,340,000 m3, including;

• 45,000 m3 topsoil removed, stockpiled and re-spread

• 975,000 m3 cut to fill

• 320,000 m3 landscape fill & unsuitables

Topography The main alignment generally traverses the gently sloping

northern flanks of the adjacent headland between the ridgeline

to the south and the low lying areas to the North. Due to this

the topography is generally undulating with the highway

crossing numerous minor ridges and stream gullies, some of

which are in excess of 10m deep.

Catchments The majority of the route drains to the Kyle, Orwell, and

Greenhithe catchments. These catchments discharge to the

Lucas Creek Estuary / Paremoremo Estuary, and finally to the

Upper Waitemata Harbour. The catchments are generally

steep with gradients typically in excess of 10%.

The western end of the Greenhithe Deviation between Tauhinu

Road, and the Upper Harbour Bridge discharges directly to the

Waitemata Harbour.




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Drainage Features Much of the Albany-Greenhithe component of the route

traverses the relatively steep flanks of a hill with the old Upper

Harbour Drive defining the ridge. Several small streams

descend these flanks, crossing the main alignment before

joining Lucas Creek at the Upper Waitemata Harbour. Where

the main alignment crosses the natural hillside channels

culverts were installed to maintain, as best practicable, the

natural flow regime of the streams.

In total 17 culverts were installed on the project. Where

culverts serve a watercourse that was identified as containing

important ecological values provision was made for fish

passage.

To mitigate erosion at the interface between the natural stream

channels and the culvert headwall/wing-wall structures, rip-rap

protection, and other energy dissipation measures were

installed.

Geotechnical

limitations/opportunities

Due to the underlying soils and associated low permeability

and high winter groundwater levels soakage was deemed to be

impractical.

The numerous high cut and fill embankments and historic

geotechnical instabilities presented significant geotechnical

limitations on the placement of stormwater management

devices.

Soils The soils on the project site are derived from the weathering of

the underlying inter-bedded sandstones and siltstones of the

East Coast Bays Formation forming Silty-Clays. The East

Coast Bays Formation is a subset of the Waitemata Group

soils and rock that is common in the Auckland Region.

Soft alluvial soils are typically found at the base of stream

gullies. In several areas these deposits were in excess of 12m

deep and required undercutting before forming the road

embankment.

Both soil types are generally characterised by high plasticity,

high winter groundwater levels, and low permeability. Alluvial

soils generally have a high water content and consequently

tend to consolidate if surcharged.

Erosion potential Due to the rolling topography of the site and the numerous

gullies which concentrate overland flows the potential for

erosion from raindrop impact, sheet flow, and concentrated

flow on the site is moderate. As such robust erosion control

measures were required during construction.




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Site flooding history Previous studies undertaken within the catchment area

indicated that flooding issues were not critical due to the

deeply incised streams with no significant floodplains. North

Shore City Council has no records of flooding problems within

the main drainage area.

Design storm event The designed/constructed erosion & sediment control systems

were based on ARC TP90 criteria.

TP90 generally uses the 5% AEP rainfall event for sizing of

devices (although some devices such as silt fences do not use

a specified design storm for sizing).

VKT at time of

completion

The most recent data for total traffic flow along SH18 gives

approximately 25,000vpd (Transit New Zealand 2006). The

total route length of the Greenhithe Deviation is approximately

5.5km. This gives an approximate value of 137,500 vehicle

kilometres travelled per day along the route. However this

does not take into account induced traffic on the route due to

the new motorway nor does it account for the reduction in

traffic along the old route due to the construction of the

motorway itself.

The 2011 modelled traffic flows for the Upper Harbour Corridor

along the Greenhithe Section is approximately 43,000vpd.

This gives a value of 236,500 vehicle kilometres per day

travelled. The true number of vehicle kilometres travelling on

the route at the time of opening (or soon after) is expected to

fall somewhere between these two values.

Catchment

classification1

The project catchment can be classified as peri-urban,

according to the NSHS-2007 document.

Sensitivity of receiving

environments2

Attribute Sensitivity Score

Type Sensitivity High 30

Ecological Value Moderate 10

Human Use Value Moderate 5

Overall Sensitivity Rating (Sum) High 45

The Draft Erosion and Sediment Control Standards for State

Highway Infrastructure classifies the Lucas Creek Estuary in

the highest risk category due to the highly depositional nature

of the estuary.

1 As per “National State Highway Strategy” (Transit NZ 2007)

2 As per “Identifying Sensitive Receiving Environments at Risk from Road Runoff - Research Report 315” (Land Transport New Zealand 2007)




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3 Designed Solutions

This section provides a comparison between the Erosion and Sediment Control Measures

designed for and constructed on the SH18 Greenhithe Deviation project designed and constructed

in accordance with the regional requirements to meet the ARC Consent requirements appropriate

at the time and the requirements that would need to be met under the Draft NZTA Standard with

respect to the following general aspects:

• Design philosophy;

• Project Requirements (e.g. consents, phasing etc.);

• Erosion and Sediment Control practices;

• Cost; and

• Time.

Each general aspect has been divided as appropriate into designed/constructed (Existing Design)

and the anticipated outcomes under the Draft NZTA Standard

3.1 Design philosophy

3.1.1 Objectives

Existing Design

Under the Auckland Regional Plan: Sediment (November 2001), the following objectives are stated

relating to earth disturbing activities;

7.1.1 To reduce the exposure of land to the risk of surface erosion leading to sediment

generation.

7.1.2 To minimise sediment discharge to the receiving environment

The design objectives for Erosion and Sediment Control on the SH18 Greenhithe Deviation project

were in accordance with regional guidelines (Auckland Regional Council Technical Publication 90

(ARC TP90), 1999) TP90 is a guideline that is a means to compliance with the Auckland Regional

Plan; Sediment control.

The objectives of this document are summarised as follows:

• Diverting clean water before it flows onto the disturbed area and discharging this water

untreated;

• Minimising sediment generation by minimising the disturbed area and stabilising disturbed

surfaces as soon as possible by hydro-seeding, mulching or sealing.

• Conveying sediment laden water to treatment systems before discharge to the receiving

environment;

• Erosion and sediment controls should be in place before earthworks commence and should be

removed only after the site has been fully stabilised




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Draft NZTA Standard

The above objectives would still have been applicable if the Draft NZTA Standard had been adopted

for the project. However, in addition to those regional requirements, the objectives outlined in

Section 2.4 of the NZTA Environmental Plan (2008) would also have applied, namely;

ES1 Ensure construction and maintenance activities avoid, remedy or mitigate effects of soil erosion, sediment run-off and sediment deposition.

ES2 Identify areas susceptible to erosion and sediment deposition and implement erosion and sediment control measures appropriate to each situation with particular emphasis on high- risk areas.

ES3 Use bio-engineering and low-impact design practices where practicable

The fundamental design principles/objectives of the Draft NZTA standard (section 4.6) are virtually

identical to those of ARC TP90 (“ten commandments” given in section 2). Consequently, the design

objectives for the SH18 Greenhithe Deviation project under the Draft NZTA Standard would be

similar if not identical to the Existing Design case. No significant change in terms of design

objectives would be anticipated.

3.1.2 Criteria

Existing Design

Sediment Retention Pond:

The basic design criteria used for Erosion and Sediment Control practices on the SH18 Greenhithe

Deviation project are those contained in ARC TP90. The design volume of sediment retention ponds

prescribed in this standard is not dependent on the characteristics of the receiving environment.

They are only dependent on the characteristics of the earthworks site.

The topography along the route is generally quite steep and consequently the following design

criteria was applied; 3% pond sizing used for site slopes steeper than 10% and/or greater than

200m in length (i.e. 300m3 per hectare of catchment area. Maximum catchment area of 5.0 ha.

Maximum depth of 2 m)

Note; Even though the size of pond is not dependant on the receiving environment the Air Land and

Water Plan as implemented by the Auckland Council is more than likely to require flocculation of

ponds for sites that drain to sensitive receiving environments. This is not a prescribed requirement.

Decanting Bunds:

Volume sizing as per ponds. Maximum 0.3 ha catchment area.

Diversion Channels:

Conveyance provided for the 5% AEP storm event (20yr ARI) with nominal 300mm freeboard.

Maximum longitudinal grade of 2% unless armoured.

Silt Fences:

Catchment size generally limited to 0.5 ha.




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Draft NZTA Standard

Design Storm Selection:

The selection of the design storm under the Draft NZTA Standard is based on the risk based

approach outlined in the Australia New Zealand Environment and Conservation Council (ANZECC)

guidelines.

The sediment control devices that were located east of the Tauhinu Road interchange discharge to

the Lucas Creek Estuary. Due to the highly sensitive nature of estuarine receiving environments a

low risk approach is proposed the in NZTA guidelines. The guidelines suggest that a 1 hour 100

year average return interval (ARI) storm be adopted for the design of a majority of the erosion and

sediment control devices. This exceeds the current regional standards that require a 20 year ARI

design storm to be adopted for all projects in the Auckland Region.

The erosion and sediment control devices that were located west of the Tauhinu Road interchange

drained to the Upper Waitemata Harbour. The NZTA guideline prescribes a more lenient design

standard for devices that discharge to harbours as tidal action is greater and rates of deposition of

sediments are lower compared to estuaries. The guideline indicates that a 1 hour, 5 year ARI storm

is appropriate for this receiving environment; This is lower than the 20year ARI storm required by

the Auckland Council.

Sediment Retention Ponds:

The sizing criterion for sediment retention ponds is based on the design storm event and the runoff

characteristics of the catchment using the Rational Formula to estimate runoff volumes. This means

that storage volumes are based on more project specific parameters than those obtained using the

sizing methodology in ARC TP90.

On the SH18 Greenhithe Deviation project this would have increased the required storage volumes

by approximately 80% for all devices located east of the Tauhinu Road interchange. Sediment

retention ponds located west of the Tauhinu Road would need to be approximately the same size as

that prescribed in TP90 even though the NZTA standard indicates a less stringent design storm (ie

5 year ARI storm for NZTA’s standard, compared to the 20 year ARI storm). This is primarily due to

the more conservative design methodology outlined in the NZTA standard.

Decanting Earth Bunds;

Volume sizing as per sediment retention ponds, but limited to a maximum catchment size of 0.3 Ha.

Decant Rate:

The decant rate calculation used in the Draft NZTA Standard is based on the total live storage

volume released over 24-hours (as opposed to 3 L/s/ha used in ARC TP90). On the Greenhithe

Deviation project this would have generally led to a 50% reduction in decant rates in sediment

retention ponds and decanting earth bunds. This would also have led to longer retention times and

potentially greater sediment removal efficiency.




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Diversion Channels:

The design criteria for diversions are dependent on the sensitivity of the receiving environment, and

the risk associated with the project. On the SH18 Greenhithe Deviation project this would have

impacted on the design and size of diversions.

Diversion channels located east of the Tauhinu Road Interchange with catchments greater than 2Ha

would need to be designed for the 1 hr - 100 year ARI storm. Diversion channels located west of the

Tauhinu Road interchange with catchments greater than 2Ha would need to be designed for the

1hr-5 year ARI storm

A nominal 300mm freeboard would be required for all diversions.

Silt Fences:

The NZTA design criterion for silt fences and supersilt fences is identical to the Auckland Council

requirements; however they need to be cleared of sediment more frequently. The NZTA standards

require that sediment is removed from behind silt fences when it has accumulated to 20% of the

fabric height. The Auckland Council requires that silt is removed once it reaches 50% of the fabric

height.

3.1.3 References

Existing Design

References used for the design of the SH18 Greenhithe Deviation Erosion and Sediment Control

practices included the following;

• Proposed Auckland Regional Plan: Sediment Control (September 1995)

• Auckland Regional Plan: Sediment Control (November 2001)

• Erosion & Sediment Control Guidelines for Land Disturbing Activities in the Auckland Region

(March 1999) Auckland Regional Council Technical Publication 90

• Beca Carter Hollings & Ferner Ltd. (1998) SH18 – Upper harbour highway (Greenhithe) Route

Selection & Environmental Effects Investigation

Draft NZTA Standard

Under the Draft NZTA Standard, the reference documents used for the design and development of

the Erosion and Sediment Control measures would generally remain the same, with the addition of

the following documents;

• NZTA Environmental Plan

• NZTA Draft Erosion and Sediment Control Standard for State Highway Infrastructure

• Australia New Zealand Environment and Conservation Council (ANZECC) Guidelines for Fresh

and Marine Water Quality.

These documents would be used in conjunction with the relevant regional policies and guidelines to




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establish the more stringent requirements and objectives for erosion and sediment control. The

most stringent guidelines would need to be followed and complied with.

The NZTA Environmental Plan is complimentary to the Auckland Regional Plan: Sediment Control

and the design objectives in both documents would have applied to the project.

The ANZECC guidelines would serve as a reference for risk assessment of the receiving

environment as the methodology used in the Draft NZTA Standard is based on these guidelines.

In the case of the SH18 Greenhithe Deviation project, the Draft NZTA guidelines would take

precedence as they provide more stringent criteria for design and sizing of storage devices and at

least equal criteria for all other erosion and sediment control practices used on the project.

3.2 Project Requirements

3.2.1 Local Consent Requirements

Existing Design

A Land Use: Earthworks Consent was required from the Auckland Regional Council under the

Proposed Regional Plan: Sediment Control for all earthworks activities > 1.0 ha for land slopes less

than 15o or > 0.25 ha for sites with slopes greater than 15

o outside the Sediment Control Protection

Area3. For sites within the Sediment Control Protection Area any earthworks greater than 0.25 ha in

area require a consent.

Accordingly the earthworks for the SH18 Greenhithe Deviation project required resource consent

from the Auckland Regional Council. The consents granted with specific conditions relating to

erosion and sediment control, were;

25620 – Earthworks, Vegetation Clearance and Roading (Albany Interchange)

25621 – Works in Watercourse (Albany to Bridge)

25622 – Earthworks, Vegetation Clearance and Roading (Albany to Bridge)

25624 – Works in Watercourse (Albany Interchange)

5829 – Diversion of Tributaries (Albany Interchange)

25853 – Diversion of Tributaries (Greenhithe section)

The conditions of the above consents required Erosion and Sediment Control practices to be

constructed and maintained in accordance with ARC TP90 guidelines. However the consent

conditions do allow for alternative standards to be included in the Project Environmental

Management Plan subject to approval from the Auckland Regional Council coastal

3 The Sediment Control protection Area is defined as 100m either side of a foredune or 100m landward of

the coastal marine area (whatever is the more landward of mean high water springs); or 50m landward of the

edge of a watercourse or wetland of 1000 m2 or more.




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Draft NZTA Standard

Adoption of the Draft NZTA standard would have not changed the regulatory environment in which

the project occurred. As such the required resource consents and associated consent conditions

would not have altered significantly.

3.2.2 Phasing

Existing Design

Earthworks were not phased during the construction of the Greenhithe Deviation. The Resource

Consents for earthworks did not impose any conditions that restricted the exposed earthwork area

but did require submission of an “Earthworks Staging Plan” and “Flocculation Management Plan” for

ARC approval.

The total volume of earthworks needed to build the highway was more than 1 Million cubic metres

and consequently earthworks were completed over several earthworks seasons. Earthworks in

some areas (e.g. Albany / Greenhithe / Tauhinu Interchanges) had to be staged with associated

traffic diversions, to allow construction of culverts / retaining walls, and progressive cutting or filling

of earthworks.

Relocation of ESCP controls were required to accommodate the earthworks as the road

embankments were constructed.

Draft NZTA Standard

While some discussion on construction sequencing and staging is given in the Draft NZTA Standard

(in particular Sections 4.6 and 7.2.3) no direct requirements are given.

Under the Draft NZTA Standard the requirements for staging of works and maximum exposed areas

would have been left to the discretion of the consenting authority, as was the case on the SH18

Greenhithe Deviation project via, consent conditions.

3.2.3 Winter Works

Existing Design

Construction continued on a reduced basis over the winter periods but was generally limited by

access to the specific sites being worked on. Earthworks were completed on a limited extended

time basis outside the ‘normal’ October to April earthworks season. No particular ‘additional

practises’ were undertaken apart from mandatory use of mulch on exposed areas for surface

stabilisation during these extension periods.

Generally site shutdown of earthworks areas was limited to the construction of diversion channels /

contour drains to ensure run-off was directed to ponds / decanting bunds, and all exposed




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earthwork areas were grassed / hydroseeded.

“Late finish” areas were generally mulched as the primary cover, and grass seed was added in the

mulch spraying operation, although germination was limited.

Draft NZTA Standard

The approval for winter works comes under the jurisdiction of the consenting authority (Auckland

Council). The adoption of the Draft NZTA Standard would therefore have had no impact on the

approval of winter works or the limitations/procedures applied on site.

3.2.4 Conversion of Temporary to Permanent Facilities

Existing Design

Permanent stormwater controls were not specifically designed for use as temporary controls;

However the Contractor took advantage of the size and location of some permanent stormwater

ponds in the latter stages of the earthworks to use them as temporary sediment ponds, before

converting them to permanent ponds.

The regional council typically discourages this practice as it is difficult to remove all of the sediment

from sediment retention ponds and it often causes a disjoint between the measures required during

construction to those required for the operation of the highway.

Draft NZTA Standard

The Draft NZTA Standard does not contain any guidance for the conversion of temporary practices

to permanent stormwater management devices. As such it is anticipated that the adoption of the

Draft NZTA Standard would have no bearing on this aspect of the design.

3.2.5 Dewatering, Sediment Removal and Disposal

Existing Design

Sediment retention ponds were the primary mechanism used to limit the release of sediment from

the site. Most of the temporary sediment ponds constructed were flocculated to increase sediment

removal efficiency. Sediment from de-commissioned ponds was disposed of in unsuitable earthwork

disposal areas on site; none was removed from site to other disposal areas / dumps.

The secondary temporary controls used on the project were decanting earth bunds in small / difficult

areas. Diversion bunds, channels and silt fences were used on site perimeters as necessary.

TP90 recommends that sediment retention ponds should be cleared of sediment once 20% of the

ponds storage volume has been filled with sediment. Silt fences were cleared once sediment

accumulation reached 50% of the height of the fabric.




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Draft NZTA Standard

The basic procedures recommended in the Draft NZTA Standard for dewatering, removal and

disposal of sediment from storage practices are essentially in line with the guidance given in ARC

TP90.

No significant change in procedures or equipment used would be anticipated had the Draft NZTA

Standard been applied to the SH18 Greenhithe Deviation project.

The NZTA guidelines recommend that sediment retention ponds are cleaned of accumulated

sediment once 20% of the ponds storage volume has been filled with sediment. This is identical to

the guidelines outlined in TP90. The NZTA guidelines recommend that silt fences are cleared of

sediment more frequently than TP90. The NZTA standards recommend that sediment is cleared

from silt fences once accumulated sediment reaches 20% of the fabric height, or when the silt

fences start to bulge.

3.2.6 Decommissioning

Existing Design

The Contractor was required to obtain approval from ARC to commence de-commissioning of

temporary ESC works in any ESCP area, particularly temporary sediment ponds. Ponds were

generally de-watered by pumping out with a floating intake to ensure only the cleanest surface

water was removed.

Sediment was excavated and carted to unsuitable disposal areas, and mixed / spread in the

disposal area within the motorway designation. Other storage controls such as decanting earth

bunds were de-commissioned in a similar manner. Adequate stabilisation by permanent or

temporary grass / hydroseed / mulch of areas controlled by silt fences was a prerequisite to removal

of such controls.

Draft NZTA Standard

The Draft NZTA Standard contains explicit guidance on decommissioning procedures for the

various erosion and sediment control practices (e.g. Section 6 of the Field Guide Document). No

such guidance is given in ARC TP90, but approval to decommission Erosion and Sediment Controls

was generally a condition of the earthworks consents issued by ARC.

The inclusion of the relevant decommissioning procedures within the Draft NZTA Standard provides

the contractor with a single point of reference when decommissioning individual and site wide

erosion and sediment control measures. This would ensure consistency between state highway

construction projects.

However, in general, the guidance given in the Draft NZTA Standard is in accordance with industry

best practice and it is likely that the exact procedures followed would have been similar had the




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Draft NZTA Standard been used as the main guidance document. In addition the same levels of

approval and control would still have been required by the consenting authority.

3.3 Erosion and Sediment Control Practices

3.3.1 Erosion Control

Existing Design

• Clean water diversion drains upslope of earthwork areas installed to prevent off-site runoff from

flowing onto the disturbed area and to minimise the contributing catchment areas;

• Minimised on site flow velocities by the use of contour drains;

Draft NZTA Standard

The Draft NZTA Standard design methodologies and procedures for the erosion control measures

used on the SH18 Greenhithe Deviation project do not differ significantly from those given in ARC

TP90 to which the erosion controls were designed. Accordingly it is anticipated that there would be

no significant change to the design or implementation of the erosion control measures used.

The Draft NZTA Standard contains some minor differences in fertiliser types/application rates for

temporary/permanent grassing and improved guidance on hydro-seeding and mulching procedures

over that given in ARC TP90. However, these differences would have had only minor effects, if any,

on the methods, techniques and materials used on the site.

3.3.2 Sediment Control

Existing Design

• A specimen erosion and sediment control plan was developed to gain resource consent. These

plans proposed that 30 sediment control ponds be formed to limit the discharge of sediment to

the receiving environment.

Once the contract was awarded, Works Infrastructure developed an Environmental

Management plan, and a new set of erosion and sediment control plans that addressed their

works methodology. These plans proposed 51 silt ponds, many of which needed to be

reformed or relocated as construction of the road embankment proceeded. The sediment

retention ponds typically ranged in size from 100 m3 to 1,500 m

3.

For the purpose of this assessment the specimen design ESCP has been used as the base

case for comparison with the draft NZTA ESC guidelines.

• Silt fences were installed at the base of fill slopes and around watercourses in areas where

work was located in close proximity to watercourses. Silt fences were installed to provide

additional treatment rather than as sole treatment devices. In total, approximately 11,000 m of

silt fences were installed including 1,200 m of super silt fence (steel mesh reinforced).




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• Works in or adjacent to watercourses, such as the installation of culverts, were considered high

risk activities and particular attention was paid to erosion and sediment control in these areas to

meet consent conditions.

• Chemical flocculation was required by the Auckland Regional Council to improve the

effectiveness of the sediment retention pond and to help reduce damage to the receiving

environment. Chemical flocculation is not a requirement of TP90 but is often imposed as a

condition of consent for large earthworks sites that have sensitive receiving environments.

Draft NZTA Standard

Sediment Retention Ponds and Decanting Earth Bunds;

The location and layout of sediment retention pond and decants may have varied significantly in

topographically difficult areas if they had been designed based on the NZTA ESC guidelines instead

of TP90.

The NZTA guideline prescribes a more conservative design scenario than TP90 as it takes the

sensitivity of the receiving environment into consideration. Ponds and decants located east of the

Tauhinu Interchange discharge to the Lucas Creek Estuary and consequently would require about

80% more storage than the equivalent devices based on TP90 design standard.

Sediment retention ponds located west of the Tauhinu Interchange discharge directly to the Upper

Waitemata Harbour. A lower design standard can be adopted for these devices because the

receiving environment is considered to be less sensitive. The ponds location west of Tauhinu Road

would be about the same size as a TP90 compliant device.

In areas where the designation is narrow, or where the topography is steep, the increased storage

needed under the NZTA standards would have to be resolved by;

• Increasing the width of the designation to accommodate the larger sediment retention ponds.

• Additional ponds would have to be constructed to limit encroachment past the designation

• Use of temporary retaining to support steeper temporary pond batters

The designation was hugely constrained by bush preservation requirements and so could not have

been extended. Temporary works had to be built within the footprint of the road embankment in

many locations forcing the contractor to work around them and rebuild the ponds as the road

embankment was raised. Making ponds 80% larger would have a huge imposition to their work at a

big cost.

Each additional pond requires a level spreader, outlet manhole, decants, and a stabilised outfall and

so the cost of additional ponds would have been very significant.

Silt Fences and Super Silt Fences;

The design of silt and super silt fences are generally identical to the requirements outlined in TP90,

and consequently adoption of the NZTA standards would have not impacted on the size or location

of silt fences.

The NZTA standards require a higher level of maintenance than prescribed in TP90. The cleanout

trigger for silt fences under the Draft NZTA Standard is when accumulated sediment reaches 20%

of the fabric height versus 50% of the fabric height in ARC TP90. The increased maintenance




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frequency is considered good practice.

Chemical Flocculation

Chemical flocculation is discussed in the NZTA guidelines, but is not a requirement. Consequently

the regional requirements would be more demanding that the NZTA guidelines in this case.

3.3.3 Non-sediment Contaminants

Existing Design

The contractors environmental management plan (CEMP) addresses the following non sediment

related contaminants;

• Lime Stabilisation

• Fuel and Oil Spills

• Construction Material / Chemical Loss

• Litter Control

The CEMP outlines procedures for handling, storage, monitoring and recording procedures of these

potential non-sediment contaminates.

Draft NZTA Standard

Section 10 of the Draft NZTA Standard outlines the potential non-sediment contaminants on state

highway construction projects. This section also outlines the specific measures to be taken to

prevent discharge of non-sediment contaminants from the site.

Generally, the contaminants discussed in the Draft NZTA Standard and the associated control

measures do not differ significantly from those outlined in the SH18 Greenhithe Deviation project

EMP. Accordingly, the management methods relating to non-sediment contaminants would still be

appropriate if the project was designed under the Draft NZTA Standard.

However it is worth noting that the inclusion of the contaminants that require consideration and the

defined management protocols within the Draft NZTA Standard may have streamlined the

preparation of the project EMP.

3.4 Cost

3.4.1 Resource Consents

Existing Design

The resource consents for the Greenhithe Deviation were combined with those of the wider Upper

Harbour Corridor Project including the Upper Harbour Bridge and Causeway, and the Hobsonville

Deviation section (including SH18 and SH16 extension). As such, the costs for consent preparation

and lodgement of the individual project sections are unavailable.




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Draft NZTA Standard

As the adoption of the Draft NZTA Standard would not have changed the consenting process, it is

anticipated that it would have had no impact on the costs of resource consents.

3.4.2 Final Design

Existing Design

The final detailed design was undertaken by Opus and Maunsell consultants (Maunsell have

recently changed their name to Aecom).

It is not clear from the available data whether this figure included design of the Erosion and

Sediment Control measures for the project or if so, what proportion of this cost can be attributed to

Erosion and Sediment Control.

Draft NZTA Standard

As the design of the various practices would not have varied significantly under the Draft NZTA

Standard, it is expected that there would have been no significant change to the overall cost of

designing the erosion and sediment control measures on the SH18 Greenhithe Deviation project.

However an increase in design costs would be likely as it is likely a complimentary design based on

TP90 would need to be developed to demonstrate that the NZTA standard is more conservative to

the consenting authority. In addition the site constraints on this project would have made it

incrementally more difficult to construct the larger sediment retention ponds. To address this issue a

larger number of small footprint devices would have been required and consequently the design

cost would have been higher.

3.4.3 Construction

Existing Design

The total cost of erosion and sediment control related project costs was $3.86 million.

This cost can be broken down as follows;

• Temporary Ponds ~ $970K;

• Flocculation stations ~$90K, plus floc chemical (PAC) ~$47K;

• Decanting Bunds ~$42K;

• Grassing / Hydroseeding / Mulching ~$525K;

• Diversion Channels / Flumes ~$1.18 million (incl. diversion channels associated with culverting

streams);

• Silt Fences / Hay Bale Dams ~$195K.

• CEMP & ESCPs plus management / monitoring / reporting ~$135K.

• Maintenance of controls ~ $475K.




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• Maintenance / monitoring / testing / reporting of Floc Dosing ~200K.

Draft NZTA Standard

The draft NZTA standard would affect the following ESC items that were involved with construction

of the Greenhithe Deviation;

• Temporary Ponds

• Flocculation stations, and floc chemicals

• Decanting Bunds;

The construction cost of these components was $1.35 million.

The cost of the additional storage required to meet the NZTA guidelines would have likely doubled

the cost of devices in topographically constrained areas. In areas where there were less constraints

the cost of the sediment retention ponds / decants would likely have increased by about 40%.

Silt fences would need to be cleared of sediment more frequently based on the standards, but the

cost of completing this is expected to be minimal.

The total cost of erosion and sediment control based on the NZTA standards is estimated at

$4.9million. This represents an increased cost of about $750,000 or about 20% over a standard

TP90 compliant management strategy.

3.4.4 Proportion of Total Cost

Existing Design

The total cost of constructing the Greenhithe Deviation was $107 million

The cost of the erosion and sediment control for the project was $3.68 million

Consequently the cost of the erosion and sediment control was about 3.4% of the total project cost.

Draft NZTA Standard

The projected cost of the erosion and sediment control devices based on the NZTA standard is $4.6

million.

This represents about 4.2% of the total project cost.




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3.4.5 Monitoring Costs

Existing Design

The Auckland Regional Council construction monitoring costs for the SH18 Greenhithe Deviation

project were approximately $150,000. The ARC monitoring costs covered weekly site walkovers by

the ARC representative, and processing of the resource consents.

Maintenance, monitoring, testing and reporting of the flocculation devices associated with sediment

retention ponds cost approximately $200,000. Therefore the total monitoring cost including

inspections, testing, and reporting was $350,000.

Draft NZTA Standard

The Draft NZTA Standard requires, at a minimum, water quality monitoring of total suspended

solids, turbidity and pH (when chemical flocculation is used) of the discharge from sediment control

practices. The monitoring protocols are set out in Appendix A of the Draft Standard and reiterated in

the accompanying Field Guide for Contractors. No such requirements are contained in the ARC

consents where monitoring of Erosion and Sediment Control measures was limited to visual

qualitative monitoring only.

The cost of monitoring of controls would likely be greater than TP90 to meet the new NZTA

standards. The draft standard requires monitoring of total suspended solids, turbidity, and pH.

Testing of water quality was very limited during construction of the Greenhithe Deviation and was

basically confined to the measurement of the pH of flocculated ponds and the watercourses directly

downstream. Consequently it is expected that a significant increase in monitoring costs would be

required to comply with the draft guidelines. As the Draft NZTA Standard requires more stringent

water quality monitoring than the ARC resource consents. It is estimated that adopting the NZTA

standards would increase the water quality monitoring costs from $200k to approximately $400k.

The new Auckland Council construction monitoring costs may have increased marginally if the

design was based on the NZTA standard. Additional ponds may have been required to be

constructed to accommodate the higher storage requirements outlined in NZTA’s draft guideline.

The time required by Council representative to inspect and approve the additional ponds would be

longer and consequently a higher monitoring cost would be likely.

It is suggested that monitoring costs could be reduced by using lab testing to relate total suspended

solids (TSS) to turbidity of a sediment laden stormwater at the start of a project. Turbidity and pH

can be measured by field equipment, but TSS requires laboratory analysis. This approach would

minimise the amount of lab testing that would be required on a project and consequently reduce

monitoring related costs.




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3.5 Time

3.5.1 Resource Consents

Existing Design

Resource consent applications were lodged in July 2001 with the consents being granted in

February 2002. The consenting period of approximately 7 months was due to the scale and

complexity of the project.

Draft NZTA Standard

The processing of resource consents and the approval of the Site Specific Erosion and Sediment

Control Plans may have taken slightly longer had the Erosion and Sediment Control measures been

designed using the Draft NZTA Standard. This would be mainly due to the fact that the consenting

authority is familiar with the current design guidelines used (ARC TP90) but not with the Draft NZTA

Standard.

The Auckland Council would need to be satisfied that the proposed measures were at least

equivalent to those designed under ARC TP90. Consequently an alternative design based on the

TP90 may be necessary to demonstrate that the NZTA compliant erosion and sediment control plan

meets the requirements of the regional authority. As the analysis is relatively straight forward the

additional time to prepare the TP90 compliant calculations would not be significant.

3.5.2 Final Design

Existing Design

The initial professional services contract for detailed design of the project was awarded to Opus. It

took 3 years to develop the construction plans from the preliminary design. All stormwater related

design elements (including Erosion and Sediment Control) were undertaken concurrently with the

other design disciplines during this period.

The cost of designing the erosion and sediment control devices on the Greenhithe Deviation was

$312,000.

Draft NZTA Standard

The design methodology for storage practices given in the Draft NZTA Standard is slightly more

involved than that used by TP90. In addition a second design based on TP90 may be required to

demonstrate compliance with the regional design standards.

Developing a TP90 design for comparison to the NZTA standard would not have taken a significant

amount of time to develop as the analysis is relatively simple.




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3.5.3 Construction

Existing Design

Temporary ponds were generally constructed within 2-4 days, with a further day for set-up and

commissioning of flocculation stations. The time taken to construct the sediment retention ponds

was mostly dependent on the size and location of the devices.

Other controls such as decanting bunds and silt fences were generally constructed within 1 day,

and diversion bunds / channels were dependent on the length, but again could usually be

completed within 1-2 days.

Construction of the Greenhithe Deviation was commenced in September 2004 and was opened in

February 2008. Enabling works were completed during the 2003-2004 earthworks season before

the main contract was commenced. Enabling works focussed on stabilising the foundations of the

Ashby Place fill embankment.

Draft NZTA Standard

The number and size of sediment retention ponds would have been significantly larger if the draft

NZTA standards had been adopted instead of the regional requirements, but it would have had little

impact on the overall contract period.

The guideline requires a 100 year ARI design standard to be adopted. This equates to an 80%

increase over what in the storage needed in decanting earth bunds and sediment retention ponds. It

would have taken the contractor longer to install the erosion and sediment control devices as they

would need to be bigger, and a larger number of them would have been required to confine their

footprint within the designation. This would have been difficult to achieve.

Irrespective of the size of the ESC devices adoption of the draft NZTA standard would not have had

a major impact on the duration of the project in general as the time taken to install the erosion and

sediment control facilities is relatively insignificant compared to the overall contract period.

3.5.4 Monitoring

Existing Design

The site was monitored by the ARC consultants on a weekly basis during the construction phase,

particularly for the purpose of assessing the ARC Compliance Score for the Site

Water quality testing was limited to the measurement of pH in flocculated sediment retention ponds

and the watercourses directly downstream from them.

Draft NZTA Standard

The draft NZTA ESC standard requires measurement of pH, TSS, and turbidity (section 7.3). This

testing needs to be undertaken regardless of the regional requirements. Subsequently adoption of

the Draft NZTA Standard would not have significantly altered the conditions of the resource

consent, and therefore minimal impact on the overall project timeframe.




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4 General Comments and Discussion

4.1 Design/Construction Issues

There were no significant problems with approvals of ESCPs by ARC, or approvals for de-

commissioning ESC works.

There were several cases of discharges to the receiving environment during construction

that required the Contractor to undertake remedial work.

Some time and discussion was required with ARC to reach agreement on the Flocculation

Management Plan, however there was no delay to construction during this period.

4.2 Project Completion

Project was completed late, after allowing for Time Extensions granted, and approx 30%

(excluding Escalation) above the original Tender Price, although variations and changes of

scope account for most of this increase. ESC works were the subject of variation claims for

the extended contract period.

Flocculation work was a Provisional Sum of $40,000 in the original Contract, but Final

contract cost was approx $340,000. Approx $200,000 of the $340,000 amount was for

personnel time involved in Maintenance and Monitoring of the temporary flocculant dosing

stations, and sampling and testing of the pond discharges.

Some delays to the Contract were due to failure of the Contractor’s ESC facilities/practices,

which required re-building. Generally the effect of the above was insignificant to the overall

construction time-frame, but had some effect on the project cost.

4.3 General Comments

• The NZTA Draft requires a baseline measurement of pH, turbidity, and TSS is

recommended prior to commencing construction. This should allow NZTA and the regional

consenting authority to assess the impact of the works on the receiving environment. In

some cases existing stream conditions may not meet the regional requirements irrespective

of the impact of construction works.

• Further guidance on the classification of receiving bodies is recommended. For example

the upper limit of many harbours could be considered estuaries due to their depositional

nature; However there is a massive difference between the ARC and NZTA Draft in the

relevant design requirements (i.e. 5 year ARI design requirement for harbours versus a 100

year ARI design requirement for estuaries)




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Appendix A:

Design Erosion and Sediment Control Plans




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Appendix B: Design Information

NZTA Erosion and Sediment Control Guidelines

Test Drive of the Draft Erosion and Sediment Control Standards for State Highway Infrastructure

Upper Harbour Corridor Motorway; Greenhithe Section

Comparison between the TP90 specimen design, and an equivalent system that has been designed to meet the draft NZTA erosion and sediment control guidelines

1/02/2011

Sediment Retention

Pond IdentifierLocation Receiving Environment

Design Criteria

(NZTA Standards)

Catchment

Area

(Ha)

Required

Volume

(m3)

Live Storage

(m3)

Decant Rate

(L/s)

Number of

10mm Ø

holes

Inflow Rate

Q (m3/s)

Required

Volume

(m3)

Live Storage

(m3)

Decant Rate

(L/s)

Number of

10mm Ø

holes

Required

Volume

(m3)

% DifferenceDecant Rate

(L/s)% Difference

1(a) Sta 1400m - 2300m Lucas Estuary 100 year ARI Storm 2.8 840 588 8 373 0.426 1533 1073 12 552 693 82% 4.0 48%

1(b) Sta 1400m - 2300m Lucas Estuary 100 year ARI Storm 1.5 450 315 5 200 0.228 821 575 7 296 371 82% 2.2 48%

1(c ) Sta 1400m - 2300m Lucas Estuary 100 year ARI Storm 0.7 210 147 2 93 0.106 383 268 3 138 173 82% 1.0 48%

A1 Sta 2300m - 2820m Lucas Estuary 100 year ARI Storm 6.8 2040 1428 20 907 1.034 3723 2606 30 1341 1683 82% 9.8 48%

A1(a) Sta 2820m - 3000m Lucas Estuary 100 year ARI Storm 1.6 480 336 5 213 0.243 876 613 7 315 396 82% 2.3 48%




2(c ) Sta 3700m - 4160m Lucas Estuary 100 year ARI Storm 8.8 2640 1848 26 1173 1.338 4818 3373 39 1735 2178 82% 12.6 48%

A3(a) Sta 4160m - 4460m Lucas Estuary 100 year ARI Storm 5.3 1590 1113 16 707 0.806 2902 2031 24 1045 1312 82% 7.6 48%

A3(b) Sta 4460m - 4640m Lucas Estuary 100 year ARI Storm 2.3 690 483 7 307 0.350 1259 881 10 453 569 82% 3.3 48%




A5 Sta 6150m - 6700m Waitemata Harbour 5 year ARI Storm 2.9 870 609 9 387 0.248 892 624 7 321 22 2% -1.5 -17%

Design Parameters

TP90

Pond Sizing Criteria 300 m3/ha of catchment area (3% Pond)

Design Decant Rate 3 L/s/ha

Draft NZTA Standard

Rainfall intensity (100 year ARI storm) 73 mm/hr (100yr-1hr storm NSCC intensity, duration, frequency curve)

Rainfall intensity (5 year ARI storm) 41 mm/hr (5yr-1hr storm NSCC intensity, duration, frequency curve)

C Factor 0.75 Clay >10% slope (refer Table 6.3)

Design Decant Rate 0.0225 L/s/10mm dia hole

Draft NZTA Standard DifferenceExisting Design

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