SH18 Greenhithe Deviation - NZ Transport Agency · Harbour Bridge in Hobsonville. Starting at the...
Transcript of 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
Erosion and Sediment Control
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
SH18 Greenhithe Deviation
Erosion and Sediment Control
Comparison with NZTA Draft Erosion & Sediment Control Standard Requirements
1-T0063.00/01AE
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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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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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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Environmental Factor Discussion
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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Environmental Factor Discussion
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%
A2 Sta 3000m - 3220m Lucas Estuary 100 year ARI Storm 1.9 570 399 6 253 0.289 1040 728 8 375 470 82% 2.7 48%
2(a) Sta 3220m - 3700m Lucas Estuary 100 year ARI Storm 4.2 1260 882 13 560 0.639 2299 1610 19 828 1039 82% 6.0 48%
2(b) Sta 3400m - 3700m Lucas Estuary 100 year ARI Storm 1.8 540 378 5 240 0.274 985 690 8 355 445 82% 2.6 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%
A4 Sta 4680m - 5500m Lucas Estuary 100 year ARI Storm 6.8 2040 1428 20 907 1.034 3723 2606 30 1341 1683 82% 9.8 48%
4(a) Sta 5500m - 5850m Lucas Estuary 100 year ARI Storm 2.4 720 504 7 320 0.365 1314 920 11 473 594 82% 3.4 48%
4(b) Sta 5850m - 6150m Lucas Estuary 100 year ARI Storm 2.7 810 567 8 360 0.411 1478 1035 12 532 668 82% 3.9 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