SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge

SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Stormwater Management Report

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge NZ Transport Agency Contents 1. Executive Summary... 1 2. General Project Description... 3 3. Introduction - Stormwater... 4 4. Background... 5 4.1 Related Documents... 5 4.2 Flood Risk... 6 4.3 Land Use... 6 4.4 Soils / Geology... 6 4.5 Hydraulic Modelling... 7 4.6 Waterway Classification... 8 4.7 Catchment Run-off Models... 8 4.8 Consents... 8 5. Design Principles... 9 5.1 Stormwater Treatment Philosophy... 9 5.2 Construction Effects... 10 6. Stormwater System Design Criteria... 11 6.1 Motorway Surface... 11 6.2 Culverts... 11 6.3 Open Channels... 12 7. Stormwater Treatment... 13 7.1 Treatment Devices... 13 7.2 Permanent Water Quality Management... 14 7.3 Specific Device Details... 15 Status 09 Final (Rev 9) Page ii October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 8. Maintenance Issues... 22 8.1 Durability... 22 8.2 Erosion Protection... 23 9. Conclusions... 24 Appendices Appendix A Stormwater Calculations (Water Quality) Appendix B Cartridge Filter Information Drawings (included in Volume 4 Plans and Drawings) PD - 2 08 11 Rev D Preliminary Stormwater Design Plan Sheet 1 of 3 PD - 2 08 12 Rev D Preliminary Stormwater Design Plan Sheet 2 of 3 PD - 2 08 13 Rev E Preliminary Stormwater Design Plan Sheet 3 of 3 PD - 2 08 14 Rev A Preliminary Stormwater Design Existing and Proposed Catchments Adjacent to Wetland D PD - 2 08 15 Rev A Preliminary Stormwater Design Wetland D Layout Plan PD - 2 08 16 Rev A Preliminary Stormwater Design Wetland D (West) Long Section and Typical Section PD - 2 08 17 Rev A Preliminary Stormwater Design Wetland D (East) Long Section and Typical Section PD - 2 08 18 Rev A Preliminary Stormwater Design Wetland E Layout Plan PD - 2 08 19 Rev A Preliminary Stormwater Design Wetland E Long Section and Typical Section PD - 2 08 20 Rev A Preliminary Stormwater Design Outfall K Long Section PD - 2 08 21 Rev A Preliminary Stormwater Design Outfall Device A and B Long Sections PD - 2 08 22 Rev A Preliminary Stormwater Design Existing 900mm Outfall to CMA Long Section Status 09 Final (Rev 9) Page iii October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Glossary AEE Assessment of Environmental Effects ARC Auckland Regional Council BPO Best Practical Option CEMP Contractor s Environmental Management Plan CMA Coastal Marine Area COPTTM Code of Practice for Temporary Traffic Management CPA1 Coastal Protection Area 1 CPA2 Coastal Protection Area 2 EMG Environmental Monitoring Guidelines MHWS Mean High Water Springs NOR Notice of Requirement SH16 State Highway 16 SHGDM State Highway Geometric Design Manual TP10 Auckland Regional Council Technical Publication #10; 2003 Stormwater Management Devices: Design Guidelines Manual TP108 Auckland Regional Council Technical Publication #108; April 1999; Guidelines for stormwater run-off modelling in the Auckland Region. TP90 Auckland Regional Council Technical Publication No.90; March 1999; Erosion and Sediment Control Guidelines for Land Disturbing Activities in the Auckland Region WCC Waitakere City Council WRR Western Ring Route Status 09 Final (Rev 9) Page iv October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 1. Executive Summary The proposed SH16 improvement project area includes works to SH16 between Waterview and Westgate, of which this section between Henderson Creek and Huruhuru Road Bridge forms a separable portion of works. The project is located within Waitakere City. The Project encompasses approximately 2.0km of motorway carriageway widening and enhancement, including widening of the Henderson Creek Bridges, replacement and duplication of the existing Lincoln Bridges, new interchange ramps, realignment of Selwood Road and replacement of the Selwood Road Bridge 1, and the construction of a shared use path adjacent to the motorway carriageway. The works could have a range of short and long term stormwater management related effects on the receiving environment. Best practicable measures have been adopted in this assessment in order to avoid, remedy or mitigate any resultant adverse environmental effects. The following are the principal conclusions of the stormwater assessment: The project results in a total impermeable pavement surface area of approximately 11.1ha, which represents 5.3ha of new impermeable area added to the existing motorway of 5.8ha. The general principles to provide the Best Practicable Option (BPO) approach in order to achieve sustainable long term stormwater management have been followed. Stormwater management and treatment will be consistent with Auckland Regional Council s Technical Publication #10; Stormwater Management Devices: Design Guideline Manual (TP10). The stormwater from existing pavement areas currently receives no active treatment. It is proposed that stormwater from 91% of existing pavement areas will be treated. It is proposed to treat 100% of the stormwater from the new road pavement areas created by the project. This will equate to more than 95% of the stormwater from all pavement areas within the project area being treated. Stormwater treatment devices will be sized to treat the run-off from the impermeable road pavement surfaces. The principles of TP10 have been used to undertake this assessment, however the drainage design and treatment devices will be further assessed during the detailed design phase. Preliminary design has determined that the treatment devices deemed most suitable for use include wetlands, proprietary filter cartridges and grassed swales. Treatment devices will be sized to achieve a 75% TSS removal efficiency for stormwater entering the treatment devices. As the receiving environment is in close proximity to coastal receiving waters, attenuation is not considered necessary therefore devices will treat for water quality only. 1 Selwood Road Bridge replacement works subject to separate ARC resource consent approval and Outline Plan to WCC. Status 09 Final (Rev 9) Page 1 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge An allowance for climate change has been made in the design of the stormwater collection, conveyance and treatment networks. In the context of stormwater management, an overall positive environmental outcome will be achieved by the proposed stormwater treatment. Status 09 Final (Rev 9) Page 2 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 2. General Project Description The upgrade of the existing motorway between Henderson Creek and Huruhuru Road Bridge forms part of the Western Ring Route, one of the NZ Transport Agency s (NZTA) Roads of National Significance (RoNS). The Project encompasses approximately 2.0km of carriageway widening and enhancement, including improvements to the Lincoln Road Interchange, the replacement Lincoln Bridges (crossing Huruhuru Creek), and, strengthening/widening to Henderson Creek Bridges. A location plan (PD-20210) and general arrangement drawings (PD-20211 to PD-20213) are included in Volume 4 Plans and Drawings. The existing motorway cross section comprises of two general purpose 3.5m wide traffic lanes in each direction, and a grassed median dividing the two carriageways. The improvement works include widening the motorway to provide three general-purpose 3.5m wide traffic lanes and a 3.5m bus shoulder in each direction, with a 3.0m wide shared use path adjacent to the westbound carriageway. The overall width of the motorway is expected to increase by approximately 16 to 18 metres. The estuarine area surrounding Henderson Creek Bridges and Lincoln Bridges (crossing Huruhuru Creek) are identified in the Auckland Regional Plan: Coastal (ARPC) as Coastal Protection Area 2 (CPA 2). Construction work is proposed within this area at both creeks. At Huruhuru Creek, this includes new bridge construction, ground improvements and reclamation to accommodate the new eastbound off-ramp and replacement of the existing motorway bridges. At Henderson Creek, this includes motorway bridge widening and strengthening works, ground improvements, and some associated reclamation at the abutments for both bridges. The existing Lincoln Road Interchange layout will be upgraded and improved to meet predicted future traffic growth demands, whilst improving the safe operation of the interchange. This interchange gives access to Henderson and Waitakere City town centre and is currently operating over capacity causing severe congestion in both the AM and PM peak travel times along SH16 and Lincoln Road. The upgrade of the Lincoln Road Interchange forms a significant component of the project with improvements to the westbound and eastbound on- and off-ramps. The improvements include relocating the existing eastbound on-ramp and removing the existing eastbound off-ramp loop to create a standard Diamond type interchange. The Selwood Road bridge replacement works (Selwood Road Bridge replacement works subject to separate ARC resource consent approval and Outline Plan with WCC) will increase the existing capacity across the motorway from two lanes to seven lanes, and improve the alignment of Selwood Road and the vertical clearances to the motorway. New signalised intersections on either side of the interchange will provide safer and more efficient movements between the motorway ramps and the local roads. Status 09 Final (Rev 9) Page 3 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 3. Introduction - Stormwater This report is prepared as supporting information for alterations to an existing designation and applications for resource consents to Auckland Regional Council (ARC) and Waitakere City Council (WCC), and for stormwater management consents for the proposed improvement works to the motorway and associated interchange between Henderson Creek and Huruhuru Road Bridge. Refer to the Location Plan (PD-20210) in Volume 4 Plans and Drawings. This report describes the general stormwater design principles and criteria adopted in the preliminary design, and provides indicative details of the proposed discharges and treatment devices. All stormwater details are subject to detailed design. The existing motorway surface run-off is currently collected through a combination of kerb and channel and swales / ditches, discharging to the nearest watercourse or culvert. This existing arrangement has developed over time on an ad-hoc basis as the motorway has been developed. In the existing situation, aside from any treatment and attenuation provided by the grass-lined swales/ditches, the surface run-off is discharged directly to the receiving environment. The stormwater network will be upgraded as part of the motorway improvements, and the proposed works will meet the general guidelines of TP10. The proximity of residential and commercial properties to the motorway corridor imposes significant constraints to the motorway improvements in general, and this includes the stormwater network. Therefore, a Best Practicable Option (BPO) approach has been adopted for the design of the stormwater network. Status 09 Final (Rev 9) Page 4 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 4. Background 4.1 Related Documents Design and evaluation of the Stormwater Drainage Management system for the motorway project will take account of the following standards and documents: Stormwater Management Devices: Design Guidelines Manual; ARC Technical Publication No.10 (TP10), Second Edition, 2003 Guidelines for Stormwater Run-off Modelling in the Auckland Region, ARC Technical Publication No.108 (TP108), April 1999 NZTA Stormwater Treatment Standards for State Highway Infrastructure 2010, AS/NZS 3725: Loads on Buried Concrete Pipes, 1989 Bridge Manual (SP/M/022) Second Edition, Transit New Zealand, June 2003 A Guide to Road Design Part 5: Drainage Design, AUSTROADS 2009 Highway Surface Drainage Design Guide for Highways with a Positive Collection System, NRB, November 1977 Waterway Design A Guide to the Hydraulic Design of Bridges, Culverts and Floodways, AUSTROADS, 1994 Integrated Catchment Management Plan in Waitakere City Council Riparian Zone Management, ARC Technical Publication No. 148, 2001 Climate Change Effects and Impacts Assessment A Guide Manual for Local Government in New Zealand, Ministry for the Environment, Second Edition, 2008 EcoWater Draft Stormwater Management Unit Comprehensive Catchment Management Plan URS - Comprehensive Catchment Management Plan Henderson Creek Stormwater Management Unit, Version 1 URS Central Park Drive Stormwater Management Unit Comprehensive Catchment Management Plan, Version 1.0, June 2003 URS Massey Stormwater Management Unit Comprehensive Catchment Management Plan, Version 1.0, March 2001 URS Massey East Stormwater Management Unit Comprehensive Catchment Management Plan, Version 1.0, March 2001. Status 09 Final (Rev 9) Page 5 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 4.2 Flood Risk There are no known current issues with flooding along this section of the motorway carriageway. The additional impermeable surface is not considered to have an impact on flooding as the surface run-off from the motorway is proposed to be discharged, following treatment where applicable, directly into the adjacent coastal marine environment. As the receiving environment is in close proximity to coastal receiving waters, attenuation is not considered necessary. 4.3 Land Use The existing motorway is located in an urban environment with established residential and commercial development immediately adjacent to the motorway. These catchments have a high factor of impermeable run-off which is generally directed north under the motorway through culverts or into adjacent creeks / waterways. The run-off from these catchments is generally conveyed to its discharge point as a separate network to the motorway run-off. The proposed stormwater system associated with the project will maintain this arrangement and will generally only collect run-off from the motorway. Existing culverts crossing the motorway will be extended and the existing diameters maintained where possible. Standard headwalls will be provided where space permits or replaced with manholes/chambers should they require location under the carriageway. This is discussed in greater detail in Section 6. Stormwater management and treatment of these catchments outside the motorway are not within the scope of this project. As discussed further in Section 7.3.4, there is a land-locked low area of privately owned land around CH7085 with no defined overland flowpath that has been observed to have a shallow water table, with an underlying peat soil type. Catchment modelling indicates that the existing 900mm dia culvert (Culvert C) is unable to accommodate all flows from the contributing catchments. As part of the widening works it is proposed to provide a new culvert (Culvert K) in order to accommodate the 100 year flows from the contributing catchments. These flows will be piped eastwards via Culvert K (1800mm dia max), which will be located under the westbound bus shoulder, and discharge via an outfall on the southwestern side of the Henderson Creek bridge. The proposed works will, on a BPO basis, collect and treat the existing motorway run-off in addition to the new run-off where practicable, as discussed in Section 5.1. 4.4 Soils / Geology Two main geological units appear within the project area: The underlying geological unit appears to comprise of Waitemata Group weathered mudstone and sandstone comprising primarily clay and silt. This material outcrops along the motorway alignment from Henderson Creek to Lincoln Road. Status 09 Final (Rev 9) Page 6 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge West of Lincoln Road along the motorway alignment, to Huruhuru Road Bridge, the Waitemata Group appears to be overlain by Tauranga Group Alluvium. This material consists of firm to stiff silty clays and clayey silts. The soil classification adopted for the purposes of TP108 calculations has been determined to be Group C. 4.5 Hydraulic Modelling Hydraulic modelling has been undertaken to simulate flood flows to provide the basis for design of the proposed stormwater management regime. Hydraulic models were developed using HEC-HMS 2 to identify the effects of the proposed bridge replacement works on stormwater discharge and to assess options for mitigation of those effects (refer calculations provided in Appendix A). The ARC Guidelines for Stormwater Runoff Modelling in the Auckland Region 1999, Technical Publication No. 108 (TP108) were used to determine inputs into the flood model. The rainfall data used for the calculation of the design stormwater flows was derived from the 24-hour rainfall depths provided in TP108. The rainfall depths derived from these maps were checked against the summary statistics for the NIWA Owairaka and Onehunga gauges, and the HIRDS databases. The TP108 rainfall data was factored by the recommended increment identified in Climate Change Effects and Impacts Assessment (MfE 2008), determined assuming a mean predicted temperature increase for the Auckland region of up to 2.1 C to 2090. This resulted in an increase in the 24-hour rainfall depth of 9% for 2 year ARI, 13% for 10year ARI and for the 17% for the 100 year ARI event. The following rainfall depths have been used to calculate catchment stormwater run-off for the project area (Refer Table 4.5-1). Table 4.5-1 Design 24-hour Rainfall Depths based on TP108 Adjusted for Climate Change to 2090 Description Rainfall Depth (mm) 2 year ARI 10 year ARI 100 year ARI WQ Event² EDV 1 Current Rainfall Depths 80 130 204 26.7 34.5 Rainfall Adjusted to 2090 For Climate Change 87 147 239 29 34.5 1. EDV = Extended Detention Volume. 2. WQ = Water Quality Event. 2 Hydraulic Engineering Centre Hydraulic Modelling System Status 09 Final (Rev 9) Page 7 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 4.6 Waterway Classification An Ecological Assessment (report reference BR-2-RPT-EC-001) has been undertaken to determine the extent of waterways within the project area. It has been confirmed that there are no streams affected by the proposed physical works and the existing culverts are all cross drainage. Therefore, the proposed works will not impact riparian vegetation and there is no need for fish passage to be included within the design. 4.7 Catchment Run-off Models The procedures outlined in TP108 will be applied to determine catchment run-off characteristics for the motorway. The majority of catchments outside the motorway extents are not within the scope of this project. The only instance where this is not the case is the upstream catchment to Wetland D. Although this is not to be treated the surface water run-off will drain partly to the existing culvert (Culvert C), and partly to the proposed culvert (Culvert K) discharging to the Henderson Creek (as discussed in Item 4.3 above). The modelling includes this upstream catchment area and consideration of the overflow from an existing stormwater pond located in Tony Street, adjacent to the catchment. Plans of the existing and proposed catchments adjacent to Wetland D are shown on drawing PD-20814 (Volume 4 Plans and Drawings ). 4.8 Consents All resource consents that shall be submitted for the Henderson Creek to Huruhuru Road Bridge project are described in the Assessment of Environmental Effects (Volume 2). Status 09 Final (Rev 9) Page 8 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 5. Design Principles The design principles are not intended to be a constraint to innovative design where such detailed design can achieve equivalent environmental outcomes. The approach for the stormwater management system design is to provide a BPO to avoid, remedy or mitigate adverse environmental effects and shall include the following principals: Maintain the natural flow regime, Protect ecological value, Provision of effective drainage, Achieve hydrologic neutrality and connectivity, Treatment to meet or exceed TP10 guidelines, Integrated approach to manage stormwater quality and detention issues, Ensure efficient and effective maintenance both from earthworks sites and road construction, Erosion and sediment control. 5.1 Stormwater Treatment Philosophy The quality of stormwater treatment is measured by the amount of Total Suspended Solids (TSS). The existing impermeable area (carriageway) within the project area is approximately 5.8 hectares. Upon completion of the project this area will increase by approximately 5.3 hectares, to 11.1 hectares. Currently stormwater from the existing impermeable areas does not receive any form of treatment or active TSS removal. It is proposed to treat 100% of the stormwater which is generated from the new impermeable areas (approx 5.3 ha) created by the project. In addition to this treatment, it is proposed that stormwater from 5.3 hectares of the existing 5.8 hectares (91%) of existing pavement areas will also be treated. This will equate to more than 95% of the total stormwater from all impermeable areas within the project area (being 10.6 ha of the total 11.1 ha) being treated. Given that the stormwater treatment facilities proposed are designed to remove 75% of the TSS from treated stormwater, the introduction of the stormwater treatment will provide a large improvement over the existing situation. Status 09 Final (Rev 9) Page 9 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Based on stormwater treatment of 95% of impermeable areas it has been calculated that the total TSS (kg/year) into the receiving environment from the project area will reduce from the existing estimate of approximately 4100kg/year, to approximately 1879kg/year, as shown in Table 5.2-1 below. These calculations are based on the estimated contaminant loading ranges for roads in TP10, utilising a previously accepted estimate for TSS of 708.93 kg per hectare per annum. Table 5.2-1: Indicative TSS removal Impermeable area to be treated ha TP10 TSS estimate for motorways kg/ha/yr Total TSS without treatment kg/yr Removal of TSS % Total TSS from project area kg/yr Existing 5.8 708.93 4112 0% 4112 Proposed 10.6 708.93 7515 75% 1879 5.2 Construction Effects A detailed Construction and Environmental Management Plan (CEMP) will be prepared prior to construction commencing to address matters including, but not limited to, temporary stormwater diversions, staging, removal/modification of existing reticulation, construction traffic, pedestrian access and safety. Status 09 Final (Rev 9) Page 10 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 6. Stormwater System Design Criteria 6.1 Motorway Surface Subject to NZTA requirements for the detail design the drainage design for the motorway surface may provide for the following: The run-off across the road surface shall meet NZTA standards for aquaplaning for vehicle safety. No stormwater from the uphill side of the carriageway shall be allowed to flow across the pavement of the opposing lanes of traffic. For design flood events the motorway shall remain serviceable by limiting the flooded extent of the carriageway. Provision for all overland flow paths will be made to enable safe discharge to the receiving environment of large storm events. Wherever possible, clean stormwater arising from outside of the paved area should be kept separate from contaminated road surface run-off. 6.2 Culverts Culvert works within the project includes extending existing culverts where affected by the motorway widening, as well as new culverts as required for reticulations. Each new culvert and culvert extension will be assessed individually to determine any practicable improvements that may mitigate existing flooding issues. The design of new culverts and culvert extensions shall be in accordance with the requirements of NZS/AS 3725 2007 Australian Standards Loads on Buried Concrete Pipes and TNZ F/3: 2000 TNZ Specification for Pipe Culvert Construction. The above issues will be assessed further during detailed design. A condition assessment of existing culverts will be undertaken to determine any serious or significant risk to the operation of the culvert. Pipe capacity has been calculated using the Colebrook White method, with a roughness factor of 0.6m. Pipes are sized to prevent flooding traffic lanes during the 100 year storm event. For indicative details of the stormwater catchment areas contributing run-off to the proposed motorway, refer to drawings PD-20811 to PD-20814 (Volume 4 Plans and Drawings ). These drawings also show the approximate location and extent of new and extended culvert works. Table 6.2-1 following shows the preliminary schedule of existing culverts and the proposed lengths. Status 09 Final (Rev 9) Page 11 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Table 6.2-1: Preliminary Schedule of Culverts Culvert ID Location (at alignment centreline) Existing Length (m) 1 Proposed Length (m) 2 Size Comments A CH 7085 53 78 Ø900mm Existing Culvert to be extended to suit proposed Motorway Works B CH 7110 35 30 Ø450mm Existing Culvert to be shortened and incorporated into Motorway reticulation. C CH 7120 99 105 Ø900mm Existing Culvert outside Motorway Works D CH 7360 30 66 Ø600mm Existing Culvert to be extended to suit proposed Motorway Works E CH 7360 (crossing westbound off-ramp) 18 45 Ø600mm Existing Culvert to be extended to suit proposed Motorway Works H CH 8150 30 47 Ø450mm Existing Culvert to be extended to suit proposed Road Works I CH 8285 41 48 Ø600mm Existing Culvert to be extended to suit proposed Motorway Works K CH 6615 CH6915 (under motorway shoulder) n/a 300 Ø1800mm max Proposed outfall culvert to discharge existing u/s catchment to Wetland D and bypass flows from motorway 1 Existing culvert lengths are based on as-built information and some survey, and will be confirmed by detailed survey. 2 Both existing and proposed lengths of culverts are measured from toe of the earthworks on either side of the motorway. 6.3 Open Channels The design criteria of open channels used for cleanwater diversion and conveyance shall be generally in accordance with TP10 s for swales, and traversable where necessary in accordance with design standards as per the following requirements: Subsoil drains will be provided unless the permeability of the in-situ soils is shown to be sufficient that saturation of the ground would not occur. Check-dams and or erosion protection shall be provided where the flow velocity may be considered to cause erosion. This will be assessed against the appropriate standards and guidelines. Status 09 Final (Rev 9) Page 12 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 7. Stormwater Treatment The design approach for this project is to select the BPO for stormwater treatment to address the direct effects of the motorway extension but also to consider providing betterment by treating the run-off from the existing pavement. The areas from which run-off may be treated have been identified as: Existing motorway pavement surfaces, Proposed motorway pavement surfaces, Ancillary pavement areas on local roads affected by the works or which are part of the motorway catchment, Cut batter slopes from which run-off will be managed in conjunction with pavement run-off. Treatment devices will be designed in accordance with TP10 and / or approved proprietary devices may be used. 7.1 Treatment Devices Stormwater treatment devices will be provided in line with the guidelines of TP10. A BPO approach has been adopted and the design principles for the treatment devices include the following: Wetlands will generally be adopted should available space and topography allow. Wetlands are ARC s preferred treatment device (refer TP10), Proprietary treatment cartridge filters will be used where other treatment devices cannot be used due to incompatible levels or available space, As the receiving environment lies in close proximity to coastal receiving waters attenuation is not required. Treatment devices will be designed and sized to treat run-off for the design stormwater quality (WQ) event, being one third of the two-year event, and not extreme flows. Flows in excess of the Water Quality (WQ) event shall bypass the treatment device through a high flow by-pass, or overflow diversion and outfall directly to the coastal receiving waters. This will avoid flooding of treatment devices by rain storms exceeding the design criteria, and also helps to maintain the natural flow regime. Status 09 Final (Rev 9) Page 13 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 7.2 Permanent Water Quality Management 7.2.1 General The stormwater management approach will be generally consistent with TP10. The full extent of stormwater works required will be determined during detailed design. Proposed devices will be designed to meet or exceed the stipulated treatment target of 75% TSS removal efficiency required by TP10. Preferred management solutions that are considered suitable for stormwater quality control include wetlands, ponds, filters and swales. The preliminary Proposed Treatment Devices are discussed further in Section 7.3, and summarised in Table 7.3-1. 7.2.2 Stormwater Management Hierarchy The general philosophy for the stormwater management is to utilise a treatment train approach. The idea of the treatment train is to acknowledge the fact that individual stormwater management tools are unlikely to achieve all of the stormwater management objectives for any given site. Several devices may be needed to achieve multiple objectives such as stormwater quantity, quality and aquatic ecosystem protection, etc. The general concepts for addressing stormwater management include the following: Stormwater falling on road surfaces shall be collected and conveyed by kerb, channel and swales where space and/or local topography permits for initial treatment. Where open channels are shown to provide adequate treatment they may outfall to the coastal receiving environment if this is practicable. Otherwise stormwater shall be collected at intervals by catchpits and directed into pipes for conveyance to appropriate treatment devices (for primary or secondary treatment, and for flow attenuation), and discharged to the receiving network. Treatment devices will be designed in accordance with TP10 to treat run-off for the water quality event and not extreme flows. Flow in excess of the Water Quality event shall be bypassed around the treatment devices and outfall through new or existing culverts and swales, or directly into the coastal receiving waters. Wherever possible, clean stormwater arising from outside of the paved area will be kept separate from contaminated road surface run-off. Surface run-off from adjacent catchments shall remain separate from road surface run-off. The Indicative Stormwater Design drawings (PD-20811 to PD-20814, within Volume 4 Plans and Drawings ) show the proposed clean water run-off diversion channels for the motorway project. Status 09 Final (Rev 9) Page 14 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 7.3 Specific Device Details TP10 will be used as the design guideline for the design of treatment devices. The design of stormwater devices shall provide for the following: The minimum target adopted for treatment has been set out in TP10, i.e. to remove at least 75% of the total suspended solids on a long term average basis of the existing and proposed motorway surface, Stormwater devices will have facility for outlets to be manually shut through the use of penstocks or gate valves to enable capture of spilt fluids from the motorway. These elements will be developed through the detailed design phase, Maintenance access to each device will be provided via the road network or proposed access tracks. The Indicative Stormwater Design drawings (PD-20811 to PD-20813, within Volume 4 Plans and Drawings ) show the preliminary location of the stormwater treatment devices proposed for the motorway project. A summary schedule of preliminary proposed stormwater treatment devices for the project is presented in Table 7.3-1 (in Section 7.3.1 below). 7.3.1 Proposed Treatment Devices Table 7.3-1: Summary of Preliminary Proposed Treatment Devices Device ID Location (at alignment centreline) Device A CH 6460 Westbound Device B CH 6460 Eastbound Device C CH 7110 Eastbound Wetland D CH 7150 Westbound Wetland E CH 7900 Westbound Contaminant Bearing Area 1 Treatment Device Details 0.7 hectares Cartridge Filters (14 No) & treatment trench 0.6 hectares Cartridge Filters (12 No) & treatment trench 0.5 hectares No treatment devices to be installed 3.75 hectares 1. Sediment Forebay 2. Water Quality Volume Wetland 5.57 hectares 1. Sediment Forebay 2. Water Quality Volume Wetland 1 Approximate total contaminant bearing surface area includes the Motorway, access ramps and other impermeable areas. 2 >75% TSS removal efficiency shall be achieved (in accordance with TP10). Device Size Filter chamber: 1.5m wide x 4.2m long x 1.5m deep Filter Chamber: 1.5m wide x 4.2m long x 1.5m deep TSS Removal Efficiency approx 75% 2 approx 75% 2 In-situ concrete chamber 0% 1.5m wide x 4.2m long x 1.5m deep 2No. 1 @ 53m³ and 1@ approx 75% 2 86m³ Total area approximately 2300m² x 0.5m deep (permanent wet depth) with a maximum depth of 1m 1 @ 206m³ approx 75% 2 Approximately 2800m² x 0.5m deep (permanent wet depth) with a maximum depth of 1m Status 09 Final (Rev 9) Page 15 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 7.3.2 Devices A & B - Cartridge Filters (east of Henderson Creek) Run-off from the widened motorway for both eastbound and westbound lanes, from the high point at CH 6750 to CH 6460, is an approximately 290m long section. Two cartridge filter chambers have been identified as the most viable treatment option for the two small catchments in this location. The chambers will be located on both sides of the motorway at approximately CH 6460. These catchments are identified as subcatchments 9 and 10 on drawings PD-20811 to PD-20813 (Volume 4 Plans and Drawings ). Run-off will be intercepted by grated/slotted channel drains and catchpits and piped eastwards to the treatment device located on the verge of the motorway east of the Henderson Creek Bridge abutments at approximately CH 6460. Rip rap will be provided downstream for erosion control. The treatment devices will discharge the treated stormwater directly into the Henderson Creek. Flows above the WQ event will bypass the devices. Preliminary design and information from manufacturers indicates that one cartridge filter is required to treat approximately 500m² of carriageway surfacing to the TP10 standards (refer to Appendix B). In accordance with preliminary design information received from a cartridge filter manufacturer, removal efficiency by the proposed 14 no. cartridge filters on westbound side and 12 no. cartridge filters on the eastbound side would achieve a relative treatment performance greater than 75% TSS removal. The proposed chambers will be located outside of the carriageway below the motorway embankments. Maintenance access to the filter chambers will be gained from the bus shoulders and shared use path on the motorway. Localised traffic management will be required during maintenance operations. Consideration of Alternatives These cartridge filter devices have only been considered for use in this location because low impact devices (e.g. wetlands or ponds) cannot be provided in the area required due the proximity with the Coastal Marine Area, meaning unacceptable encroachment into the CMA would result, and also due to the low lying nature of the land in this area, resulting in incompatible levels. The design allows for future connection of run-off from these two subcatchments (9 and 10) to be directed to proposed softer treatment devices east of Henderson Creek, which will be incorporated into motorway improvements in the wider area. At this time these cartridge filter devices may be decommissioned. 7.3.3 Device C - Chamber Due to a low point on the carriageway occurring at CH 7080 it will not be practical due to level constraints and constructability to drain approximately 5900m² of the eastbound carriageway into Wetland D. As such it is proposed to collect the run-off via kerb channel and catchpits and discharge into a new insitu concrete chamber located under the eastbound shoulder at CH 7120. This is shown as Subcatchment 7 on drawings PD-20811 to PD-20813 (Volume 4 Plans and Drawings ). Status 09 Final (Rev 9) Page 16 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge The chamber will act as an oversized manhole, combining the downstream outlets from the existing 900mm Culvert A and 450mm Culvert B, where they pass under the motorway. The final outlet from the main chamber will be connected to the upstream end of the existing 900mm diameter culvert C and will discharge via this culvert directly to the Coastal Marine Area (CMA). It is impracticable to fit typical TP10 devices at this location. Treatment cartridge filters in this chamber could provide the necessary water treatment, however, because of land constraints, the chamber would need to be located under the motorway bus shoulder. The practicality of accessing and maintaining the chamber makes this unfeasible. Filter cartridges require regular maintenance, and if installed in this location access would need to be gained from the bus shoulder introducing a safety conflict with the motorway on-ramp. Access to the chambers would require significant traffic management which would be disruptive to motorway traffic. The non-treatment of this small impervious area is not deemed significant given that the proposed stormwater management system for the overall project will achieve treatment of 95% of the final pavement areas. 7.3.4 Wetland D (Westbound Off ramp) A proposed wetland area, would be located on the southern side of the motorway adjacent to westbound offramp at between CH 6960 and CH 7180. The wetland will treat carriageway run-off from 3.75 ha, shown as subcatchments 5, 6 and 8 on drawings PD-20811 to PD-20814 (Volume 4 Plans and Drawings ). This encompasses the motorway area between the high point at CH 7550 (under the Selwood Road Bridge) to the high point at approximately CH 6740 (west of the Henderson Creek Bridges). It will also treat the westbound off-ramp, southeastern side of Selwood Road, and eastbound on-ramp. This total area equates to 4.34 ha, Subcatchment 7, which is 0.59 ha of this motorway carriageway catchment area, cannot be treated by Wetland D due to incompatible levels (refer Section 7.3.3 above). Water quality will be managed by providing storage and residence time within the proposed stormwater wetland. The required treatment volumes have been derived using 1/3 of the two year 24 hour storm event in accordance with ARC Technical Publication TP 108 (refer stormwater calculations provided in Appendix A). The wetland will be designed in accordance with TP10 and will require permanent water storage to ensure that proper treatment of road run-off is achieved. The minimum target adopted for treatment is consistent with ARC TP10 - i.e. to remove at least 75% of the total suspended solids (TSS) on a long-term average basis. The wetland will be constructed by means of cut below existing ground levels, thereby only minimal fill for shaping and bunding will be required. A soils investigation has been undertaken in this area which has identified that the wetland will be located within a zone of Tauranga group silts/clays. Therefore the ground is considered to be suitable to maintain a wetland. If during construction areas of unsuitable material are encountered these will be excavated and replaced with clay. Any issues with infiltration or seepage around installed components can be addressed with at this stage. To minimise the surface area and any redundant or excessive storage volume, Wetland D will be split into two discrete sections - east and west. This reduces the land area required and subsequent property impacts. The base level of each section of the wetland will be at different levels to match the contours of the existing Status 09 Final (Rev 9) Page 17 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge land and receiving outlet pipes/culverts. This arrangement has the further advantage of providing overland flowpaths for the existing upstream catchments around and between the wetlands. A plan and typical sections through the proposed wetland are shown on drawings PD-20815 to PD-20817 (Volume 4 Plans and Drawings ). Wetland Discharge and Outfall Discharge from the west wetland will be into existing Culvert A, then via Culvert C on the northern side of The Concourse, to an existing outfall to the CMA. During preliminary design it has been determined that the existing outfall has a peak discharge of 1.37m³/s. The rate at this outfall will not change. Flows over and above the WQ event, together with untreated run-off from the upstream catchment, will discharge to both the existing culvert (Culvert A) and an additional proposed culvert (Culvert K) via an open channel located between the wetland and the motorway embankment. Discharge from the east wetland will also be through this culvert (Culvert K). The new 1800mm (max) outfall culvert is proposed as the existing culverts, Culvert A (900mm dia) and Culvert B (450mm dia), which combine to outfall into Culvert C (900mm dia) currently receive discharge from 1.4 ha of motorway run-off together with an upstream catchment of agricultural and industrial land measuring 6.7 ha. Calculations indicate the existing culverts are insufficient in size to discharge unrestricted flows in the existing scenario and proposed. Calculations indicate that run-off from a 1 in 100 year event from the existing motorway and upstream catchment could generate 4.5m³/s of flow, whereas the capacity of the downstream Culvert C is only 1.37m³/s. The proposed widened motorway and contributing catchments could generate a 100 year peak flow of approximately 5.5m³/s. Plans of the existing and proposed catchments adjacent to the wetland are shown on drawing PD-20814 (Volume 4 Plans and Drawings ). The proposed Culvert K will be located beneath the westbound bus shoulder, between approximate chainage CH 6615 and CH 6915, and the flows will be piped eastwards over a length of 300m to discharge into the Henderson Creek at approximate CH 6615m. Preliminary design indicates that the new outfall at Henderson Creek may be required to discharge up to approximately 4.5m³/s. Scour protection will be provided at the outfall (refer to Appendix A for calculations). Consideration of Alternatives Due to minimal level differences between the finished motorway, the adjacent land and sea levels in this area, the options for the location of any treatment device are limited. The facility needs to be sized to accommodate the volume of stormwater to treat the largest possible area of the adjacent carriageway (as discussed in paragraph 7.3.4 above). Both the physical size of the facility required and the levels in the area have dictated the proposed location of the wetland. The location at the natural low point of the motorway alignment maximises the catchment area that can be treated whilst taking best advantage of the topography and levels. This location also enables the re-use of the existing culvert outfall (Culvert C) to the CMA which in turn keeps the secondary outfall (Culvert K) as small as possible. Status 09 Final (Rev 9) Page 18 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Although wetlands are the preferred treatment solution by ARC (TP10) over ponds and other treatment devices, such as filtration-type treatment, other options have been considered for the necessary treatment device at this location:- A single larger wetland was initially investigated but through design development it was found to be more effective and also possible to reduce land impact, by splitting the wetland into two sections for the reasons discussed earlier. The total land area of an equivalent pond would be larger than that of the sectionalised wetland, or very similar to that of a single larger wetland, but under TP10 a pond is less desirable than a wetland as outlined above. Cartridge filters were also considered as an alternative option to a pond or wetland. However, as one filter can only treat 500m² of carriageway area, approximately seventy five cartridges would be needed to provide the equivalent treatment. A cartridge facility of this scale would be disproportionately expensive to construct and maintain. A wetland has been deemed the most appropriate treatment device for this area for several reasons including: The highest range of contaminants can be effectively removed by a wetland, The necessary footprint is achievable at this location, A wetland provides some benefits in terms of being naturally sustainable, A wetland provides enhanced habitat value, With a proposed operating depth of between 0.5 and 1 metre the hazards associated with deep bodies of water (i.e. ponds) are reduced, A wetland can be an attractive amenity and enhance urban design. A total contributing catchment of 3.75 hectares will be treated for the WQ event. The wetland will be located and designed to ensure that the longest possible drainage path is achieved, to maximise the retention times and optimise the treatment efficiency. Access for maintenance purposes will be provided from the shared use path adjacent to the westbound offramp. The location of the access is shown on drawing PD-20812, included in Volume 4 Plans and Drawings. 7.3.5 Wetland E (Huruhuru Creek) This treatment device, a proposed wetland, would be located south of the motorway carriageway near CH 7900 on the western side of Huruhuru Creek. The device will treat run-off from the carriageway from the motorway high point at CH 7550 (under Selwood Road Bridge), through to the motorway high point at CH 8450 (approximately 220m west of Huruhuru Road Bridge). The catchment also includes the Westbound On-ramp and Eastbound Off-ramp, the western side of Lincoln Road, the northern part of Selwood Road including the Status 09 Final (Rev 9) Page 19 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge bridge. This is a total catchment area of 5.57 hectares, and is shown as subcatchments 1-4 on drawings PD-20811 to PD-20813 (Volume 4 Plans and Drawings ). Water quality will be managed by providing storage and residence time within the proposed stormwater wetland. The required treatment volumes have been derived using 1/3 of the two year 24 hour storm event in accordance with ARC Technical Publication TP 108 (refer stormwater calculations provided in Appendix A). The wetland will be designed in accordance with TP10 and will require permanent water storage to ensure that proper treatment of road run-off is achieved. The minimum target adopted for treatment is consistent with ARC TP10 - i.e. to remove at least 75% of the total suspended solids (TSS) on a long-term average basis. The wetland will be constructed by means of cut below existing ground levels, and only minimal fill for shaping and bunding will be required. A soils investigation has been undertaken in this area which has identified that the wetland will be located within a zone of Tauranga group silts/clays. Therefore the ground is considered to be suitable to maintain a wetland. If during construction areas of unsuitable material are encountered these will be excavated and replaced with clay. Any issues with infiltration or seepage around installed components can be addressed with at this stage. A plan and typical sections through the proposed wetland are shown on drawings PD-20818 and PD-20819 (Volume 4 Plans and Drawings ). Wetland Discharge and Outfall For this section of the motorway the low point is at about CH 7900, adjacent to the proposed wetland location. Final discharge will be piped to the existing channel located along the southern boundary of the revised designation, currently outfalling into the Huruhuru Creek. During preliminary design it has been determined that the outlet from the wetland to the existing outfall will have a peak discharge of 0.25 m³/s Flows in excess of the WQ event will bypass the wetland to a new proposed outfall in the CMA near Lincoln Bridge No. 1. Peak discharge at this proposed outfall may be approximately 2.0 m³/s. Refer to Appendix A for calculations. Some formalisation and reshaping may be required together with the installation of energy dissipation and local rip rap erosion protection at the wetland and pipe outfall points. Consideration of Alternatives Similarly to Wetland D assessment, other options considered for treatment at this location included filtration type treatment systems such as a sand filter. Because no extended detention or flood control is considered necessary, filtration type systems would be viable treatment solutions. However, during preliminary design it has been deemed that wetland treatment will be more appropriate here for several reasons including: That a wider range of contaminants would be effectively removed by a wetland, The treatment device footprint is achievable at this location, A wetland provides some benefits in terms of being naturally sustainable, A wetland provides enhanced habitat value, A wetland can be an attractive amenity. Status 09 Final (Rev 9) Page 20 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge A total contributing catchment area of approximately 5.57ha impermeable area will be treated for the Water Quality Event. Landscaped/planted embankments and other permeable areas will drain to the existing culverts by swales, or clean water channels. Flows over and above the WQ event will be diverted upstream of the wetland to a bypass pipe discharging directly into the Huruhuru Creek. Detail design is required to confirm the final extent and shape of the wetland. The wetland will be located and designed to ensure that the longest possible drainage path is achieved, to maximise the residence times and optimise the treatment efficiency. Access for maintenance purposes will be provided from the shared use path adjacent to the westbound onramp. The location of the access is shown on drawing PD-20813, included in Volume 4 Plans and Drawings. Status 09 Final (Rev 9) Page 21 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 8. Maintenance Issues The proposed drainage network predominantly consists of commonly adopted elements, namely piped drainage, swales, kerb & channel and wetlands. This familiarity should assist greatly in establishing an effective maintenance regime. NZTA s agent responsible for the maintenance and operation of the Auckland motorway network is currently Auckland Motorways, or the Auckland Motorway Alliance (AMA). Auckland Motorways will be responsible for maintenance of the systems upon the completion of the construction contract period. The following maintenance provisions will be included within the drainage design requirements, and detailed in the Asset Owners Manual for future reference: Pipework/Catchpits/Channels Maintenance will be carried out in accordance with NZTA standard requirements. Wetlands - will be inspected frequently during the first three years during both growing and nongrowing seasons to observe plant species presence, abundance, and condition, bottom contours, and water depths relative to plans, sediment, outlet, and buffer conditions. Plants may require watering, physical support, mulching, weed removal, or replanting during the first three years. Nuisance plant species should be removed and desirable species should be replanted. Swales where ground conditions cannot be confirmed to be free draining, and for all swales with a longitudinal slope less than 2%, subsoil drains will be provided. Cartridge Filters will need to be inspected every six months for the first two years and then cleaned at yearly intervals. Isolating valves fitted to Stormwater devices which provide the ability for containment of hazardous substances, should they be spilt on the motorway, will need to be inspected every six months. 8.1 Durability All new culverts and culvert extensions and component members shall be designed to provide adequate durability in accordance with the requirements of the material design codes in NZTA Bridge Manual or relevant design codes, except where specific requirements are included: The design working life of proposed culvert structures is assumed to be 100 years in normal circumstances. The culvert and associated structures shall be designed with consideration of minimising maintenance requirements during the design life. Status 09 Final (Rev 9) Page 22 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Where culverts or piping material, other than concrete, are proposed (e.g. PVC, PE, etc.), demonstration of adherence to the longevity criteria will be needed by design. Structures shall be sufficiently durable to ensure that, without reconstruction or major renovation, they continue to fulfil their intended function throughout their design life. A condition assessment of existing culverts is proposed in order to assess any significant or serious risks to ongoing operation and maintenance of the culverts. 8.2 Erosion Protection To mitigate erosion at the interface between natural streams and culvert headwall / wing-walls, appropriate erosion control and energy dissipation measures shall be incorporated into the site specific design during detailed design. These shall be considered for the 10 year and 100 year design flows. Adequate erosion protection will be provided to facilitate the transition from pipe flow to natural channel flow. There is currently little or no erosion protection and it is not proposed to extend the designation boundary to accommodate extensive erosion protection, which calculations to recognised design standards could show to be required. Therefore, erosion protection will be provided within the available space. Status 09 Final (Rev 9) Page 23 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge 9. Conclusions The project includes motorway pavement widening, widening and duplication of bridges, new on-and offramps, and the construction of a shared use path separate to the motorway carriageway. The proposal could have a range of short term and long term stormwater management related effects on the receiving environment. Best practicable measures will be taken to avoid, remedy or mitigate any resultant adverse environmental effects. A consistent approach to stormwater treatment has been adopted and consists of the use of treatment devices prior to the outfall to the coastal marine waters. No attenuation is required as the stormwater network outfalls to the coastal marine waters. The stormwater treatment proposes to treat 91% of the existing and 100% of the proposed motorway. This equates to treatment of a total impermeable catchment area of 10.6ha; in excess of 95% of the total motorway impermeable area (11.1ha). The overall benefit to stormwater quality as a result of the works is considered to be significant. Status 09 Final (Rev 9) Page 24 October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Appendix A Stormwater Calculations Status 09 Final (Rev 9) October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc

NZ Transport Agency SH16 Western Ring Route Henderson Creek to Huruhuru Road Bridge Appendix B - Cartridge Filter Information Status 09 Final (Rev 9) October 2010 Document Reference No. AC-2-RPT-SW-001 AC2RPSW001-Rev9 Final (Oct 2010).doc