Appendix C Stormwater Management Report

Transcription

1 Appendix C Stormwater Management Report

2 SH20:Manukau Harbour Crossing REPORT S92 Response to ARC Stormwater Report Prepared for: Qantas House 191 Queen Street Auckland 14 September 2006 PHONE FAX WEB OFFICE College Hill, Auckland POST PO Box 821, Auckland, 1140

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4 Contents Executive Summary ES-1 1 Introduction Background Design Philosophy Information Provided Amended Treatment Device Layout Proposed Treatment Devices Tararata Pond West swale: East swale: West swale: East swale: East swale: Waterfront Park wetland Gloucester Park wetland with partial pre-treatment East swale: East Gloucester Park Interchange filter Orpheus Drive South Filter West: and East: causeway continuous sand filters East swale: Beachcroft Road wetland Proposed Outfalls Untreated Catchment Areas West Carriageway and motorway reserve: West Carriageway and motorway reserve: Summary General Design Issues Drawings TP 108 Runoff Calculations TP 10 Treatment Calculations Outfall Locations: Photographs i

5 List of Tables Tables Table 2-1: Amended Integrated Stormwater Treatment Device System Table 2-2: Schematic Comparison of Original Devices with Refined Device Layout Table 3-1: Proposed treatment device catchment characteristics Table 3-2: Proposed treatment device hydrological design parameters Table 3-3: Proposed pond and wetland dimensions and treatment efficiency Table 3-4: Proposed swale dimensions and treatment efficiency Table 4-1: Existing Outfalls for Integrated Stormwater Treatment Device System Table 4-2: New Outfalls for Integrated Stormwater Treatment Device System Table 5-1: Proposed untreated areas ii

6 Executive Summary This report has been prepared in response to: The Auckland Regional Council (ARC) Section 92 response seeking further information (ref. ARC 19092, dated 29 June 2006) with regard to the resource consent applications submitted by Transit NZ (May/June 2006). The additional request (per Barker and Associates, dated 12 July 2006) regarding the condition and levels of the existing outfall from the Gloucester Park saltwater marsh, and the Auckland City rain gauge in Gloucester Park. This report presents the detail of the revised design of the proposed motorway drainage and runoff treatment systems. This is based on the design originally presented with the Application, but refined on the basis of the greater detail of information now available. Of the total catchment contributing to the proposed motorway and off-motorway drainage systems (40.8 ha), runoff from 96 % (39.1 ha) will be treated before discharge to Manukau Harbour. This includes not only new pavement (10.6 ha) created as part of the motorway widening but also 13.3 ha existing pavement, and the contributing catchment areas of the pervious motorway reserve and urban development along the motorway alignment (16 ha). Runoff from 95 % existing pavement area and over 99 % new pavement areas will be treated. The main and most significant feature of the refined systems is that in all cases the treatment efficiency achieved for the total runoff directed to the proposed devices (i.e. from new and existing pavement areas, and the contributory motorway reserve and urban catchment areas) will be at least 75 %, consistent with the guidelines of ARC TP10. In this regard a detailed options analysis has not been carried out for the proposed devices, as they meet and/or exceed the TP % TSS removal efficiency criterion. Overall it is expected that the proposed integrated treatment system will prevent the transportation of at least 4 tonnes of sediment to the harbour annually, and up to 18 tonnes. Details of the devices and preliminary background design information are provided in tables in Section 3. The specific issues raised in the s92 response are addressed in the following sections: Re-suspension, Section 6 Levels, Section s 3 and 6 Feasibility, Section 6 Existing Gloucester Park saltmarsh outfall, Section 3.8 Existing Auckland City rain gauge, Section 3.8 ES-1

7 1 Introduction SECTION 1 Introduction In response to the resource consent applications submitted by Transit NZ (May/June 2006), the Auckland Regional Council (ARC) issued a Section 92 response to the Application seeking further information (ref. ARC 19092, dated 29 June 2006). With regards to stormwater, the response included the following: 37. Stormwater and Earthworks The PARP:ALW (Policy (e) (ii)) requires that that when considering a Resource Consent to Divert and Discharge Stormwater the ARC requires the applicant to adopt the Best Practicable Option (BPO) for the diversion and discharge of stormwater. This shall be determined by having regard to, the level of stormwater quality treatment provided for existing impervious areas to ensure the greatest extent practicable of TSS removal is achieved on an average annual basis. Whilst it may well be that the current level of TSS removal is the Best Practicable Option for stormwater management, without a thorough options analysis there is no way for the ARC to make this assessment. In this regard, please provide an options analysis of the proposed stormwater treatment devices to demonstrate the differing levels of TSS removal that can be achieved (in accordance with Policy 5.4.4). This analysis should include at least 3 options for different levels of contaminant removal. The benefit to the receiving environment, cost and practical implementation of each option should be considered and compared. Please when providing maps / plans show representative sizes of the devices. The Application must have sufficient detail to provide a level of confidence that the proposed stormwater management devices can be constructed. This should include information about: re-suspension issues considered; in areas where levels are an issue further details should be provided; feasibility of device specifically looking at key issues of each device (if any). Constructability / feasibility of the proposed measures. It is understood that the erosion and sediment control plans are indicative, however additional information is required. Please provide catchment boundaries for the proposed sediment control devices. An additional request for further information (per Barker and Associates, dated 12 July 2006) sought information regarding the condition and levels of the existing outfall from the Gloucester Park saltwater marsh, and the Auckland City rain gauge in Gloucester Park. This report sets out the additional information provided to address the issues raised by ARC. 1.1 Background The Application was based on a drainage system design, which included treatment devices for stormwater before discharge to the receiving environments. The design of the system devices was generally in accordance with ARC TP 10. The devices were sized for a water quality volume based on the runoff from the new pavement surface area to be created as part of the motorway widening and ancillary works, i.e. to provide 75 % efficiency with regard to removal of Total Suspended Sediments (TSS) for that new pavement runoff. 1-1

8 SECTION 1 Introduction However, as the total runoff from the motorway (including both new and existing pavement areas and the greater catchments) will be directed to the same conveyance system the efficiency for the treatment of the actual catchment runoff volume would be less. Nonetheless, in all cases the anticipated efficiency for treatment of total runoff was expected to be greater than 35 % and in effect significantly greater than this, although detailed analyses were not submitted at the time of the Application. An objective of the treatment system design, as presented in the Application, was that Best practicable measures will be taken to avoid, remedy or mitigate any resultant adverse environmental effects. It was anticipated that refinements would be made to the design of treatment devices during detailed design, and that these would improve the treatment efficiency as far as practicable. ARC, however, sought in its initial s92 response (and as clarified in a later meeting, 30 June 2006) to confirm that the treatment device options presented in the Application, where they do not achieve 75 % treatment efficiency for all runoff directed to them, represent the best practicable option for each location - particularly in the context of comparison and assessment against other options. Furthermore, aspects of feasibility of devices and operational issues were also raised for clarification. 1.2 Design Philosophy With more detailed information, particularly in terms of analysis of catchment areas and survey information now available, the systems proposed as part of the Application have been reviewed. This review focussed on: Location and space constraints on device selection The treatment efficiencies of proposed devices with respect to treatment of runoff from the total contributing catchment Feasibility of conveyance of flow both to and through the proposed treatment devices Catchment boundaries have been checked and refined as necessary with additional information and as required by alignment revisions. The main features of the refined integrated treatment system now proposed along the alignment of the motorway comprise: Layout and sizing of treatment devices in accordance with the guidelines of ARC TP 10 to achieve at least 75 % treatment efficiency with respect to removal of TSS from the total contributing catchment (including both new and existing pavement areas) Comprehensive treatment of runoff from all catchment surfaces except for: 0.72 ha existing motorway carriageway and reserve at the Rimu Road interchange 0.99 ha existing motorway carriageway and reserve at the Queenstown Road off-ramp Maximisation of runoff from the motorway directed to wetlands and ponds for treatment New wetland layouts developed at this stage to be over-sized with respect to minimum water quality volume required, recognising that in detailed design the actual volume may differ from that identified at concept stage 1-2

9 SECTION 1 Introduction Treatment of runoff in swales that provide residence times greater than the minimum required to achieve 75 % reduction in TSS Swale design parameters: Bottom width less than 2 m Side slopes no steeper than 5H:1V Longitudinal slope generally greater than 1 % Minimum hydraulic residence time of 9 minutes Maximum water quality flow depth of 100 mm Maximum water quality flow velocity of 0.8 m/s Maximum 1 % AEP flow velocity of 1.5 m/s Treatment devices to discharge to existing stormwater systems and outfalls where possible 1.3 Information Provided This report presents the detail of the revised design of the proposed motorway drainage and runoff treatment systems. This is based on the design originally presented with the Application, but refined on the basis of the greater detail of information now available. The amended treatment device layout is presented in Section 2 below, with summary tables and crossreferences to the original application layout. Drawings of the motorway alignment identifying treatment devices, contributory catchments and outfalls are included in Section 7. Discussion of the features of each device is included in Section 3. Locations of outfalls for the proposed devices are identified in Section 4. Untreated catchment areas are discussed in Section 5. Aspects to be considered in further detail during the design process are identified in Section 6. Sections 8 and 9 include TP 108 runoff calculations for the various catchments and TP 10 water quality treatment calculations and treatment analysis for the proposed swales. Section 10 includes photographs of all existing outfalls, and at locations of proposed new outfalls. 1-3

10 2 Amended Treatment Device Layout SECTION 2 Amended Treatment Device Layout Of the total catchment contributing to the motorway drainage systems (40.8 ha), runoff from 39.1 ha will be treated before discharge to Manukau Harbour, i.e. approximately 96 %. This includes not only new pavement (10.6 ha) created as part of the motorway widening but also 13.3 ha existing pavement, and the contributing catchment areas of the pervious motorway reserve and urban development along the motorway (16 ha). Runoff from over 95 % of existing pavement and over 99 % new pavement will be treated. The main and most significant feature of the refined systems is that in all cases the treatment efficiency achieved for the total runoff directed to the proposed devices (i.e. from new and existing pavement areas, and the contributory motorway reserve and urban catchment areas) will be at least 75 %, consistent with the guidelines of ARC TP10. The devices proposed along the motorway alignment are listed below in Table 2.1. The general layout of the devices is shown on the attached drawings (Section 7). Table 2.2 indicates schematically a comparison between the devices as included in the Application and the amended layout. The main feature of the proposed integrated treatment system along the alignment of the motorway compared to that in the Application is the deletion of Pond 2 (Tararata North). Runoff from this section of the motorway catchment will be directed to the (existing) Tararata Pond (Pond 1). The proposed motorway treatment system will require three new harbour outfalls, and one upgraded Auckland City stormwater outfall. 2-1

11 SECTION 2 Amended Treatment Device Layout Table 2-1: Amended Integrated Stormwater Treatment Device System Motorway catchment Treatment Device chainage (m) West East Tararata Pond Comment An existing pond at Chainage 760, in the reserve between the motorway and Coronation Road off-ramp West swale Existing outfall East swale West swale Includes some non-motorway catchment Includes significant non-motorway catchment New shared outfall East swale New shared outfall East swale New shared outfall Waterfront Park wetland Gloucester Park wetland Includes existing bridge New outfall Includes existing bridge With some swale pre-treatment New shared outfall East swale New shared outfall East Gloucester Park Interchange filter device Orpheus Drive South filter device Discharges to existing stormwater system and outfall New Outfall West filter East filter Discharge to existing cross drainage systems and lagoon outfalls East swale Discharges to upgraded existing drainage system outfall Beachcroft Road wetland Existing wetland, at Chainage

12 SECTION 2 Amended Treatment Device Layout Table 2-2: Schematic Comparison of Original Devices with Refined Device Layout Devices proposed in Application Motorway chainage (m) Not included Pond 1 (Tararata South) Pond 2 (Tararata North) and swales Swales and offset mitigation South >> Refined device layout for s92 response Motorway chainage (m) West East Tararata Pond Swale Untreated Swales Tararata Pond Swale Swale Comment Existing pond, retrofitted to provide water quality volume for greater catchment. Deletion of Pond 2, full stormwater treatment provided by swales. West carriageway pavement already exists Pond Pond 5 (Gloucester Park) Sand filters << North Waterfront Reserve Wetland Gloucester Park Wetland East Gloucester Park Interchange Proprietary filter Causeway sand filters Gloucester Park Wetland Swale Orpheus Drive South proprietary filter Causeway sand filters Extended catchment on south bridge abutment to maximise runoff capture to wetland. Part catchment pre treatment in swale. Off-motorway interchange and reclamation carriageways. West causeway filter catchments do not include existing Orpheus Drive Swale on east, connection from west Pond 6 (Beachcroft Road) Untreated Swale Beachcroft Wetland Existing pond 2-3

13 3 Proposed Treatment Devices SECTION 3 Proposed Treatment Devices Each device is discussed separately in the following sections, identifying in detail the contributing catchments and outfall arrangements. Summary information is provided in tables at the end of this section, including: Table 3.1: Proposed treatment device catchment characteristics Table 3.2: Proposed treatment device hydrological design parameters Table 3.3: Proposed pond and wetland dimensions and treatment efficiency Table 3.4: Proposed swale dimensions and treatment efficiency 3.1 Tararata Pond This pond already exists as a treatment device receiving runoff from SH20 south as far south as Chainage 370 beyond Walmsley Road. It is located between the motorway and the Coronation Road offramp at Chainage 760. To provide treatment for the greater contributing catchment the pond weir will be raised by approximately 300 mm maximum. The retro-fitted pond will provide retention of the required water quality volume (determined in accordance with TP 10) from the total contributing catchment. The pond will receive runoff from: the west carriageway between Chainage 370 and Chainage 1070 the east carriageway between Chainage 370 and Chainage 900, piped across motorway part on the Coronation Road off-ramp and embankment Runoff from the north of Tararata Creek will be piped across the bridge. Tararata Pond discharges directly to Tararata Creek 3.2 West swale: The swale has been designed in accordance with TP 10 to treat stormwater from the total contributing catchment, including: the west carriageway from Chainage 1070 to Chainage 1350 limited residential catchment to the west The swale will discharge to the existing stormwater pipe and outfall to Tararata Creek. This pipe is 900 mm diameter, with an invert approximately 4.5 m below the carriageway level. 3.3 East swale: The swale has been designed in accordance with TP 10 to treat stormwater runoff from the total contributory catchment, being the east carriageway and motorway strip from Chainage 900 to Chainage The swale will discharge directly by overflow to Tararata Creek, at the location untreated runoff from the existing motorway currently discharges. 3-1

14 SECTION 3 Proposed Treatment Devices 3.4 West swale: The swale has been designed in accordance with TP 10 to treat stormwater from the total contributing catchment, including: the west carriageway and motorway reserve from Chainage 1350 to Chainage 2050 a relatively significant urban residential subcatchment to the west Due to the additional catchment, the initial analysis indicates that peak velocities for the 1 % AEP event will be greater than 1.5 m/s. This will require attention in design, with possible reinforcement of the swale invert required. It is noted that the capacity of the existing Manukau City drainage infrastructure in the catchment has been neglected in the initial analysis, and thus actual peak flows for design may be less than assumed at this stage. In design it is likely that the hydraulic loading on the swale can be reduced. The swale will discharge to a new stormwater 1050 mm pipe, located at Chainage This pipe will be required as the existing stormwater at this location has insufficient capacity to convey peak runoff flows (at 1 % AEP). A new outfall to the harbour will be constructed in the vicinity of the off-ramp junction with Mahunga Drive and Rimu Road. The new pipe and outfall will also convey runoff from: East swale: West Carriageway and motorway reserve: East swale: East swale: The swale has been designed in accordance with TP 10 to treat stormwater from the total contributing catchment, being: the east carriageway and motorway reserve from Chainage 1350 to Chainage 2050 part of the marae reserve on Mahunga Drive The swale will discharge to the new stormwater pipe and outfall to be constructed to the harbour, located at Chainage 2050 (refer Section 3.4 above). 3.6 East swale: The swale has been designed in accordance with TP 10 to treat stormwater from the total contributing catchment, being: the east carriageway from Chainage 2050 to Chainage 2200 part of the embankment to the Rimu Road on-ramp The swale will discharge to the new stormwater pipe and outfall to be constructed to the harbour, located at Chainage 2050 (refer Section 3.4 above). 3.7 Waterfront Park wetland A new wetland will be constructed as a treatment device in the existing open space in Waterfront Park in the vicinity of the south-western abutment of the existing bridge. The new wetland has been designed in 3-2

15 SECTION 3 Proposed Treatment Devices accordance with TP 10 to store and treat the water quality volume from the total contributing catchment, being: the west carriageway between Chainage 2200 to the crest of the existing bridge at Chainage 2800 the Rimu Road on-ramp and part of the on-ramp embankment the east carriageway from Chainage 2200 to the crest of the new bridge at Chainage 2800 the Rimu Road off-ramp and part of the off-ramp embankment Runoff from the eastern catchment will be conveyed by pipes across the motorway. A bypass system will be provided to convey flows in excess of the water quality event around the wetland system. A new harbour outfall will be constructed for flow discharge from the wetland and bypass systems, to be located adjacent to an existing Manukau City Council outfall. 3.8 Gloucester Park wetland with partial pre-treatment A new wetland will be constructed in Gloucester Park, in close proximity to but separate from the existing salt marsh. The new wetland has been designed in accordance with TP 10 to store and treat the water quality volume from the total contributing catchment, being: the west carriageway between the crest of the existing bridge at Chainage 2800 and Chainage 3750 the east carriageway from the crest of the new bridge at Chainage 2800 to Chainage 3520 part of the north-bound motorway on and off-ramps at the Gloucester Park interchange part of Onehunga Harbour Road Two swales will provide pre-treatment for part of the catchment: from Chainage 3300 to Chainage 3480 in the west reserve by Gloucester Park which conveys runoff from the northern bridge carriageways from Chainage 3420 to Chainage 3500 between the off ramp and the motorway carriageway. Flow from this swale will be piped to the wetland. Runoff from the eastern subcatchment swale will be conveyed by a pipe across the motorway. As noted the new wetland and contributory drainage systems will be hydraulically separate from the existing saltmarsh and Auckland City stormwater infrastructure. The wetland will discharge to a new harbour outfall, to be located in the existing harbour sea wall revetment to the north of the Auckland City outfall. In this regard, there will be no connection to the existing outfall, which is presently below harbour bed level. Also the motorway works will not affect access to the existing Auckland City rain gauge. 3.9 East swale: The swale has been designed in accordance with TP 10 to treat stormwater from the total contributing catchment, being: the east carriageway from Chainage 3520 to Chainage 3670 the Gloucester Park Road on-ramp 3-3

16 SECTION 3 Proposed Treatment Devices This swale will connect to the new pipe system from the Gloucester Park wetland to the new harbour outfall East Gloucester Park Interchange filter This filter device will receive runoff and treat from the east approaches to the Gloucester Park interchange. Due to space constraints it will comprise a proprietary system, such as an Ingal Stormfilter or similar, to meet the TP 10 standard. The filter will connect to the existing Auckland City stormwater network that discharges to the tidal inlet on the east side of the motorway Orpheus Drive South Filter This filter device will receive runoff from the reclamation area at the junction with Onehunga Harbour Road. It will comprise of a proprietary system similar to the East Gloucester Park Interchange filter above. The filter will discharge to the harbour through a outlet to be constructed in the new reclamation revetment sea wall West: and East: causeway continuous sand filters Filter devices as proposed in the Application will be constructed to treat stormwater runoff from the motorway carriageway across the Hugh Watt Drive causeway. The filters will be placed at the edge of the motorway reserve on the east side, and on the west side at the edge of Orpheus Drive. Runoff from the existing pavement of Orpheus Drive north of the Manukau Cruising Club will not be treated. Filters will receive runoff continuously along the length of the filter, and have been sized in accordance with TP10 guidelines to treat a unit width of contributing catchment. Discharge from the filters will be conveyed by collector pipes to existing cross drainage structures under the motorway. In total there will be 12 filter outfall connections on the west side, and 11 on the east side. On the west side, conventional catchpits will be integrated with the filter system to provide bypass capacity for flows in excess of the water quality event to achieve required road drainage standards for Orpheus Drive and the motorway. On the east side, runoff from the eastern motorway carriageway in excess of the water quality event will be shed to Onehunga Lagoon East swale: This swale has been designed in accordance with TP 10 to treat stormwater runoff from the total contributory catchment, being the east carriageway and motorway reserve from Chainage 4620 to Chainage 5120 The swale will discharge via a connection to the existing Auckland City stormwater system of the west side of the motorway. The existing stormwater line may require upgrading downstream to accommodate flows between the connection and the existing outfall beside the boat ramp at the north end of Orpheus Drive. 3-4

17 SECTION 3 Proposed Treatment Devices 3.14 Beachcroft Road wetland This is an existing wetland, designed as a treatment device to receive runoff from SH20 north of Queenstown Road as far as Hillsborough Road. The design parameters for the wetland include for the limited catchment of the widened motorway south of Queenstown Road, being: the west and east carriageways from Chainage 5120 to Chainage 5200 the Queenstown Road off-ramp embankment the Queenstown Road on-ramp and embankment The design catchment to the existing wetland is 8.9 ha. The contributory catchment of the widened motorway is 0.96 ha. No further treatment measures for runoff from this area are proposed as part of this project, as runoff is already treated in accordance with TP10 guidelines. New drainage infrastructure will be provided as necessary to convey runoff to the pond. 3-5

18 SECTION 3 Proposed Treatment Devices Table 3-1: Proposed treatment device catchment characteristics Device Motorway catchment chainage (m) Catchment Area (ha) Catchment Length (m) West East Total Pavement Existing New Urban Pervious Tararata Pond Waterfront Park wetland Gloucester Park wetland Beachcroft Road wetland Sub catchment 1. West swale East swale West swale East swale East swale East swale East swale East Gloucester Park Interchange filter Orpheus Drive South filter Gloucester Park interchange overbridge eastern approach Reclamation at junction with Onehunga Harbour Road West causeway filters (12) East causeway filters (11) Total treated catchment area Notes Existing wetland has design catchment beyond project limit to Hillsborough Road of 8.9 ha 20 for treatment 175 for discharge 3-6

19 SECTION 3 Proposed Treatment Devices Device Table 3-2: Proposed treatment device hydrological design parameters 1. Motorway catchment chainage (m) West East Total Water Quality Volume (m 3 ) Pavement Existing New Urban Pervious Water Quality Event Peak flow (l/s) Tararata Pond Waterfront Park wetland Gloucester Park wetland Beachcroft Road wetland West swale East swale West swale East swale East swale East swale East swale East Gloucester Park Interchange filter Orpheus Drive South filter Gloucester Park overbridge eastern approach Reclamation at junction with Onehunga Harbour Road 1 % AEP West causeway filters (12) East causeway filters (11) Notes Determined using TP 108 Peak flows determined for largest single filter catchment 3-7

20 SECTION 3 Proposed Treatment Devices Table 3-3: Proposed pond and wetland dimensions and treatment efficiency Device Motorway catchment chainage (m) West East Device volume (m 3 ) Treatment volume required for TP10 compliance (m 3 ) Treatment Efficiency Device Dimensions Area (m 2 ) Average depth (m) Notes Tararata Pond % Existing pond, retrofitted to provide extra volume. Waterfront Park wetland Up to At least 75 % Gloucester Park wetland Up to At least 75 % Runoff from part of catchment treated in swales before flow to wetland Beachcroft Road wetland % Existing pond, includes catchment to Hillsborough Road 3-8

21 SECTION 3 Proposed Treatment Devices Table 3-4: Proposed swale dimensions and treatment efficiency Device Swale Length (m) Minimum Treatment Length 1. (m) 50 mm grass 150 mm grass Treatment Efficiency Water quality flow depth (m) Flow Velocity 2. (m/s) Water quality event 1 % AEP Notes West swale: > 75 % East swale: > 75 % West swale: > 75 % infrastructure neglected (refer Contribution of urban drainage Section 3.4) East swale: > 75 % East swale: > 75 % East swale: > 75 % East swale: > 75 % Notes Minimum length calculated for 9 minute residence time in swale in accordance with ARC TP 10 Flow velocity generally calculated for 50 mm grass length except as noted Flow parameters determined for grass length 150 mm length, noting also significant non-motorway catchment (refer Section 3.4) 3-9

22 4 Proposed Outfalls SECTION 4 Proposed Outfalls The concept design of the proposed integrated treatment system has sought to connect to existing outfall structures where possible. However, in some cases the capacity of existing infrastructure is not sufficient to carry the expected design flows. This is an issue particularly for catchment with no secondary flow path. In these, the stormwater infrastructure must have capacity to carry the 1% AEP flows. Generally in this instance a new outfall is required. Existing and proposed new outfalls are summarised in Tables 4.1 and 4.2. Section 10 includes photographs of all existing outfalls, and at locations of proposed new outfalls. 4-1

23 SECTION 4 Proposed Outfalls Table 4-1: Existing Outfalls for Integrated Stormwater Treatment Device System Outfall Location Drainage system Diamete Level me mn r (mm) OE1 Existing pond to Tararata Creek Tararata Pond Weir RL 2.2 m OE2 Existing pipe to Tararata Creek West swale: RL 2.8 m OE3 OE4 Upgraded existing overland flow path to Tararata Creek Existing Auckland City pipe and outfall to Local Lockup tidal inlet East swale: East Gloucester Park Interchange filter device 450 RL 1.92 m OE RL 0.13 m OE RL m OE RL 0.02 m West East OE8 Eight existing pipes and outfalls RL 0.70 m causeway causeway across Hugh Watt and Orpheus filters: filters: OE9 Drives RL m OE RL m OE RL m OE12 OE13 Existing pipe and outfall at northern Orpheus Drive boat ramp East swale: Beachcroft Road wetland 1850 RL m 1050 RL m Comments Pipe under Coronation Road on-ramp To be extended with new reclamation works To be extended with new reclamation works Southern outfall from Onehunga Bay Lagoon Northern outfalls from Onehunga Bay Lagoon Possible upgrading to increase diameter of outfall pipe 4-2

24 SECTION 4 Proposed Outfalls Table 4-2: New Outfalls for Integrated Stormwater Treatment Device System Outfall ON1 New pipe to upper Manukau Harbour in vicinity of Rimu Road off-ramp Location me mn Drainage system West swale: East swale: No Treatment: East swale: Diameter (mm) 1500 Comments ON2 New pipe to harbour Waterfront Park wetland Located beside existing outfall ON3 New pipe to harbour Gloucester Park wetland To be confirmed in design To be constructed in sea wall revetment, with reclamation works ON4 New pipe to harbour Orpheus Drive at Onehunga Harbour Road To be constructed with new reclamation works 4-3

25 5 Untreated Catchment Areas SECTION 5 Untreated Catchment Areas 5.1 West Carriageway and motorway reserve: This is a relatively small catchment (0.72 ha), with limited space in the motorway reserve to construct a treatment facility. There is not sufficient space on the west motorway reserve to construct a pond or swale within the bridge abutment. The surface cover of this catchment will not be changed, comprising already motorway carriageway and the reserve incorporating the Rimu Road on-ramp embankment. For these reasons, no specific treatment measures are proposed. Runoff from this area will discharge to the new stormwater pipe and outfall to be constructed to the harbour, located at Chainage 2050 (refer Section 3.4). 5.2 West Carriageway and motorway reserve: This is a relatively small catchment (0.99 ha) of existing motorway at the Queenstown Road off-ramp. It presently drains to existing stormwater infrastructure and is conveyed to the harbour at the northern end of Orpheus Drive. There is not sufficient space on the west motorway reserve to construct a pond or swale within the bridge abutment. Consideration was given to conveying runoff from this catchment across the motorway to the east swale: However, due to the flat grades of the vertical alignment this was not feasible. The surface cover of this catchment will not be changed, comprising already motorway carriageway and the reserve incorporating the off-ramp embankment. For these reasons, no specific treatment measures are proposed. 5.3 Summary The proposed untreated areas comprise 1.7 ha in total, compared to the 75 % treated catchment area of 39.1ha. The untreated areas comprise over 46 % pervious areas (e.g. motorway reserve) and only 0.01 ha of new pavement. The characteristics of the untreated catchments are summarised in Table 5.1. In this context it is noted that as part of the proposed motorway widening, stormwater treatment will be provided for runoff from: the equivalent of 13.3ha of paved carriageway on the existing motorway up to 6.4 ha of urban runoff in addition to the new impervious surface cover to be created (10.6 ha). Overall it is expected that the proposed integrated treatment system will prevent the transportation of at least 4 tonnes of sediment to the harbour annually, and up to 18 tonnes. It is submitted that this is adequate mitigation for the non-treatment of runoff from the 4 % of project catchment that will not be treated. 5-1

26 SECTION 5 Untreated Catchment Areas Location Total Table 5-1: Proposed untreated areas Catchment Area (ha) Pavement Urban Existing New Pervious 1 % AEP peak flow (l/s) Comments West Carriageway: Shared new outfall West Carriageway: Shared (upgraded) outfall Total