Appendix C Stormwater Management Report

SH20:Manukau Harbour Crossing REPORT

S92 Response to ARC Stormwater Report Prepared for:

Qantas House 191 Queen Street Auckland

14 September 2006

PHONE 09 355 1300 FAX 09 355 1333 EMAIL [email protected] WEB www.northerngateway.co.nz OFFICE 13 – 15 College Hill, Auckland POST PO Box 821, Auckland, 1140

Contents Executive Summary ------------------------------------------------------------------------------------------------------- ES-1 1

Introduction-------------------------------------------------------------------------------------------------------------1-1 1.1 1.2 1.3

Background Design Philosophy Information Provided

1-1 1-2 1-3

2

Amended Treatment Device Layout ----------------------------------------------------------------------------2-1

3

Proposed Treatment Devices -------------------------------------------------------------------------------------3-1 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14

Tararata Pond West swale: 1070-1350 East swale: 900-1350 West swale: 1350-2050 East swale: 1350-2050 East swale: 2050-2200 Waterfront Park wetland Gloucester Park wetland with partial pre-treatment East swale: 3520-3670 East Gloucester Park Interchange filter Orpheus Drive South Filter West: 3750-4800 and East: 3750-4620 causeway continuous sand filters East swale: 4620-5000 Beachcroft Road wetland

3-1 3-1 3-1 3-2 3-2 3-2 3-2 3-3 3-3 3-4 3-4 3-4 3-4 3-5

4

Proposed Outfalls ----------------------------------------------------------------------------------------------------4-1

5

Untreated Catchment Areas---------------------------------------------------------------------------------------5-1 5.1 5.2 5.3

West Carriageway and motorway reserve: 2050-2200 West Carriageway and motorway reserve: 4800-5120 Summary

5-1 5-1 5-1

6

General Design Issues ----------------------------------------------------------------------------------------------6-1

7

Drawings-----------------------------------------------------------------------------------------------------------------7-1

8

TP 108 Runoff Calculations ---------------------------------------------------------------------------------------8-1

9

TP 10 Treatment Calculations ------------------------------------------------------------------------------------9-1

10

Outfall Locations: Photographs ------------------------------------------------------------------------------- 10-1

i

List of Tables Tables Table 2-1: Amended Integrated Stormwater Treatment Device System .................................................. 2-2 Table 2-2: Schematic Comparison of Original Devices with Refined Device Layout................................ 2-3 Table 3-1: Proposed treatment device catchment characteristics ........................................................... 3-6 Table 3-2: Proposed treatment device hydrological design parameters1. ................................................. 3-7 Table 3-3: Proposed pond and wetland dimensions and treatment efficiency.......................................... 3-8 Table 3-4: Proposed swale dimensions and treatment efficiency ............................................................. 3-9 Table 4-1: Existing Outfalls for Integrated Stormwater Treatment Device System................................... 4-2 Table 4-2: New Outfalls for Integrated Stormwater Treatment Device System ........................................ 4-3 Table 5-1: Proposed untreated areas........................................................................................................ 5-2

ii

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 10 75 % 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

SECTION 1

Introduction 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 5.4.4 (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

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

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:



1.3



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

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

SECTION 2

Amended Treatment Device Layout 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

SECTION 2

Amended Treatment Device Layout

Table 2-1: Amended Integrated Stormwater Treatment Device System

Treatment Device

Motorway catchment chainage (m) West East

Tararata Pond

370-1070

West swale

1070-1350

East swale

West swale

Comment •

An existing pond at Chainage 760, in the reserve between the motorway and Coronation Road off-ramp



Existing outfall



Includes some non-motorway catchment

• •

Includes significant non-motorway catchment New shared outfall

370-900

900-1350

1350-2050

East swale

1350-2050



New shared outfall

East swale

2050-2200



New shared outfall

• •

Includes existing bridge New outfall

2800-3520

• • •

Includes existing bridge With some swale pre-treatment New shared outfall

3520-3750



New shared outfall

East Gloucester Park Interchange filter device



Discharges to existing stormwater system and outfall

Orpheus Drive South filter device



New Outfall



Discharge to existing cross drainage systems and lagoon outfalls



Discharges to upgraded existing drainage system outfall



Existing wetland, at Chainage 5100

Waterfront Park wetland Gloucester Park wetland

2200-2800

2800-3750

East swale

West filter

3750-4800

East filter

East swale Beachcroft Road wetland

3750-4620

4800-5120

4620-5120

5120-5200

2-2

SECTION 2

Amended Treatment Device Layout

Table 2-2: Schematic Comparison of Original Devices with Refined Device Layout Devices proposed in Application

Refined device layout for s92 response Motorway chainage (m) West East 370-900 370-1070 Tararata Tararata Pond Pond

370-540 Not included 540-880 Pond 1 (Tararata South) 880-1350 Pond 2 (Tararata North) and swales 1350-2270 Swales and offset mitigation

… South >>

Motorway chainage (m)

75 %

0.09

0.22

1.15

East swale: 900-1350

450

106

91

> 75 %

0.08

0.20

0.94

West swale: 1350-2050

700

175

150

> 75 %

0.13 3.

0.78 3.

2.08 3.

East swale: 1350-2050

700

114

89

> 75 %

0.10

0.21

1.05

East swale: 2050-2200

150

54

49

> 75 %

0.07

0.10

0.50

East swale: 3520-3680

160

36

31

> 75 %

0.10

0.07

0.61

East swale: 4620-5120

500

92

72

> 75 %

0.10

0.17

0.94

Notes

1. 2. 3.

Notes

Contribution of urban drainage infrastructure neglected (refer Section 3.4)

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

SECTION 4

Proposed Outfalls

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

SECTION 4

Proposed Outfalls Table 4-1: Existing Outfalls for Integrated Stormwater Treatment Device System

OE1

Existing pond to Tararata Creek

Location mE mN 302641 691804

OE2

Existing pipe to Tararata Creek

302630

691846

West swale: 1070-1350

302702

694883

East swale: 900-1350

301549

694580

East Gloucester Park Interchange filter device

OE5

301461

694408

900

RL 0.13 m

OE6

301461

694408

1350

RL -0.04 m

OE7

301260

694525

1200

RL 0.02 m

301211

694560

375

RL 0.70 m

301107

694624

900

RL -0.14 m

OE10

300998

694700

1750

RL -0.76 m

OE11

300755

694861

1600

RL -0.46 m

OE12

300755

694861

1850

RL -0.66 m

300755

694861

1050

RL -0.53 m

Outfall

OE3 OE4

OE8 OE9

OE13

Upgraded existing overland flow path to Tararata Creek Existing Auckland City pipe and outfall to Local Lockup tidal inlet

Eight existing pipes and outfalls across Hugh Watt and Orpheus Drives

Existing pipe and outfall at northern Orpheus Drive boat ramp

Drainage system

Diamete r (mm)

Level

Tararata Pond

Weir

RL 2.2 m

1350

RL 2.8 m

450

RL 1.92 m

West causeway filters: 3750-4800

East causeway filters: 3750-4620

East swale: 4620-5120 Beachcroft Road wetland

4-2

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

SECTION 4

Proposed Outfalls

Table 4-2: New Outfalls for Integrated Stormwater Treatment Device System Location mE mN

Outfall

Drainage system

ON1

New pipe to upper Manukau Harbour in vicinity of Rimu Road off-ramp

302409

693153

West swale: 1350-2050 East swale: 1350-2050 No Treatment: 2050-2200 East swale: 2050-2200

ON2

New pipe to harbour

302054

693408

Waterfront Park wetland

ON3

New pipe to harbour

301564

694200

Gloucester Park wetland

ON4

New pipe to harbour

301517

694275

Orpheus Drive at Onehunga Harbour Road

4-3

Diameter (mm)

Comments

1500

Located beside existing outfall To be confirmed in design

To be constructed in sea wall revetment, with reclamation works To be constructed with new reclamation works

SECTION 5

Untreated Catchment Areas 5

Untreated Catchment Areas

5.1

West Carriageway and motorway reserve: 2050-2200

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: 4800-5120

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: 4620-5120. 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

SECTION 5

Untreated Catchment Areas Table 5-1: Proposed untreated areas Location Total

Catchment Area (ha) Urban Pavement Existing New

Pervious

1 % AEP peak flow (l/s)

Comments

West Carriageway: 2050-2200

0.72

0.33

0.01

0.26

0.13

96

Shared new outfall

West Carriageway: 4800-5120

0.99

0.33

0

0

0. 66

92

Shared (upgraded) outfall

Total

1.71

0.66

0.01

0.26

0.79

5-2