northern beaches hospital connectivity and network

Job No: AM351 File:/Reports/NBH_V1_006_Report.doc

Date: October 2014 Rev No: 3.3

Principal: SAB Authors: SAB/BJN

NORTHERN BEACHES HOSPITAL CONNECTIVITY AND NETWORK ENHANCEMENT PROJECT

PROPOSED CONSTRUCTION AND OPERATIONAL PHASE

SURFACE WATER MANAGEMENT STRATEGY CONCEPT PROPOSAL AND STAGE 1 PROJECT

VOLUME 1 – MAIN REPORT AND APPENDICES

OCTOBER 2014

FINAL REPORT

Northern Beaches Hospital Connectivity and Network Enhancement Project Proposed Construction and Operational Phase Surface Water Management Strategy

Concept Proposal and Stage 1 Project

NBH_V1_006_Report.doc Page i Lyall & Associates October 2014 Rev. 3.3 Consulting Water Engineers

TABLE OF CONTENTS Page No.

S1 SUMMARY .................................................................................................................... 1

S1.1 Background and Study Objectives .................................................................... 1 S1.2 Existing Environment ........................................................................................ 1 S1.3 Surface Water Hydrology – Present Day Conditions ......................................... 2 S1.4 Impact of Proposed Road Works on Surface Water Hydrology .......................... 3 S1.5 Surface Water Management Considerations ..................................................... 3 S1.6 Construction Phase Surface Water Management Strategy ................................ 3 S1.5 Operational Phase Surface Water Management Strategy .................................. 3 S1.7 Key Findings in Relation to Concept and Project Approval Requirements ......... 4

1 INTRODUCTION ........................................................................................................... 1

1.1 Study Tasks...................................................................................................... 1 1.2 Outline of Report .............................................................................................. 1 1.3 Available Data .................................................................................................. 2

2 CONCEPT PROPOSAL................................................................................................. 3

3 EXISTING ENVIROMENT .............................................................................................. 5

3.1 General ............................................................................................................ 5 3.2 Catchment Description ..................................................................................... 5

3.2.1 Middle Creek ......................................................................................... 5 3.2.2 Bantry Bay and Curl Curl Creek Catchments ......................................... 6

3.3 Water Quality Control Measures ....................................................................... 6 3.4 Major Waterways .............................................................................................. 7

3.4.1 Narrabeen Lagoon ................................................................................. 7 3.4.2 Manly Dam ............................................................................................ 7 3.4.3 Bantry Bay ............................................................................................. 8

3.5 Catchment Biodiversity ..................................................................................... 9

4 SURFACE WATER HYDROLOGY - PRESENT DAY CONDITIONS ............................ 10

4.1 Water Quantity ............................................................................................... 10 4.1.1 General ............................................................................................... 10 4.1.2 DRAINS Model Development ............................................................... 10 4.1.3 Concept Proposal ................................................................................ 11 4.1.4 Stage 1 Project .................................................................................... 12

4.2 Water Quality .................................................................................................. 15 4.2.1 General ............................................................................................... 15 4.2.2 Concept Proposal ................................................................................ 17 4.2.3 MUSIC Model Development ................................................................. 17 4.2.4 Stage 1 Project .................................................................................... 17

5 IMPACT OF PROPOSED ROAD WORKS ON SURFACE WATER HYDROLOGY ....... 19

5.1 Water Quantity Related Impacts ..................................................................... 19 5.1.1 Concept Proposal ................................................................................ 19 5.1.2 Stage 1 Project .................................................................................... 19

5.2 Water Quality Related Impacts ....................................................................... 19 5.2.1 Concept Proposal ................................................................................ 19 5.2.1 Stage 1 Project .................................................................................... 22

6 SURFACE WATER MANAGEMENT CONSIDERATIONS ........................................... 26

6.1 Water Quantity Related Considerations .......................................................... 26 6.1.1 Design Development ........................................................................... 26



NBH_V1_006_Report.doc Page ii Lyall & Associates October 2014 Rev. 3.3 Consulting Water Engineers

6.1.2 Design Standards ................................................................................ 26 6.1.3 Potential Blockage of the Road Drainage System ................................ 26 6.1.4 Utilities ................................................................................................ 26 6.1.5 Potential to Impact Flooding Conditions in Existing Development ......... 26 6.1.6 Future Development Upslope of the Road Corridor .............................. 27 6.1.7 Future Climate Change ........................................................................ 27

6.2 Water Quality Related Considerations ............................................................ 28 6.2.1 Construction Phase Stormwater Runoff ............................................... 28 6.2.2 Operational Phase Stormwater Pollution Reduction Targets ................ 28 6.2.3 Water Sensitive Urban Design ............................................................. 29 6.2.4 Spill Containment ................................................................................ 30

7 CONSTRUCTION PHASE SURFACE WATER MANAGEMENT STRATEGY .............. 31

7.1 General .......................................................................................................... 31 7.2 Key Elements ................................................................................................. 31

7.2.1 Erosion Control .................................................................................... 31 7.2.2 Sediment Control ................................................................................. 32

7.3 Concept Proposal ........................................................................................... 32 7.4 Stage 1 Project ............................................................................................... 33

8 OPERATIONAL PHASE SURFACE WATER MANAGEMENT STRATEGY ................. 35

8.1 Water Quantity Related Aspects ..................................................................... 35 8.1.1 Concept Proposal ................................................................................ 35 8.1.2 Stage 1 Project .................................................................................... 35

8.2 Water Quality Related Aspects ....................................................................... 38 8.2.1 Concept Proposal ................................................................................ 38 8.2.2 Stage 1 Project .................................................................................... 38

9 REFERENCES ............................................................................................................ 40

APPENDICES A. Detailed Description of Drainage Lines 1, 2 and 3

B. Plates Showing Key Features of Drainage Lines 1, 2 and 3

C. Tables Summarising Estimated Average Annual Soil Loss from Disturbed Areas along Stage 1 Project

D. Detailed Description of Proposed Operational Surface Water Management Strategy - Stage 1 Project

E. Table Summarising Impact of Stage 1 Project on Peak Flows



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LIST OF FIGURES (BOUND IN VOLUME 2)

1.1 Location Plan 2.1 Layout Plan 3.1 Catchment Plan 3.2 Existing Stormwater Drainage System – Stage 1 Project Area (4 Sheets) 4.1 Concept Proposal DRAINS Model Layout 4.2 Extent of Sub-Catchments Comprising Stage 1 Project MUSIC Model 7.1 Extent of Disturbed Areas – Stage 1 Project Area (4 Sheets) 8.1 Operational Phase Surface Water Management Strategy – Stage 1 Project Area (4 Sheets)

ABBREVIATIONS

AHD Australian Height Datum

ALS Airborne Laser Scanning (survey data)

ARI Average Recurrence Interval (years)

ARR Australian Rainfall and Runoff, 1998 Edition

DRC Design Road Chainage

DPI NSW Department of Primary Industries

DFEC Duffys Forest Ecological Community

GIS Geographic Information System

LESCA Local Erosion and Sediment Control Area

LGA Local Government Area

ML Megalitres

RCP Reinforced Concrete Pipe

RL Reduced Level

Roads and Maritime Roads and Maritime Services

RUSLE Revised Universal Soil Loss Equation

SWMP

TN

TP

Soil and Water Management Plan

Total Nitrogen

Total Phosphorus

WC Warringah Council



NBH_V1_006_Report.doc Page S1 Lyall & Associates October 2014 Rev. 3.3 Consulting Water Engineers

S1 SUMMARY

S1.1 Background and Study Objectives

Roads and Maritime Services (Roads and Maritime) is proposing to carry out road works that are classified as State Significant Infrastructure (SSI), thereby requiring approval from the Minister administering the Environmental Planning and Assessment Act 1979 (EP&A Act). The road works comprising the SSI are aimed at enhancing the connectivity of the arterial and sub-arterial road network in the vicinity of the proposed Northern Beaches Hospital (NBH) at Frenchs Forest and are to be known as the Northern Beaches Hospital – Connectivity and Network Enhancement Project (Concept Proposal). Figure 1.1 bound in Volume 2 of this report shows the extent of the road works in relation to the NBH site.

The Concept Proposal has been divided into two projects, as shown on Figure 2.1. The first project comprises road works directed toward facilitating enhanced road network connectivity to the NBH site (Stage 1 Project), while the second project provides broader network capacity enhancement that has been identified at a strategic level (Stage 2 Project). Chapter 2 of this report provides a brief description of the key features which comprise the Concept Proposal.

Roads and Maritime is currently seeking Concept and Project Approval for the Concept Proposal and Stage 1 Project, respectively. This report will form an Appendix to the Environmental Impact Statement (EIS) that is being prepared as part of the approval process.

The objectives of the present study in regards the Concept Proposal and more specifically the Stage 1 Project were to identify the impact the projects will have on:

surface water hydrology in terms of its quantity and quality during both the construction and operational phases of the road upgrade works;

flooding conditions in existing development; and

the condition of the receiving drainage lines.

The findings of the study were then used as the basis for developing a strategy which is aimed at mitigating both the construction and operational related impacts of the road works on flooding conditions in existing development and on the quantity and quality of stormwater runoff discharging to the receiving drainage lines. S1.2 Existing Environment

The Concept Proposal is located in the headwaters of the following three catchments:

Middle Creek which drains north into Narrabeen Lagoon;

an unnamed watercourse which drains south to Bantry Bay in Middle Harbour; and

Curl Curl Creek which drains south to Manly Dam. Figure 1.1 shows the extent of these catchments relative to the Concept Proposal. The drainage lines which control runoff from the upper reaches of the aforementioned catchments have been piped along most of their length as a result of urbanisation. Figure 3.1 shows the layout of the existing stormwater drainage system in the vicinity of the Concept Proposal, while Figure 3.2 (3 Sheets) shows details of the pit and pipe arrangement in the vicinity of the Stage 1 Project.




Stormwater generated by the urbanised portion of the catchments discharges to several drainage lines that are in either a semi-natural or natural state. The bed slope of these receiving drainage lines is relatively steep, with exposed rock observed in their inverts at several locations. Figure 2.1 shows the location of these receiving drainage lines and the naming convention adopted for the purpose of this report.

A biodiversity study undertaken as part of the EIS process identified a number of endangered flora and fauna species which could potentially be impacted by the Concept Proposal. For example, Red-crowned Toadlet habitat was identified in the headwaters of Drainage Line 2 immediately north of Frenchs Forest Road East and along Drainage Line 6 south of Warringah Road.

Further discussion on the existing environment which could potentially be impacted by the Concept Proposal and more specifically the Stage 1 Project is contained in Chapter 3 of this report.

S1.3 Surface Water Hydrology – Present Day Conditions

Hydrologic and hydraulic modelling was undertaken using the DRAINS software to assess the hydrologic standard of the local stormwater drainage system and to assess peak flows in the receiving drainage lines. Figure 4.1 shows the layout of the DRAINS model that was developed as part of the present study. It was found that the local stormwater drainage system has a hydrologic standard which is generally less than 5 year average recurrence interval (ARI). Excessive widths of gutter flow were also found to be present within the local road network which will be upgraded as part of the Stage 1 Project (refer Table 4.1 in Chapter 4 for details).

The study also identified the following three locations where existing flood prone development will be impacted by the Stage 1 Project should appropriate mitigation measures not be implemented as part of the road works:

northern side of Frenchs Forest Road immediately east of Rabbett Street;

western side of Nandi Avenue north of Frenchs Forest Road East; and

between Patanga Road and Jones Street north of Frenchs Forest Road East. Further discussion on the existing hydrologic standard of the local stormwater drainage system is contained in Chapter 4 of this report, while a table containing peak flows at the locations shown on Figure 3.2 (4 Sheets) is contained in Appendix E.

Water quality sampling has been undertaken as part of a local government area (LGA) wide creek management study which was prepared on behalf of Warringah Council (WC) in 2004. Table 4.2 in Chapter 4 of this report summarises the results of the sampling as they relate to the Middle Creek, Bantry Bay and Curl Curl Creek catchments.

A pollutant load generating model was developed as part of the present study of the upper Middle Creek catchment. The MUSIC software was used for this purpose. Figure 4.2 shows the extent of the sub-catchments which comprise the MUSIC model of the upper Middle Creek catchment. The MUSIC model was used to estimate the average annual weight of pollutants which is presently being generated within the catchments through which the Stage 1 Project runs. Table 4.3 in Chapter 4 summarises the results of the MUSIC modelling for present day catchment conditions.




S1.4 Impact of Proposed Road Works on Surface Water Hydrology

The widening of the existing road network and the upgrade of the existing pavement drainage system within the road corridor will increase peak flows in the local stormwater drainage system and the receiving drainage lines. The resulting increase in peak flows has the potential to exacerbate flooding conditions in existing development and to cause scour and bank instability in the semi-natural and natural reaches of the drainage system. Further details on the water quantity related impacts of the Concept Proposal and more specifically the Stage 1 Project are contained in Section 5.1 of this report (refer also Table 5.1 in Chapter 5 which summarises the water quantity related impacts of the Concept Proposal) .

Increases in pollutant load attributed to both the construction and operational phases of the road upgrade works has the potential to impact existing ecological systems located along the receiving drainage lines. Further details on the water quality related impacts of the Concept Proposal and more specifically the Stage 1 Project are contained in Section 5.2 of this report (refer also Table 5.2 in Chapter 5 which summarises the major ecosystem management issues associated with pollutants present in road runoff and Tables 5.3 to 5.5 which quantify the impact the Stage 1 Project will have on the pollutant load in the receiving drainage lines absent any mitigation measures.

S1.5 Surface Water Management Considerations

A number of water quantity and quality related criteria were considered when developing the construction and operational phase surface water management strategy for the Concept Proposal and more specifically the Stage 1 Project, details of which are set out in Chapter 6 of this report.

S1.6 Construction Phase Surface Water Management Strategy

The construction of the road upgrade has the potential to impact existing ecosystems by increasing sediment loads and turbidity levels in stormwater runoff. A strategy has therefore been developed which is aimed at mitigating the adverse impacts of construction activities on the quality of water discharging to the receiving drainage lines. A description of the key elements comprising the construction phase surface water management strategy for the Concept Proposal and more specifically the Stage 1 Project is presented in Chapter 7 of this report. Figure 7.1 (4 Sheets) shows the areas which will be disturbed during construction of the Stage 1 Project.

S1.5 Operational Phase Surface Water Management Strategy

A strategy has been developed as part of the present study which is aimed at mitigating the impacts of the road works on both the quantity and quality of stormwater runoff discharging to the receiving drainage lines. The strategy involves the provision of temporary stormwater de tention along Warringah Road as well as the rerouting and upgrade of the local stormwater drainage system in the affected areas.

Mitigation measures incorporated in the strategy for the Stage 1 Project are aimed at ensuring:

the hydrologic regime is not altered in areas identified as Red-crowned Toadlet habitat;

flooding conditions are not exacerbated in existing development;

scour within the receiving drainage lines immediately downstream of upgraded sections of the local stormwater drainage system is prevented; and

bank instability is not exacerbated as a result of the road works.




An outline of the water quantity related aspects of the operational surface water management strategy for the Concept Proposal and more specifically the Stage 1 Project is provided in Section 8.1 of this report. A detailed description of the stormwater upgrade requirements for the Stage 1 Project is contained in Appendix D. The operational surface water management strategy developed for the Stage 1 Project also ensures that gross pollutant and suspended solid loads will not be increased in the receiving drainage lines when compared to present day conditions. An outline of the water quality related aspects of the operational surface water management strategy for the Concept Proposal and more specifically the Stage 1 Project is provided in Section 8.2 of this report. Tables 5.3 to 5.5 in Chapter 5 of this report quantify the impact the installation three in-line pollution control devises as part of the Stage 1 Project will have on the pollutant load in the receiving drainage lines. Figure 8.1 (4 Sheets) shows the strategy for managing stormwater runoff during the operational phase of the Stage 1 Project. S1.7 Key Findings in Relation to Concept and Project Approval Requirements

The key findings of the investigation as they relate to the Director General’s Environmental Assessment Requirements (DGEAR’s) that have been issued for the Concept Proposal and Stage 1 Project are respectively set out in Tables S1 and S2 over page. Also presented are the measures which are aimed at mitigating the surface water related impacts of the road upgrade projects on existing development and the receiving environment.




TABLE S1 SUMMARY OF DIRECTOR GENERAL’S ENVIRONMENTAL ASSESSMENT REQUIREMENTS

CONCEPT PROPOSAL

Issue Key Issues and Requirements Description of Potential Project Related Impacts and Mitigation Measures

Soils, Water and Waste

Potential erosion, sediment and water quality and quantity impacts, including assessment of:

potential water quality and quantity impacts and likely mitigation and manage measures;

Assessed Impact(s) The road works have the potential to increase peak flows downstream of the road corridor due to an increase in impervious are a and improvements in

the efficiency of the road drainage system. The road works have the potential to increase pollutant loads discharging to the various drainage lines.

Potential Mitigation Measures Measures aimed at mitigating the impacts of increased flow on existing development could include diversion of flow away from flood prone property and

temporary containment of stormwater runoff in Warringah Road. Scour protection measures will need to be provided where upgraded sections of the local stormwater drainage system discharge to the receiving

drainage lines. Measures aimed at mitigating the impact of the project on pollutant loads could include the provision of in -line pollution control devices and storage

facilities for spill containment.

potential impacts on groundwater recharge and flow path; Assessed Impact(s) Assessment to be undertaken by others.

Potential Mitigation Measures Assessment to be undertaken by others.

potential impacts of road widening and decreased permeability on downstream catchments and wetlands.

Assessed Impact(s) The increase in impervious area associated with the road works will increase the average annual volume of runoff discharging to the receiving drainage

lines. The increase in the runoff potential of the catchment will assist in maintaining wet areas, such as wetlands.

Potential Mitigation Measures No mitigation measures are proposed to manage the increased volume of water discharging to the receiving drainage lines given the benefit this would

have on the receiving environment.




TABLE S2 SUMMARY OF DIRECTOR GENERAL’S ENVIRONMENTAL ASSESSMENT REQUIREMENTS

STAGE 1 PROJECT

Issue Key Issues and Requirements Description of Potential Project Related Impacts and Proposed Mitigation Measures

Visual Amenity, Built Form and Urban Design

Incorporation of water sensitive urban design. Water sensitive urban design is not well suited to the Concept Proposal, which involves the upgrade of arterial and sub -arterial roads with higher speed

limits and limited landscaped areas within the road corridor.


Erosion, sediment and water quality impacts, including an assessment of:

potential water quality impacts and mitigation measures to manage water pollution, with reference to relevant public health and environmental water quality criteria, including those specified in the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC/ARMCANZ 2000), and any applicable regional, local or site-specific guidelines;

Assessed Impact(s) The Stage 1 Project will increase the pollutant load discharging to the receiving drainage lines.

Proposed Mitigation Measures Given the confined nature of the road corridor there is insufficient space to incorporate measures which could achieve the ca tchment-based reduction

targets which have been adopted by WC for the Middle Creek catchment. As a result, measures are limited to the installation of three in -line pollution control devices that have been strategically located within the catchment to offset the incremental increase in gross polluta nt and suspended solid load generated by the project.

proposed storm water management system and management measures for the containment of pollutants and minimisation of leachate and sediment mobilisation;

The in-line pollution control devices referred to above will contain pollutants and minimise leaching and sediment remobilisation.

impacts on soil and slope stability resulting from vegetation clearing;

Assessed Impact(s) The road works principally involve only minor widening of existing carriageways, with the exception of the upgrade of the Wakehurst Parkway and

Frenchs Forest Road (East and West) intersection which will involve major cut activities. Proposed Mitigation Measures

Cut batters will be stabilised following road widening works.

impacts on watercourses, including bed and bank stability; Assessed Impact(s) The road works have the potential to significantly increase peak flows in the receiving drainage lines, resulting in increase d scour and bank instability.

Proposed Mitigation Measures No new drainage paths will be created in bushland areas as part of the road works. Upgraded sections of piped drainage system will be extended to locations where the incremental increase in peak flow attribut able to the project will not

cause scour or bank instability problems in the receiving drainage lines. Due to the presence of rock in the bed of Drainage Lines 1 and 3 at the point of discharge of the upgraded drainage system, t he project will not cause

scour at the location of the outlet headwalls. Scour protection and energy dissipation measures will be provided on the outlet of the new trunk drainage line which will discharge to the main arm of

Drainage Line 2 downstream of the location where it crosses the Wakehurst Parkway.

potential erosion and sediment controls consistent with Managing Urban Stormwater – Soils and Construction (Landcom 2004); and

Assessed Impact(s) While the road works principally involve minor widening of existing carriageways, a large volume of sediment could potentiall y be deposited in the

receiving drainage lines if suitable erosion and sediment controls are not implemented during the construction phase of the p roject. There is also the potential for relatively high sediment loads to be generated during excavation of the proposed roa d cuttings at the intersection of

Wakehurst Parkway and Frenchs Forest Road (East and West). Proposed Mitigation Measures

Effective local erosion and sediment control measures will be implemented which are aimed at minimising the volume of sedimen t which is transported from disturbed areas.

Given the close proximity of bushland to the Wakehurst Parkway and Frenchs Forest Road (East and West) intersection, it is re commended that sediment retention basins (or sumps) be installed adjacent to the stormwater pipes controlling runoff from the cut areas immediately after commencement of the initial excavation works. Furthermore, excavation for the cuttings should not be undertaken in parallel, but rather staged so that work on a new cutting does not commence until the surface of the completed cutting is stabilised.

management of and disposal strategies for acid sulphate soils, in accordance with the Acid Sulphate Soils Manual (ASSMAC).

Assessed Impact(s) Assessment to be undertaken by others.

Potential Mitigation Measures Assessment to be undertaken by others.

Cont’d Over




TABLE S2 (Cont’d) SUMMARY OF DIRECTOR GENERAL’S ENVIRONMENTAL ASSESSMENT REQUIREMENTS

STAGE 1 PROJECT

Issue Key Issues and Requirements Description of Project Related Impacts and Proposed Mitigation Measures


Water quantity and stormwater impacts, including:

a hydraulic/hydrological study of the impacts on Council’s stormwater drainage system;

Assessed Impact(s) The road works have the potential to result in a reduction in the hydrologic standard of WC’s stormwater drainage system due to an increase in the rate

of runoff generated by the widened sections of road. Proposed Mitigation Measures

The operational phase surface water management strategy which has been developed for the Stage 1 Project incorporates requirements to upgrade sections of the existing stormwater drainage system. The aim of the upgrades is to prevent adverse flooding conditions being experienced in existing development.

impacts on water flows into and surrounding The Forest High School;

Assessed Impact(s) The road works will not increase either the rate or volume of runoff discharging to The Forest High School.

Proposed Mitigation Measures None.

details of any works likely to intercept, connect with or infiltrate groundwater services and of any proposed groundwater extraction; and

Assessed Impact(s) Assessment to be undertaken by others.

Proposed Mitigation Measures Assessment to be undertaken by others.

measures to mitigate or prevent an increase in downstream stormwater flows.

Assessed Impact(s) The road works have the potential to significantly increase peak flows in the receiving drainage lines, resulting in increase d scour and bank instability.

Proposed Mitigation Measures The operational phase surface water management strategy which has been developed for the Stage 1 Project incorporates requirements to upgr ade

sections of the existing stormwater drainage system. The aim of the upgrades is to prevent adverse flooding conditions being experienced in existing development and to discharge road runoff to the receiving drainage lines at locations where the incremental increase in peak flow attribu table to the project is insignificant.



NBH_V1_006_Report.doc Page 1 Lyall & Associates October 2014 Rev. 3.3 Consulting Water Engineers

1 INTRODUCTION

1.1 Study Tasks

The study tasks broadly as they relate to the Concept Proposal and more specifically the Stage 1 Project comprised the following:

Investigate and describe the existing environment which could potentially be impacted by the proposed road works.

Assess the quantity and quality of surface water runoff which presently discharges to the receiving drainage lines downstream of the road corridors.

Assess the impact the proposed road works will have on the quantity and quality of surface water runoff discharging to the receiving drainage lines.

Assess the impact any changes in the quantity and quality surface water runoff of will have on existing development and the receiving drainage lines.

Broadly identify surface water hydrology related issues which will need to be considered during the development of the construction and operational phase surface water management strategy for the proposed road works. This includes ascertaining water quantity and quality control criteria relevant to the catchments through which the Concept Proposal runs.

Scope measures which are aimed at mitigating the impacts of the proposed road works on the quantity and quality of surface water runoff discharging to the receiving drainage lines.

Formulate a construction and operational phase surface water management st rategy for the proposed road works.

1.2 Outline of Report

Section 2 provides a brief overview of the Concept Proposal and its key features. Section 3 deals with the existing environment in the vicinity of the Concept Proposal. Included in this section of the report is a brief description of the catchments through which the Concept Proposal runs and the receiving drainage lines and major waterways into which runoff from the road network presently discharges. Also presented in this section of the report is a brief description of the existing flora and fauna which could be impacted upon by changes in the quantity and quality of surface water runoff. Section 4 deals with the investigation that was carried out to assess the quantity and quality of surface water runoff which presently discharges to the receiving drainage lines downstream of the road corridors. This section of the report also includes an assessment of the hydrologic standard of the local stormwater drainage system and sets out indicative widths of gutter flow along the sections of road which will be upgraded as part of the Stage 1 Project. Section 5 describes the impact the proposed road works will have on the quantity and quality of surface water runoff discharging to the receiving drainage lines. Also presented in this section of the report is a description of the impact these changes will have on existing development and the receiving drainage lines.




Section 6 provides a brief description of the surface water hydrology related issues associated with both the construction and operational phases of the Concept Proposal. Also included in this section of the report is a list of measures which could be implemented to mitigate the surface water hydrology related impacts of the Concept Proposal.

Section 7 outlines the strategy which is to be adopted for reducing the erosion potential in areas which will be disturbed during the construction of the proposed road works. This includes requirements for local erosion and sediment control measures along the length of the road upgrade, including where feasible sediment retention basins.

Section 8 presents the recommended strategy for managing operational stormwater runoff along the upgraded lengths of road both in terms of its quantity and quality. This includes requirements for energy dissipation and scour protection at the outlet of several new piped drainage lines, as well as the installation of three in-line pollution control devices which will mitigate the impact of the Stage 1 Project on the gross pollutant and suspended solid load discharging to the receiving drainage lines.

Section 9 contains a list of documents referenced in this report.

Appendix A contains a detailed description of the local stormwater drainage system which presently controls runoff from the road network. Appendix B contains a series of plates showing key features of the drainage system which could potentially be impacted by the proposed road works. Appendix C contains tables setting out the estimated average annual soil loss from areas which will be disturbed during the construction of the Stage 1 Project. Appendix D contains a detailed description of the proposed upgrade requirements for the local stormwater drainage system as part of the Stage 1 Project. Appendix E contains a table which gives peak flows at key locations in the Middle Creek catchment under both present day and post -Stage 1 Project conditions.

Figures referred to in this report are bound separately in Volume 2.

1.3 Available Data

The following data were made available by Roads and Maritime for this present investigation:

Aerial photography covering the study area.

Airborne laser scanning (ALS) survey data covering the study area.

Detailed ground survey information along the route of the proposed road works , including the location and level of drainage pits.

Current concept road design models for the proposed road works.

Drawings showing the indicative location of existing utilities within the road corridor.

GIS datasets including property boundary information.

The following additional data were obtained during the course of the investigation:

GIS data sets were obtained from WC showing details of the existing stormwater drainage systems, water quality devices and natural drainage lines in the upper reaches of the Middle Creek Bantry Bay and Curl Curl Creek catchments.

Rainfall data were obtained from Manly Hydraulics Laboratory for the pluviographic stations at Glen Street Belrose (Station Number 566149), Belrose West (Station Number 566143), Middle Creek (Station Number 566147) and North St Ives (566084).




2 CONCEPT PROPOSAL

The strategic design of the Concept Proposal broadly includes the following:

Widening of Warringah Road and the provision of a centralised “slot” (Slot) to increase capacity and to enable through traffic to avoid signalised intersections at Forest Way and Wakehurst Parkway through grade separation.

Widening, intersection upgrades (including new signals) and potential changes to access along sections of Forest Way, Naree Road, Frenchs Forest Road West, Wakehurst Parkway, Frenchs Forest Road East, Allambie Road and Aquatic Drive.

Traffic management measures along local roads. Table 2.1 over page lists the key features of the Concept Proposal separated into the Stage 1 and Stage 2 Projects, while Figure 2.1 shows the extent of the arterial and sub-arterial road network which will be upgraded as part of the project. The Concept Proposal will also include drainage works, landscaping, property acquisition and adjustments, utility relocations (which may be required to extend into surrounding streets), as well as ancillary works during construction including but not limited to, construction compounds, sedimentation basins, batch plants, and stockpile sites. For the purposes of this report, the scope of the Concept Proposal does not include the provision of ongoing maintenance works. Note that a separate design package has been prepared for the widening of the southbound left hand turn slip lane from Wakehurst Parkway into Frenchs Forest Road East. This package included the extension of an existing 825 mm diameter pipe which crosses Wakehurst Parkway about 40 m north of Frenchs Forest Road West and the provision of outlet scour protection and energy dissipation measures.




TABLE 2.1 KEY FEATURES OF THE CONCEPT PROPOSAL

Key Feature Stage 1 Project

Stage 2 Project

Widening and intersection upgrades along sections of Forest Way between Warringah Road and the vicinity of Adams Street.

X

Upgrade of the existing bus stop fronting the Forest Way shopping centre on Forest Way, to accommodate two buses within the bay.

X

Widening of Naree Road, Frenchs Forest Road West and a section of Frenchs Forest Road East from Wakehurst Parkway intersection to Skyline Place.

X

Provision of a new signalised intersection on Naree Road and Frenchs Forest Road West, on Frenchs Forest Road West as a new access to the hospital, on Frenchs Forest Road East and Romford Road, and on Frenchs Forest Road East, Patanga Road and Allambie Road.

X

Provision of dedicated kerb side bus lanes along approaches and departures to the Frenchs Forest Road West and East intersection with Wakehurst Parkway.

X

Widening and upgrades to the intersection of Frenchs Forest Road and Wakehurst Parkway.

X

Widening of Wakehurst Parkway from about 450 metres north of the intersection with Frenchs Forest Road to the intersection with Warringah Road.

X

Widening of Allambie Road to the north of the intersection with Warringah Road. X

Widening and upgrade to the Warringah Road and Allambie Road intersection. X

Widening of Warringah Road around 500 metres west of the intersection with Allambie Road to the vicinity of Courtley Street in the east.

X

Widening and upgrades along Warringah Road, including grade separated pedestrian facilities.

X

Grade separation of Warringah Road via an underpass at the intersection with Forest Way.

X

Grade separation of Warringah Road via an underpass at the intersection with Wakehurst Parkway.

X

Connecting Aquatic Drive with Wakehurst Parkway.

X

Grade separation of Warringah Road via an underpass at the intersection with Hilmer Street.

X

Widening and upgrades along sections of Wakehurst Parkway from about 140metres north of the intersection with Warringah Road to about 500 metres south of the intersection.

X

Widening and upgrades of Allambie Road between Warringah Road and Rodborough Road.

X

Minor upgrades of Rodborough Road between Warringah Road and Allambie Road.

X




3 EXISTING ENVIROMENT

3.1 General

The following chapter provides a brief description of the catchments through which the Concept Proposal runs and their associated drainage systems. Also included in this chapter are the findings of a review which was undertaken to identify existing measures which are aimed at controlling runoff from the road corridor, a brief description of the major waterways into which runoff from the road network discharges and the flora and fauna which could be impacted by changes in surface water hydrology. 3.2 Catchment Description

3.2.1 Middle Creek

The Stage 1 Project is located within the headwaters of the Middle Creek catchment, runoff from which discharges into Narrabeen Lagoon on its western edge. Figure 1.1 shows the extent of the catchment which drains to Narrabeen Lagoon. Figure 3.1 shows the extent of the fifteen (15) catchments which drain to existing transverse drainage structures located along the sections of rod which will be upgraded as part of the Stage 1 Project (refer catchments labelled with prefixes 1, 2 and 3). Land use within these catchments is predominantly residential in nature, but also includes commercial and industrial development on the southern side of Frenchs Forest Road East and Warringah Road. The Forest High School is located on the southern side of Frenchs Forest Road West, immediately west of the proposed NBH site. The prevailing terrain within the road corridor is moderate to steep, with maximum longitudinal grades up to about 5 per cent, except for the area in the vicinity of Wakehurst Parkway where grades of up to about 8 per cent are present. Runoff from the sections of road which will be upgraded as part of the Stage 1 Project presently discharges to three receiving drainage lines (denoted herein as Drainage Lines 1, 2 and 3). Figure 3.2 (3 Sheets) shows details of the catchments and associated road drainage systems along the length of the Stage 1 Project which presently discharge to Drainage Lines 1, 2 and 3. Drainage Line 1 forms the main arm of Middle Creek and drains the section of road corridor between Adam Street and Gladys Avenue. The section of the road corridor between Gladys Avenue and Romford Road drains to Drainage Line 2, which is referred to as Trefoil Creek and is a tributary Middle Creek. Drainage Line 3 is an unnamed tributary of Middle Creek that drains the section of road corridor between Romford Road and Courtley Road. The inbank area of Drainage Lines 1 and 3 immediately downstream of the piped outlets is characterised by exposed rock and a steeply graded channel. Drainage Line 2 is fed by several piped drainage lines which discharge to the steep sided valley north of Frenchs Forest Road East. Drainage Line 2 also appears to have been realigned where it runs along the rear of several residential properties that are located along the western side of Nandi Avenue north of Frenchs Forest Road East. A detailed description of Drainage Lines 1, 2 and 3 is provided in Appendix A.




3.2.2 Bantry Bay and Curl Curl Creek Catchments

The Stage 2 Project is located principally within the headwaters of the Bantry Bay and Curl Curl Creek catchments (refer Figure 1.1). Figure 3.1 shows the extent of the six catchments which drain in a southerly direction and contribute to flow in the existing road drainage systems along the Stage 2 Project (refer catchments labelled with prefixes 4, 5 and 6). Land use within these catchments comprises residential development and the Frenchs Forest Public School west of Forest Way, a portion of the NBH site and Skyline Business Park. Runoff from a portion of a vacant parcel of land which is located on the western side of Wakehurst Parkway also discharges south where it contributes to flow in the road drainage system. The prevailing terrain along Warringah Road is moderate in nature east of Wakehurst Parkway, becoming steeper further to the west, while Wakehurst Parkway has a grade of about 3 per cent immediately south of Warringah Road.1 Runoff from the sections of road which are to be upgraded as part of the Stage 2 Project discharges south into three receiving drainage lines (denoted herein as Drainage Lines 4, 5 and 6). Drainage Lines 4 and 5 form part of the unnamed watercourse which drains south into Bantry Bay in Middle Harbour, while Drainage Line 6 is an unnamed tributary of Curl Curl Creek. Drainage Line 4 has been partially modified where its runs south from Parni Place within a narrow landscaped reserve, while Drainage Line 5 is in a more natural state where it runs through bushland south of same road. Drainage Line 6 is also in a more natural state where it runs through the Brick Pit Reserve and bushland which is located on the eastern (downstream) side of Wakehurst Parkway. 3.3 Water Quality Control Measures

Review of WC’s stormwater database shows that there are presently no water quality control measures within the road corridor. However, there is a sediment pond located on Drainage Line 1 in Rabbett Reserve, approximately 140 m downstream of the road corridor. The location of the Rabbett Reserve sediment pond is shown on Figure 3.2, Sheet 1 of 4, while plates showing key features of the structure are contained in Appendix B (refer Plates 1 to 3). The Rabbett Reserve sediment pond comprises a permanent storage zone that is approximately 40 square metres in surface area. A screen is also provided across the outflow weir to retain gross pollutants. There are presently no water quality measures controlling runoff from the urban catchments discharging to Drainage Lines 2 to 6 in the immediate vicinity of the road corridor.

1 Warringah Road has a grade of about 7 per cent immediately west of its intersection with Forest Way.




3.4 Major Waterways

3.4.1 Narrabeen Lagoon2

Narrabeen Lagoon is the largest of four coastal lagoons within the WC LGA with a surface area of approximately 2 km2 and a catchment area of about 55 km2.

The lagoon comprises a fragile and diverse aquatic and terrestrial ecosystem. It has been identified by the NSW Department of Primary Industries (DPI) as key fish habitat with significant seagrass meadows being a key contributor to the quality of this habitat. The seagrass meadows provide nursery habitat for economically important juvenile fish species. Seagrass meadows also provide habitat for the Hairy Pipefish (Urocampus carinirostris) which is a listed marine species under Section 248 of the Environment Protection and Biodiversity Conservation Act 1999 .

The lagoon also provides habitat for bird life and the islands within the lagoon have vegetation communities that are listed as endangered under the NSW Threatened Species Conservation Act 1995. Narrabeen Lagoon is owned by the State of NSW and is administered as a reserve under the Crown Lands Act 1989 by the DPI.

Water quality in the lagoon ranges from good at the entrance (due to effective tidal flushing) to poor at the western basin, with elevated concentrations of nutrients and algae (WBM Oceanics Australia, 2000). Water quality in the lagoon, particularly in the central and western basins, is dominated by the quality of catchment runoff as tidal flushing in these sections of the lagoon is very poor. Typical flushing times are more than 90 days. As such, the lagoon has a ‘low pollutant assimilation capacity’ (WBM Oceanics Australia, 2000).

The lagoon is dominated by freshwater during and immediately following catchment runoff events, Salinity is low, pH is neutral and the water temperature is generally cooler (WBM Oceanics Australia, 2000). Higher turbidity is also associated with such events due to stirring of the bed sediments, as well as suspension of fine sediment washed off the catchment.

WBM Oceanics Australia, 2001 investigated the quality of stormwater discharging into the lagoon. It found that Total Nitrogen (TN), Total Phosphorus (TP) and Total Suspended Solids (TSS) are the main constituents of stormwater to the lagoon.

3.4.2 Manly Dam3

Manly Dam has a capacity of approximately 2,000 megalitres (ML) and its water body is sheltered and deep (in most parts) with a surface area of approximately 30 ha. The dam wall was first constructed across Curl Curl Creek in the late 1800’s to supply drinking water to the local settlement. The role of the dam in the supply of drinking water continued until 1936, after which time it became an important recreational facility for the local area and beyond.

The water body currently provides a valued facility for swimming, fishing, water skiing, canoe/kayaking and boating, among other activities. Significantly Manly Dam offers the largest, and only, sheltered freshwater “lake” on the northern beaches. A growing population within the area, and the Sydney region, is placing more and more demand on the water body as a recreational venue. 2 Source of Description: SMEC, 2011 3 Source of Description: Gondwana Consulting Pty Ltd, 2014




WC is currently in the process of developing a Plan of Management for the Manly Warringah War Memorial Park in which Manly Dam is located. In regards the dam, the Plan of Management has the following stated objectives:

“Management Intent – To manage the waterbody as both a recreational and flood management asset, ensuring that water quality is suitable for swimming, and providing for safe, equitable and sustainable recreational use.

Primary management objectives:

o To collaborate with other agencies involved in the dam’s management, and manage water levels in the dam to meet flood management requirements while also catering for recreational use and visual appeal.

o To manage water quality in the dam suitable for swimming and other “primary contact” uses, and to provide for equitable and fair recreational access to and use of the waterbody catering for a range of sustainable water-based activities and uses.

o To provide for visitor safety.

Secondary management objectives:

o To manage impacts from recreational use of the waterbody on surrounding Park values and uses.

o To accommodate occasional “special use” recreational, community or educational activities, with prior approval.”

3.4.3 Bantry Bay

4

Bantry Bay is located in the Garigal National Park in Middle Harbour and is the site of the former government operated Explosives Magazines. The Magazines were operated on a commercial basis by the State Government and used chiefly for the receiving, storage and distribution of explosives used for civil and commercial purposes. The Bantry Bay Explosives Magazines played an important role in supplying explosives for major 1930s public works – including building of the Sydney Harbour Bridge, underground tunnels and railways in the City, and the Newcastle Highway – as well as in serving an expanding mining industry. The NSW National Parks and Wildlife Service has developed a small walk-in picnic area on the bay’s eastern foreshore; however, this site is little-known and receives only limited visitor use. The bay itself is, at times, a popular destination and refuge for the boating community. Together, the area’s closure, difficult access and limited public knowledge have combined to maintain the Explosives Magazines’ seclusion and isolation. Today Bantry Bay remains a largely unknown part of Sydney, despite its location right behind people’s backyards. Due to its deep nature and close proximity to Middle Harbour, the bay is subject to regular tidal flushing.

4 Source of Description: Fact Sheet accessible on Manly Council’s web site.




3.5 Catchment Biodiversity5

The portion of the Middle Creek, Bantry Bay and Curl Curl Creek catchments through which the Concept Proposal runs has been assessed as having high biodiversity value with the majority of vegetation being Duffys Forest Ecological Community (DFEC) which is listed as endangered under the NSW Threatened Species Conservation Act 1995. The Concept Proposal also runs through a Priority 1 Wildlife Corridor as mapped by WC and contains habitat for a number of threatened flora and fauna species. Five threatened fauna species and one marine migratory species were recorded in the study area during field surveys: the Red-crowned Toadlet (TSC Act: vulnerable), Powerful Owl (TSC Act: vulnerable), Eastern Bentwing-bat (TSC Act: vulnerable), Grey-headed Flying-fox (TSC Act: vulnerable, EPBC Act: vulnerable), Swift Parrot (TSC Act: endangered, EPBC Act: endangered) and White-bellied Sea-eagle (EPBC Act: marine migratory). No threatened flora species were observed in the study area during targeted seasonal surveys. Red-crowned Toadlet habitat has been identified on the northern side of Frenchs Forest Road East in the upper reaches of the Drainage Line 2 catchment and on the southern side of Warringah Road in the upper reaches of the Drainage Line 6 catchment. Increases in peak flow associated with the proposed road works has the potential to impact the Red-crowned Toadlet habitat due to increased energy in the flow and potential scour of the inbank area of the receiving drainage lines.

5 Source: SMEC, 2014




4 SURFACE WATER HYDROLOGY - PRESENT DAY CONDITIONS

4.1 Water Quantity

4.1.1 General

WC advised that no detailed investigations have been undertaken to define the capacity of the local stormwater drainage system and the patterns of overland flow in the vicinity of the Concept Proposal. A model was therefore developed at part of the present investigation to assess the hydrologic standard of the existing stormwater drainage system and the magnitude of flow in the various overland flow paths and receiving drainage lines under present day conditions. The DRAINS software was used for this purpose.

The DRAINS model comprised a greater level of detail in the Middle Creek catchment given the need to develop a comprehensive strategy for managing the impacts of the Stage 1 Project on the receiving drainage lines. However, a sufficient level of detail was incorporated into the model to allow an assessment to be made of the approximate hydrologic standard of the local stormwater drainage system that discharges south (i.e. the drainage system that will be impacted by the Stage 2 Project).

The following sections of the report provide background to the development of the DRAINS model and a description of the assessed capacity of the local stormwater drainage system which will be impacted by the proposed road works.

4.1.2 DRAINS Model Development

DRAINS is a simulation program which converts rainfall patterns to stormwater runoff, and then routes flows through networks of piped drainage systems, culverts, storages and open channels. The software develops hydrographs and calculates hydraulic grade lines throughout the drainage network, enabling users to analyse the magnitude of overflows and stored water for established drainage systems. DRAINS was originally developed for the purpose of analysing urbanised catchments, and is therefore well suited to this present investigation.

Figure 4.1 shows the layout of the pit and pipe network comprising the DRAINS model which was developed as part of the present investigation (Concept Proposal DRAINS Model).

Pit and pipe drainage networks were defined using GIS based data obtained from WC. This information included dimensions of pipes and the location and level of pits and headwalls. Details of several pits (e.g. location and grate level) within the road corridor were updated based on Roads and Maritime’s detailed survey information. Pits were categorised into kerb inlets, grated inlets, letterbox pits or junction pits according to WC’s GIS data and visual inspection. An assumed cover of 600 mm was adopted for those drainage elements where inverts levels were not available. This assumed cover was adjusted to ensure that the dra inage system had positive fall in the downstream direction.

Adopted DRAINS hydrologic model parameters comprised initial losses of 1 and 5 mm for paved and grassed areas, respectively. An antecedent moisture condition of 3 was adopted, reflecting rather wet conditions prior to the occurrence of storm events and the soil type was set equal to 3, which corresponds with a soil of comparatively high runoff potential.

Rainfall intensities for design storms ranging between 1 and 100 year ARI, and for storm durations ranging between 20 minutes and 3 hours, were derived using procedures outlined in Australian Rainfall and Runoff (ARR) (IEAust, 1998).




4.1.3 Concept Proposal

Middle Creek Catchment

A detailed description of the key findings of the investigation as they re late to the Middle Creek catchment is provided in Section 4.1.4.

Bantry Bay Catchment

Excessive gutter flows occur along Warringah Road west of The Forest High School for events as frequent as 2 year ARI due to limited inlet capacity in the existing pavement drainage system. As a result, a large number of the pipes located both in the road reserve and downstream in the residentially developed areas of the catchment were found to run only partially full for events up to 100 year ARI.

Flow in the southern kerbline of Warringah Road discharges in a westerly direction, crossing Fitzpatrick Avenue before turning into Maxwell Parade. From Maxwell Parade gutter flows turn into Parni Place where they initially pond at the sag in the local road before discharging s outh into a landscaped reserve through which Drainage Line 4 runs.

Flow in the northern kerbline of Warringah Road west of Forest Way discharges in a westerly direction where it ponds at the location of a sag which is located about 70 m west of Parni Pla ce. Flow surcharging the sag in Warringah Road discharges in an easterly direction where it runs through several residential properties before entering onto Maxwell Parade. It then turns south and ponds in a sag in the local road which is located about 80 m north of Tyalla Avenue. Flow surcharging the sag in Maxwell Parade discharges in an easterly direction where it joins flow in Drainage Line 4. While a dedicated overland flow path runs from the sag in Maxwell Parade to the landscaped reserve through which Drainage Line 4 runs, flooding may be experienced in existing residential development during intense storm events.

More detailed investigations would need to be undertaken to determine whether the existing residential development which borders the reserve through which Drainage Line 4 runs is impacted by flows which surcharge the watercourse.

Runoff from a relatively small portion of the road corridor on the southern side of Warringah Road between the extension of Rangers Retreat Road and Hilmer Street contributes to flow in Drainage Line 5. The area is presently landscaped and drains toward the rear of several residential properties which are located on Rangers Retreat Road.

Curl Curl Creek Catchment

Runoff from the section of Warringah Road which extends west from Wakehurst Parkway to the crest is controlled by a pavement drainage system which discharges to Drainage Line 6. Runoff originating from a large portion of the NBH site is also controlled by this section of pavement drainage system.

Flooding at the location of the sag in Warringah Road immediately west of Wakehurst Parkway occurs for storms as frequent as 2 year ARI due to insufficient capacity in the transverse drainage. Stormwater which surcharges the sag in the road discharges in a southerly direction directly into Brick Pit Reserve.

The pavement drainage system which controls runoff originating from the southbound carriageway of Wakehurst Parkway near its intersection with Warringah Road contributes to flow




in Drainage Line 6. Runoff captured by the piped drainage system contributes to flow in Drainage Line 6 on the eastern (downstream) side of the transverse drainage of Wakehurst Parkway, while flows which surcharge the minor piped drainage system cross the road and contribute to its in let. This piped drainage system has a hydrologic standard of about 5 year ARI on the northern side, and less than 2 year ARI on the southern side of Warringah Road.

Due to limited inlet capacity the pavement drainage system which controls runoff from a 560 m length of Warringah Road immediately east of Wakehurst Parkway has a hydrologic standard of less than 5 year ARI. Runoff which surcharges the sag in this section of Warringah Road discharges south through a commercial development before flowing onto Aquatic Drive. The local stormwater drainage system in Aquatic Drive has a hydrologic standard of less than 2 year ARI, again due to limited inlet capacity. Runoff which surcharges the sag in Aquatic Drive discharges south where it flows through Aquatic Reserve before joining flow in Drainage Line 6.

4.1.4 Stage 1 Project

Peak flows at key locations in the upper Middle Creek catchment are given in Table E1 in Appendix E (refer Columns [D] to [I]).

The investigation identified the following three locations where flooding is a concern given the proposed road works have the potential to significantly increase peak flows in the local stormwater drainage system:

Residential development located on the northern side of Frenchs Forest Road West immediately east of Rabbett Street (refer to Peak Flow Location Identifier F11 on Figure 3.2, Sheet 1 of 4). The hydrologic standard of the adjacent transverse road drainage structure and also the local stormwater drainage system which runs through these properties has a hydrologic standard of between 1 and 2 year ARI. Any increases in peak flow at this location will therefore exacerbate flooding conditions in these properties.

Residential development located on the western side of Nandi Avenue north of Frenchs Forest Road East (between Peak Flow Location Identifiers F24c and F24d on Figure 3.2, Sheet 3 of 4). Available ALS survey data shows that the low point in the valley lies along Nandi Avenue to the east of the main arm of Drainage Line 2, indicating that surcharge flows will discharge through these properties. Any increase in peak flow along the main arm of Drainage Line 2 where it runs along the western side of the Nandi Avenue properties has the potential to exacerbate flooding conditions in existing residential development.

Residential development located east of Patanga Road (north (downstream) of Peak Flow Location Identifier F37a on Figure 3.2, Sheet 4 of 4). Presently stormwater ponds across the sag in Warringah Road which lies to the south (upstream) of these properties. Upgrading the pavement drainage system in Warringah Road has the potential to increase peak flows in the local stormwater drainage system where it runs through existing residential development north of Frenchs Forest Road East.

The study also identified that due to inlet capacity constraints on two transverse drainage structures that cross the Wakehurst Parkway about 40 m and 280 m north of Frenchs Forest Road West, both the northbound and southbound lanes of the road are subject to flooding during relatively frequent storm events. Further details on this issue are provided in the following discussion.




The key findings of the investigation in regards the hydrologic standard of the local stormwater drainage system which drains north and discharges to Drainage Lines 1, 2 and 3 were as follows:

Catchment 1A – The existing drainage system on the western side of Forest Way has a hydrologic standard of between 1 and 2 year ARI (refer to Peak Flow Location Identifier F1 on Figure 3.2, Sheet 1 of 4). As a result, flows in excess of the capacity of the drainage system will pond at the low point on the western side of Forest Way before overtopping the crown of the road and discharging east.

The existing 450 mm and 525 mm diameter pipes which drain east from the road corridor at Forest Way have a hydrologic standard of between 2 and 5 year ARI (Location F2).

Catchment 1B - The existing drainage system on the western side of Forest Way has a hydrologic standard of about 5 year ARI (Location F5). Flows in excess of the capacity of the drainage system will pond at the low point on the western side of Forest Way before overtopping the crown of the road and discharging east.

The existing 600 mm diameter pipe which drains east from the road corridor at Forest Way has a hydrologic standard of between 10 and 20 year ARI (Location F6).

The existing 525 mm diameter pipe drainage line in Rabbett Street, south of Frenchs Forest Road West has a hydrologic standard of about 1 year ARI (Location F7b). Stormwater runoff which surcharges the kerb inlet pits in Rabbett Street discharges into the adjoining catchment (i.e. Catchment 1C) where it flows onto Frenchs Forest Road West at the intersection of the two roads.

The existing piped drainage system along the southern side of Naree Road has a hydrologic standard of about 1 year. There is presently no piped drainage system along the northern side of Naree Road. As a result, concentrated flow discharges along the northern kerb line where it is captured by the drainage system that runs along Rabbett Street. The existing 750 mm diameter pipe in Rabbett Street, north of Naree Road has a hydrologic standard of about 2 year ARI (Location F8).

Catchment 1C – The existing 900 mm diameter pipe that drains the sag in Frenchs Forest Road West (Location F11) has a hydrologic standard of between 1 and 2 year ARI. As a result, flows which surcharge the drainage system presently pond at the sag in the road before travelling overland through existing residential development toward Rabbett Reserve (Location F12).

Catchment 1D - There is presently no piped drainage system along the northern side of Frenchs Forest Road West at Catchment 1D. As a result, concentrated flow discharges along the northern kerb line where it is captured by the drainage system that runs in a northerly direction along Bluegum Crescent.

The existing drainage line in Bluegum Crescent has a hydrologic standard of about 5 year ARI (Location F20). [Note that the hydrologic standard is controlled by the capacity of a single letterbox type inlet pit, with the analysis showing that the piped drainage system does not flow full for storms up to 100 year ARI.]

Catchment 2A – The existing 750 mm diameter pipe that drains across Wakehurst Parkway, approximately 280 m north of Frenchs Forest Road West has a hydrologic standard of less than 1 year ARI (Location F21a and F21b). The hydrologic standard of the drainage line is controlled by the capacity of the 0.9 m letter box type inlet pit. As a result, the pipe does not flow full for storms up to 100 year ARI.




Flows in excess of the capacity of the drainage line travel north along Wakehurst Parkway where the road is in cut. While a shallow table drain runs along the western side of Wakehurst Parkway, flows which surcharge the inlet of the 750 mm diameter cross the road due to its superelevated nature. Flows discharging in a northerly direction from the road cutting are diverted into the adjacent bushland on both the western and eastern sides of the road corridor, where it joins flow in the main arm of Drainage Line 2 on the southern (upstream) side of the Wakehurst Parkway crossing (refer location F24e on Figure 3.2, Sheet 3 of 4)).

Catchment 2B – The existing 825 mm diameter pipe that drains across Wakehurst Parkway about 40 m north of Frenchs Forest Road West has a hydrologic standard of less than 1 year ARI (Location F22a and F22b on Figure 3.2, Sheet 2 of 4). [Note that the hydrologic standard is controlled by the capacity of a single grated surface inlet pit, with the analysis showing that the pipe system does not flow full for storms up to 100 year ARI.]

Catchment 2C – The existing 375 mm diameter pipe that discharges north of the intersection of Wakehurst Parkway and Frenchs Forest Road East has a hydrologic standard of less than 1 year ARI (refer Location F23).

Catchment 2D – The existing 750 mm diameter pipe that drains across Frenchs Forest Road East about 80 m east of Wakehurst Parkway has a hydrologic standard of between 1 and 2 year ARI (Location F24). Flow in excess of the capacity of the 750 mm diameter pipe initially pond at the sag in Frenchs Forest Road East before surcharging the kerb line and discharging north along Drainage Line 2.

Catchment 2E – Runoff from Catchment 2E travels along the northern side of Frenchs Forest Road East to a kerb inlet pit which drains north to Bimbadeen Crescent via a 375 mm diameter pipe (refer Location F27a and F27b). The 375 mm diameter pipe drainage system has a hydrologic standard of about 1 year ARI, with overflows discharging west along Frenchs Forest Road East. [Note that the hydrologic standard is controlled by the capacity of the inlet pits, with the analysis showing that the pipe system does not flow full for storms up to 100 year ARI.]

Catchment 3A – The piped drainage system draining the low point in Warringah Road, west of Allambie Road (refer location F34b on Figure 3.2, Sheet 4 of 4)) has a hydrologic standard of between 2 and 5 year ARI. Flows in excess of the capacity of the piped drainage system discharge through the Allambie Grove Business Park.

The piped drainage system that approaches Frenchs Forest Road East from the industrial development to the south of Inverness Avenue has a hydrologic standard of about 10 year ARI. Stormwater runoff which surcharges this pipe drainage system discharges onto Frenchs Forest Road East at the location of the sag in the road immediately east of Inverness Avenue.

The 1200 mm diameter pipe drainage line that discharges from Frenchs Forest Road East at Inverness Avenue has a hydrologic standard of about 10 year ARI.

Catchment 3B – The 750 mm diameter pipe that drains the sag in Frenchs Forest Road East, east of Inverness Avenue has a hydrologic standard of between 2 and 5 year ARI (Location F30 on Figure 3.2, Sheet 4 of 4).

Catchment 3C – The piped drainage system along Allambie Road between Warringah Road and Frenchs Forest Road East has a hydrologic standard of about 2 year ARI.




Stormwater runoff that surcharges the kerb inlets on the southern side of the Allambie Road and Warringah Road intersection discharges west along Warringah Road. The peak flow in Warringah Road is about 0.3 m3/s in the 10 year ARI design storm event.

The piped drainage system that discharges from Frenchs Forest Road East at Patanga Road has a hydrologic standard of between 1 and 2 year ARI.

Catchment 3D – Stormwater runoff that surcharges the inlet pits in Warringah Road will pond at the sag in the road to a depth of 0.7 m before discharging in a westerly direction along Frenchs Forest Road East (refer Location F36). The peak ponding depth in Warringah Road is about 0.5 m for the 10 year ARI design storm event, with ponding extending across its full width.

Catchment 3E – The existing piped drainage system in Warringah Road controlling runoff from Catchment 3E has a hydrologic standard of about 2 year ARI. The hydrologic standard of the drainage system is controlled by the capacity of the kerb inlet pits which are located along Warringah Road, with the analysis showing that the pipe line does not flow full for storms up to about 20 year ARI. Stormwater runoff that surcharges the kerb inlet pits in Warringah Road will discharge into Catchment 3F.

Catchment 3F – The existing piped drainage system in Warringah Road controlling runoff from Catchment 3F has a hydrologic standard of about 2 year ARI. The hydrologic standard of the drainage system is controlled by the capacity of the kerb inlet pits which are located along Warringah Road, with the analysis showing that the pipe line does not flow full for storms up to about 20 year ARI. Stormwater runoff that surcharges the kerb inlet pits in Warringah Road discharges east along Warringah Road.

Excessive widths of gutter flow were found to be present at a number of locations along the sections of road which will be upgraded as part of the Stage 1 Project.6 Table 4.1 gives the maximum width of gutter flow in the network of streets for a 10 year ARI design storm.

4.2 Water Quality

4.2.1 General

An LGA wide study was commissioned by WC to assess the condition of the creeks into which runoff from urban areas discharge and to provide both qualitative and quantitative information which could be used to develop and implement effective long-term creek management strategies and development controls. The findings of the study are documented in a report entitled “Warringah Council Creek Management Study” (MWH, 2004).

The results of water quality sampling undertaken as part of MH, 2004 show that urbanisation in the upper reaches of the Middle Creek, Bantry Bay and Curl Curl Creek catchments has had an impact on water quality in the receiving drainage lines. The sampling data, which are reproduced in Table 4.2 over page, show that levels of TN and Nitrates/Nitrites exceed ANZECC, 2000 default trigger levels at all four sampling sites, while TP levels exceed ANZECC, 2000 default trigger levels at sampling location WQS2.7

6 Note that Roads and Maritime would typically limit the encroachment of flow onto the road such that the trafficable lane width is not reduced to less than 2.5 m. In the situation where a 3.5 m wide lane is provided the maximum allowable width of flow would be 1.5 m allowing for a 0.5 m wide gutter. 7 The sampling undertaken for MWH, 2004 was based on a single monitoring program during fine weather conditions reflecting low flow conditions.




Another key fining of MH, 2004 was surprisingly high nitrogen concentrations in Curl Curl Creek, which the author states may be reflecting upper catchment fertiliser use or polluted groundwater.

TABLE 4.1

MAXIMUM WIDTHS OF GUTTER FLOW 10 YEAR ARI

Catchment Location Maximum Width of Flow Along Kerb Line (m)

1A Forest Way Southbound 5.1

1B Forest Way Southbound 5.5

1C Frenchs Forest Road West Westbound 5.4

1D Frenchs Forest Road West Eastbound 0.5

2A Wakehurst Parkway Northbound 6.0

2B Wakehurst Parkway Northbound 3.1

2C Frenchs Forest Road East Westbound 2.3

2D Frenchs Forest Road East Westbound 6.1

2E Frenchs Forest Road East Eastbound 0.8

3A Frenchs Forest Road East Westbound 4.0

3B Frenchs Forest Road East Westbound 2.8

3C Frenchs Forest Road East Westbound 3.9

3D Warringah Road Westbound 4.8

3E Warringah Road Eastbound 1.5

3F Warringah Road Eastbound 2.8

TABLE 4.2 SUMMARY OF WATER QUALITY SAMPLING IN UPPER MIDDLE CREEK (1)

Sampling ID Location Description Suspended

Solids (mg/L)

Total Nitrogen

(mg/L)

Nitrate and Nitrite (mg/L)

Total Phosphorus

(mg/L)

WQS1(2) Drainage Line 1 - Carnarvon Drive near Peppercorn Park

17 0.7 0.22 <0.01

WQS2(2) Drainage Line 3 - Dreadnought Road near Oxford Falls Grammar School

15 0.4 0.08 0.05

WQS3 Drainage Line 5 – Pildra Place near Forestville Park

7 0.4 0.13 <0.01

WQS4(3) Curl Curl Creek – Upstream of Manly Dam

7 1.5 1.13 0.01

ANZECC, 2000 trigger levels Not specified 0.35 0.04 0.025

(1) Water quality sampling data has been reproduced from MWH, 2004. (2) Refer Figure 4.2 for approximate location of water quality sampling sites. (3) Location of sampling site lies to the south of the Aquatic Reserve Fields in the Manly Warringah War Memorial

Park.





Whilst the lengths of road which will be upgraded as part of the Concept Proposal represent a small portion of the total catchment from which the pollutant load in the receiving drainage lines is being generated, road runoff is a known source of pollutants such as gross pollutants and suspended solids to which toxicants such as heavy metals are absorbed. Road runoff is also a source of hydrocarbons in receiving waterways (EA, 2006). While the water quality sampling undertaken as part of MH, 2004 identified that both suspended and dissolved pollutant loads in catchment runoff generally exceed the ANZECC, 2000 default trigger levels (refer Table 4.2), no data are available on the nature of toxicants that are present in the bed sediments of the receiving drainage lines. Furthermore, while the results of the sampling undertaken as part of MH, 2004 only show that urbanisation has had an impact on water quality in the upper reaches of the Middle Creek, Bantry Bay and Curl Curl Creek catchments, they cannot be used as a benchmark for assessing the impact the proposed road works will have on the quality of runoff discharging to the receiving drainage lines. It was therefore necessary to develop a pollutant load generation model which could be used to set baseline conditions and also assess the relative impact the proposed road works will have on the quality of stormwater runoff. The model was also used to assess measures aimed at mitigating the impact of the Stage 1 Project on water quality. The MUSIC software was used for this purpose. The following sections of the report provide background to the development of the MUSIC model, as well as the average annual weight of pollutants that have been assessed as discharging to the receiving drainage lines in the Middle Creek catchment under present day conditions.

4.2.3 MUSIC Model Development

To estimate the average annual weight of pollutants which presently enter the receiving drainage lines downstream of the Stage 1 Project, a MUSIC model was established to reflect the contributing catchments discharging into Drainage Lines 1, 2 and 3 under present day conditions (Stage 1 Project MUSIC Model). The model included the details of the Rabbett Reserve sediment pond that presently controls runoff discharging into Drainage Line 1. Figure 4.2 shows the extent of the sub-catchments which comprise the Stage 1 Project Music Model. Rainfall records from the pluviographic recorder at Glen Street, Belrose for the period 1994 to 2014 were selected for use in the investigation. A one hour time step was adopted, taking into consideration the response time of the catchment, residence time of the sediment pond and model run times.


Table 4.3 over page gives the estimated average annual weight of gross pollutants, TSS, TP and TN in Drainage Lines 1, 2 and 3. The results of the analysis have been used to assess requirements for the control of stormwater runoff from the Stage 1 Project, further details on which are contained in Chapter 8 of this report.




TABLE 4.3 ESTIMATED AVERAGE ANNUAL WEIGHT OF POLLUTANTS IN DRAINAGE LINES 1, 2 AND 3

PRESENT DAY CONDITIONS

Drainage Line Location Identifier and Description(1) Catchment

Area (ha)

Impervious Area (%)

Runoff Volume

(ML/year)

Gross Pollutants (kg/year)

TSS (kg/year)

TP (kg/year)

TN (kg/year)

1

W1A - Downstream of Epping Drive 41.5 45 282 4,010 34,400 85.1 705

W1B - Upstream of Tallowood Way 76.0 40 484 9,260 59,600 147 1,210

2

W2A - Downstream of Frenchs Forest Road East

10.7 51 73.9 1,880 10,100 23.6 184

W2B - Downstream of Nandi Avenue 49.4 32 287 6,890 35,300 83.8 677

3

W3A - Downstream of Winslea Crescent 29.3 56 210 5,680 29,400 69.1 537

W2B - Upstream of Barnes Road 59.9 43 376 9,780 49,600 120 952

(1) Refer Figure 4.2 for water quality location identifiers.




5 IMPACT OF PROPOSED ROAD WORKS ON SURFACE WATER HYDROLOGY

5.1 Water Quantity Related Impacts


The widening of the existing road network and the upgrade of the existing pavement drainage system within the road corridor has the potential to increase peak flows in the receiving drainage lines. Given the relatively low hydrologic standard of the local stormwater drainage system (assessed at generally less than 5 year ARI), any increase in flow could exacerbate flooding conditions in existing development. The increase in peak flows also has the potential to cause scour and destabilise the banks of the natural and semi-natural reaches of the receiving drainage lines. Table 5.1 over page summarises the potential impacts of the Concept Proposal as they relate to water quantity.

The criteria which have been taken into account when developing the strategy for mitigating the water quantity related impacts of the Concept Proposal on existing development and the receiving drainage lines are outlined in Chapter 6, while details of the recommended management strategy are contained in Chapter 8.


Adverse flooding conditions will be experienced in a number of properties as a result of the proposed road works unless appropriate measures are incorporated in the design of the Stage 1 Project. Areas of concern include residential properties located on the northern side of Frenchs Forest Road immediately east of Rabbett Street, on the western side of Nandi Avenue north of Frenchs Forest Road East and between Patanga Road and Jones Street north of Frenchs Forest Road East.

Section 8.1.2 and Appendix D of this report contain details of the measures which will need to be incorporated in the design of the Stage 1 Project in order to mitigate the impact the proposed road works will have on flooding conditions in existing development.

5.2 Water Quality Related Impacts


The widening of the existing roads as part of the Concept Proposal and the increased traf fic volume that will occur over time will increase the weight of pollutants which will be conveyed in stormwater runoff to the receiving drainage lines. Table 5.2 over page lists the management issues increased pollutant loads from road corridors can have on major ecosystems. Included in Table 5.2 are several key indicators, external drivers and potential modifiers of processes associated with each management issue. The information presented in Table 5.2 is taken from Table 2.3 in EA, 2006 and is adapted from ANZECC, 2000.

Given the already high pollutant load level identified in runoff discharging to the receiving drainage lines (MWH, 2004), it will be important that the management strategy for the Concept Proposal be aimed at mitigating as far as is practicable the impacts of the proposed road works on the quality of road runoff. Chapters 7 and 8 of this report respectively set out the water quality control measures which are to form part of the construction and operational phase surface water management strategy for the Concept Proposal.




TABLE 5.1 SUMMARY OF POTENTIAL WATER QUANTITY RELATED IMPACTS OF THE CONCEPT PROPOSAL

Potential Impact Cause Key Locations

Adverse flooding conditions within existing development.

Increases in peak flows which can be attributed to proposed improvements to the pavement drainage system within the road corridor and the increase in paved area associated with the widening of the existing roads.

Middle Creek Catchment - Residential development located on the northern side of Frenchs Forest Road West immediately east of Rabbett Street, on the western side of Nandi Avenue adjacent to the main arm of Drainage Line 2 and on the northern side of Frenchs Forest Road East between Patanga Road and Jones Street.

Bantry Bay Catchment – Residential development located on the eastern side of Warringah Road south of Fitzpatrick Avenue.

Curl Curl Creek Catchment – Commercial and industrial development located on the southern side of Warringah Road east of Wakehurst Parkway.

Increased scour potential at the outlet of the local stormwater drainage system.


Middle Creek Catchment - Rabbett Reserve on Drainage Line 1 and the upper reaches of Drainage Line 2 north of Frenchs Forest Road East.

Bantry Bay Catchment – Drainage Line 4 where it runs south of Parni Place.

Curl Curl Creek Catchment – South of Warringah Road in Brick Pit Reserve on Drainage Line 6.

Bank instability in the natural and semi-natural reaches of the receiving drainage lines.


Middle Creek Catchment - Rabbett Reserve on Drainage Line 1 and the upper reaches of Drainage Line 2 north of Frenchs Forest Road East.

Bantry Bay Catchment – Drainage Line 4 where it runs south of Parni Place.

Curl Curl Creek Catchment – South of Warringah Road in Brick Pit Reserve on Drainage Line 6.




TABLE 5.2 MAJOR ECOSYSTEM MANAGEMENT ISSUES ASSOCIATED WITH POLLUTANTS PRESENT IN ROAD RUNOFF

Management Issue Indicators External Drivers or Stressors and Potential Modifiers of Processers

Impacts on ecosystem and use values due to toxicants in water column.

Species composition and abundance, biomarkers, toxicants.

Stressors: Heavy metals, hydrocarbons and pesticides, ammonia.

Modifiers: Reducing conditions (high organic loads), suspended solids (SS), dissolved oxygen (DO), hardness, pH.

Impacts on ecosystem and use values due to toxicants in sediments.

Species composition and abundance, biomarkers, toxicants.

Stressors: As above.

Modifiers: As above, plus composition of sediments and bioturbation.

Impacts on ecosystem and use values due to nuisance plant growth.

Cell numbers, chlorophyll ‘a’, species composition, TP, TN.

Drivers: TP, TN direct stressors, organic load (biochemical dissolved oxygen (BOD)) indirect driver.

Modifiers: Detention time, temperature, light modifiers, SS adsorption of nutrients, mixing regimes, composition of sediments.

Asphyxiation of respiring organisms due to depletion of oxygen.

DO concentrations, species compositions and abundance.

Drivers: Organic load (BOD), ammonia, hydrocarbons.

Modifiers: Re-aeration and mixing (flow, wind), temperature, photosynthesis.

Modified primary production as a result of reduction in light by suspended particles.

SS levels, plant and animal species composition and abundance, turbidity.

Drivers: Suspended particulate material load, organic load indirect driver.

Modifiers: Flow, detention time (sedimentation), total dissolved solids (TDS) (coagulation).

Smothering of benthic organisms by sedimentation. Sediment depths, plant and animal species composition and abundance, SS.

Drivers: Sediment load.

Modifiers: Flow (transport capacity, sedimentation, re-suspension), detention time.

Impairment on aesthetic values. Visible debris, scums, turbidity.

Unpleasant odours. Drivers: Gross contaminants, SS, hydrocarbons, organic loads, ammonia, nutrients.

Source: Table 2.3 in EA, 2006.





Improvements to the road network as part of the Stage 1 Project will result in an increase in the build up and transport of contaminants such as suspended sediments, heavy metals, litter, nutrients, oils and greases. In order to quantify the impact the proposed road works will have on the weight of pollutants entering the receiving drainage lines, the MUSIC model that was developed to assess pollutant loads under present day conditions (refer Section 4.2.3 for details) was adjusted to reflect post-road upgrade conditions. This was done by adjusting catchment boundaries and per cent impervious. The changes in pollutant loads which will result from the road upgrade works are summarised in Tables 5.3, 5.4 and 5.5 over page for Drainage Lines 1, 2 and 3 respectively. Following construction of the proposed road works, it is predicted that TSS will increase by 3 to 11 per cent within the downstream drainage lines. The exception to this is in Drainage Line 2, immediately downstream of Frenchs Forest Road East, where the diversion of pavement runoff in a northerly direction along Wakehurst Parkway will result in a reduction in TSS of 14 per cent (refer Location W2A on Figure 4.2). Similar relative changes are predicted to occur in TN and TP loads. Gross pollutants are predicted to increase within Drainage Line 1 by up to 42 per cent. This is due to the increase in impervious area associated with the proposed road widening, as well as a decrease in the catchment area draining to Rabbett Reserve as a result of the proposed drainage line diversion along Cobb Street. In Drainage Line 2, immediately downstream of Frenchs Forest Road East, gross pollutant loads are predicted to decrease due to the diversion of pavement runoff in a northerly direction along Wakehurst Parkway. Downstream of the diversion along Wakehurst Parkway gross pollutant loads in Drainage Line 2 are predicted to increase by up to 4 per cent (refer Location W2C on Figure 4.2). Gross pollutants within Drainage Line 3 are predicted to increase by up to 2 per cent.




TABLE 5.3 IMPACT OF STAGE 1 PROJECT ON POLLUTANT LOADS IN DRAINAGE LINE 1

Location Identifier and Description(1)

Scenario Catchment

Area (ha)

Impervious Area (%)

Runoff Volume

(ML/year)


TSS (kg/year)

TP (kg/year)

TN (kg/year)

W1A Downstream of Epping Drive

Present Day Conditions 41.5 45 282 4,010 34,400 85.1 705

Post Stage 1 Project Conditions 41.7 52 300 [6%]

5,700 [42%]

38,200 [11%]

93.2 [10%]

757 [7%]

Post Stage 1 Project with In-Line Pollutant Control Devices(3)

41.7 52 300 [6%]

3,920 [-2%]

33,200 [-3%]

93.2 [10%]

757 [7%]

W1B Upstream of

Tallowood Way

Present Day Conditions 76.0 40 484 9,260 59,600 147 1,210


10,900 [+18%]

63,300 [6%]

155 [5%]

1,260 [4%]


76.0 43 500 [3%]

9,130 [-1%]

58,300 [-2%]

155 [5%]

1,260 [4%]


(2) Value in [ ] relates to percentage change relative to present day conditions. A positive va lue represents an increase in runoff volume or pollutant load compared to present day conditions.

(3) Refer Section 8.2.2 for discussion on proposed water quality control measures.






Scenario Catchment

Area (ha)

Impervious Area (%)

Runoff Volume

(ML/year)


TSS (kg/year)

TP (kg/year)

TN (kg/year)

W2A Downstream of Frenchs Forest

Road East

Present Day Conditions 10.7 52 73.9 1,880 10,100 23.6 184

Post Stage 1 Project Conditions 7.6 67 60

[-19%] 1,580 [-16%]

8,720 [-14%]

19.9 [-16%]

152 [-17%]


7.6 67 60

[-19%] 1,580 [-16%]

8,720 [-14%]

19.9 [-16%]

152 [-17%]

W2B Downstream of Nandi Avenue



[-6%] 6,550 [-6%]

33,700 [-5%]

79.5 [-6%]

639 [-6%]


45.7 38 271

[-6%] 6,550 [-6%]

33,700 [-5%]

79.5 [-6%]

639 [-6%]

W2C Downstream of

Wakehurst Parkway



[+2%] 7,570 [+4%]

40,400 [+4%]

93.8 [+4%]

766 [+3%]


63.0 32 348

[+2%] 6,910 [-5%]

38,500 [-1%]

93.8 [+4%]

766 [+3%]


(2) Value in [ ] relates to percentage change relative to present day conditions. A positive value represents an increase in runoff volume or pollutant load compared to present day conditions.







Scenario Catchment

Area (ha)

Impervious Area (%)

Runoff Volume

(ML/year)


TSS (kg/year)

TP (kg/year)

TN (kg/year)

W3A Downstream of

Winslea Crescent



5,800 [2%]

30,400 [3%]

71.2 [3%]

551 [3%]


29.3 58 215 [2%]

1,620 [-71%]

18,200 [-38%]

71.2 [3%]

551 [3%]

W3B Upstream of Barnes Road

Present Day Conditions 59.9 43 376 9,780 49,600 120 952


9,940 [2%]

50,900 [3%]

122 [2%]

970 [2%]


59.9 44 383 [2%]

5,760 [-41%]

38,700 [-22%]

122 [2%]

970 [2%]


(2) Value in [ ] relates to percentage change relative to present day conditions. A positive value represents an increase in run off volume or pollutant load compared to present day conditions.





6 SURFACE WATER MANAGEMENT CONSIDERATIONS

6.1 Water Quantity Related Considerations

6.1.1 Design Development

Assessment of drainage requirements was undertaken based on concept road design models that were provided by Roads and Maritime in June 2014. As part of the detailed design process, the designers of the road upgrade works will need to review the concept road design models which have been developed by Roads and Maritime in light of the recommendations contained in this report to ensure the intent of the recommended drainage strategy is maintained in the final design of the road works.

6.1.2 Design Standards

Stormwater runoff from the widened sections of road pavement will be controlled by new or upgraded pavement drainage systems which will typically discharge to WC’s existing piped stormwater drainage system. Roads and Maritime advised that the new and upgraded road drainage systems, including transverse crossings of the road corridor, are to have a minimum hydrologic standard of 10 year ARI. However, a key finding of this present investigation is that due to constraints imposed by one particular piped drainage system into which new road drainage system will be connected, this hydrologic standard cannot be achieved along the full length of the road upgrade. As a result, the recommended operational surface water management strategy will only provide a minimum hydrologic standard of about 2 year ARI at the location of the sag in the road corridor within Catchment 1C (refer Section D1.2.3 in Appendix D for further details).

6.1.3 Potential Blockage of the Road Drainage System

The following pit blocking factors were adopted for pit spacing purposes to account for the potential reduction in inlet capacity that may occur as a result of a partial blockage due to litter and other debris:

On-grade pits – 20% blockage

Sag pits – 50% blockage

6.1.4 Utilities

The location and depth of utilities and their impact on the proposed drainage system have not been considered as part of the present investigation. Detailed investigations will need to be carried out during the preparation of the detailed design to ensure that there is no conflict between the proposed pavement drainage systems and either existing or proposed utilities. As part of these investigations it will be necessary to collect detailed field survey of existing piped drainage networks, as the present investigation is based on GIS based data provided by WC.

6.1.5 Potential to Impact Flooding Conditions in Existing Development

The increase in impervious area associated with the widening of the existing arterial road network has the potential to increase the rate of flow discharging through existing development located downstream of the road corridor. The raising of road levels also has the potential to exacerbate




flooding conditions in existing development located upstream of the road corridor. Measures aimed at mitigating the impacts of the proposed road works on flooding conditions in existing development will need to from part of the management strategy for the Concept Proposal and Stage 1 Project.

6.1.6 Future Development Upslope of the Road Corridor

Figure 3.1 shows that the NBH site lies partially within Catchments 1C, 2C and 6A, and therefore contributes to runoff in the piped drainage systems along the road corridor. It has been assumed that on-site detention will be provided to limit peak discharges from the NBH site to no larger than under present day conditions. Similarly, for the developed catchments which lie along the upstream side of the road corridor, i t was assumed that measures will be incorporated into any future redevelopment which will control the rate of flow discharging to the adjacent road drainage system to no larger than occurs under present day conditions. As shown in Figure 3.1, the Stage 2 Project extends into Catchments 1C, 3A and 3C. Consideration has therefore been given to the potential impacts of these works on the drainage strategy for the Stage 1 Project. This is discussed further in Sections D1.2.3, D1.4.1 and D1.4.3 in Appendix D.

6.1.7 Future Climate Change

Scientific evidence shows that climate change will lead to sea level rise and potentially increase flood producing rainfall intensities. The significance of these effects on flood behaviour will vary depending on geographic location and local topographic conditions. Climate change impacts on flood producing rainfall events show a trend for larger scale storms and resulting depths of rainfall to increase, whilst future impacts on sea levels are likely to result in a continuation of the rise which has been observed over the last 20 years. Current projections of the impact climate change will have on sea levels are for it to raise by about 0.4 m by 2050 and 0.9 m by 2100. As the Concept Proposal is located on a ridgeline which lies more than 100 m above mean sea level, projected rises in sea level linked to future climate change will not impact flooding behaviour along the road corridor. CSIRO prepared reports for the NSW Government on the impacts of climate change on rainfall intensities. In the Sydney metropolitan catchments, the 40 year ARI, 1 day rainfall was predicted to increase by up to 10% over the period 2030 to 2070. These results suggest that the impacts of climate change in the catchments which contribute runoff to the receiving drainage lines will lie somewhere between present day conditions and conditions existing with a 10% increase in rainfall intensities. As increases in rainfall intensity of up to 10% will result in only a minor decease in the hydrologic standard of the local stormwater drainage system, the potential impact of future climate change on flood producing rainfall was not taken into account when assessing measures for managing stormwater runoff from the upgraded sections of road.




6.2 Water Quality Related Considerations

6.2.1 Construction Phase Stormwater Runoff

Large-scale disturbance associated with the construction of the proposed road works has the potential to increase the volume of sediment deposited in the receiving drainage lines and the level of turbidity present in stormwater discharging to the downstream waterways of Narrabeen Lagoon, Bantry Bay and Manly Dam.

Given the sensitivity of the receiving drainage lines and downstream waterways, it will be necessary to design and implement effective erosion and sediment control measures during construction of the proposed road works.

6.2.2 Operational Phase Stormwater Pollution Reduction Targets

The DGEAR’s include an assessment of water quality impacts and mitigation measures to manage water pollution, with reference to relevant environmental water quality criteria, including the “Australian and New Zealand Guidelines for Fresh and Marine Water Quality” (ANZECC, 2000) and any applicable regional, local or site specific guidelines.

The ANZECC, 2000 guidelines provide a nationally consistent approach to water quality management based on the principle of ecological sustainable development of water resources. The guidelines contain a set of tools for the assessment and management of water quality across a range of water resource types and based on designated environmental values.

Within the broader catchment context, the ANZECC, 2000 guidelines have been used in the development of the following catchment management studies and plans:

Northern Beaches Stormwater Management Plan (PBP, 1999)

Warringah Council Creek Management Study (MWH, 2004)

The primary objective of PB, 1999 was to establish a plan for the co-ordinated management of stormwater within the Northern Beaches catchments to promote ecologically sustainable development and the social and economic benefits of sound stormwater management practices. The report was prepared under the requirements of Section 12 of the Protection of the Environmental Administration Act 1991, in co-ordination with relevant local councils and government authorities.

PBP, 1999 includes a set of stormwater pollution reduction targets (in terms of gross pollutants, TSS, TP, TN, oil and grease) for all new development within the Northern Beaches catchments to achieve best management practice in terms of controlling pollutant loads in stormwater runoff. These pollutant reduction targets are reproduced in Table 6.1 over page and are divided into short term and long term reduction targets. Note that the short term targets set out in PBP, 1999 were based on a timeframe of 5 years. The long term targets are more general in nature and reflect the broader objectives of stormwater management throughout the catchments.

Stormwater pollution reduction targets recommended in “Managing Urban Stormwater: Council Handbook (Revised Draft)” (EPA, 2007) are also shown in Table 6.1. These targets are commonly adopted for new development across NSW in the absence of local guidelines such as that provided by PBP, 1999. For the purpose of the present investigation, the PBP, 1999 targets are considered more appropriate as they have been based on a location specific assessment of the Northern Beaches catchments.




TABLE 6.1 STORMWATER POLLUTION REDUCTION TARGETS

Stormwater Pollutant

PBP, 1999 EPA, 2007

Short Term Objectives(1) Long Term Objectives

Gross Pollutants 100% retention of litter

and coarse sediment up to 3 month ARI peak flow

70% reduction in gross solid loads in creeks and streams

100% retention of litter and coarse sediment up to 0.25 times the 1 year

ARI peak flow

Total Suspended Solids (TSS)

50% reduction in total suspended solid loads

Achieve suspended sediment loads which protect

ecosystems and maintain natural creek bed regime

85%

Total Phosphorus (TP)

45% reduction in nutrient loads

30% reduction in nutrient loads 65%

Total Nitrogen (TN)

45% reduction in nutrient loads

30% reduction in nutrient loads 45%

Oil and Grease No visible oils up to 3 month ARI peak flow

No visible oils on all waterways No visible oils up to 0.25

times the 1 year ARI peak flow

(1) The short term objectives set out in PBP, 1999 were based on a 5 year timeframe. Note that the ability to install water quality arrangements to meet current best practice guidelines for treatment of all runoff from the road corridor (not just from the additional paved area associated with the upgrade works) may be constrained by the prevailing topography and limited corridor area as well as the configuration of the existing road drainage system. Furthermore, the piped drainage systems that presently controls runoff along the road corridor form part of a larger stormwater network controlling runoff from the urban areas that lie to upstream and downstream of the road corridor. It is therefore not practicable to treat runoff from the road corridor in isolation of the broader catchment. A more holistic approach to water quality within the broader stormwater network is therefore required. Based on the above, an assessment has been made of the potential pollutant loads at select locations within the broader catchment under present day and post-upgrade conditions. A set of management measures have been identified to offset increases in pollutant loads as a direct result of the Stage 1 Project. Where feasible, further measures to meet the BPB, 1999 targets have also been considered within the broader catchment and are discussed further in Section 8.2.2.

6.2.3 Water Sensitive Urban Design

Water sensitive urban design measures that have been successfully installed within streetscapes include rain gardens, porous paving, grassed buffer strips and bioretention trenches. However, these measures are more suited to local roads with low speed zones and sufficient road reserve space, and are therefore not well suited to the proposed road works which involves the upgrade of arterial and sub arterial roads with higher speed limits and limited road corridor.




6.2.4 Spill Containment

Depending on the sensitivity of the receiving environment, measures may need to be incorporated in the design of a road drainage system to intercept spill material before it can discharge to the receiving drainage system. These measures typically comprise an end-of-line facility that is capable if intercepting and temporarily storing up to 25 m3 of spill material.

Careful consideration will need to be given to the impact a potential spill could have on the Red -crowned Toadlet habitat when developing the operational surface water management strategy for the proposed road works. Options for mitigating the impact of the road works on the Red-crowned Toadlet habitat include the provision of temporary containment structures, or alternatively the rerouting of the road drainage system around sensitive areas.




7 CONSTRUCTION PHASE SURFACE WATER MANAGEMENT STRATEGY

7.1 General

As set out in Chapter 5, the construction of the Concept Proposal has the potential to impact existing ecosystems by increasing sediment loads and turbidity levels in stormwater runof f. A strategy will therefore need to be developed which is aimed at mitigating the adverse impacts of construction activities on the quality of water discharging to the receiving drainage lines. . It is recommended that the strategy presented in this section of the report be used as a starting point for the preparation of the Soil and Water Management Plan (SWMP) that will need to be developed as part of final design and/or construction documentation for the proposed road works. However, it should be recognised that ultimate requirements for controlling erosion and sediment during construction will be dictated by final design of the road upgrade works, proposed construction methods, staging and site management practices, all of which are yet to be determined. The strategy presented in this section of the report has been developed based on the principles and design guidelines set out in the following documents:

Soils and Construction – Managing Urban Stormwater series (herein referred to as the “Blue Book”), comprising:

o Volume 1 (Landcom, 2004)

o Volume 2D – Main Roads (DECC, 2008).

Roads and Maritime Erosion and Sedimentation Management Procedure (Roads and Maritime, 2008).

Roads and Maritime QA Specification G38 (Roads and Maritime, 2011). 7.2 Key Elements

7.2.1 Erosion Control

The key objectives of an effective erosion and sediment control strategy is to firstly minimise erosion, and then to capture sediment from disturbed areas where erosion cannot be prevented. A range of erosion control principles that should be incorporated into the future SWMP for the Concept Proposal include:

appropriate location and treatment of site access and stockpile sites;

conservation of existing topsoil for later site rehabilitation;

minimisation of disturbed areas, and stabilisation using batter blanketing, surface mulching or vegetation;

scour protection along any temporary drainage lines through the site;

separation of “clean” and “dirty” water wherever possible;

diversion of “clean” water from upslope areas through the construction corridor;

site maintenance requirements; and

progressive site rehabilitation.




7.2.2 Sediment Control

While the implementation of effective erosion control measures will reduce the volume of sediment which is available for transport during a rainfall event, measures are still required which are aimed at removing as much sediment as is practicable from stormwater runoff prior to its discharge to the receiving drainage lines. Sediment control measures which could be implemented as part of the strategy for managing runoff during construction of the Concept Proposal include:

sediment retention basins and “sumps”;

sediment barriers (eg geotextile fabric used as fencing);

kerbside filter rolls;

earth banking, sand bagging, straw bales and rock check dams; and

cleared vegetation rows. Space limitations within the road corridor and in areas adjacent to the receiving drainage lines can mean that only “at-source” sediment control measures such as earth banking and sediment fencing can be implemented during construction. However, where space permits, “end-of-line” sediment control measures such as appropriately sized sediment retention basins should always be implemented as they reduce the restrictions that “at-source” controls impose on road construction activities. 7.3 Concept Proposal

Given the sensitivity of the receiving environment and the impact increased sediment loads and turbidity levels will have on the ecosystems within the receiving drainage lines, it will be necessary to develop and implement an effective erosion and sediment control strategy for the Concept Proposal. Construction activities along Warringah Road will principally involve the construction of the Slot, which will be excavated below existing road levels. It will be necessary for the contractor to excavate localised sumps within the slot in which to temporarily store runoff prior to pumping it to the receiving drainage lines. Depending on the composition of the insitu material and the volume of runoff which is generated by each rainfall event, i t may be necessary for the contractor to flocculate the stored runoff to reduce turbidity levels prior to pump out. Opportunities should be investigated for locating sediment retention basins along the southern side of Warringah Road, where clearing and grubbing and excavation activities will be associated with the widening of the existing westbound carriageway. These basins would reduce the volume of sediment and turbidity levels in runoff discharging to Drainage Lines 4 and 6. Details on measures which should be implemented to reduce the volume of sediment and turbidity levels in runoff discharging to Drainage Lines 1, 2 and 3 are presented in the following section of the report.




7.4 Stage 1 Project

Figure 7.1 (4 Sheets) shows the extent of land which will be disturbed during the construction phase of the Stage 1 Project (excluding areas of existing pavement and trenching required to install upgraded sections of pipe drainage system) and identifies the receiving drainage lines to which runoff from these areas will discharge.

Given the relatively confined nature of the construction works (i.e. narrow sections of new pavement combined with resurfacing of existing pavement), the close proximity of existing development and the relatively steep topography at the location where the local stormwater drainage system discharges to Drainage Lines 1, 2 and 3, the installation of appropriately sized sediment retention basins is not considered feasible for all but one location.

The Blue Book allows for localised erosion and sediment control measures to be used in the absence of sediment retention basins where the average annual soil loss from a disturbed area, as derived by application of the Revised Universal Soil Loss Equation (RUSLE) 8, is less than 150 m3.

Tables C1, C2 and C3 in Appendix C respectively give the estimated average annual soil loss from each individual disturbed area along the Stage 1 Project, as well as the combined values within each of the catchments contributing runoff to Drainage Lines 1, 2 and 3 .

By inspection of values presented in Tables C1, C2 and C3, the estimated average annual soil loss from the individual areas will not exceed the threshold value of 150 m 3, with the exception of the cuttings located at the intersection of Wakehurst Parkway and Frenchs Forest Road (East and West). Given the close proximity of the receiving drainage line (i.e. Trefoil Creek), it is recommended that sediment retention basins (or sumps) be installed adjacent to the stormwater pipes controlling runoff from the cut areas immediately after commencement of the initial excavation works. Furthermore, excavation for the cuttings should not be undertaken in parallel, but rather staged so that work on a new cutting does not commence until the surface of the completed cutting is stabilised.

Depending on the staging of the works, there is the potential for the sediment load discharging to the receiving drainage lines to exceed the threshold value of 150 m3 (refer total for each drainage line given at the bottom of Tables C1, C2 and C3). The implementation of effective localised erosion and sediment control measures aimed at minimising the volume of sediment which is transported from individually disturbed areas will therefore be key to the control of sediment from the road corridor in the absence of sediment retention basins. Measures would include use of the following practices and smaller scale elements:

staging of works to minimise the extent of disturbance at any one time;

temporary stabilisation or revegetation/rehabilitation works to reduce the extent of disturbed surfaces;

application of temporary surface treatments or blanketing on exposed earth surfaces;

sediment barriers;

vegetative buffer strips; and

stabilised drainage lines incorporating rock check dams at regular intervals.

8 For further details of the RUSLE, refer Appendix A of Landcom, 2004.




Measures such as temporary diversion channels and bunding will also need to be implemented during the construction phase of the project to prevent concentrated flow, which presently discharges onto the road corridor from causing scour of disturbed surfaces. The difficulty which will be faced by the contractor is that in several locations concentrated flow discharges onto the road corridor via ill-defined drainage paths (e.g. down driveways and along boundary fences). In order to minimise scour potential during the construction phase of the project, it will be necessary to carefully plan, implement and maintain measures which are aimed at intercepting this concentrated flow and diverting it toward the existing piped drainage system.




8 OPERATIONAL PHASE SURFACE WATER MANAGEMENT STRATEGY

8.1 Water Quantity Related Aspects


As previously mentioned, the widening of the existing road network and the implementation of a more effective pavement drainage system will result in an increase in peak flows in the receiving drainage lines. This increase in peak flows has the potential to impact flooding conditions in existing development and cause scour and bank instability in the receiving drainage lines. One measure which could be implemented to mitigate the impact of the Concept Proposal on peak flows in the receiving drainage lines which run south into Bantry Bay and Manly Dam is the provision of stormwater retention tanks within the Slot. These tanks would temporarily store surface runoff which is generated from within the Slot before it is then pumped to the adjacent at-grade pavement drainage system. Through the adoption of suitably sized storage tanks and appropriate pump rates, the tanks would perform the function of a more traditional on-site detention system which is aimed at reducing the incoming flow to a permissible discharge rate. Similar to the works described below for the Stage 1 Project, it will be necessary to upgrade the existing stormwater drainage system located in the road reserves of Warringah Road, Wakehurst Parkway and Allambie Road as part of the Stage 2 Project. These works will require trenching for new sections of reinforced concrete pipe and possible upgrades of existing outlet headwalls to incorporate larger diameter pipes. Improved scour protection measures will also need to be provided at the outlet of upgraded sections of the piped drainage system.


A strategy aimed at mitigating the adverse impacts of the road upgrade works on existing development and the drainage lines into which the upgraded pavement drainage system will discharge was developed as part of the present investigation. The potential impact of the road upgrade works on both nuisance and major flooding was taken into account when developing the strategy. Figure 8.1 (4 Sheets) shows the local stormwater drainage upgrade requirements associated with the operational phase surface water management strategy for the Stage 1 Project. Note that Roads and Maritime will need to undertake further design development to confirm the size of pipes and number of pits required to control runoff from the upgraded sections of road, as those shown on Figure 8.1 (4 Sheets) are indicative only. The key features of the proposed operational surface water management strategy for the Stage 1 Project are summarised in Table 8.1 over page, while a detailed description of the individual measures comprising the strategy is contained in Appendix D. Table E1 in Appendix E of this report shows the impact the proposed road works will have on peak flows at selected locations along the receiving drainage lines for storms with ARI’s of 2, 10 and 100 years.




TABLE 8.1 KEY FEATURES OF THE PROPOSED OPERATIONAL SURFACE WATER MANAGEMENT STRATEGY

STAGE 1 PROJECT

Catchment Key Feature Aim of Measure

Drainage Line 1 (Middle Creek)

Upgrade of the existing pavement drainage system within the affected roads.

To provide a minimum 10 year ARI hydrologic standard in the upgraded road network with the exception of the sag in Frenchs Forest Road West immediately east of Rabbett Street.

Provision of a new section of pavement drainage line in Forest Way near its intersection with Rabbett Street.

To prevent adverse flooding conditions being experienced in existing residential development located on the eastern side of Forest Way opposite Forestway Shopping Centre.

Upgrade of the local stormwater drainage system in Rabbett Street.

To reduce the rate of overland flow discharging to the sag in Frenchs Forest Road West immediately east of Rabbett Street.

Upgrade of the transverse drainage line at the sag in Frenchs Forest Road West immediately east of Rabbett Street.

To facilitate the possible future upgrade of the downstream pipe drainage system by WC aimed at providing a minimum 10 year ARI hydrologic standard in both the road corridor and downstream residential development.

Upgrade of the local stormwater drainage system in Cobb Street.

To prevent adverse flooding conditions being experienced in existing residential development located on the northern side of Frenchs Forest Road West immediately east of Rabbett Street.

Drainage Line 2 (Trefoil Creek)

Upgrade of the existing pavement drainage system within the upgraded sections of road.

To provide a minimum 10 year ARI hydrologic standard in the upgraded road network.

Diversion of flow away from the two existing transverse drainage structures that cross Wakehurst Parkway north of Frenchs Forest Road West via the provision of new trunk drainage line.

To prevent adverse flooding conditions being experienced in existing residential development located on the western side of Nandi Avenue north of Frenchs Forest Road East.

Prevent scour and bank instability along those drainage lines where Red-crowned Toadlet habitat has been identified.

Reduce the frequency of flooding along Wakehurst Parkway north of Frenchs Forest Road West.

Provision of scour protection and energy dissipation measures on the outlet of upgraded stormwater drainage line controlling runoff in the sag in Frenchs Forest Road East east of Wakehurst Parkway.

Prevent scour of the receiving drainage line at the point of discharge of road runoff from the road corridor.

Drainage Line 3

Upgrade of the existing pavement drainage system within the affected roads.

To provide a minimum 10 year ARI hydrologic standard in the upgraded road network.

Upgrade of the local stormwater drainage system in Patanga Road, Iris Street and Winslea Avenue

To prevent adverse flooding conditions being experienced in existing residential development located to the north (downstream) of Frenchs Forest Road East.




While it is feasible to prevent adverse flooding conditions being experienced in existing development by upgrading the local stormwater drainage system discharging to Drainage Lines 1 and 3, existing inlet capacity constraints associated with the transverse drainage structures located along Wakehurst Parkway north of Frenchs Forest Road West mean that improvements to the drainage system in this area have the potential to exacerbate flooding conditions in existing development located adjacent to Drainage Line 2. Increases in the magnitude of flow discharging to the two transverse drainage structures also has the potential to impact on Red-crowned Toadlet habitat.

While the provision of temporary stormwater detention at the headwaters of Drainage Line 2 was investigated as part of the present study, the volume of water which would need to be stored in order to mitigate the impacts of the road works on peak flows in the receiving drainage line was found to be too great (preliminary analyses indicated a tank with a plan area of greater than 200 m2 would be required to reduce peak flows to no greater than present day conditions).

Following a review of the available options it was decided that the preferred strategy for mitigating the impact of the Stage 1 Project on the hydrologic regime of Drainage Line 2 would be to divert flow north within the road reserve of Wakehurst Parkway via a new trunk drainage line. The route of the new trunk drainage line, which is about 840 m in length and comprises both pipe and channel reaches, is shown on Figure 8.1, Sheet 3 of 4.

While a large percentage of the flow generated by catchments 2A, 2B and 2C would be di verted north within the new trunk drainage line, it will be important to maintain as close as is practicable the existing hydrologic regime in the minor drainage lines which have been identified as Red-crowned Toadlet habitat (i.e. in those drainage lines which run down the relative steep slope on the eastern side of Wakehurst Parkway). This can be achieved by:

discharging runoff from the left hand turn slip lane in Wakehurst Parkway to the existing 825 mm diameter pipe which crosses the road approximately 40 m north of Frenchs Forest Road West (refer Location F22a and F22b on Figure 8.1, Sheet 3 of 4);9 and

diverting runoff from the urban catchment which lies to the west of Wakehurst Parkway (i.e. catchment 2A) to the inlet of the existing 750 mm diameter pipe which crosses the road approximately 280 m north of Frenchs Forest Road West (refer Location F21a on Figure 8.1, Sheet 3 of 4).

Blind pits are to be provided along the new trunk drainage line where it runs along the eastern side of Wakehurst Parkway north of Location F21a (refer Figure 8.1, Sheet 3 of 4). This will allow direct runoff from the 230 m section of Wakehurst Parkway north of Location F21a to discharge along the eastern side of Wakehurst Parkway at Location F25 as occurs under present day conditions. 10

Further details on the proposed strategy for managing surface water runoff discharging to Drainage Line 2 is contained in Section D1.3.1 in Appendix D.

9 Note this approach has already been incorporated into the detail design of the left hand slip lane project (refer Chapter 2 for details). 10 Under present day conditions Location F25 also receives flows that surcharge the inlet to the 750 mm pipe at Location F21a and travel north along Wakehurst Parkway during storms greater than about a 3 month ARI. Under the proposed strategy flows that surcharge the inlet to the 750 mm pipe at Location F21a will be conveyed along the new trunk drainage line to Location F26. This will result in a reduction in peak flows discharging to Location F25 for storms greater than about 3 month ARI, which in total generate less than 5 per cent of the average annual runoff volume discharging to this location.




8.2 Water Quality Related Aspects


While a quantitative analysis has only been undertake for the Stage 1 Project as part of the present investigation, it is expected that the Stage 2 Project will result in an increase in the weight of pollutants entering the receiving drainage lines which drain south into Bantry Bay and Manly Dam. A strategy will therefore need to be developed during the preparation of the EIS for the Stage 2 Project which is aimed at mitigating the water quality related impacts of the proposed road works. The strategy, which will also need to address requirements for spill containment, could comprise the following:

The installation of in-line pollution control devices at strategic locations within the local stormwater drainage system. The devices would principally reduce the gross pollutant and TSS loads in surface water runoff. Similar to the approach adopted for the control of pollutants from the Stage 1 Project, it may only be feasible to offset the impact of the Stage 2 Project on the quality of surface water runoff, rather than meet the catchment -based reduction targets set out in Table 6.1.

The provision of spill containment measures in the form of temporary storage facilities. Requirements for spill containment will likely focus on those drainage lines which discharge to Brick Pit Reserve, where investigations have identified Red-crowned Toadlet habitat. It is noted that the sub-surface tanks which will need to be provided in the Slot could be designed to intercept and store spill material, similar to those which are present in the tunnel systems around Sydney.

Provisions for the temporary containment of water used to wash down the walls of the Slot. It is noted that the sub-surface tanks which will need to be provided in the Slot could be designed to intercept and temporarily store wash down water. The water stored in the tanks could then be pumped out and disposed of off-site.


The proposed strategy for mitigating the impacts of the Stage 1 Project on the quality of surface water runoff discharging to the receiving drainage lines requires the installation of three in-line pollutant control devices at the following locations:

Drainage Line 1 – Cobb Street (refer Figure 8.1, Sheet 1 of 4 for location)

Drainage Line 2 – Outlet of piped section of new trunk drainage line in Wakehurst Parkway (refer Figure 8.1, Sheet 3 of 4 for location)

Drainage Line 3 – Patanga Road (refer Figure 8.1, Sheet 4 of 4 for location) The locations for the in-line devices have been selected to rationalise the number of water quality control structures by taking a more holistic approach to water qualit y management within the urbanised catchments which contribute runoff to Middle Creek. The performance of the in-line devices in reducing pollutant loads in Drainage Lines 1, 2 and 3 is summarised in Tables 5.3, 5.4 and 5.5 respectively.




The proposed strategy will result in a reduction in gross pollutant and TSS loads in the receiving drainage lines. However, it does not address increases in TN and TP, which would require large areas for the installation of bio-retention systems capable of absorbing nutrients in the stormwater runoff.




9 REFERENCES

ARUP (2011). “Northern Beaches Hospital Campus Project Stormwater Management Strategy and Plan” DECC (2008). “Managing Urban Stormwater – Soils and Construction”. Volume 2D – Main Road Construction. Gondwana Consulting Pty Ltd (2014). “Manly Warringah War Memorial Park Plan of Management” IEAust (1998). “Australian Rainfall and Runoff”. Volumes 1 and 2. Published by the Institution of Engineers, Australia. EA (2006). “Australian Runoff Quality – A Guide to Water Sensitive Urban Design” Landcom (2004). “Managing Urban Stormwater – Soils and Construction”. Volume 1, 4th Edition. MWH (2004). “Warringah Council Creek Management Study”. Patterson, Britton and Partners (PBP), (1999). “Northern Beaches Stormwater Management Plan.” Roads and Maritime (2011). “QA Specification G38”, Edition 1, Revision 3. Roads and Maritime (2008). “Erosion and Sedimentation Management Procedure”. Roads and Maritime Procedure PN143P, Version 2 dated November 2008. SMEC (2011). “Narrabeen Lagoon Plan of Management”. Prepared for the Narrabeen lagoon Reserve Trust June 2011 SMEC (2014). “Biodiversity Assessment Report: Northern Beaches Hospital Connectivity and Network Enhancements”. WBM Oceanics Australia (2000). “Narrabeen lagoon Estuary Processes Study N0433 for the Warringah Council” WBM Oceanics Australia (2001). “Narrabeen lagoon Estuary Processes Study Prepared for Warringah and Pittwater Councils”

APPENDIX A

DETAILED DESCRIPTION OF DRAINAGE LINES 1, 2 AND 3



NBH_V1_006_Report.doc Page A1 Lyall & Associates October 2014 Rev. 3.3 Consulting Water Engineers

A1 DETAILED DESCRIPTION OF DRAINAGE LINES 1, 2 AND 3 A1.1 General This appendix provides a detailed description of the receiving drainage lines which are located in the upper reaches of the Middle Creek catchment in the vicinity of the Stage 1 Project. Table A1 located at the end of this appendix provides a summary of the key features of each drainage line. Plates referred to in the following discussion are contained in Appendix B. A1.2 Drainage Line 1 (Middle Creek) Figure 3.2, Sheet 1 of 4 shows that runoff from the sections of Forest Way, Naree Road and Frenchs Forest West which are to be upgraded as part of the Stage 1 Project drain north towards Drainage Line 1. This drainage line flows generally to the north via a piped drainage system before emerging as a vegetated channel within Rabbett Reserve. A sediment pond is located at the outlet of the piped drainage system, treating runoff before it discharges to a vegetated channel that runs through Rabbett Reserve. The channel section which runs through Rabbett Reserve is densely vegetated on its overbank bank and has a bed slope of about 5 per cent. From Rabbet Reserve, the drainage line continues in a northerly direction across Epping Drive and through Jindabyne Reserve. The bed of the channel immediately downstream of Epping Drive comprises exposed bedrock, which then forms a waterfall a short distance to the north of the road corridor Plates 1 to 8 show details of Drainage Line 1, from the sediment pond in Rabbett Reserve to the pipe outlet downstream of Epping Drive. A1.3 Drainage Line 2 (Trefoil Creek) A 750 mm diameter pipe outlet discharges into Drainage Line 2 immediately north of Frenchs Forest Road East, as shown in Figure 3.2, Sheet 2 of 4. Trefoil Creek forms the main arm of Drainage Line 2. From this location the drainage line runs in a northerly direction via a vegetated channel through a nature reserve which is bounded by Wakehurst Parkway to its west and residential development to its east. Plate 9 shows the general landform and vegetation along Drainage Line 2 immediately downstream of Frenchs Forest Road East. A minor piped drainage system comprising a series of 375 mm diameter pipes also controls runoff from the road intersection and discharges to Drainage Line 2 north of Frenchs Forest Road East (refer Plate 10). Runoff from two transverse drainage lines which cross Wakehurst Parkway about 40 m and 280 m north of Frenchs Forest Road West also discharge to the headwaters of Drainage Line 2. Existing inlet capacity constraints on these two lines result in significant bypass flow occurring along Wakehurst Parkway during relatively minor storm events. Stormwater discharging from these drainage lines flows down the relatively steep vegetated slope on the eastern side of Wakehurst Parkway before joining flow in the main arm of Drainage Line 2. A large amount of gross pollutants was observed downstream of the northern of the two transverse drainage lines. Scour was also observed in the receiving drainage line at this location (refer Plates 11 to 17 which show key features of the two transverse drainage lines).




A section of Drainage Line 2 appears to have been straightened where it runs along the western boundary of several residential properties located along Nandi Avenue. A narrow grassed reserve separates the channel and the rear of the Nandi Avenue properties as shown in Plate 18. Of note is that natural surface levels fall away from the eastern bank of the channel toward the residential properties, with the adjacent low point in the valley generally aligned with Nandi Avenue. The channel is generally trapezoidal in shape where it runs closest to the properties, with a base width of about 1 m, a top width of about 3 m and a depth of about 1.5 m (refer Plate 19). Based on these dimensions and an assessed bed slope of 5%, the channel has a hydraulic capacity of about 7 m3/s.11,12 Flow in Drainage Line 2 is joined by runoff which discharges from a 450 mm and 1350 mm diameter pipe, the outlets of which are located at the northern end of Nandi Avenue (refer Figure 3.2, Sheet 3 of 4). The western bank of the channel at this location is heavily vegetated, while the eastern overbank is grassed (refer Plates 20 and 21).13 Drainage Line 2 runs in a northerly direction over a distance of about 180 m where it crosses Wakehurst Parkway via a 1650 mm diameter pipe.14 Plate 22 shows the condition of Drainage Line 2 immediately downstream of the Wakehurst Parkway. Drainage Line 2 runs north over a distance of about 200 m where it joins the main arm of Middle Creek. Runoff which presently surcharges the inlet of the transverse drainage line which crosses Wakehurst Parkway about 280 m north of Frenchs Forest Road West (refer Location F21c on Figure 3.2, Sheet 3 of 4) flows across the carriageway due to its superelevated nature. It then runs in a northerly direction along the eastern side of the southbound carriageway where the road is in cut (refer Plates 23 and 24). A tail-out drain is located at the northern end of the road cutting which diverts flow into the adjacent bushland on the eastern side of the road corridor (refer Location F25 on Figure 3.2, Sheet 3 of 4). Flow discharging from the tail-out drain joins the main arm of Drainage Line 2 on the southern (upstream) side of the Wakehurst Parkway road culvert. Runoff discharging in an easterly direction toward the aforementioned road cutting is intercepted by a series of shallow catch drains which are located along an access track which follows the route of a buried high pressure gas line. The shallow catch drains run in a northerly direction and divert surface runoff into the adjacent bushland to the west at two locations (refer Figure 3.2, Sheet 3 of 4). Several mass concrete diversion structures have also been installed along the access track north of an existing access gate (refer Figure 3.2, Sheet 3 of 4 for location of gated entry and also Plates 25 and 28). These structures divert runoff discharging along the access track into a shallow catch drain which runs along its eastern side. 11 Based on the hydrologic modelling undertaken as part of the present study, the channel has a hydrologic standard of about 100 year ARI where it runs along the rear of the Nandi Avenue properties (refer peak flows given in Table E1 in Appendix E). 12 A resident advised that he had observed floodwater discharging through several properties located at the northern end of Nandi Avenue on three occasions over the past 40 years. However, he did not advise the source of the floodwater (i.e. whether it originated from the channel which runs along the western side of the Nandi Avenue properties or due to surcharge of the local stormwater drainage system). 13 The abovementioned resident also advised that he had observed floodwater backing up along Drainage Line 2 from the Wakehurst Parkway culvert. 14 Pipe dimensions taken from WC’s stormwater asset database.




A1.4 Drainage Line 3 (Tributary of Middle Creek) Two drainage lines comprising 525 mm and 900 mm diameter pipes at their outlets discharge into Drainage Line 3 at Iris Street, west of Oxford Falls Road as shown in Figure 3.2, Sheet 4 of 4. From Iris Street the drainage line runs generally in a northerly direction via a vegetated channel along the western side of Oxford Falls Road. Plate 29 shows details of the channel immediately downstream of Iris Street. From Iris Street, Drainage Line 3 continues in a north westerly direction and joins Middle Creek approximately 150 m downstream of Wakehurst Parkway. Figure 3.2, Sheet 4 of 4 shows a 900 mm diameter pipe and a 1050 mm diameter pipe discharging to Drainage Line 3 from the cul -de-sac at Winslea Crescent toward Drainage Line 3. Discharge from the pipe outlets is conveyed over a series of rock ledges that fall approximately 14 m over a distance of 25 m before draining to a vegetated channel that runs in a northerly direction to join Drainage Line 3. Plates 31 and 32 show details of the pipe outlet in Winslea Crescent.




TABLE A1 STAGE 1 PROJECT CATCHMENT SUMMARY

Drainage Line Catchment ID Catchment

Area (Hectares)

Brief Description

Drainage Line 1 (Middle Creek)

Refer Figure 3.2,

Sheet 1 of 4

1A 2.5

Catchment 1A comprises a 380 m long section of Forest Way extending from Adams Street to Naree Road, as well as residential d evelopment on the western side of the road corridor.

The existing piped drainage system in Catchment 1A drains toward a sag in Forest Way which is located 150 m north of its intersection with Naree Road. Two 450 mm diameter pipes convey ru noff across the road from west to east.

Runoff from Catchment 1A discharges from the road corridor via a 450 mm and 525 mm diameter pipes. The 525 mm diameter pipe runs through Wareham Reserve, while the 450 mm diameter pipe runs through a drainage easement within the property to the north of Wareham Reserve. The two pipes combine in Wareham Crescent and continue along Adams Street and Epping Drive to discharge into Drainage Line 1 west of Rabbett Street.

1B 5.9

Catchment 1B covers Naree Road as well as sections of Rabbett Street and Forest Way. The catchment includes residential prop erties bounded by Naree Street, Rabbett Street and Forest Way, as well as commercial development south of Forest Way.

The existing piped drainage system in Forest Way drains to a sag in the road, approximately 190 m south of Naree Road, before discharging through the residential properties via a 600 mm diameter pipe to join the existing drainage system in Rabbett Street.

Piped drainage systems in Rabbett Street and Naree Road combine and discharge from the road corridor north along Rabbett Stre et via a 675 mm diameter pipe before outletting into the vegetated channel in Rabbett Reserve.

1C 14.5

Catchment 1C covers Holland Crescent as well as sections of Rabbett Street, Frenchs Forest Road and Warringah Road. The catc hment includes residential development west of Rabbett Street, The Forest High School and a section of the future NBH site.

The existing piped drainage system in Warringah Road drains through The Forest High School and the residential properties in Holland Crescent before joining the piped drainage system in Holland Crescent. From Holland Crescent, the piped drainage system continues through to the low point in Frenchs Forest Road West, approximately 50 m ea st of Rabbett Street.

Runoff from The Forest High School discharges into a piped drainage system that runs along the western bounda ry of the school, through the Frenchs Forest Police Station and west along Frenchs Forest Road West to the sag in the road, approximately 50 m east of Rabbett Street. The piped drainage system comprises a 67 5 mm diameter pipe where it runs through the pol ice station and along Frenchs Forest West. Note that there are no inlet pits along Frenchs Forest Road West and therefore this piped drainag e line does not collect runoff from the road corridor.

Runoff from the 510 m long section of Frenchs Forest Road West within Catchment 1C drains to a series of kerb inlet pits located at the low point in the road, approximately 50 m east of Rabbett Street. These inlet pits connect to the piped drainage systems from Holland Crescent and The Forest High School before di scharging from the road corridor via a 900 mm diameter pipe than runs in a northerly direction through the rear of properties in Rabbett Street before outletting into the vegetated channel in Rabbet t Reserve.

1D 0.1 Catchment 1D comprises a relatively small area along the northern side of Frenchs Forest Road between Gladys Avenue and Bluegum Crescent. The catchment drains to a piped drainage system

in Bluegum Crescent, which runs in a northerly direction and ultimately discharges into Drainage Line 1 dow nstream of Jindabyne Reserve.


Refer Figure 3.2,

Sheets 2 and 3 of 4

2A 6.5

Catchment 2A covers an area bounded by Gladys Avenue, Wakehurst Parkway and Frenchs Forest Road West.

Runoff from Gladys Avenue drains toward the eastern end of the road and is controlled by a kerb inlet pit and a 375 mm diameter pipe. The 375 mm diameter pipe discharges into a drainage line which runs in a north westerly direction to a 750 mm diameter pipe that conveys runoff across Wakehurst Parkw ay. The 750 mm diameter pipe also collects runoff from a 240 m section of Wakehurst Parkway which is drained via a concrete dish drain that runs in a northerly direction along the western side of the road. The inlet of the pipe comprises a letterbox type structure which has limited inlet capacity (refer Plate 13). The 750 mm diameter pipe outlets into a vegetated channel that runs in an easterly direction to join Drainage Line 2. Plates 14 to 16 shows details of the outlet to the 750 mm diameter pipe. A significant amount of gross pollutants were observed immediately downstream of the 750 mm diameter pipe.

2B 0.2 Catchment 2B comprises a relatively small area along the western side of Wakehurst Parkway at its intersection with Frenchs F orest Road East. The catchment drains to an 825 mm diameter pipe

that runs in an easterly direction across Wakehurst Parkway before discharging into a drainage line that runs in an easterly direction to join Drainage Line 2. The inlet of the pipe comprises a single surface inlet grate which has limited capacity and was partially blocked by leaf litter at the time of its inspection (refer Plate 11). Plate 12 shows details of the outlet to the 825 mm diameter pipe.




TABLE A1 (cont’d) STAGE 1 PROJECT CATCHMENT SUMMARY

Drainage Line(1) Catchment ID(1)

Catchment Area

(Hectares) Brief Description


Refer Figure 3.2,

Sheets 2 and 3 of 4

2C 3.1

Catchment 2C covers sections of Frenchs Forest Road East, Wakehurst Parkway and Bantry Bay Road. Th e catchment also includes a north east portion of the proposed NBH site, which currently contains undeveloped areas either side of the Bantry Bay Road reserve. [Note that residential development which previously e xisted along the eastern side of Bantry Road has been demolished to make way for the future hospital.]

Runoff from Catchment 2C is controlled by piped drainage systems in Frenchs Forest Road West and Wakehurst Parkway. A headwa ll inlet connected to a 375 mm diameter pipe controls runoff from the proposed NBH site east of Bantry Bay Road. The piped drainage systems combine and discharge from the road corridor via a 375 mm diameter pipe. The 375 mm diameter pipe outlets on the relatively steep slope, where stormwater discharging from the pipe flows in a northerly direction to join Drainage Line 2. Plate 10 in Appendix B shows details of the outlet to the 375 mm diameter pipe.

2D 6.4

Catchment 2D comprises a 440 m section of Frenchs Forest Road East, to the east of Wakehurst Parkway. The catchmen t includes the Skyline Business Park and other commercial development south of Frenchs Forest Road East, as well as an undeveloped area immediately east of Wakehurst Parkway.

Runoff from Catchment 2D is controlled by a piped drainage system that drains to the sag in Frenchs Forest Road East, 80 m east of Wakehurst Parkway. A 750 mm diameter pipe conveys runoff across the road from south to north. Runoff from Catchment 2D then discharges from the road corridor via a 750 mm diameter pipe into a semi-natural channel that forms Drainage Line 2.

2E 0.3 Catchment 2E covers a 180 m section of Frenchs Forest Road East, west of Romford Road. The catchment drains to a 675 mm diam eter pipe that runs in a northerly direction through residential

properties to Bimbadeen Crescent. The piped drainage system then continues in a northerly direction and ultimately discharges into Drainage Line 2 at the northern end of Nandi Avenue.

Drainage Line 3 (Tributary to

Middle Creek)

Refer Figure 3.2, Sheet 4 of 4

3A 9.2

Catchment 3A covers Frenchs Forest Road East to the north and Warringah Road to the south. The catchment also includes commercial de velopment located between Frenchs Forest Road East and Warringah Road.

The existing piped drainage system in Warringah Road drains toward a sag in the road located opposite Rodborough Road. A 525 mm diameter pipe conveys runoff across the road from south to north. Runoff from Warringah Road discharges through the commercial development to the north via a 675 mm diameter pipe, whi ch increases to a 750 mm diameter pipe where it connects into the piped drainage system in Frenchs Forest Road East.

Runoff along Frenchs Forest Road East is controlled by a piped drainage system that discharges from the road corridor via a 1 200 mm diameter pipe that runs in a northerly direction along Inverness Avenue. WC’s GIS data shows that the pipeline decreases in size to a 900 mm diameter pipe where it runs through Patanga Park, and cont inues as a 900 mm diameter pipe to the outlet into the natural channel north of Winslea Crescent. From Winslea Crescent the natural channel continues in a north easterly direction to join Dra inage Line 3 at Barnes Road.

3B 0.2

Catchment 3B comprises a relatively small area that covers Frenchs Forest East between Inverness Avenue and Patanga Road.

Runoff from Catchment 3B drains to a piped drainage system at the sag in Frenchs Forest Road East, 30 m east of Inverness Ave nue, where it is discharged via a 750 mm diameter pipe through residential property to join the piped drainage system in Patanga Park to the north. [The drainage system in Catchment 3B also receives runoff that bypasses the dr ainage system in Catchments 3A and 3C and travels along Frenchs Forest Road East from the east and west respectively].

3C 2.1 Catchment 3C covers a section of Allambie Road, Frenchs Forest Road East and Warringah Road. Runoff from the catchment is contr olled by a piped drainage system that runs in a northerly

direction along Allambie Road and across Frenchs Forest Road East before discharging from the road corridor via a 600 mm diameter pipe at Patanga Road. The drainage line continues along Patanga Road to join the 900 mm diameter pipe downstream of Patanga Park.

3D 3.0

Catchment 3D covers a 370 m section of Warringah Road between Allambie Avenue and Jones Street.

The existing piped drainage system in Warringah Road drains toward a sag which is located opposite Frenchs Forest Road East w here it is discharged to a 450 mm diameter pipe in Frenchs Forest Road East. The pipeline then continues through residential properties to connect into the existing pavement drainage system in Dareen Street. Flows in excess of the piped drainage system capacity in Frenchs Forest Road East would travel west along the roadway toward Patanga Road.

3E 0.4 Catchment 3E covers a 130 m section of Warringah Road east of Jones Street. Runoff from the catchment is controlled by a 375 mm diameter pipe that discharges from the road corridor in a

northerly direction through the residential properties to join the piped drainage system in Dareen Street. From Dareen Street the piped drainage system continues in a northerly directi on via a 525 mm diameter pipe and ultimately discharges into Drainage Line 3 north of Iris Street.

3F 0.4 Catchment 3F comprises a 110 m section of Warringah Road west of Courtley Road. Runoff from the catchment is controlled by a 375 RCP that discharge s from the road corridor in a northerly

direction. The pipeline increases to a 450 mm diameter pipe where it runs through the resi dential properties to join the piped drainage system in Dareen Street that also drains Catchment 3E.

APPENDIX B

PLATES SHOWING KEY FEATURES OF DRAINAGE LINES 1, 2 AND 3



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Plate 1 – Sediment pond at outlet to piped drainage system in Rabbett Reserve. Note the vertical rack on the spillway to provide retention of gross pollutants.

Plate 2 – Piped drainage outlets into sediment pond in Rabbett Reserve. Note the orange discolouration at the outlets, indicating the presence of iron. This suggests the presence of sewer overflows or groundwater in the stormwater runoff.




Plate 3 – Sediment pond at outlet to piped drainage system in Rabbett Reserve. Photo is taken from the access ramp used for cleaning out the pond. Note the vertical rack on the outlet weir to retain gross pollutants.

Plate 4 – Immediately downstream of sediment pond in Rabbett Reserve. Note the orange discolouration indicating high iron content in runoff discharging from the sediment pond.




Plate 5 – Natural channel draining though Rabbett Reserve. Note the presence of rock along the base of the channel.

Plate 6 – Outlet of Drainage Line 1 downstream of Epping Drive. The area between the headwall and roadway has been concrete lined to protect against scour during flows that overtop the roadway.




Plate 7 – Drainage Line 1 immediately downstream of Epping Drive. Runoff discharges onto exposed bedrock layer.

Plate 8 – Waterfall at end of rock ledge downstream of Epping Drive on Drainage Line 1.




Plate 9 – Downstream of 750 mm diameter pipe outlet into Drainage Line 2 at Frenchs Forest Road East. Area was overgrown with both native and non-native vegetation such as Lantana. The exact location of the pipe outlet could not be determined during the site visit.

Plate 10 – Outlet of 375 mm diameter controlling runoff from intersection of Wakehurst Parkway and Frenchs Forest Road (East and West) on Drainage Line 2.




Plate 11 – Single grated inlet to 825 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 40 m north of Frenchs Forest Road West on Drainage Line 2. Note build up of leaf litter on grate at time of inspection.

Plate 12 – Outlet to 825 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 40 m north of Frenchs Forest Road East on Drainage Line 2. Note build up of sediment at outlet.




Plate 13 – Inlet of 750 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 280 m north of Frenchs Forest Road West on Drainage Line 2.

Plate 14 – Outlet to 750 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 280 m north of Frenchs Forest Road East on Drainage Line 2. A concrete apron has been installed to minimise scour downstream of the outlet.




Plate 15 – Outlet to 750 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 280 m north of Frenchs Forest Road East on Drainage Line 2. Concrete apron has been installed to minimise scour at outlet.

Plate 16 – Outlet to 750 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 280 m north of Frenchs Forest Road East on Drainage Line 2. Ground conditions at end of concrete apron.




Plate 17 – Downstream of 750 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 280 m north of Frenchs Forest Road East on Drainage Line 2.

Plate 18 – Grass reserve running between rear of Nandi Avenue properties and main arm of Drainage Line 2. Fall in natural surface levels is from left to right. Photo taken looking in the downstream direction.




Plate 19 – Inbank area of main arm of Drainage Line 2 adjacent to Nandi Avenue properties. Photo taken looking in the downstream direction.

Plate 20 – Eastern overbank area of main arm of Drainage Line 2 immediately downstream of Nandi Avenue properties. Photo taken looking in the upstream direction.




Plate 21 – Inbank area of main arm of Drainage Line 2 immediately downstream of Nandi Avenue properties. Photo taken looking in the downstream direction.

Plate 22 – Photo looking downstream along main arm of Drainage Line 2 at location of access track crossing.




Plate 23 – North of 750 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 280 m north of Frenchs Forest Road East on Drainage Line 2. Photo taken looking north from within road cutting.

Plate 24 – North of 750 mm diameter transverse drainage structure located in Wakehurst Parkway approximately 280 m north of Frenchs Forest Road East on Drainage Line 2. Photo taken looking south from northern limit of road cutting.




Plate 25 – Photo looking south-east at gate across access track on western side of Wakehurst Parkway north of road cutting.

Plate 26 – Photo looking north along access track on western side of Wakehurst Parkway immediately north of access gate shown in Plate 24.




Plate 27 – Photo looking north along access track on western side of Wakehurst Parkway approximately mid-way between access gate and main arm of Drainage Line 2. Low flow mass concrete diversion structure across track in foreground.

Plate 28 – Photo looking north at access track crossing of main arm of Drainage Line 2 on western (downstream) side of Wakehurst Parkway. Low flow mass concrete diversion structure across track in foreground.




Plate 29 – Channel downstream of piped drainage outlet in Iris Street on Drainage Line 3. The base of the channel is rock. Note the concrete blocks that have been installed to contain runoff in the channel.

Plate 30 – Kerb inlet pit at end of Winslea Crescent on Drainage Line 3. Note the concrete dish drain to control flows that overtop the road.




Plate 31 – Outlet to 1050 mm diameter pipe and 900 mm diameter pipe in Winslea Crescent on Drainage Line 3.

Plate 32 – Immediately downstream of 1050 mm diameter pipe and 900 mm diameter pipe outlets in Winslea Crescent on Drainage Line 3. Runoff from the pipe outlets discharges down a series of rock ledges.

APPENDIX C

TABLES SUMMARISING ESTIMATED AVERAGE ANNUAL SOIL LOSS FROM DISTURBED AREAS ALONG STAGE 1 PROJECT



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TABLE C1

ESTIMATED AVERAGE ANNUAL SOIL LOSS FROM DISTURBED AREAS IN CATCHMENT CONTRIBUTING RUNOFF TO DRAINAGE LINE 1 STAGE 1 PROJECT

Description of Disturbed Area(1) Soil Landscape Slope

Length (m)

Gradient (%) LS Factor Sediment Load

(tonnes/ha/yr)

Area Contributing to On-site Water

(ha)

Average Annual

Sediment Load

(m3/yr)

Disturbed area on western side of Forest Way near Naree road draining to existing pipe north into drainage line A Lucas Heights 110 2 0.44 103 0.108 7.4

Disturbed area on eastern side Forest Way near Naree road Lucas Heights 70 2 0.39 91 0.028 1.7

Disturbed area on western side of Forest Way north of Russell Avenue draining to existing pipe at corner of Russel avenue and Forest Way Lucas Heights 40 1 0.31 73 0.040 1.9

Disturbed area on western side of Forest Way south of Russell Avenue draining to existing pipe Lucas Heights 80 2 0.41 96 0.106 6.8

Disturbed area on eastern side of Forest Way south of Naree Road draining to existing pipe Lucas Heights 145 1 0.23 54 0.075 2.7

Disturbed area on southern side of Naree Road draining to existing pipe along Rabbett Street Lucas Heights 80 11 3.7 866 0.177 102.0

Disturbed area on northern side of Naree Road draining to existing pipe along Rabbett Street Lucas Heights 220 11 4.94 1156 0.125 96.4

Disturbed area on eastern side of Rabbett Street south of Naree Road draining to existing pipe along Rabbett Street Lucas Heights 20 6 0.42 98 0.009 0.6

Disturbed area on eastern side of Rabbett Street north of Naree Road draining to existing pipe along Rabbett Street Lucas Heights 18 7 0.64 150 0.012 1.2

Disturbed area on western side of Rabbett Street north of Naree Road draining to existing pipe along Rabbett Street Lucas Heights 25 6 0.81 189 0.012 1.6

Disturbed area on southern side of Frenchs Forest Road West, east of Rabbett Street draining to existing pipe along Rabbett Street Lucas Heights 120 12 4.33 1013 0.197 133.3

Disturbed area on northern side of Frenchs Forest Road West, east of Rabbett Street draining to existing pipe along Rabbett Street Lucas Heights 40 7 0.97 227 0.043 6.6

Disturbed area on southern side of Frenchs Forest Road West, east of Rabbett Street Lucas Heights 180 5 2 468 0.166 51.8

Disturbed area on eastern side of Cobb Street north of Frenchs Forest Road West draining along Cobb Street Lucas Heights 12 2 0.18 42 0.007 0.2

Disturbed area on northern side of Frenchs Forest Road West between Cobb Street and Sylvia Place Lucas Heights 110 7 2.37 554 0.063 23.2

Disturbed area on northern side of Frenchs Forest Road West between Sylvia Place and Bluegum Crescent Lucas Heights 100 14 5.42 1268 0.052 43.7

Disturbed are on northern side of Frenchs Forest Road West between Bluegum Crescent Lucas Heights 100 7 2.37 554 0.051 19.0

Disturbed area on eastern side of Bluegum Crescent north of Frenchs Forest Road West draining along Bluegum Crescent Lucas Heights 10 8 0.53 124 0.005 0.4

Disturbed area on northern side of Frenchs Forest Road West between Bluegum Crescent and Gladys Avenue draining along Bluegum Crescent Lucas Heights 90 5 1.27 297 0.053 10.6

TOTAL ESTIMATED AVERAGE ANNUAL SEDIMENT LOAD 510.9




TABLE C2 ESTIMATED AVERAGE ANNUAL SOIL LOSS FROM DISTURBED AREAS IN CATCHMENT CONTRIBUTING RUNOFF TO DRAINAGE LINE 2

STAGE 1 PROJECT


Length (m)


(tonnes/ha/yr)


(ha)

Average Annual

Sediment Load

(m3/yr)

Disturbed area on corner of Frenchs Forest Road West and Gladys Avenue Lucas Heights 10 4 0.36 84 0.005 0.3

Disturbed area on northern side of Frenchs Forest Road West between Gladys Avenue and Wakehurst Parkway Lucas Heights 200 4 1.47 344 0.145 33.2

Disturbed area on eastern side of Wakehurst Parkway north of Frenchs Forest Road West Lucas Heights 10 50 3.33 779 0.194 101.0

Disturbed are on corner of Frenchs Forest Road West and Wakehurst Parkway Lucas Heights 45 9 1.51 353 0.033 7.7

Disturbed are on southern side of Frenchs Forest Road West between Gladys Avenue and Bantry Bay Road Lucas Heights 10 50 3.33 779 0.184 95.8

Disturbed area on southern side of Frenchs Forest Road West between Bantry Bay Road and Wakehurst Parkway Lucas Heights 10 50 3.33 779 0.338 175.7

Disturbed area on northern side of Frenchs Forest Road West between Bantry Bay Road and Wakehurst Parkway Lucas Heights 10 50 3.33 779 0.027 14.2

Disturbed area on eastern side of Wakehurst Parkway south of Frenchs Forest Road East Lucas Heights 10 50 3.33 779 0.020 10.2

Disturbed are on northern corner of Frenchs Forest Road East and Wakehurst Parkway Lucas Heights 10 50 3.33 779 0.029 15.2

Disturbed area on eastern side of Wakehurst Parkway north of Frenchs Forest Road East Lucas Heights 160 7 3.07 718 0.152 72.5

Disturbed area on northern side of Frenchs Forest Road East between Wakehurst Parkway and Nandi Avenue Lucas Heights 70 8 1.88 440 0.021 6.2

Disturbed area on southern side of Frenchs Forest Road East west of Wakehurst Parkway Lucas Heights 10 50 3.33 779 0.321 166.8

Disturbed area on northern side of Frenchs Forest Road East between Wakehurst Parkway and Nandi Avenue Lucas Heights 10 50 3.33 779 0.042 21.8

Disturbed area corner of Frenchs Forest Road East and Romford Road Lucas Heights 10 10 0.68 159 0.006 0.6

Disturbed area eastern corner of Frenchs Forest Road East and Romford Road Lucas Heights 10 3 0.24 56 0.005 0.2

Disturbed area western corner of Frenchs Forest Road East and Romford Road Lucas Heights 17 2 0.24 56 0.009 0.3





TABLE C3 ESTIMATED AVERAGE ANNUAL SOIL LOSS FROM DISTURBED AREAS IN CATCHMENT CONTRIBUTING RUNOFF TO DRAINAGE LINE 3

STAGE 1 PROJECT


Length (m)


(tonnes/ha/yr)


(ha)

Average Annual

Sediment Load

(m3/yr)

Disturbed area on southern side of Frenchs Forest Road East west of Allambie Road Lucas Heights 90 4 0.97 227 0.060 9.1

Disturbed area eastern corner of Frenchs Forest Road East and Patanga Road Lucas Heights 15 4 0.44 103 0.007 0.4

Disturbed area on eastern side of Allambie Road between Warringah Road and Frenchs Forest Road East Lucas Heights 90 4 0.97 227 0.051 7.7

Disturbed area on northern side of Warringah Road east of Allambie Road Lucas Heights 130 2 0.52 122 0.168 13.6

Disturbed are on northern side of Frenchs Forest Road East between Patanga Road and Warringah Road Lucas Heights 125 2 0.52 122 0.089 7.2

Disturbed area on eastern side of Allambie Road south of Warringah Road Lucas Heights 100 3 0.72 168 0.078 8.8

Disturbed area on western side of Allambie Road between Warringah Road and Frenchs Forest Road East Lucas Heights 80 5 1.19 278 0.242 44.9

Disturbed area on southern side of Warringah Road between Allambie Road and Rodborough Road Lucas Heights 120 2 0.44 103 0.079 5.4

Disturbed area on souhern side of Warringah Road east of Rodborough Road Lucas Heights 80 2 0.41 96 0.031 2.0

Disturbed area on northern side of Frenchs Forest Road East and Warringah Road Lucas Heights 130 2 0.52 122 0.077 6.3

Disturbed area on southern side of Warringah Road between Allambie Road and Jones Street Lucas Heights 90 6 1.58 370 0.420 103.4

Disturbed area on southern side of Warringah Road east of Jones Street Lucas Heights 90 6 1.58 370 0.063 15.5

Disturbed area on southern side of Warringah Road between Jones Street and Courtley Road Lucas Heights 150 6 2.14 501 0.215 71.7


APPENDIX D

DETAILED DESCRIPTION OF PROPOSED OPERATIONAL SURFACE WATER MANAGEMENT STRATEGY

STAGE 1 PROJECT



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D1 DETAILED DESCRIPTION OF PROPOSED OPERATIONAL SURFACE WATER MANAGEMENT STRATEGY - STAGE 1 PROJECT

D1.1 General

This appendix provides a detailed description of the operational surface water management strategy which has been developed for the Stage 1 Project.

D1.2 Drainage Line 1 (Middle Creek)

D1.2.1 Catchment 1A

The Stage 1 Project would involve adjustments over an 80 m section of Forest Way, north of Naree Road. Given the limited nature of the works there will be no change in contributing catchment area, with only a minor increase in impervious area of less than 0.1 ha.

Table E1 in Appendix E shows that there will be a minor increase in flow discharging from Forest Way of 2 per cent or less for the range of storm events assessed. Upgrades to the piped drainage system along Forest Way improves the capture of runoff in this area. As a result, there will be a slight reduction in gutter flows discharging from Forest Way for the range of storms assessed.

Further downstream in Wareham Crescent, there is a minor change in peak flows of 1 per cent or less across the range of storms assessed.

The recommended drainage works along Forest Way would provide a hydrologic standard of 10 year ARI for the upgraded section of road. Note that the hydrologic standard of the drainage system immediately north of the road upgrade would be between 2 and 5 year ARI, as the capacity of the system is constrained by the existing 525 mm diameter pipe that discharges from Forest Way through Wareham Reserve.

D1.2.2 Catchment 1B

The recommended upgrade of the existing drainage system along Forest Way and Naree Road to accommodate the proposed road widening within Catchment 1B are shown on Figure 8.1, Sheet 1 of 4).

The recommended drainage works in Forest Way involve the replacement of the existing pipe drainage system along the western side of Forest Way with a new drainage systems running along its western kerb line. A new pavement drainage system has also been provided along the eastern kerb line of the widened road to connect into the existing 600 mm diameter pipe that drains the sag in Forest Way (refer Location F6 on Figure 8.1, Sheet 1 of 4).

A 50 m length of 375 mm diameter pipe is also recommended along the eastern side of Forest Way to connect to the existing drainage system in Rabbett Street. This drainage diversion has been sized to offset increased flows that would otherwise discharge from the sag in Forest Way as a result of the increase in paved area attributed to the widening of Forest Way (refer Location F6).15

15 Note that an increase in flow discharging from the low point in Forest Way has the potential to impact on adjacent private property.




Table E1 shows that the recommended drainage works along Forest Way, including the 375 mm diameter pipe diversion into Rabbett Street, will result in a slight reduction in peak flows discharging from the road corridor over the full range of design storm events. For example, the peak overland flow discharging from the sag in Forest Way (Location F6) would be reduced from 0.60 m3/s to 0.54 m3/s in a 100 year ARI event.

Peak flows discharging north from Forest Way along Rabbett Street would be increased over the full range of design storm events as a result of the 375 mm diameter pipe diversion (Location F7a and F7b).

The existing piped drainage system which is located beneath the western kerb line of Rabbett Street north of Holland Crescent is presently surcharged at the 2 year ARI level, with overland flow contributing to flooding at the low point in Frenchs Forest Road West (Location F11). It is recommended that this section of piped drainage system be upgraded to prevent its surcharge for events up to 10 year ARI. A preliminary analysis indicates that the existing piped drainage system would need to be upgraded from a location immediately north of Holland Crescent to its point of discharge in Rabbett Reserve. The upgraded drainage line would likely comprise pipes of between 900 and 1200 mm diameter.

The recommended drainage works in Naree Road involve new piped drainage lines under the kerb line of the widened road connecting into the upgraded section of piped drainage system in Rabbett Street. These new piped drainage lines would provide a hydrologic standard of 10 year ARI in Naree Road.

Table E1 shows that there will be a reduction in peak flows discharging north along Rabbett Street (Location F8), and east along Frenchs Forest Road West (Location F16) into Catchment 1C for all storms up to 100 year ARI. This is due to the proposed upgrade of the piped drainage system in Rabbett Street between Frenchs Forest Road West and Rabbett Reserve.

D1.2.3 Catchment 1C

Recommended adjustments to the existing road drainage system along the length of Frenchs Forest Road West within Catchment 1C are shown on Figure 8.1, Sheet 1 of 4. These works involve the installation of new pavement drainage lines along the northern and southern kerb lines.

New kerb inlet pits along the northern kerb line will intercept runoff before it discharges into Sylvia Place and Bluegum Crescent (refer Locations F18 and F19). This will offset the potential for increases in the rate of flow discharging into these local roads as a result of the road widening. An increase in peak flows discharging into Sylvia Place and Bluegum Crescent will have the potential to impact on the properties adjacent to the sags in these roads.

The recommended drainage strategy includes a new piped drainage line along Cobb Street, where it will discharge into Drainage Line 1 at the existing pipe outlet downstream (north) of Epping Drive. This new piped drainage line has been sized to offset increased flows at the sag in Frenchs Forest Road West which would otherwise have the potential to adversely impact flooding conditions is residential properties located downstream of Frenchs Forest Road West through to Epping Drive. A preliminary analysis indicates that the new piped drainage line would likely comprise a 900 mm diameter pipe. Whilst a new headwall arrangement would need to be constructed on the northern side of Epping Drive, there is no requirement for scour protection at the outlet of the new system due to flow from the pipe discharging directly onto exposed rock.




Table E1 shows that the installation of a new piped drainage line in Cobb Street will result in a reduction in peak overland flows discharging north from the Frenchs Forest Road West low point (Location F11) through Rabbett Reserve (Location F12) and across Epping Drive (Location F13). Downstream of the outlet to the Cobb Street pipeline (Location F15) peak flows would be increased by 8 per cent during a 2 year ARI design storm. Increases would be 1 per cent or less for larger storm events.

Whilst constraints imposed by the existing 900 mm diameter pipe which runs through private property north (downstream) of the low point in Frenchs Forest Road West mean that the upgraded section of road will have a hydrologic standard of only 2 year ARI, preliminary analysis indicates that the duplication of this pipe would prevent inundation of the road for events up to 10 year ARI. Whilst drainage improvement works within private property would need to be undertaken by WC, it is recommended that the existing 900 mm diameter pipe which crosses Frenchs Forest Road West at the low point be duplicated and sufficient inlet capacity be provided within the road reserve to control flooding up to the 10 year ARI. 16 In this way, works within the road reserve would not be required as part of any future upgrade of the downstream drainage system by WC.

Note that The Forest High School is located within Catchment 1C and contributes to runoff along Frenchs Forest Road West. As shown in Figure 8.1, Sheet 1 of 4 the extent of works proposed under Stage 1 lies downstream of the Forest High School. As a result drainage patterns in the school grounds will not be impacted by the proposed road works.

The Slot which will form part of the Stage 2 Project will reduce the catchment area draining to Catchment 1C. However, a preliminary assessment found that there will still be an increase in peak flows at the sag in Frenchs Forest Road West following completion of the second phase of the road upgrade works. As a result, measures to mitigate the impact of the proposed road works on flooding conditions in existing development need to be incorporated in the Stage 1 Project.

As mentioned in Section 8.1.1, it is envisaged that runoff from the Slot will be collected in sub-surface storage tank and discharged via a pump out system to the adjacent at-grade pavement drainage system. The temporary storage of surface runoff generated from within the Slot will assist in reducing peak flows in the receiving drainage system. The reduction in peak flows that this approach to managing stormwater runoff provides will need to be investigated further during the next stage of the EIS process.

D1.2.4 Catchment 1D

A new piped drainage system is proposed along Frenchs Forest Road West to provide a 10 year ARI hydrologic standard, as shown in Figure 8.1, Sheet 1 of 3. New inlet pits along the northern kerb line of Frenchs Forest Road will increase the capture of runoff and offset increased flows along Bluegum Crescent (Location F20) that would otherwise occur as a result of the road widening.

There will be a slight reduction in peak overland flows discharging into Bluegum Crescent from Catchment 1D for all design storm events up to 100 year ARI.

16 Note the sizing of the kerb inlet structures would need to assume that the downstream drainage system has a similar 10 year ARI hydrologic standard.




D1.3 Drainage Line 2 (Trefoil Creek)

D1.3.1 Catchments 2A, 2B and 2C

The recommended upgrade of the existing road drainage system along Wakehurst Parkway is shown in Figure 8.1, Sheets 2 and 3 of 4. The works involve the installation of a new trunk drainage line within the road reserve of Wakehurst Parkway, extending over a distance of about 840 m from Frenches Forest Road West to the location where the main arm of Drainage Line 2 crosses the road corridor. As mentioned in Section 8.1.2, the strategy is aimed at not increasing peak flows in the minor drainage lines which discharge toward the main arm of Drainage Line 2 on the eastern side of the Wakehurst Parkway given the impact this would have on Red-crowned Toadlet habitat.

The first section of the new trunk drainage line will run over a distance of about 280 m along the western side of Wakehurst Parkway and comprises a 750 mm diameter pipe. While flow in the new trunk drainage line will be diverted past the inlet of the existing 375 mm, 750 mm and 825 mm diameter pipes which presently cross Wakehurst Parkway near its intersection with Frenchs Forest Road West, it will be necessary to maintain flow discharging to the receiving drainage lines due to the presence of the Red-crowned Toadlet habitat.

A modified letterbox type opening will need to be fitted to the inlet of the existing 750 mm diameter pipe in order to limit the rate of flow discharging to the receiving drainage line to no greater than present day conditions. Diverting road runoff away from the existing 750 mm diameter pipe will also reduce the high gross pollutant load which presently discharges to the receiving drainage line.

Flow which surcharges the modified inlet pit arrangement will be diverted into the new trunk drainage line which will run in a northerly direction through the existing road cutting. Due to the increase in flow at this location, the pipe will increase to 1200 mm in diameter. It will be necessary to trench across Wakehurst Parkway immediately north of the existing 750 mm diameter transverse drainage pipe due to space limitations on the western side of the road cutting. A 3.5 m wide shoulder runs along the eastern side of the two southbound climbing lanes which will allow trenching for the trunk line to proceed while maintaining traffic f low (refer Plates 23 and 24 in Appendix B).

It is proposed to route the trunk drainage line along the western side of the road corridor north of the road cutting given the already disturbed nature of the area. This will require a second transverse crossing of Wakehurst Parkway immediately north of the road cutting. Rather than pipe the flow to the main arm of Drainage Line 2, it is proposed to excavate a channel on the eastern side of the existing access track which runs parallel with Wakehurst Parkway. The channel would need to be about 1 m deep and have a top width of about 4 m in order to convey flows discharging from the 1200 mm diameter pipe. It would also need to be lined with rock -riprap to prevent scour.

A drop structure would need to be provided at the (northern) downstream end of the channel where it joins the main arm of Drainage Line 2, along with suitable scour protection measures in the form of dumped rock rip rap.

Blind pits are to be provided along the new trunk drainage line where it runs along the eastern side of Wakehurst Parkway north of Location F21a (refer Figure 8.1, Sheet 3 of 4). Direct runoff




from the 230 m section of Wakehurst Parkway north of Location F21a will discharge along the eastern side of Wakehurst Parkway at Location F25 as occurs under present day conditions. 17

The provision of the trunk drainage line will reduce peak flows along the main arm of Drainage Line 2 between Frenchs Forest Road East and Wakehurst Parkway (refer comparison of peak flows at Locations F24b to F24e in Table E1), with the greatest reductions occurring immediately upstream of the 1650 mm diameter pipe which crosses Wakehurst Parkway. The reduction of peak flow in the main arm of Drainage Line 2 at this location will increase the hydrologic standar d of the existing transverse drainage structure, thus reducing the frequency of flooding at the low point in the road.

Minor reductions in peak flow also occur immediately downstream of the confluence between Drainage Line 2 and the new trunk drainage line for events up to about 10 year ARI, due principally to a de-synchronisation of the flow in the two drainage lines. There will be a minor increase in peak flow at the 100 year ARI level. However, this represents a 1 per cent increase in peak flow when compared to present day conditions (refer comparison of peak flows at Locations F24f in Table E1).

D1.3.2 Catchment 2D

The proposed upgrade of the existing road drainage system along the section of Frenchs Forest Road East within Catchment 2D is shown in Figure 8.1, Sheet 2 of 4. The works will essentially involve a new piped drainage line along the southern kerb line of the widened Frenchs Forest Road East. The existing 750 mm pipe crossing at the low point in Frenchs Forest Road East (refer Location F24) will need to be upgraded to a 1200 mm diameter pipe in order to provide a 10 year ARI hydrologic standard.

Table E1 shows that there will be a minor change in flow discharging from the road corridor at Catchment 2D for all design storms up to 100 year ARI (refer Location F24a). Outlet scour protection, such as a rock rip rap apron with an energy dissipation structure will be required to prevent scour from occurring in Drainage Line 2 immediately downstream of the outlet of the upgraded piped drainage system.

D1.3.3 Catchment 2E

The proposed works within Catchment 2E will involve only minor adjustments to the intersection of Romford Road and Frenchs Forest Road East. The change in impervious area and therefore runoff behaviour will therefore be minor.

D1.4 Drainage Line 3

D1.4.1 Catchment 3A

Recommended upgrades to the existing drainage system to accommodate the proposed widening of Warringah Road, west of Allambie Road are shown in Figure 8.1, Sheet 4 of 4. The existing 17 Under present day conditions Location F25 also receives flows that surcharge the inlet to the 750 mm pipe at Location F21a and travel north along Wakehurst Parkway during storms in excess of about a 3 month ARI. Under the proposed strategy flows that surcharge the inlet to the 750 mm pipe at Location F21a will be conveyed along the new trunk drainage line to Location F26. This will result in a reduction in peak flows discharging to Location F25 for storms in excess of about a 3 month ARI, which generate less than 5 per cent of the average annual runoff volume discharging to this location.




piped drainage system draining to the sag in Warringah Road will be extended to accommodate the widened road.

A 70 m length of piped drainage line is also proposed along the northern side of Warringah Road to connect to the drainage system in Allambie Road. This drainage diversion, along with upgrades to the existing drainage system along Allambie Road within Catchment 3C, will offset the potential for increased flows that would otherwise discharge from the sag in Warringah Road as a result of the increase in paved area from the widened road (refer Location F36b on Figure 8.1, Sheet 4 of 4). Table E1 shows that peak overland flows discharging north from the sag in Warringah Road will be reduced by the Stage 1 Project for storms with ARI’s up to 100 years. Figure 3.1 shows that the works proposed as part of the Stage 2 Project will extend the widening of Warringah Road along the full length of the road corridor within Catchment 3A. Increases in peak flows arriving at the sag in Warringah Road due to the proposed road works would be offset by the proposed pipe diversion into Allambie Road and upgrades to the drainage system in Allambie Road described in the Catchment 3C summary. As part of the drainage investigation for the Stage 2 Project it will be necessary to confirm the arrangement of these drainage works to manage the potential for increases to occur in overland flow discharging from the sag in Warringah Road.

D1.4.2 Catchment 3B Figure 8.1, Sheet 4 of 4 shows the recommended adjustments to the existing piped drainage system within Catchment 3B to accommodate the proposed slip lane between Allambie Road and Frenchs Forest Road East. A new piped drainage line along the western side of Allambie Road will be connected into the new drainage system in Patanga Road, thus offsetting the potential increase in flows discharging into Catchment 3B from the proposed slip lane. Table E1 shows that the peak 100 year ARI overland flow discharging from the road corridor at Catchment 3B will be slightly reduced as a result of the proposed road works (Location F28 and F30). There will be no change in peak overland flows during a 2 and 10 year ARI storm.

D1.4.3 Catchment 3C Recommended upgrades to the drainage system within Catchment 3C to accommodate the proposed road works are shown on Figure 8.1, Sheet 4 of 4. The recommended drainage works include new piped drainage lines along Allambie Road and Frenchs Forest Road East, as well as amplification of the existing drainage system that runs north along Patanga Road and Iris Street before it outlets at the end of the cul-de-sac in Winslea Crescent. The amplification of the drainage system in Patanga Road is required to provide the required 10 year ARI hydrologic standard along the upgraded section of road, and to offset potential increases in discharges through existing development to the north of the road corridor. A preliminary analysis indicates that the new drainage line would likely comprise pipes ranging in diameter between 750 and 1200 mm. Whilst a new headwall arrangement would need to be constructed at the northern end of Winslea Crescent, there is no requirement for scour protection at the outlet of the new system due to flow from the pipe discharging directly onto exposed rock.




Table E1 shows that the amplification of the piped drainage line along Patanga Road will result in a slight reduction in peak overland flows discharging north from Frenchs Forest Road East (Location F31), along both Patanga Road (Location F32) and Iris Street (Location F33). Downstream of the pipe outlet in Winslea Crescent (Location F35), peak flows will be increased by 10 per cent during a 2 year ARI design storm. Increases in peak flows will be 2 per cent or less for larger storm events.

D1.4.4 Catchment 3D Figure 8.1, Sheet 4 of 4 shows the recommended upgrades to the existing drainage system to accommodate the proposed widening of Warringah Road. These works involve new piped drainage lines along the alignment of the new median and southern kerb line. The existing 450 mm diameter pipe crossing at the low point in Warringah Road will need to be upgraded to a 675 mm diameter pipe to provide a 10 year ARI hydrologic standard for the upgraded section of Warringah Road. Table E1 shows that peak overland flows discharging from Warringah Road into Frenchs Forest Road East will be reduced as a result of the proposed road works and associated upgrades to the drainage system (Location F38).

D1.4.5 Catchments 3E and 3F New piped drainage lines are recommended along the southern kerb line of the widened Warringah Road within Catchments 3E and 3F. These piped drainage lines will connect to the existing drainage systems that discharge from Warringah Road at Location F39 and F40. The hydrologic standard of the upgraded section of Warringah Road is constrained by the capacity of the existing drainage lines discharging from Warringah Road. Table E1 shows that the peak 10 year ARI flow along Warringah Road is 0.1 m3/s and 0.25 m3/s at Locations F39 and F38 respectively, which results in a flow width of between 1.6 m and 2.6 m. Table E1 shows that there will be a minor decrease in peak flow discharging in an easterly direction from the limit of proposed works along Warringah Road (Location F38b) for all design storms up to 100 year ARI.

APPENDIX E

TABLE SUMMARISING IMPACT OF STAGE 1 PROJECT ON PEAK FLOWS



NBH_V1_006_Report.doc Page E1 Lyall & Associates October 2014 Rev. 3.3 Consulting Water Engineers

TABLE E1 IMPACT OF ROAD UPGRADE ON PEAK FLOWS(1)

Catchment

Location

Location Identifier

(2)

Pre-Upgrade Conditions Post-Upgrade Conditions Difference(3)

Piped Overland Piped Overland Piped Overland

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

[A] [B] [C] [D] [E] [F] [G] [H] [I] [J] [K] [L] [M] [N] [O] [P] [Q] [R] [S] [T] [U]

1A

Flow across low point in Forest Way adjacent to Wareham Reserve.

F1 0.44 0.48 0.52 0.19 0.56 0.95 0.44 0.49 0.53 0.21 0.66 1.13 0.00 0.01 0.01 0.02 0.10 0.18

Flow discharging from Forest Way at Wareham Reserve. F2 0.71 0.84 0.91 0.00 0.36 0.81 0.73 0.96 1.02 0.00 0.27 0.80 0.02 0.12 0.11 0.00 -0.09 -0.01

Flow in Wareham Crescent. F3a 1.08 1.39 1.39 - - - 1.10 1.39 1.39 - - - 0.02 0.00 0.00 - - -

F3b - - - 0.00 0.33 1.17 - - - 0.00 0.35 1.12 - - - 0.00 0.02 -0.05

Discharging from Wareham Crescent into Adams Street. F4 0.51 0.51 0.53 1.05 1.73 2.29 0.51 0.51 0.53 1.07 1.73 2.35 0.00 0.00 0.00 0.02 0.00 0.06

1B

Flow across low point in Forest Way south of Russell Avenue. F5 0.55 0.75 0.76 0.00 0.21 0.74 0.54 0.78 0.78 0.00 0.16 0.72 -0.01 0.03 0.02 0.00 -0.05 -0.02

Flow discharging from Forest Way south of Russell Avenue. F6 0.62 0.99 1.00 0.00 0.00 0.60 0.62 0.95 1.02 0.00 0.00 0.54 0.00 -0.04 0.02 0.00 0.00 -0.06

Flow along Rabbett Street, south of Naree Road. F7a 0.20 0.29 0.39 0.00 0.00 0.01 0.23 0.33 0.45 0.00 0.00 0.01 0.03 0.04 0.06 0.00 0.00 0.00

F7b 0.60 0.81 0.88 0.48 0.85 1.50 0.97 1.42 1.75 0.15 0.27 0.67 0.37 0.61 0.87 -0.33 -0.58 -0.83

Flow along Rabbett Street, north of Naree Road. F8 1.15 1.17 1.17 0.25 0.73 1.11 1.79 2.51 3.25 0.04 0.25 0.55 0.64 1.34 2.08 -0.21 -0.48 -0.56

Flow discharging from Rabbett Street into Adams Street. F9 - - - 0.18 1.11 1.76 - - - 0.17 0.42 0.91 - - - -0.01 -0.69 -0.85

1C

Flow discharging into the low point in Frenchs Forest Road West from the south.

F10 1.57 1.58 1.59 0.12 1.51 3.23 1.57 1.59 1.58 0.18 1.50 3.24 0.00 0.01 -0.01 0.06 -0.01 0.01

Discharging from low point in Frenchs Forest Road West. F11 2.59 2.64 2.66 0.29 2.68 5.87 2.06 2.57 2.60 0.00 1.30 3.61 -0.53 -0.07 -0.06 -0.29 -1.38 -2.26

Flow in Rabbett Reserve. F12 - - - 4.02 6.50 9.71 - - - 3.91 5.77 9.12 - - - -0.11 -0.73 -0.59

Flow across Epping Drive north of Rabbett Reserve. F13 6.25 7.39 7.82 2.00 5.67 11.87 7.12 8.70 9.49 1.82 4.77 10.34 0.87 1.31 1.67 -0.18 -0.90 -1.53

Discharging from Frenchs Forest Road West into Cobb Street. F14 - - - 0.03 0.04 0.06 0.72 0.96 1.15 0.00 0.00 0.00 - - - -0.03 -0.04 -0.06

Discharging from Epping Drive into Drainage Line 1. F15 - - - 8.08 12.94 19.57 - - - 8.74 13.16 19.13 - - - 0.66 0.22 -0.44

Flow travelling west along Frenchs Forest Road West toward low point adjacent to Rabbett Street.

F16 - - - 0.33 0.86 1.60 - - - 0.00 0.03 0.13 - - - -0.33 -0.83 -1.47

Discharging from Frenchs Forest Police Station into Frenchs Forest Road West.

F17 0.50 0.65 0.70 0.05 0.24 0.61 0.51 0.65 0.70 0.05 0.24 0.61 0.01 0.00 0.00 0.00 0.00 0.00

(1) Peak flows have been quoted to more than one decimal place for comparative purposes only.

(2) Refer Figures 3.2 (4 Sheets) and Figures 8.1 (4 Sheets) for reference to Location Identifier.

(3) Note that a positive value represents an increase in peak flow when compared to present day conditions.




TABLE E1 (Cont’d) IMPACT OF ROAD UPGRADE ON PEAK FLOWS(1)

Catchment

Location

Location Identifier

(2)



2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI


1C Discharging from Frenchs Forest Road West into Sylvia Place. F18 - - - 0.02 0.04 0.05 - - - 0.00 0.01 0.01 - - - -0.02 -0.03 -0.04

Discharging into western end of Bluegum Crescent from Frenchs Forest Road West.

F19 - - - 0.03 0.04 0.06 - - - 0.00 0.01 0.01 - - - -0.03 -0.03 -0.05

1D Discharging into eastern end of Bluegum Crescent from Frenchs Forest Road West.

F20 0.03 0.05 0.06 0.01 0.01 0.02 0.02 0.03 0.04 0.00 0.00 0.01 -0.01 -0.02 -0.02 -0.01 -0.01 -0.01

2A

Flow in 750 mm diameter pipe crossing Wakehurst Parkway about 280 m north of Frenchs Forest Road West.

F21a 0.36 0.45 0.45 - - - 0.27 0.35 0.36 - - - -0.09 -0.10 -0.09 - - -

Flow discharging from 750 mm diameter pipe. F21b - - - 0.38 0.50 0.52 - - - 0.30 0.40 0.43 - - - -0.08 -0.10 -0.09

Flow surcharging inlet of 750 mm diameter pipe and discharging north along Wakehurst Parkway.

F21c - - - 1.30 2.71 4.60 - - - 0.00 0.00 1.94 - - - -1.30 -2.71 -2.66

2B

Flow in 825 mm diameter pipe crossing Wakehurst Parkway about 40 m north of Frenchs Forest Road West.

F22a 0.15 0.26 0.36 - - - 0.13 0.20 0.28 - - - -0.02 -0.06 -0.08 - - -

Flow discharging from 825 mm diameter pipe. F22b - - - 0.15 0.26 0.36 - - - 0.13 0.20 0.28 - - - -0.02 -0.06 -0.08

Flow surcharging inlet of 825 mm diameter pipe and discharging north along Wakehurst Parkway.

F22c - - - 0.28 0.72 1.26 - - - 0.00 0.01 0.11 - - - -0.28 -0.71 -1.15

2C Discharge from 375 mm diameter pipe at intersection of Wakehurst Parkway and Frenchs Forest Road.

F23 0.31 0.31 0.31 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 -0.31 -0.31 -0.31 0.00 -0.01 -0.01

2D

Discharge in new 1200 mm diameter pipe discharging to Drainage Line 2

F24 1.45 1.50 1.51 0.70 1.25 2.54 1.97 3.06 4.10 0.00 0.00 0.00 0.52 1.56 2.59 -0.70 -1.25 -2.54

Discharge from low point in Frenchs Forest Road East into Drainage Line 2.

F24a - - - 2.14 2.72 4.05 - - - 1.97 3.06 4.10 - - - -0.17 0.34 0.05

Main arm of Drainage Line 2 downstream of confluence with flow discharging from 2B and 2C.

F24b - - - 2.72 3.45 5.04 - - - 2.17 3.40 4.66 - - - -0.55 -0.05 -0.38

Main arm of Drainage Line 2 downstream of confluence with flow discharging from 2A, 2B and 2C.

F24c - - - 3.51 5.36 6.97 - - - 2.84 4.64 6.96 - - - -0.67 -0.72 -0.01

Main arm of Drainage Line 2 at northern end of Nandi Avenue. F24d - - - 8.55 15.41 21.69 - - - 8.23 14.24 21.34 - - - -0.32 -1.17 -0.35

Main arm of Drainage Line 2 immediately upstream of Wakehurst Parkway crossing.

F24e - - - 10.96 18.77 29.39 - - - 9.55 16.10 25.70 - - - -1.41 -2.67 -3.69

Main arm of Drainage Line 2 immediately downstream of Wakehurst Parkway crossing.

F24f - - - 11.21 19.04 30.01 - - - 11.20 19.00 30.40 - - - -0.01 -0.04 0.39

Flow discharging into bushland on eastern side of Wakehurst Parkway north of existing road cutting.

F25 - - - 1.40 2.93 4.90 - - - 0.22 0.36 2.14 - - - -1.18 -2.57 -2.76

Flow discharging into bushland on western side of Wakehurst Parkway north of existing road cutting. (4)

F26 - - - - - - - - - 2.13 3.84 4.24 - - - 2.13 3.84 4.24




(4) Note that flows discharging to bushland on the western side of Wakehurst Parkway will be controlled by a new channel which will run along the eastern side of the existing acces s track.




TABLE E1 (Cont’d) IMPACT OF ROAD UPGRADE ON PEAK FLOWS(1)

Catchment

Location

Location Identifier

(2)



2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI

2 year ARI

10 year ARI

100 year ARI


2E Flow in Frenchs Forest Road East opposite Skyline Business Park.

F27a 0.06 0.09 0.12 - - - 0.06 0.09 0.12 - - - 0.00 0.00 0.00 - - -

F27b - - - 0.02 0.04 0.07 - - - 0.02 0.04 0.07 - - - 0.00 0.00 0.00

3A

Discharging from Frenchs Forest Road East into Inverness Avenue.

F28 2.16 3.36 3.67 0.04 0.18 1.80 2.11 3.18 3.67 0.04 0.18 1.58 -0.05 -0.18 0.00 0.00 0.00 -0.22

Flow in Patanga Park. F29 2.77 2.93 3.02 0.09 1.59 4.95 2.73 2.94 3.02 0.09 1.45 4.67 -0.04 0.01 0.00 0.00 -0.14 -0.28

3B Discharge from low point in Frenchs Forest Road East, east of Inverness Avenue.

F30 0.08 0.12 0.81 - - - 0.09 0.13 0.75 - - - 0.01 0.01 -0.06 - - -

3C Discharge from Frenchs Forest Road East into Patanga Road. F31 0.45 0.53 0.53 0.10 0.32 1.04 1.15 1.59 1.86 0.04 0.29 0.53 0.70 1.06 1.33 -0.06 -0.03 -0.51

Flow in Patanga Road, north of Dareen Street. F32 2.75 2.75 2.76 1.11 3.19 6.25 3.78 4.15 4.16 0.70 2.78 5.59 1.03 1.40 1.40 -0.41 -0.41 -0.66

3C

Flow along Iris Street, west of Winslea Crescent. F33 3.51 3.56 3.58 1.06 3.42 6.71 3.85 4.23 4.35 0.94 3.29 6.59 0.34 0.67 0.77 -0.12 -0.13 -0.12

Discharge from Winslea Crescent. F34 5.09 5.34 5.66 0.43 3.47 7.16 5.41 6.11 6.39 0.36 2.66 6.66 0.32 0.77 0.73 -0.07 -0.81 -0.50

Flow in natural watercourse downstream of Winslea Crescent. F35 - - - 5.43 8.96 13.01 - - - 6.02 9.06 13.17 - - - 0.59 0.10 0.16

Flow along Allambie Road. F36a 0.25 0.31 0.32 0.00 0.00 0.01 0.36 0.60 0.67 0.01 0.03 0.04 0.11 0.29 0.35 0.01 0.03 0.03

Flow discharging from low point in Warringah Road, west of Allambie Road.

F36b 0.54 0.72 0.70 0.00 0.48 1.11 0.51 0.71 0.66 0.00 0.31 0.82 -0.03 -0.01 -0.04 0.00 -0.17 -0.29

3D Flow in Frenchs Forest Road East, west of Warringah Road.

F37a 0.39 0.46 0.57 - - - 0.06 0.16 0.21 - - - -0.33 -0.30 -0.36 - - -

F37b - - - 0.02 0.04 0.68 0.51 0.74 0.88 0.02 0.03 0.06 - - - 0.00 -0.01 -0.62

Discharge from low point in Warringah Road, west of Frenchs Forest Road East.

F38 0.34 0.37 0.40 0.00 0.00 0.67 0.52 0.82 0.98 0.00 0.00 0.00 0.18 0.45 0.58 0.00 0.00 -0.67

3E Flow along Warringah Road, east of Jones Street. F39a 0.11 0.16 0.17 - - - 0.16 0.18 0.18 - - - 0.05 0.02 0.01 - - -

F39b - - - 0.03 0.06 0.15 - - - 0.00 0.10 0.22 - - - -0.03 0.04 0.07

3F Flow along Warringah Road, adjacent to Courtley Road. F40a 0.13 0.16 0.19 - - - 0.11 0.19 0.22 - - - -0.02 0.03 0.03 - - -

F40b - - - 0.15 0.23 0.39 - - - 0.10 0.25 0.37 - - - -0.05 0.02 -0.02




(4) Overland flow at Location F30 combines with flow at Location F28 where it discharges in a northerly direction from the low point in Frenchs Forest Road east of Inverness Avenue.

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