Northern Beaches Hospital Connectivity and Network Enhancements Stage 2 Environmental Impact Statement: Surface Water Assessment

SMEC Northern Beaches Hospital Connectivity and Network Enhancements Stage 2 Environmental Impact Statement: Surface Water Assessment

-XO\ 2015

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | i

Table of contents

1. Introduction ...... 10 1.1 Introduction ...... 10 1.2 Background...... 10 1.3 Project location ...... 11 1.4 Project Description ...... 11 1.5 Secretary’s Environmental Assessment Requirements...... 12 1.6 Purpose and scope ...... 15 2. Regulatory context ...... 17 2.1 Secretary’s Environmental Assessment Requirements...... 17 2.2 Water Management Act 2000...... 17 2.3 Environmental Planning and Assessment Act 1979 ...... 18 2.4 Protection of the Environment Operations Act 1997 ...... 18 2.5 Policy and guidelines ...... 19 3. Methodology ...... 26 3.1 Data collection and field work...... 26 3.2 Existing conditions and impact assessment ...... 27 3.3 Surface water management strategy ...... 29 3.4 Assumptions and limitations...... 31 4. Existing environment ...... 33 4.1 Catchment overview ...... 33 4.2 Major waterways ...... 33 4.3 Ecology ...... 34 4.4 Geology...... 34 4.5 Stormwater drainage network...... 35 4.6 Erosion, sediment and stormwater quality ...... 40 4.7 Water quantity and stormwater ...... 45 4.8 Water sources and supply catchments ...... 46 4.9 Groundwater and surface water interactions ...... 46 5. Proposed drainage description ...... 48 5.1 Slot drainage ...... 48 5.2 Cross-drainage upgrades ...... 49 5.3 Detention storages ...... 50 5.4 Other network upgrades ...... 51 6. Impact assessment ...... 54 6.1 Overview of changes to catchment hydrology ...... 54 6.2 Erosion, sediment and water quality impacts ...... 55 6.3 Water quantity and stormwater impacts ...... 63 6.4 Cumulative impacts on water sources ...... 65

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6.5 Interaction with the Stage 1 Project and the Northern Beaches Hospital ...... 69 6.6 Climate change...... 69 7. Surface water management strategy ...... 71 7.1 Measures to manage erosion, sediment and water quality impacts ...... 71 7.2 Measures to mitigate water quantity and stormwater impacts ...... 74 7.3 Cumulative impact management...... 75 8. References ...... 77

Table index

Table 1: SEARs related to the Stage 2 Project Surface Water Assessment ...... 13 Table 2 WMA 2000 principles relating to the Stage 2 Project ...... 18 Table 3: Stormwater pollution reduction targets as set in the Northern Beaches Stormwater Management Plan (1999) ...... 20 Table 4: Water quality objectives (Engineers Australia, 1999) ...... 22 Table 5 River flow objectives (Engineers Australia, 1999)...... 23 Table 6: Interpreted geological conditions (eastbound slot location) ...... 35 Table 7: Existing stormwater quality treatment devices ...... 37 Table 8: Removal rates for Trash Racks SWS0018 and SWS0041 ...... 38 Table 9: Results of 2004 water quality sampling ...... 45 Table 10: Proposed culvert upgrades ...... 50 Table 11 Proposed below-ground detention storages as part of the Stage 2 Project ...... 51 Table 12 Summary of changes to existing drainage lines under the proposed Stage 2 Project ...... 52 Table 13: Summary of catchment areas under existing and proposed conditions ...... 55 Table 14 MUSIC model results under existing and proposed conditions ...... 60 Table 15: Potential impacts of changes to groundwater seepage flows, dry periods...... 65 Table 16: Music Modelling Average Flow Outputs ...... 68 Table 17: CSIRO indicative change in rainfall one-day totals (CSIRO, 2007) ...... 69 Table 18: Fraction impervious assumptions by land use zone ...... 81 Table 19: Rainwater tank properties...... 82 Table 20: DRAINS model overland flow paths ...... 83 Table 21: Antecedent Rainfall Depths ...... 84

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | iii Figure index

Figure 1: Stage 1 and Stage 2 Network Enhancement Location ...... 16 Figure 2: Study process overview ...... 26 Figure 3 Assessment of potential erosion hazard ...... 28 Figure 4: Sediment trap/trash rack downstream of Currie Road (looking upstream) ...... 39 Figure 5: Sediment trap/trash rack downstream of Currie Road (looking downstream) ...... 39 Figure 6: Sediment trap/trash rack at Rabbett Reserve (looking downstream) ...... 39 Figure 7: Sediment trap/trash rack at Rabbett Reserve (looking upstream) ...... 39 Figure 8: Bioretention system (western)...... 40 Figure 9: Bioretention system (eastern) ...... 40 Figure 10: Waterway upstream of Currie Road in Catchment 4 ...... 41 Figure 11: Undermining of maintenance access track at the sediment pond and trash rack downstream of Currie Drive in Catchment 4 ...... 41 Figure 12: Trash rack with evidence of bank and bed erosion, downstream of the existing piped culvert under Aquatic Drive culvert in Catchment 6 (refer to culvert SPI00369 in Map 2 of Appendix C) ...... 42 Figure 13: Nitrogen loads simulated by MUSIC for discharge from the detention storage on Aquatic Drive ...... 61 Figure 14: Nitrogen concentrations simulated by MUSIC for discharge from the detention storage on Aquatic Drive ...... 62 Figure 15: MUSIC model daily flows for Curl Curl Creek for an average year (1959)...... 67 Figure 16: Intensity-Frequency-Duration table for the Northern Beaches Hospital Site ...... 84

Appendices

Appendix A – MUSIC Model Development Appendix B – DRAINS model development Appendix C – Maps Appendix D – Detailed tables Appendix E – Water quality sampling Appendix F – Proposed Stage 2 Project Surface Water Drainage Design

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This report: has been prepared by GHD for SMEC and may only be used and relied on by SMEC for the purpose agreed between GHD and the SMEC as set out in Section 1.6 of this report. GHD otherwise disclaims responsibility to any person other than SMEC arising in connection with this report. GHD also excludes implied warranties and conditions, to the extent legally permissible. The services undertaken by GHD in connection with preparing this report were limited to those specifically detailed in the report and are subject to the scope limitations set out in the report. The opinions, conclusions and any recommendations in this report are based on conditions encountered and information reviewed at the date of preparation of the report. GHD has no responsibility or obligation to update this report to account for events or changes occurring subsequent to the date that the report was prepared. The opinions, conclusions and any recommendations in this report are based on assumptions made by GHD as described in the following section of this report. GHD disclaims liability arising from any of the assumptions being incorrect. GHD has prepared this report on the basis of information provided by SMEC and others who provided information to GHD (including Government authorities)], which GHD has not independently verified or checked beyond the agreed scope of work . GHD does not accept liability in connection with such unverified information, including errors and omissions in the report which were caused by errors or omissions in that information. The opinions, conclusions and any recommendations in this report are based on information obtained from specific field observations. Site conditions at other parts of the site may be different from the site conditions found at the specific points of observation. Investigations undertaken in respect of this report are constrained by the particular site conditions, such as the location of buildings, services and vegetation. As a result, not all relevant site features and conditions may have been identified in this report. Site conditions (including the presence of hazardous substances and/or site contamination) may change after the date of this Report. GHD does not accept responsibility arising from, or in connection with, any change to the site conditions. GHD is also not responsible for updating this report if the site conditions change.

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Abbreviations

AEP Annual exceedance probability AHD Australian Height Datum BOM Bureau of Meteorology CMS Cubic metres per second EIS Environmental Impact Statement EP&A Act Environmental Planning and Assessment Act 1979 EPBC Act Environment Protection and Biodiversity Conservation Act 1999 GPT Gross Pollutant Trap L&A Lyall and Associates Pty Ltd LGA Local Government Area LPI Land and Property Information L/s Litres per second MUSIC Model for Urban Stormwater Improvement Conceptualisation NOW New South Wales Office of Water NSW New South Wales SEARs Secretary’s Environmental Assessment Requirements TN Total Nitrogen TP Total Phosphorous TSS Total Suspended Solids WM Act Water Management Act 2000

Glossary

Activity A scheduled or non-scheduled activity within the meaning of the Protection of the Environment Operations Act 1997. Australian Height Datum A common national plane of level approximately equivalent to the (AHD) height above sea level. Annual exceedance The annual exceedance probability is a measure of the frequency probability (AEP) of a rainfall event. It is the probability that a given rainfall total accumulated over a given duration will be exceeded in any one year. A 1 per cent event is a rainfall event with a 1 per cent chance of being exceeded in magnitude in any year. In accordance with current Australian Rainfall and Runoff recommendations (Institute of Engineers, Australia, 1987), annual exceedance probability terminology has been used in this document. Average recurrence The average recurrence interval, like the annual exceedance interval (ARI) probability, is also a measure of the frequency of a rainfall event. The average, or expected, value of the periods between exceedances of a given rainfall total accumulated over a given duration. For example, a 100-year average recurrence interval event occurs or is exceeded on average once every 100 years. It is important to note that the ARI is an average period and it is implicit in the definition of the ARI that the periods between exceedances are generally random. Average recurrence intervals of greater than 10 years are closely approximated by the reciprocal of the annual exceedance probability. A 1 in 100-year average recurrence interval is therefore approximately equivalent to a 1 per cent annual exceedance probability event.

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ARI terminology is not used in this document but is included in the glossary for its common usage in matters related to flooding. Catchment The area drained by a stream or body of water or the area of land from which water is collected. Consent Approval to undertake a development received from the consent authority. Datum A level surface used as a reference in measuring elevations. Discharge Quantity of water per unit of time flowing in a stream, for example cubic meters per second or megalitres per day. DRAINS modelling DRAINS is a multi-purpose software program for designing and analysing urban stormwater drainage systems and catchments. Ephemeral Stream that is usually dry, but may contain water for rare or irregular periods, usually after significant rainfall. Erosion A natural process where wind or water detaches a soil particle and provides energy to move the particle. Flood For the purposes of this report, a flood is defined as the inundation of normally dry land by water which: escapes from, is released from, is unable to enter, or overflows from the normal confines of: a natural body of water or watercourse such as rivers, creeks or lakes, or any altered or modified body of water, including dams, canals, reservoirs and stormwater channels. Flood liable land Land which is within the extent of the probable maximum flood and therefore prone to flooding. Floodplain The area of land subject to inundation by floods up to and including the probable maximum flood event. Floodway The area of the floodplain where a significant portion of flow is conveyed during floods. Usually aligned with naturally defined channels. Formation A fundamental unit used in the classification of rock or soil sequences, generally comprising a body with distinctive physical and chemical features. Geomorphology Scientific study of landforms, their evolution and the processes that shape them. In this report, geomorphology relates to the form and structure of waterways. Groundwater Subsurface water stored in pores of soil or rocks. Hazard The potential or capacity of a known or potential risk to cause adverse effects. Headwater erosion The upstream lengthening and/or cutting of a valley or gully at its head, as the stream erodes away the rock and soil at its headwaters in the opposite direction that it flows. Hydraulics The physics of channel and floodplain flow relating to depth, velocity and turbulence. Hydrograph A graph which shows how a water level at any particular location changes with time. Hydrology The study of rainfall and surface water runoff processes. Infiltration The downward movement of water into soil and rock, which is largely governed by the structural condition of the soil, the nature of the soil surface (including presence of vegetation) and the antecedent moisture content of the soil. Landform A specific feature of the landscape or the general shape of the land. LiDAR LIDAR —Light Detection and Ranging — is a remote sensing method used to examine the surface of the Earth. Meteorology The science concerned with the processes and phenomena of the atmosphere, especially as a means of forecasting the weather. MUSIC modelling MUSIC is a software program used to estimate the performance of

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stormwater quality management systems. Overbank The portion of the flow that extends over the top of waterway banks. Overland flow path The path that water can follow if it leaves the confines of the main flow channel. Overland flow paths can occur through private property or along roads. Water travelling along overland flow paths, often referred to as ‘overland flows’, may re-enter the main channel or may be diverted to another watercourse. Permeability The capacity of porous medium for transmitting water. Pluviograph A rain gauge with the capability to record data in real time to observe rainfall over a short period of time. Probable maximum flood The probable maximum flood is the maximum flood which can (PMF) theoretically occur based on the worst combination of the probable maximum precipitation and flood-producing catchment conditions that is reasonably possible at a given location. Probable maximum The probable maximum precipitation is the greatest amount of precipitation (PMP) rainfall which can theoretically occur over a given duration (period of time) for a particular geographical location. Reach Defined section of a stream with uniform character and behaviour. Riparian Pertaining to, or situated on, the bank of a river or other water body. Risk The chance of something happening that will have an impact measured in terms of likelihood and consequence. Risk assessment Systematic process of evaluating potential risks of harmful effects on the environment from exposure to hazards associated with a particular product or activity. River Styles® framework A geomorphic approach for examining river character, behaviour, condition and recovery potential which it provides a template for river management. Runoff The amount of rainfall which actually ends up as streamflow, also known as rainfall excess. Salinity The total soluble mineral content of water or soil (dissolved solids); concentrations of total salts are expressed as milligrams per litre (equivalent to parts per million). Secretary’s Requirements for an environmental assessment issued by the environmental Secretary of the NSW Department of Planning and Environment in assessment requirements accordance with the Environment Planning and Assessment Act 1979. Sediment Material of varying sizes that has been or is being moved from its site of origin by the action of wind, water or gravity. Sinuosity Extent of curvature or meandering of a stream. Highly sinuous streams meander over a low gradient and short distance. Low sinuosity streams are straighter and have a steeper gradient. Stage 1 Project Project proposed to improve traffic connectivity to the Northern Beaches Hospital, subject to a separate Environmental Impact Assessment Stage 2 Project Project proposed to improve overall network capacity and traffic flow in the vicinity of the Northern Beaches Hospital. The subject of this Environmental Impact Assessment. Stream order Stream classification system, where order 1 is for headwater (new) streams at the top of a catchment. Order number increases downstream using a defined methodology relating to the branching of streams. Surface water Water that is derived from precipitation or pumped from underground and may be stored in dams, rivers, creeks and drainage lines. Study area The subject site and any additional areas which are likely to be

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affected by the proposal, either directly or indirectly. The study area should extend as far as is necessary to take all potential impacts into account. Topography Representation of the features and configuration of land surfaces. Water quality Chemical, physical and biological characteristics of water. Also the degree (or lack) of contamination. Water sharing plan A legal document prepared under the State Water Management Act 2000 that establishes rules for sharing water between the environmental needs of the river or aquifer and water users and also different types of water use. Water table The surface of saturation in an unconfined aquifer, or the level at which pressure of the water is equal to atmosphere pressure

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1. Introduction

1.1 Introduction Roads and Maritime is now seeking approval for the Stage 2 Project Network Enhancement Works (Stage 2 Project) which forms part of the Northern Beaches Hospital – Road Connectivity and Network Enhancements Project Concept Proposal. The approval is sought under Part 5.1 (State significant infrastructure) of the Environmental Planning and Assessment Act 1979 (NSW) (EP&A Act). The Stage 2 Project comprises road upgrades directed towards broader network capacity enhancement of the existing road network surrounding the Northern Beaches Hospital at Frenchs Forest, within the Warringah local government area (LGA) on Sydney’s Northern Beaches (refer to Figure 1). These upgrades focus on Warringah Road which caters for the dominant traffic movement.

1.2 Background Roads and Maritime Services (Roads and Maritime) placed the EIS for the Concept Proposal and Stage 1 Project Hospital Connectivity Works on public exhibition between 22 October 2014 to 21 November 2014. A Submissions Report which responds to issues raised during exhibition has been prepared and is currently being considered by the Department of Planning and Environment as part of its assessment. Based on this assessment the Department will decide whether to recommend concept approval of the Northern Beaches Hospital – Road Connectivity and Network Enhancements Project, and Project approval for the Stage 1 Project. Northern Beaches Hospital is proposed by NSW Health Infrastructure (HI) and would be located at the intersection of Warringah Road and Wakehurst Parkway. The hospital concept proposal and the first stage (clearing and utility works) of the hospital project were approved on 22 June 2014, and the second stage of the Northern Beaches Hospital project (hospital construction and operation) is currently being considered by the Department of Planning and Environment. The Long Term Transport Master Plan (Transport for NSW, 2012) identifies Warringah Road as a key strategic transport corridor linking Dee Why and Chatswood. Heavy traffic flows and congestion along Warringah Road during commuter peak periods and, to a lesser degree, during business hours result in low average peak travel speeds, unreliable travel times and disruptions to traffic movements. This impacts both road users and the adjoining community. Warringah Road is already operating at or beyond capacity during peak periods and is expected to experience continued traffic growth in the future. Based on the anticipated future land use changes, both within and surrounding the Northern Beaches Hospital Precinct, under Warringah Council’s structure plan (currently being prepared by Warringah Council), and the current level of congestion on Warringah Road (a key strategic transport corridor linking Dee Why and Chatswood), the Stage 2 Project is needed at a strategic level to: y Assist in the management of journeys in connection with anticipated future intensification of medical, commercial and residential land uses surrounding the new hospital y Mitigate the worsening of traffic congestion on the Warringah Road and Wakehurst Parkway arterial road corridors and their principal feeder roads y Enhance access arrangements by car, bus and active transport for the Northern Beaches Hospital employees, patients, outpatients and visitors y Facilitate improved access to the Northern Beaches Hospital and the surrounding employment precincts y Have key infrastructure components in place for the proposed hospital opening in 2018.

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1.3 Project location The Stage 2 Project is generally proposed to be carried out in the following locations: y Warringah Road between west of Fitzpatrick Ave East to west of Allambie Road y Forest Way between Warringah Road and the Stage 1 Project tie in (about 100 metres north of the Warringah Road intersection) y Wakehurst Parkway from the intersection with Warringah Road to about 120 metres south of Aquatic Drive y Aquatic Drive for about 100 metres east from the intersection with Wakehurst Parkway y Allambie Road between Warringah Road and Rodborough Road. On Warringah Road, the Stage 2 Project is incorporated largely within the existing road-reserve to the north, and extends beyond the existing road-reserve to the south by up to 30 metres. The indicative location of the Stage 2 Project is shown in Figure 1.

1.4 Project Description The Stage 2 Project would broadly include the following key project elements (subject to detailed design): y Provision of four through lanes on Warringah Road (two lanes in each direction for east-west through traffic) within a grade separated open slot for about 1.3 kilometres y Ingress and egress points from and to the slot include: – Western extent - Warringah Road near Fitzpatrick Avenue East – Eastern extent - Warringah Road from about 350 metres east of the Wakehurst Parkway grade separated intersection – Provision of a two-lane on-ramp (merging into one lane) from Wakehurst Parkway (southbound) into the slot (westbound) y Widening of Warringah Road from west of Fitzpatrick Avenue East to west of Allambie Road to include: – Westbound travel lanes, at surface level on the southern side of the Warringah Road corridor for the length of the project – Eastbound travel lanes, at surface level on the northern side of the Warringah Road corridor (using existing road pavement), for the length of the project y The intersections of Warringah Road with Forest Way, Hilmer Street and Wakehurst Parkway to form a surface level bridge over the slot to provide all traffic movements at surface level and allow east-west through traffic in the slot to pass beneath uninterrupted. y Upgrades or adjustments to existing intersections of Warringah Road with the following local roads and approaches: – Fitzpatrick Avenue East (including the closing of the left turn into Fitzpatrick Avenue East from Warringah Road westbound) – Rodborough Road – Allambie Road y Widening of Wakehurst Parkway from the intersection of Warringah Road to south of Aquatic Drive y Provision of a new connection at Aquatic Drive including right in from Wakehurst Parkway (northbound), left in from Wakehurst Parkway and left out movements from Aquatic Drive and Wakehurst Parkway. y Provision of shared (pedestrian and cyclist) bridges at the following locations:

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– Across Warringah Road west of the intersection of Forest Way (removal and replacement of the existing pedestrian bridge). – Across Warringah Road on the western side of the intersection with Hilmer Street (new pedestrian bridge). y Removal of the existing pedestrian crossing across Warringah Road at Hilmer Street. The Stage 2 Project would also include drainage works, landscaping, property acquisition and adjustments. The ancillary works would include but not be limited to, construction compounds, and stockpile sites. For the purposes of this EIS, the scope of the Stage 2 Project would not include ongoing maintenance works. Further details of the proposed surface water features of the Stage 2 Project are provided in Section 5.

1.5 Secretary’s Environmental Assessment Requirements Roads and Maritime has a statutory responsibility under the NSW Environmental Planning and Assessment Act 1979 and Commonwealth Environment Protection and Biodiversity Conservation Act 1999 to consider the impacts of its activities on the environment. The majority of Roads and Maritime’s projects are assessed under Part 5 of the Environmental Planning and Assessment Act 1979. Projects that qualify as State Significant Infrastructure are assessed under Part 5.1 of the Act and documented in an Environmental Impact Statement (EIS). On 9 September 2014, the NSW Department of Planning and Environment issued the Secretary’s Environmental Assessment Requirements (SEARs) for the Stage 2 Project. In preparing the SEARs, State government agencies and local authorities were consulted to identify issues and environmental assessment requirements. The assessment requirements and references to relevant sections within this report are outlined in Table 1.

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Table 1: SEARs related to the Stage 2 Project Surface Water Assessment Key Issue Key Issue Items Report reference Soils, Water and Potential water quality impacts and mitigation Section 6.2 - Erosion, sediment Waste - Eros ion, measures to manage water pollution, with reference sediment and to relevant public health and environmental water and water quality impacts water quality quality criteria, including those specified in the impacts Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECCC/ARMCAMNZ, 2000), and any applicable regional, local or site specific guidelines Proposed stormwater management system and Section 7.1 - Measures to management measures for the containment of manage erosion, sediment and pollutants and minimisation of leachate and sediment water quality impacts mobilisation Impacts on soil and slope stability resulting from This has been addressed in vegetation clearing Section 6.2 - Erosion, sediment and water quality impacts, where it relates to surface water drainage rates and quality. Issues associated with slope stability itself due to vegetation clearance are not considered as part of this assessment. Impacts on water courses, including bed and bank Section 6.2.1 - Erosion, scour stability and sedimentation. Potential erosion and sediment controls consistent Section 7.1.1 - Erosion and with Managing Urban Stormwater – Soils scour Construction (Landcom, 2004) Management of and disposal strategies for acid Not part of the Stage 2 Project sulphate soils, in accordance with the Acid Sulphate Surface Water Assessment. Soils Manual (ASSMAC) Soils, Water and A hydraulic/hydrological study of the impacts on Section 6.3.2 - Impacts on Waste – Water Councils stormwater drainage system Council’s stormwater drainage Quantity and system stormwater Potential impacts of road widening and decreased Section 6.3.4 - Impacts on impacts permeability on downstream catchment and wetlands downstream catchments and wetlands Impacts on water flows into and surrounding The Section 6.3.3 - Impacts on Forest High School and Frenchs Forest Public stormwater flows into and School surrounding the Forest High School and Frenchs Forest Public School Impacts on groundwater recharge and flow path Addressed in the Northern Beaches Hospital Network Enhancement - Stage 2 Groundwater Assessment (separate report) Details of any works likely to intercept, connect with Addressed in the Northern or infiltrate groundwater services and any proposed Beaches Hospital Network groundwater extraction Enhancement - Stage 2 Groundwater Assessment (separate report) Measures to mitigate or prevent an increase in Section 7.2 - Measures to downstream stormwater flows mitigate water quantity and stormwater impacts Soils, Water and Consistency with relevant water sharing plans Section 2.5.5 - NSW Water Waste – Sharing Plan Impacts on Details of water supply sources and the purpose, Section 4.8 - Water sources and water sources, location, construction and expected annual extraction supply catchments sharing and volumes

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Key Issue Key Issue Items Report reference licensing Potential cumulative impacts on water sources and Section 6.4 - Cumulative mitigation measures to manage cumulative impacts impacts on water sources

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The SEARs for the Stage 2 Project require that an assessment is undertaken of the listed ‘key issues’ in accordance with Clause 7(1)(d) of Schedule 2 of the Environmental Planning and Assessment Regulation 2000. Clause 7 prescribes the content required in an environmental impact statement and specifies that the assessment must include (among other things): y A general description of the environment likely to be affected by the development y The likely impact on the environment y A full description of the measures proposed to mitigate any adverse effects of the development on the environment y A list of approvals required before the development can be lawfully carried out. Further to this, the SEARs also state that the assessment must consider potential cumulative impacts and the measures to avoid, minimise and if necessary, offset the predicted impacts, including detailed contingency plans for managing any significant risks to the environment.

1.6 Purpose and scope This report presents the surface water assessment and management strategy for the construction and operation of the Stage 2 Project. GHD undertook the following surface water assessment to address the surface water concerns outlined in the SEARs: y Detailed hydrologic and hydraulic (DRAINS) modelling and analysis of the existing and proposed drainage network that drains the Stage 2 Project site, and cross-comparison with the Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M (Lyalls & Associates, 2014) conceptual results y Water quality (MUSIC) modelling and analysis of subcatchments draining the Stage 2 Project site under existing and proposed conditions, including the estimation of mean annual nutrient, sediment and gross pollutant loads y Assessment of downstream water quantity and quality impacts, including the ecological and geomorphic condition of receiving waterways y Assessment of potential stormwater control measures to address water quantity and quality concerns, with particular attention paid to impacts on downstream council assets, scour and erosion along major drainage lines, and pollutant loads reaching downstream waterways. The findings formed the basis of the surface water management strategy presented in Section 7 of this report. For the purpose of this document, the following definitions have been used: y The Stage 2 Project refers to the works proposed under Stage 2 of the Northern Beaches Hospital Connectivity and Network Enhancements Project y The ‘Stage 2 Project site’ refers to the footprint directly impacted by the Stage 2 Project as well as any areas required for construction activities y The ’Stage 2 Project study area‘ refers to the catchments to which the Stage 2 Project site drains, namely Middle Creek, Curl Curl Creek and Bantry Bay (see Map 1, Appendix C).

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y a w k r a P

Forest Way k

a W New shared path and footpaths on both sides of Warringah Road Naree Road Frenchs Forest Road West New on-ramp from Wakehurst Parkway southbound into the slot westbound

Frenchs Forest Road East New shared path and footpath east side The Forest Northern of Forest Way High School Beaches Hospital Through traffic on Warringah Road separated from turning traffic Frenchs Forest at Wakehurst Parkway by way Public School of an underpass DEE WHY Widening and upgrade along Warringah Road Warringah Road at surface Widening of Allambie Road ay Road south of Warringah Road Fitzpatrick Avenue East Hilmer Street New shared path and Rodborough Road footpath east side New shared path and Proposed footpath west side Bantry B of Wakehurst Parkway CHATSWOOD shared pedestrian / of Allambie Road cyclist bridges Widening along

Wakehurst Parkway Allambie Road Remove existing Through traffic Through traffic Aquatic Drive pedestrian overbridge and on Warringah Road on Warringah Road replace with shared separated from separated from pedestrian / cyclist bridges turning traffic at turning traffic at Connecting Forest Way by way Hilmer Street by Aquatic Drive of an underpass way of an underpass with Wakehurst Parkway

rade

Wakehurst Parkway

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KEY )LJXUH6WDJH+RVSLWDO&RQQHFWLYLW\:RUNVDQG6WDJH 1HWZRUN Stage one road upgrades Stage two road upgrades (QKDQFHPHQW/RFDWLRQ Hospital connectivity works Network enhancement works Construction compound sites

Pedestrian overbridge

2. Regulatory context

2.1 Secretary’s Environmental Assessment Requirements The SEARs, discussed in Section 1, form the primary criteria for assessing the surface water impacts for the Stage 2 Project.

2.2 Water Management Act 2000 The Water Management Act 2000 (WMA 2000) is administered by the NSW Office of Water and is intended to ensure that water resources are conserved and properly managed for sustainable use benefitting both present and future generations. The WMA (2000) is also intended to provide a formal means for the protection and enhancement of the environmental qualities of waterways and their in-stream uses as well as to provide for protection of catchment conditions. Generally, this Stage 2 Project Surface Water Assessment is concerned with pavement drainage and floodplain management, as well as controlled activities under the WMA 2000. Under the WMA 2000, a controlled activity means: y The erection of a building or the carrying out of a work (within the meaning of the Environmental Planning and Assessment Act 1979) or y The removal of material (whether or not extractive material) or vegetation from land, whether by way of excavation or otherwise or y The deposition of material (whether or not extractive material) on land, whether by way of landfill operations or otherwise or y The carrying out of any other activity that affects the quantity or flow of water in a water source. Each of these controlled activities will be carried out during the construction phase of the Stage 2 Project and are therefore considered in this Stage 2 Project Surface Water Assessment. Other WMA 2000 principles relating to the Stage 2 Project are summarised in Table 2. The SEARs related to this Stage 2 Surface Water Assessment (see Table 1) cover each of the WMA 2000 principles.

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Table 2 WMA 2000 principles relating to the Stage 2 Project

WMA 2000 Relation to the Stage 2 Project Principles Drainage Drainage activities should avoid or minimise land degradation, including soil management erosion, compaction, geomorphic instability, contamination, acidity, waterlogging, decline of native vegetation or, where appropriate, salinity and, where possible, land should be rehabilitated. The impacts of drainage activities on other water users should be avoided or minimised. Floodplain Floodplain management must avoid or minimise land degradation, including management soil erosion, compaction, geomorphic instability, contamination, acidity, waterlogging, decline of native vegetation or, where appropriate, salinity and, where possible, land must be rehabilitated. The impacts of flood works on other water users should be avoided or minimised. The existing and future risk to human life and property arising from occupation of floodplains must be minimised. Controlled The carrying out of controlled activities must avoid or minimise land activities degradation, including soil erosion, compaction, geomorphic instability, contamination, acidity, waterlogging, decline of native vegetation or, where appropriate, salinity and, where possible, land must be rehabilitated. The impacts of the carrying out of controlled activities on other water users must be avoided or minimised.

2.3 Environmental Planning and Assessment Act 1979 Under section 115ZG of the Environmental Planning and Assessment Act 1979 (EP&A Act 1979), authorisations relevant to this Surface Water Assessment under the Water Management Act (ie. water use approval, water management work approval and activity approval) are not required for approved State Significant Infrastructure. Aquifer interference approval, which is required under section 91 of the Water Management Act, is addressed in the Stage 2 Project Groundwater Impact Assessment Report.

2.4 Protection of the Environment Operations Act 1997 The aim of the Protection of the Environment Operations Act 1997 (POEO Act 1997) is to protect, restore and enhance the quality of the NSW environment. Road construction, meaning the construction, widening or re-routing of roads, is a scheduled activity related to the Stage 2 Project that comes under the POEO Act 1997. Environment protection licences may be issued for the following purposes related to the Stage 2 Project: y To authorise the carrying out of scheduled activities at any premises y To authorise the carrying out of scheduled activities not related to premises y To control the carrying out of non-scheduled activities for the purpose of regulating water pollution resulting from any such activity. Relating to this Stage 2 Surface Water Assessment, the conditions of a licence may require the holder of the licence to undertake and submit to the appropriate regulatory authority studies into any aspect of the environmental impact of the activity or work authorised or controlled by the licence. In addition, the license holder may be required to develop and submit to the appropriate regulatory authority a pollution reduction program and to comply with the program as approved by the appropriate regulatory authority. The SEARs related to this Stage 2 Surface Water Assessment (see Table 1) cover these requirements.

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2.5 Policy and guidelines

2.5.1 National Water Quality Management Strategy Since 1992 the National Water Quality Management Strategy (NWQMS) has been developed by the Australian and New Zealand Governments in cooperation with state and territory governments. The NWQMS aims to protect the nation's water resources, by improving water quality while supporting the businesses, industry, environment and communities that depend on water for their continued development. The NWQMS consists of three major elements: policy, process and guidelines. The main policy objective of the NWQMS is to achieve sustainable use of water resources, by protecting and enhancing their quality, while maintaining economic and social development. The process strives to form a nationally consistent approach to water quality management through the development of high-status national guidelines. The guidelines provide the point of reference when issues are being determined on a case-by-case basis. These include guidance on regulatory and market-based approaches to managing water quality as well as regional water quality criteria. The ANZECC Guidelines (see Section 2.5.2) are most relevant to the Stage 2 Project Surface Water Assessment.

2.5.2 ANZECC Guidelines In 2000, the former Australian and New Zealand Environment and Conservation Council (ANZECC) released the Australian and New Zealand guidelines for fresh and marine water quality to 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. The objective of the ANZECC, relevant to the Stage 2 Project, is to maintain and enhance the ‘ecological integrity’ of freshwater and marine ecosystems, including biological diversity, relative abundance and ecological processes. The framework categorises ecosystems on a three point scale from high conservation and ecological value to highly disturbed systems. Indicators include biological indicators, physical and chemical stressors, toxicant and sediments. The guidelines have been referenced in this Stage 2 Surface Water Assessment to address the SEARs related to water quality. Biological indicators are not covered in this report but rather in the Stage 2 Project Biodiversity Assessment.

2.5.3 Local catchment management studies and plans As reported for the Northern Beaches Hospital Connectivity and Network Enhancement Project: Proposed Construction and Operational Phase Surface Water Management Strategy Concept Proposal and Stage 1 Project (L&A 2014, incorporated as Appendix M of the Stage 1 and Concept Proposal Environmental Impact Statement), the ANZECC 2000 guidelines were used in the development of the following catchment management studies and plans: y Northern Beaches Stormwater Management Plan (Patterson Britton and Partners, 1999) y Warringah Council Creek Management Study (Montgomery Watson Harza, 2004). Patterson Britton and Partners (1999) includes a set of stormwater pollution reduction targets for all new development within the Northern Beaches catchments as shown in Table 3.

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Table 3: Stormwater pollution reduction targets as set in the Northern Beaches Stormwater Management Plan (1999) Stormwater Short term objectives (5-year Long term objectives pollutant timeline) Gross Pollutants 100% retention of litter and 70% reduction in gross solid loads in coarse sediment up to 3 month creeks and streams ARI peak flow Total Suspended 50% reduction in total Achieve suspended sediment loads Solids (TSS) suspended solid loads which protect ecosystems and maintain natural creek bed regime Total Phosphorus 45% reduction in nutrient loads 30% reduction in nutrient loads (TP) Total Nitrogen 45% reduction in nutrient loads 30% reduction in nutrient loads (TN) Oil and Grease No visible oils up to 3 month No visible oils on all waterways ARI peak flow The water quality management objectives adopted for this study are discussed in section 3.3.2.

2.5.4 NSW Water Quality and River Flow Objectives The former Healthy Rivers Commission, which was replaced by the Natural Resources Commission in 2004, made recommendations to Government in relation to: y Suitable objectives for water quality, flows and other goals central to achieving ecologically sustainable development y The known or likely views of stakeholder groups y The economic and environmental consequences of recommendations y Strategies, instruments and changes in management practices needed to implement the recommended objectives. The objectives relevant to the Stage 2 Project for Narrabeen lagoon, Curl Curl and Manly lagoons are outlined in

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Table 4 and Table 5. The community perspective was also considered in the recommendations. Management of stormwater from existing urbanised areas, and protection of undeveloped and relatively undisturbed areas and streams of the catchments (both bush and rural) were identified as the highest priorities by the community. Flows were regarded as a matter of less concern, but issues were raised about restoring or mimicking the natural variability of flows. The community recognised that the flow regime in the creeks feeding the lagoons, and circulation within the lagoons, were modified, and a more 'natural' flow regime was prioritised by some stakeholders, including the opening of lagoon mouths to the sea. Each objective and community concern is consistent within the SEARs for this Stage 2 Project Surface Water Assessment.

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Table 4: Water quality objectives (Engineers Australia, 1999)

Objective Description Narrabeen Curl Curl Manly lagoon and Manly reservoir lagoons Aquatic Maintaining or improving the 9 9 9 ecosystems ecological condition of w aterbodies and their riparian zones over the long term. High level protection of aquatic ecosystems applies to w aters in and immediately upstream of national parks, nature reserves, state forests, drinking w ater catchments and high- conservation-value areas. Visual Aesthetic qualities of w aters 9 9 9 amenity including visual clarity, colour, surface films, debris and nuisance organisms. Secondary Maintaining or improving 9 9 9 contact w ater quality for activities recreation such as boating and w ading, w here there is a low probability of water being sw allow ed Pr imar y Maintaining or improving 9 - 9 contact w ater quality for activities recreation such as sw imming in w hich there is a high probability of w ater being sw allow ed Aquatic Protect w ater quality so that 9 9 9 foods it is suitable for the (cooked) production of aquatic foods for human consumption and aquaculture activities.

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Table 5 River flow objectives (Engineers Australia, 1999)

Objective Description Narrabeen Curl Curl Manly lagoon and Manly reservoir lagoons Maintain Maintain or restore the 9 9 w etland and natural inundation floodplain patterns and distribution inundation of floodw aters supporting natural w etland and floodplain ecosystems Mimic natural Mimic the natural 9 9 drying in frequency, duration and temporary seasonal nature of drying waterways periods in naturally (and temporary w aterways w etlands) Maintain Maintain or mimic natur al 9 9 9 natural flow flow variability in all variability streams Maintain Maintain rates of rise and 9 9 9 natural rates fall of river heights within of change in natural bounds w ater levels Manage Maintain groundw ater 9 9 groundw ater w ithin natural levels and for variability, critical to ecosystems surface flow s and ecosystems Maintain or Maintain or rehabilitate 9 9 rehabilitate estuarine processes and estuarine habitats processes and habitats

2.5.5 NSW Water Sharing Plan Water sharing plans are implemented under the NSW Water Management Act 2000 and specify the rules for the sharing of water between the environment and water users and between water users themselves. Water sharing plans also specify rules for the trade and management of water access licences. The Water Sharing Plan for the Greater Metropolitan Region Unregulated River Water Sources (the water sharing plan), which commenced in 2011, covers 87 management zones that are grouped into six water sources. The Stage 2 Project is situated in the Northern Sydney Rivers catchment or source. The Northern Sydney Rivers catchment is separated into management areas, which includes amongst other management areas, the Northern Sydney Coastal management area and the Middle Harbour management area. Middle Creek and Curl Curl Creek (See Section 4.2) are interpreted to be located within Northern Sydney Coastal management area while the Bantry Bay catchment is expected to be located within Middle Harbour management area. The water sharing plan suggests that the Sydney Catchment Authority (now Water NSW) do no obtain any water supplies from the management areas in which the Stage 2 Project is located. As part of Sydney Water Corporations (SWC) previous Water Act 1912 licence, SWC was required to develop a Manly Dam Environmental Flow Release Strategy (EFRS). SWC’s report found that infrastructure and operational constraints on the dam would only allow low flows to be released without major infrastructure spending. As a result, the plan provides for the following steps to be taken in regards to the development of environmental releases for Manly Dam over the term of the plan:

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y Stage 1: feasibility, suitability and decision on environmental flows (years 1 to 5) y Stage 2: design and decision on an implementation date (years 6 to 7) y Stage 3: any necessary infrastructure modifications needed to deliver flows (year 8 onwards). The water sharing rules listed in the water sharing plan for the Middle Harbour management and Northern Sydney Coastal management units are summarised below: y Middle Harbour management unit: – To provide environmental flow protection - pumping is not permitted when there is no visible flow at the pump site. – Trading is permitted within the management zone but is not permitted into the management zone. – High flow access licenses, which allow additional water to be accessed during high flow to offset collection during low flow, are not permitted. y Northern Sydney Coastal management unit: – To provide environmental flow protection - pumping is not permitted when there is no visible flow at the pump site. Flow classes may be established for this management zone if environmental flow rules are established for Manly Dam – Trading is permitted within the management zone but is not permitted into the management zone if trade will increase the total licensing entitlement for the zone. – High flow access licenses, which allow additional water to be accessed during high flow to offset collection during low flow, are not permitted. While the Stage 2 project will change surface run-off conditions in the catchments it intersects, which may also create minor incidental loses associated with evaporative changes, it is expected that all water generated at the site will be re-directed back to the existing surface water catchments. The design of the Stage Project stormwater management system could potentially reduce evaporative losses and provide a buffer system to capture and release water under lower flow conditions. To clarify this further, flow changes from each catchment are documented in this report. Some water may be taken from site run-off for dust suppression purposes during construction. An access licence is under some circumstances required under Section 57 of the Water Management Act. However, dust suppression is an exempt activity for the purposes of access licences under the Water Management (General) Regulation (2011). Due to changes in the Stage 2 Project drainage system there will be minor changes in catchment that will result in the movement of surface water run-off from the Sydney Coastal River management unit to the Middle Harbour management unit which are expected to be minimal. These volumes are quantified in this report. Water Sharing Plans in relation to groundwater resources are discussed in the Northern Beaches Hospital Network Enhancement - Stage 2 Groundwater Assessment (separate report).

2.5.6 Roads and Maritime policy and design guidelines Roads and Maritime has prepared the following policy and design guidelines related to the Stage 2 Project road works and stormwater management: y Erosion and Sedimentation Management Procedure y Code of Practice for Water Management Road Development and Management y Environmental Management of Construction Site Dewatering

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y Temporary stormwater drainage for road construction y Guidelines for the Management of Acid Sulfate Materials. These guidelines have been referenced to address the SEARs related to construction and operational phases of stormwater management.

2.5.7 Managing urban stormwater: soils and construction (Blue Book) The NSW Government publishes the following documents about the management of erosion and sediment control during construction and other land disturbance activities.

Managing Urban Stormwater: Soils and Construction – Volume 1 (Blue Book) The documents provide guidance for local councils and practitioners on the design, construction and implementation of measures to improve stormwater management, primarily erosion and sediment control, during the construction-phase of urban development. Managing Urban Stormwater - Volume 2D: Main road construction The document provides guidelines, principles and recommended minimum design standards for managing erosion and sediment control during the construction of main roads. The construction of main roads and highways commonly involves extensive earthworks, with significant potential for erosion and subsequent sedimentation of waterways and the landscape. The publications have been referenced in the stormwater management plan presented in Section 6.4 in this Stage 2 Project Surface Water Assessment.

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3. Methodology

The steps followed in this assessment are outlined in Figure 2:

Existing conditions Proposed conditions Data collection and assessment, including impact assessment, field work water quality and including water quality drainage modelling and drainage modelling

Identification of Development of mitigation measures surface water Residual risk under proposed management strategy assessment conditions for construction and operation phases

Figure 2: Study process overview

3.1 Data collection and field work The following information was used for the purposes of this assessment.

Site inspections A catchment walkover to observe key catchment drainage features was conducted by a drainage engineer during October 2014. The findings of the walkover are incorporated in the modelling and recommendations in this report. Site observations of the upper reaches of Trefoil Creek and Curl Curl Creek were undertaken in February 2015 to qualitatively assess flow conditions and the potential for reliance on groundwater inputs. Site investigations were undertaken following wet and dry conditions.

Land and Property Information map data Map data was sourced from New South Wales Land Property Information (LPI) 2012 dataset including: y Two-metre topographic contours y Regional transport (road) layouts y Lot boundary (cadastre) layouts y Waterways layers. 200 mm LIDAR (light and radar remote sensing) contour information (LPI, 2013) was also used for catchment delineation and modelling.

Network Enhancements Stage 2 Project Reference Design The following reference design information was available: y Preliminary reference design prepared for Roads and Maritime by Mott MacDonald (24 October 2014) y Updated reference design prepared for Roads and Maritime by Mott MacDonald (23 December 2014)

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y Northern Beaches Hospital Enabling Road Works Stage 2 EIS Construction Methodology, Primary Notes y Revised drainage design prepared by Mott MacDonald (31 March 2015)

Warringah Council data Warringah Council provided GIS information including details of: y The Council stormwater network including pits and pipes y Waterway locations y Locations of existing water quality treatment devices y Summary condition reports from two large trash racks located on Currie Road and Rabbett Street.

Stage 1 and Concept Proposal surface water reports and models The following information was available from the Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M prepared by L&A (August, 2014): y Northern Beaches Hospital Connectivity and Network Enhancement Project: Proposed Construction and Operational Phase Surface Water Management Strategy Concept Proposal and Stage 1 Project, Volumes 1 and 2 y DRAINS models y MUSIC models. In addition, CADD drawings and an associated DRAINS model of the updated reference design was prepared by Mott MacDonald (23 December 2014) and provided to GHD to inform the surface water assessment.

Flood data and reports The Geoscience Australia Flood Information Portal provided the following relevant studies in the vicinity of the Stage 2 Project: y Manly Lagoon Flood Study (BMT WBM, August 2013) y Narrrabeen Lagoon Flood Study (BMT WBM, September 2013).

Water quality data and reports The Warringah Council Creek Management Study (2004) is published on Warringah Council’s website and provides findings relating to water quality in the waterways downstream of the Stage 2 Project site. More recent information was not available.

3.2 Existing conditions and impact assessment

3.2.1 Study area and catchment characteristics The study area, shown in Figure 1 extends beyond the Stage 2 Project limit of works to include downstream council assets and drainage lines. The study area boundary was defined by the upper catchment area of the major waterways impacted by the Stage 2 Project. Subcatchment areas and impervious fractions were defined based on LiDAR data and LPI lot boundary information. Climate data and design rainfall information was obtained from the Bureau of Meteorology. Methods and datasets used for MUSIC model and DRAINS model development are discussed in more detail in Appendix A and Appendix B, respectively.

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3.2.2 Erosion, sediment and stormwater quality impacts

Erosion and sedimentation assessments The Blue Book provides guidance on Erosion Hazard Assessments and appropriate management practices. Assessments calculate annual soil loss (tonnes per hectare) based on rainfall erosivity and the typical upper slope gradient of the site landform as shown in Figure 3. For sites of high erosion hazard, the Revised Soil Loss Equation is recommended to determine the site specific soil loss classes and erosion hazards.

Figure 3 Assessment of potential erosion hazard Preliminary Erosion and Sedimentation Assessments (PESAs) are sometimes used by Roads and Maritime to determine projects that pose a high risk for erosion and sedimentation control and therefore require the preparation of an Erosion and Sedimentation Management Report (ESMR). Where a PESA and ESMR is available, the Roads and Maritime Erosion Sedimentation Management Procedure is used to determine how the findings would be incorporated in the Environmental Impact Assessment. As no PESA was available for the project, the Blue Book procedures described above were used. This approach is also consistent with the Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS, Appendix M (L&A, 2014).

Water quality modelling Stage 2 Project impacts on water quality were estimated using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC). Specifically, MUSIC was used to estimate annual pollutant loads, generated under existing and proposed conditions, for nitrogen, phosphorus, suspended solids and gross pollutants. Existing water quality treatment devices in the catchment were excluded from the modelling exercise. For the purposes of comparison with proposed conditions, this was considered a reasonable approach. Further details of MUSIC model development are provided in Appendix A.

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3.2.3 Water quantity and stormwater impacts

Hydrologic and hydraulic modelling Stage 2 Project impacts on stormwater flow regimes and drainage network capacity were modelled using DRAINS, a software program for modelling design storm events and runoff carried by piped networks and overland flow routes. A DRAINS model representing existing conditions was available from the Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M (L&A, 2014). The model was updated by GHD, using the stormwater asset information provided by Warringah Council, to provide finer detail for the catchments draining the Stage 2 Project study area. Where invert level data was missing, it was assumed that pipes had minimum 600 mm cover and 1 per cent grade, which is a standard approach to managing incomplete drainage information. A DRAINS model representing the Stage 2 Project stormwater drainage design was available from the reference design prepared by Mott MacDonald Pty Ltd (Mott MacDonald, 2015). The design model was used to update the existing conditions model to represent the catchment and drainage network under proposed conditions. DRAINS modelling was used to estimate the capacity of the stormwater drainage network and compare overland flows and velocities under existing and proposed conditions. Further details of DRAINS model development are provided in Appendix B.

3.2.4 Impacts on water sources, sharing and licensing The MUSIC modelling detailed in Section 3.2.2 was used to characterise the overall change in the flows exiting the catchments intersecting the Stage 2 Project between pre and post construction. The flow changes within each catchment simulated by MUSIC model outputs have also been assessed to characterise the cumulative flow impacts created by the Stage 2 Project. Any changes have been related to the requirements of the water sharing plan.

3.3 Surface water management strategy Based on the existing and proposed conditions assessment, a strategy to manage the potential impacts on downstream council assets and receiving environments is recommended in Section 7. The objectives and procedures adopted to form the surface water management strategy are outlined below.

3.3.1 Soil management objectives Recommendations for a Soil and Water Management Plan (SWMP) were prepared in accordance with Blue Book I (Landcom, 2004) and the Erosion and Sedimentation Management Procedure (Roads and Maritime). The guidelines provide strategies to mitigate the impacts of land disturbance, which includes construction and operational activities, on receiving waters through focussing on erosion and sediment control. SWMP objectives were determined by considering competing land use requirements, physical constraints and development opportunities. The following procedure was adopted: 1. Assessment of constraints and opportunities On-site soil, landform, ecology, pollutants and hydrology were assessed under construction and operating conditions. Also, the sensitivity of downstream aquatic ecosystems and the social and

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aesthetic values of the community were assessed with reference to the water quality and river flow objectives outlined in Section 2.5.4. 2. Erosion control Limiting exposed areas for erosion prevention were considered based on the assessment of erosion hazard. Where erosion hazard was determined to be low, a standard set of erosion control practices were recommended. Management of sites of high erosion hazard (based on the revised universal soil loss equation) was also considered. 3. Management of surface water flows Stormwater control measures were considered where site hydrology was likely to cause greater erosion, for example where changes in flow velocity or concentration were predicted. 4. Sediment control Sediment traps and basins were considered to minimise sediments entering downstream environments and council assets, while aiming to keep sediment as close to its source as possible. 5. Site stabilisation Stabilisation measures (eg. vegetation, paving, rock armouring) were also considered where changes to land-cover and hydrology were likely to expose the site or downstream drainage lines to high risk erosive forces.

3.3.2 Operational water quality management objectives Ecosystems downstream of the Stage 2 Project site are of high ecological value and require protection (see Section 3 for existing conditions assessment). To achieve the water quality targets set out in the Northern Beaches Stormwater Management Plan, outlined in Table 3, the Stage 2 Project would be required to treat stormwater beyond the level of protection provided under existing conditions. There are, however, limited options for mitigation measures within the confined corridor of the Stage 2 Project. Consistent with the Concept Proposal, The Stage 2 proposal is aimed at mitigating as far as practical the impacts from the road works. The ability to install controls to meet the northern beaches stormwater reduction targets is constrained by topography and limited corridor space. Based on input from Roads and Maritime, it was not considered practical to treat runoff from the road in isolation of the surrounding catchment and a more holistic approach to water quality was adopted The Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M (L&A, 2014) applied a set of management measures to offset potential increases in pollutant loads and where feasible further measures to meet the pollution stormwater targets were considered within the broader catchment. The Stage 1 Project avoided recommending stormwater treatment in locations constrained by road upgrades by considering treatment options further downstream. This led to the consideration of a pollutant control device located north of Frenchs Forest Road East rather than along the nature strip of the road corridor. This meant that a localised decrease in water quality was accepted near the Stage 1 Project. In this instance, water quality impacts were considered to be within the ability of existing ecosystems to accommodate the change. The SEARs require “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 (ANZECCC/ARMCAMNZ, 2000), and any applicable regional, local or site specific guidelines”. In line with Roads and Maritime drainage design objectives, the water

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quality management will reduce the post-development annual nutrient loads to existing levels where possible. Thereafter, the suitability of the Stage 1 Project approach to stormwater treatment will be assessed such that the short and long term objectives of the Northern Beaches Stormwater Management Plan (see Section 2.5.3) are not compromised.

3.3.3 Water quantity management objectives

Flood protection Water quantity objectives are set-out in the SEARs, requiring stormwater management measures to mitigate or prevent an increase in downstream stormwater flows. The impact of the Stage 2 Project was assessed by comparing: x The level of service of downstream council assets, measured by the capacity of the pipe system to carry 1-year, 5-year, 10-year and 100-year ARI flows, before and after the proposed development x Peak 100-year ARI overland flows before and after the proposed development. Where proposed conditions reduced levels of flood protection, or increased 100-year ARI overland flows, stormwater management options were considered in the Stormwater Management Plan.

Water resources The objective of the water resources assessment is to address the key items listed in regard to water resources outlined in the SEARS, which include assessing the project with regard to: y Consistency with water sharing plans. This has been completed by comparing the water sharing plan rules against the flow changes predicted by the MUSIC modelling. y Detail of the supply sources and the purpose, location, construction and expected annual extraction volumes. The project is not expected to require a surface water supply source and all surface flows generated at the site are expected to be returned to surface water other than for exempt activities such as for dust suppression. Despite this the report documents the overall changes in flows expected at the site. y Potential cumulative impacts on water resources and mitigation measures to manage cumulative impacts. This has been completed by comparing the water sharing plan rules against the flow changes predicted by the MUSIC modelling.

3.4 Assumptions and limitations The study was based on the following assumptions: y Consistent with the Stage 2 Northern Beaches Hospital EIS Appendix O: Stormwater Management Strategy and Plan (Hyder, 2014), it has been assumed that measures will be provided within the Stage 2 Project to offset impacts on water quality and quantity y The data available was assumed to be accurate. Where drainage data was incomplete, 600 mm of cover or one per cent slope was assumed. No detailed survey on the drainage structures was available y Existing stormwater quality treatment devices in the study area were not modelled due to limited data availability y No continuous stream flow monitoring or water quality monitoring data was available. Limited water quality sampling data became available in March 2015. Hydraulic (DRAINS) modelling sometimes displays instabilities (rapidly fluctuating water surfaces) or spikes (sudden rises and falls in water level and flowrates). For this study, where

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such results were present in hydraulic modelling, explanations and interpretations are presented in this report and do not invalidate the results.

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4. Existing environment

4.1 Catchment overview The Stage 2 Project site, which has an approximate footprint of 17.8 hectares, lies within the Warringah Council local government area (LGA) on a ridge at the top of three catchments: Middle Creek to the north; Bantry Bay to the south west and Curl Curl Creek to the south. The site is located at a maximum elevation of 160 metres AHD and minimum of 130 metres AHD. The natural catchment boundaries are shown on Map 1 (Appendix C). The stormwater drainage network conveys stormwater flows for minor storm events (ie less than 10-year ARI flows) throughout the Stage 2 Project site. Stormwater runoff is predominantly generated by roads, roofs and pavements, which discharges into the major drainage lines described in Section 4.2. The existing drainage network capacity is assessed further in Section 4.5 The waterways are tidal in their lower reaches but dominated by rainfall-runoff on the upper reaches in the vicinity of the Stage 2 Project. The LPI waterways layer is indicated on Map 1, together with indicative open channel drainage lines observed during the catchment walkover.

4.2 Major waterways

4.2.1 Middle Creek Under existing conditions, about 4.2 hectares (24 per cent) of the Stage 2 Project site drains north to Middle Creek via Catchment 1 and Catchment 3, which have total surface areas of about 92.6 ha and 60.8 ha, respectively. This differs to the Stage 1 Project, which in part drains Catchment 1, Catchment 2 (Trefoil Creek) and Catchment 3 to the north. Trefoil Creek is a tributary of Middle Creek with headwaters to the north-east of Wakehurst Parkway and Frenchs Forest Road East. The Stage 2 Project does not impact surface water flows received by Trefoil Creek. Groundwater interaction with Trefoil Creek is discussed in Section 4.9. Middle Creek is a major tributary of Narrabeen Lagoon and has a catchment area of around 1500 hectares. The catchment is relatively steep through the upper and middle reaches of Middle Creek, flattening to form a delta plain through the lower reaches approaching the mouth of the Narrabeen Lagoon. The catchment is mainly covered by residential land-use in the upper reaches near the Stage 2 Project site. Further downstream, land-use is a mix of residential, public use, retail and forested areas. Development includes a number of schools at Oxford Falls and the Sydney Academy of Sport and Recreation near Narrabeen Lagoon. The dominant land cover is native forest, with the majority of the catchment downstream of Oxford Falls West Road lying within the Garigal National Park.

4.2.2 Curl Curl Creek Under existing conditions, about 9.5 hectares (53 per cent) of the Stage 2 Project site drains south to Curl Curl Creek via Catchment 6, which has a total surface area of about 135 hectares. The Curl Curl Creek catchment area is about 700 hectares in total, which includes the Manly Dam catchment. Manly Dam was formerly a water supply reservoir for Manly that, since 1942, has been used entirely for recreational purposes. Downstream of the dam the creek continues in open channel before discharging into Manly Lagoon.

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Catchment grades are up to eight per cent in the upper reaches of the catchment. In the vicinity of the Stage 2 Project site, land-use is predominantly commercial. The Manly Dam catchment consists predominantly of forested areas known as the Manly Dam Reserve, as well as a golf course and residential development. Downstream of the dam, the land-use is predominantly residential with some commercial lots and open-space public reserves. According to Warringah Council information, the main creek arm upstream of the dam is known as Curl Curl Creek. Downstream of the dam it is known as Manly Creek. For simplicity, Curl Curl Creek is used throughout this report to refer to the major waterway to which the Stage 2 Project site drains.

4.2.3 Bantry Bay The catchment draining to Bantry Bay includes Bates Creek and an unnamed tributary to which about 4.2 hectares (24 per cent) of the Stage 2 Project site drains. The unnamed tributary is split into Catchments 4 and 5, which have total catchment areas of 34.2 ha and 26.7 ha, respectively. The Bantry Bay catchment is steep, with grades of around six per cent to 20 per cent. Land-use in the headwaters of Catchments 4 and 5 is largely residential and includes the Frenchs Forest Public School, a retirement village and an aquatic centre. These areas drain to Bantry Bay via drainage lines that cut through the southern catchments of Garigal National Park.

4.3 Ecology The Stage 2 Project biodiversity assessment (SMEC 2015) documents the findings of the biodiversity investigations and potential impacts. Where applicable, cross references to that study are made in this report.

4.4 Geology

4.4.1 Regional geology The 1:100000 Sydney Geological Sheet (Geological Series Sheet 9130, NSW Government, 1983) indicates that the area is predominately underlain by Hawkesbury Sandstone, of Triassic age. The Sydney Geological Sheet also documents the presence of north-west trending Jurassic aged basalt dykes in the vicinity of the Stage 2 Project, however these were not encountered during the geotechnical investigations (Roads and Maritime, 2014a). The Hawkesbury Sandstone is interpreted to be overlain by Triassic Aged Wianamatta Group Ashfield Shales in the vicinity of the site.

4.4.2 Site geology Site geology has been characterised by geotechnical investigations presented in the following reports: y Roads and Maritime (2014a): Concept Design Geotechnical Interpretation Report, Northern Beaches Hospital Enabling Roadworks, Frenches Forest y Roads and Maritime (2014b): Concept Design Geotechnical Investigation Report, Northern Beaches Hospital Enabling Roadworks, Frenches Forest. The reports document the completion of 29 bores and 17 test pits and two hand auger holes over the Stage 2 Project area to characterise the geological conditions. Roads and Maritime (2014a) provided the following summary of the geotechnical model developed from the site investigations.

34 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 Site topography has been influenced by the subsurface geology and surface geomorphological processes. Typically the site subsurface is characterised by organic topsoil (nominally 0.25 mm thick) in the near surface, overlying a thin residual soil profile (commonly less than 1 m thick, although this deepens to in excess of 1.5 metres toward the eastern extent of the Stage 2 Project). Fill materials were encountered along the existing road alignment, and in discreet pockets related to other construction activities. Aside from within the existing road alignment, the largest quantities of fill material are expected within the council depot at the western end of Aquatic Drive. Colluvial soil, (eg slope-wash) was also encountered at a discreet location within the site. Bedrock, where encountered, shows a general pattern of increasing strength and decreasing weathering effects with depth below existing ground levels. The published geology map (Geological Survey of NSW, 1983) indicates the presence of an agillaceous (ie shale, laminite, siltstone) capping layer, overlying sandstone, within the greater Hawkesbury Sandstone unit. The primary units identified within the vicinity of the road works are summarised in Table 6. Table 6: Interpreted geological conditions (eastbound slot location) Summary description Depth range of Depth range top of layer (m) of base of layer (m) Asphaltic Concrete/Road-base Gravels/GRAVEL/Silty 0 0.45 – 1.1 SAND

Silty CLAY/ Sandy CLAY/ CLAY; red brown, grey brown, 0.45 – 1.1 1.05 – 1.95 low to medium plasticity, stiff to hard, dry to moist

SANDSTONE/ Interlaminated SANDSTONE/ SHALE/ Silty 0.65 – 1.95 2.0 – 5.1 SANDSTONE; cream to white, pale grey with red brown iron cemented layers, extremely to highly weathered, extremely low to very low strength with occasional medium to high strength

SANDSTONE/ Interlaminated SANDSTONE; cream to 2.4 – 5.1 4.6 – 6.1 white with red brown iron cemented layers, extremely to highly weathered, extremely low to low strength with occasional medium to high strength SANDSTONE/ Interlaminated SANDSTONE/ SILTSTONE; 2.0 – 4.8 3.3 – 7.8 cream to white and grey, highly to slightly weathered, low to medium strength

SANDSTONE; pale grey and brown, slightly weathered to 3.3 – 7.8 11+ fresh, medium to high strength

Adapted from Table 5.1 in RMS (2014a)

4.5 Stormwater drainage network

4.5.1 Overview Map 2 (Appendix C) shows the catchment layout, stormwater drainage network and main waterways under existing conditions. Map 3 shows the hydraulic capacity of the existing stormwater drainage network modelled in DRAINS. Map 4 shows peak 100-year ARI flows for the existing pipe drainage network and overland flow paths modelled in DRAINS.

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 35 Catchment 1 (Middle Creek) About 2.3 ha (13 per cent) of the Stage 2 Project site contributes to Catchment 1 runoff, mainly via stormwater drainage infrastructure connected to Warringah Road, east of Forest Way. The existing underground network is comprised of the following infrastructure related to the Stage 2 Project site: y 375 mm to 450 mm diameter pipes, which drain north to Holland Crescent y 375 mm to 450 mm diameter pipes, which drain north to Rabbett St, via Forest Way. Overland flow is generally conveyed by kerb and channel along Warringah Road to Rabbett St, via Forest Way.

Catchment 2 (Middle Creek) The Stage 2 Project site does not contribute to runoff within Catchment 2.

Catchment 3 (Middle Creek) About 1.9 hectares (11 per cent) of the Stage 2 Project site contributes to Catchment 3 runoff, mainly via stormwater drainage infrastructure connected to Warringah Road and Rodborough Road, west of Allambie Road. The existing underground network is comprised of the following infrastructure related to the Stage 2 Project site: y 375 mm to 450 mm diameter pipes, which drain surface water runoff from Warringah Road and Rodborough Road y 450 mm to 525 mm diameter pipe culvert, connecting to Warringah Road and Rodborough Road drainage infrastructure and providing cross-drainage under Warringah Road to the Allambie Grove Business Park to the north of Warringah Road. Overland flow is generally conveyed by kerb and channel to a low point on the northern side of Warringah Road at the intersection with Rodborough Road. Flood levels on the road above the kerb height will likely spill north to connect with drainage infrastructure and overland flow paths within the Allambie Grove Business Park. DRAINS modelling indicates that surface flows drain overland from Warringah Road to the Business Park during peak 5-year, 10-year and 100-year ARI design storm events.

Catchment 4 (Bantry Bay) About 3.7 hectares (21 per cent) of the Stage 2 Project site contributes to Catchment 4 surface water runoff, mainly via stormwater drainage infrastructure connected to Warringah Road. The existing underground network is comprised of the following infrastructure related to the Stage 2 Project site: y 225 mm to 450 mm diameter pipes, which drains surface water runoff from Warringah Road y 600 mm diameter pipe culvert, connecting to Warringah Road drainage infrastructure and providing cross-drainage under Warringah Road to the south of Fitzpatrick Avenue East y 375 mm diameter pipe culvert, connecting to Warringah Road drainage infrastructure and providing cross-drainage under Warringah Road to drainage infrastructure within the Pipeline Reserve, north of Parni Place. Overland flow is generally conveyed south along Warringah Road by kerb and channel.

36 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 Kerb and channel on the northern side of Warringah Road connects overland flows to a low point on Warringah Road, west of Maxwell Parade. Flood levels on the road above the kerb height will likely spill south through residential properties to connect with drainage infrastructure and overland flow paths in Maxwell Parade. As shown in Map 4, DRAINS modelling indicates that surface water flows drain overland through these properties from Warringah Road during peak 100-year ARI design storm events. Kerb and channel on the southern side of Warringah Road connects overland flows to stormwater infrastructure and overland flow paths in the Pipeline Reserve, north of Parni Place.

Catchment 5 (Bantry Bay) About 0.5 hectares (three per cent) of the Stage 2 Project site contributes to Catchment 5 surface water runoff, mainly via kerb and channel connected to Hilmer Street. Overland flow is generally conveyed south along Hilmer Street to connect with stormwater infrastructure and overland flow paths draining south.

Catchment 6 (Curl Curl Creek) About 9.5 ha (53 per cent) of the Stage 2 Project site contributes to Catchment 6 runoff, mainly via stormwater drainage infrastructure connected to Warringah Road, to the east and west of the intersection with Wakehurst Parkway, and Wakehurst Parkway itself. The existing underground network is comprised of the following infrastructure related to the Stage 2 Project: y 375 mm diameter pipes, which drain surface water runoff from Warringah Road, Wakehurst Parkway and Bantry Bay Road, into Brick Pit Reserve to the south y 1050 mm diameter pipe culvert, which drains overland flows from Brick Pit Reserve, under Wakehurst Parkway to the south-eastern bushland y 375 mm diameter pipes, which drain surface water runoff from the intersection of Warringah Road and Wakehurst Parkway, to the south-eastern bushland y 1050 mm diameter pipe culvert, which drains overland flows from the bushland under Aquatic Drive to the south y 375 mm to 1050 mm diameter pipes, which drain surface water runoff from Warringah Road (east of Wakehurst Parkway) and properties nearby to the north y 1050 mm diameter pipe culvert, connecting to Warringah Road drainage infrastructure and providing cross-drainage under Warringah Road to the Aurora Business Park to the south of Warringah Road. Overland flows are generally conveyed by kerb and channel, following the underground network alignment, before entering natural waterways and cross-drainage infrastructure.

4.5.2 Stormwater quality control measures Council provided GIS information indicating the locations of existing stormwater quality treatment measures near the Stage 2 Project site. The measures include a range of trash racks; other gross pollutant traps (GPTs) and bioretention measures. The locations of the devices are shown on Map 2 of Appendix C. A summary is provided in Table 7. Table 7: Existing stormwater quality treatment devices Warringah asset Description Subcatchment Date of construction / number installation

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 37 Warringah asset Description Subcatchment Date of construction / number installation SWS00050 GPT (CDS Unit) Curl Curl Creek Unknown SWS00004 Unknown Middle Creek 2000 SWS00055 Trash rack Curl Curl Creek 2010 SWS00056 Trash rack Curl Curl Creek 2010 SWS00018 Combined trash Middle Creek 1999 rack/sediment trap SWS00032 Unknown South Creek 1999 SWS00041 Combined trash Bantry Bay 1998 rack/sediment trap SWS00042 Unknown Curl Curl Creek 1996 SWT00001 Sediment tank and reed Curl Curl Creek 1974 beds SWS00005 Unknown Middle Creek 2000 SWT00038 Sediment tank Middle Creek 1995 SWS00058 GPT Curl Curl Creek 2010 SWS00059 Trash rack Curl Curl Creek 2010 SWT00056 Sediment basin / Middle Creek 2010 wetland

GPTs and trash racks predominantly treat gross pollutants such as litter and large debris. Sediment basins, wetlands and reed beds remove sediments and varying amounts of nutrients such as phosphorous and nitrogen. Additional details were available for the trash rack on Rabbett Street (SWS0018) and the trash rack on Currie Road (SWS0041). Treated catchment areas and removal rates for these devices were available from Council surveys dated 2007 and are provided in Table 8. Photographs are provided in Figure 4 to Figure 7. Table 8: Removal rates for Trash Racks SWS0018 and SWS0041 Currie Road Rabbet Street Catchment treatment area 70 ha 24 ha Removal rate, floatables 0.5m3/year (0.007m3/ha/year) 0.54 m3/year (0.023 m3/ha/year) Removal rate, sediments 19 tons /year (0.3 tons/ha/year) 12.6 tons /year (0.52 tons/ha/year)

38 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 Figure 4: Sediment trap/trash rack Figure 5: Sediment trap/trash rack downstream of Currie Road (looking downstream of Currie Road (looking upstream) downstream)

Figure 6: Sediment trap/trash rack at Figure 7: Sediment trap/trash rack at Rabbett Reserve (looking Rabbett Reserve (looking upstream) downstream)

Bioretention systems in Aurora Business Park on Aquatic Drive, observed during site investigations, are shown in Figure 8 and Figure 9. Council had no data available for these systems as they are on private property. It is assumed that these systems treat flows from the business park only, with external catchment flows bypassing the systems. Site investigations also found other stormwater quality control measures in the Stage 2 Project study area that were not included in the Council GIS database. Notably, a trash rack is located downstream of the existing piped culvert (SPI00369) under Aquatic Drive in Catchment 6 (refer to Map 2 of Appendix C). However, the intended function and existing performance of the control measure is unknown.

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 39 Figure 8: Bioretention system Figure 9: Bioretention system (western) (eastern)

4.6 Erosion, sediment and stormwater quality

4.6.1 Erosion and scour Scour and erosion is a geomorphic process that mobilises and removes land surface materials (eg. soils) and increases the amount of sediment carried and deposited downstream. From the Blue Book I (Appendix B), rainfall erositivity (R-factor) in the vicinity of the Stage 2 Project is about 4000. The relatively steep terrain in the vicinity of the Stage 2 Project (slope >10% in some locations), sighted during site inspections and evident from contour information, indicates potentially high erosion hazards. The catchment walkover identified waterways downstream of the Stage 2 Project, in Catchments 1, 4, 5 and 6, which were generally stable with minimal signs of erosion or scour, as shown in Figure 10. However, there were some instances where stormwater control measures in Catchment 4 and Catchment 6 had been undermined by high flows as shown in Figure 11 and Figure 12.

40 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063

Figure 10: Waterway upstream of Currie Road in Catchment 4

Figure 11: Undermining of maintenance access track at the sediment pond and trash rack downstream of Currie Drive in Catchment 4

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 41 Figure 12: Trash rack with evidence of bank and bed erosion, downstream of the existing piped culvert under Aquatic Drive culvert in Catchment 6 (refer to culvert SPI00369 in Map 2 of Appendix C)

4.6.2 Water quality Surface water sampling was undertaken by SMEC at five locations (SW1 to SW5) across the wider catchment. The sampling locations are presented in Figure E1 (Appendix E). Four sample events have been recorded: y 12 November 2014 y 6 January 2015 y 4 February 2015 y 10 March 2015. Sampling was undertaken during low flow conditions for all events and field parameters (pH, electrical conductivity, oxidation reduction potential, dissolved oxygen, temperature) were collected. The samples were analysed at a national association of testing authorities (NATA) accredited laboratory for the following analyses: y pH y Phosphate y Electrical conductivity y Total dissolved solids y Total suspended solids

42 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 y Total and dissolved heavy metals (arsenic, cadmium, chromium, copper. iron, lead, manganese, mercury, nickel, zinc) y Total recoverable hydrocarbons y Benzene, toluene, ethyl benzene and total xylenes (BTEX) y Polycyclic aromatic hydorcarbons (PAHs) y Major cations and anions y Nutrients – ammonia, nitrate, nitrate and phosphorus.

Ba sis of a sse ssme nt The Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC guidelines) provide a management framework, guideline water quality triggers, protocols and strategies to assist water resource managers in assessing and maintaining aquatic ecosystems. The primary objective of the ANZECC guidelines is: ‘To provide an authoritative guide for setting water quality objectives required to sustain current or lik ely future environmental values for natural and semi-natural water resources in Australia and New Zealand’. The guidelines recommend numerical and descriptive water quality guidelines (water quality threshold/trigger criteria) to help managers establish water quality objectives that would maintain the environmental values of water resources. They are not standards, and should not be regarded as such (ANZECC, 2000). It should also be noted that the water quality values are not suitable for direct application to stormwater quality. Rather, the guidelines have been derived to apply to the ambient waters that receive stormwater discharges, and to protect the environmental values that they support. Of particular importance is the philosophical approach for using the ANZECC guidelines of: ‘protect environmental values by meeting management goals that focus on concerns or potential problems’ (ANZECC, 2000). That is, development of a monitoring program, including the performance objectives, standards and measurement criteria, should focus on specific issues not on pre-determined guideline values. The philosophy, management framework and guiding principles outlined in the ANZECC guidelines have formed the basis for development of Stage 2 Project specific performance standards. Surface water sampling has been undertaken at the site for the purposes of characterising the baseline water quality conditions and environmental values on which the surface water quality assessment and management measures can be based and form the basis of assessing impacts. To better understand how the surface water quality data reported for the site related to the water quality threshold criteria presented in ANZECC (2000) the surface water quality data was compared against the criteria outlined in ANZECC (2000). The surface water features receiving water from the Stage 2 Project and surrounding area are located within urbanised catchments and as such are considered to be “slightly modified fresh water systems” (ANZECC, 2000). Based on this a protection level of 95 per cent for freshwater ecosystems was used for the assessment, as specified in Table 3.4.2 of the ANZECC (2000) guidelines. The adopted criteria are listed within the analytical results tables in Table E2 (Appendix E).

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 43 The low land rivers criterion has also been used in this assessment. The ANZECC (2000) also provides screening trigger values for southeast Australia NSW low land rivers (less than 150 m in altitude).

Results

Field parameters monitored at the time of sampling are detailed in Table E1 (Appendix E). A summary of the analytical results is provided in Table E2 (Appendix E).

The TRH, BTEX and PAHs were all below the laboratory limit of detection.

Parameters that exceeded the nominated screening criteria and which highlight the differences between existing water quality and the selected ANZECC threshold criteria included:

y pH: two minor exceedences for SW3 and SW5 in November 2014, which suggest that the existing pH variation may be outside those normally present in lowland rivers within south east Australia

y Electrical conductivity: majority of results were above the nominated screening criteria suggesting that existing catchment water quality has a higher conductance

y Total suspended solids: the existing (background) total suspended solids concentration in one surface water sample (January 2015 event) at SW3 exceeded the nominated screening criteria

y Copper, iron and zinc: the existing (background) copper, iron and zinc concentrations in surface water samples exceeded the nominated screening criteria, however these exceedances could be natural background concentrations

y Nitrate: the existing (background) nitrate concentrations in surface water samples exceeded the nominated screening criteria at SW1, SW2, SW3 and SW4. Total dissolved solids results are indicative of freshwater conditions and is of drinking water quality (< 500 mg/L) for the majority of samples analysed.

Previous Investigations The Warringah Council Creek Management Study (2004) provides the most recent existing water quality information in the creeks of the Warringah LGA, including the receiving waterways of the Stage 2 Project site. Results of the 2004 water quality sampling are shown in Table 9. The sampling was undertaken over a single monitoring period during dry weather. At the time of sampling, total nitrogen (TN), total phosphorous (TP) and nitrates/nitrites exceeded ANZECC (2000) default trigger guidelines.

Table 9: Results of 2004 water quality sampling

Sampling Catchment Location Suspended Total Nitrate Total location solids nitrogen and phosphorous ID (mg/L) (mg/L) nitrite (mg/L) (mg/L) WQS11 Middle Carnarvon Drive 17 0.7 0.22 < 0.01 Creek near Peppercorn Park WQS21 Middle Dreadnought Road 15 0.4 0.08 0.05 Creek near Oxford Falls Grammar School WQS31 Bantry Bay Pildra Place near 7 0.4 0.13 < 0.01 Forestville Park WQS4 Curl Curl Upstream of Manly 7 1.5 1.13 0.01 Creek Dam

44 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 Sampling Catchment Location Suspended Total Nitrate Total location solids nitrogen and phosphorous ID (mg/L) (mg/L) nitrite (mg/L) (mg/L) ANZECC Not 0.35 0.04 0.025 2000 specified default trigger levels Source: Warringah Council Creek Management Study (2004)

4.7 Water quantity and stormwater

4.7.1 Riverine flooding

Flood studies are available for Middle Creek as part of the Narrabeen Lagoon Flood Study and Curl Curl Creek as part of the Manly Lagoon Flood Study.

The Narrabeen Lagoon Flood Study maps included in the report do not show the flood extent on Middle Creek upstream of the Sydney Academy of Sport and Recreation, which is shown to flood in a 100-year ARI flood event and in a 10-year ARI flood event. The community consultation material reproduced in the report appendices does show a wider flood extent and indicates the presence of flooding in the vicinity of Oxford Falls Grammar School, though the event shown is not confirmed. The results do not extend to the Stage 2 Project site.

The focus of the Manly Lagoon Flood Study is Burnt Bridge Creek and Brookvale Creek which, in addition to Curl Curl Creek, are tributaries of Manly Lagoon. The flood study does not include flood maps showing flood extent in Curl Curl Creek upstream of Manly Dam.

4.7.2 Stormwater network capacity and local flooding Information regarding the capacity of the existing stormwater drainage network was not available from Council.

The performance of the existing stormwater drainage network was assessed using results from DRAINS modelling for all standard durations of the 1-year, 5-year, 10-year and 100-year ARI storm events. The existing pipe network capacity is generally less than 10-year ARI design flows as shown in Map 3.

4.8 Water sources and supply catchments

As described in Section 2.5.5 the Stage 2 Project is located within the Northern Sydney Rivers water source as defined in the water sharing plan for the Greater Metropolitan Region Unregulated River Water Sources.

The Northern Sydney Rivers water source is separated into several Management Units, which includes amongst other management units, the Northern Sydney Coastal Management Unit and the Middle Harbour Management Unit. Middle Creek and Curl Curl Creek (See Section 4.2) are located within the Northern Sydney Coastal Management Unit while the Bantry Bay catchment is considered to be located within Middle Harbour Management Unit.

The water sharing plan specifies share components within the Northern Sydney Rivers water source, it does not separate the share components based on the Management Units. The allocation volumes in the water sharing plan are summarised below. y The share component for domestic and stock use is 5.5 ML/yr (15 m3/day).

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 45 y The share component for local and major water utilities is 0 ML/yr, which indicates that there is no water allocated for water utilities in the Northern Sydney Rivers water source area. To maintain environmental flows, the water sharing plan prohibits abstraction at times of no visible flows.

4.9 Groundwater and surface water interactions The Northern Beaches Hospital Network Enhancement - Stage 2 Groundwater Assessment report (February 2015) investigated the potential for groundwater seepage and its influence on surface water flows. Details are provided in that report but the key findings from the site observations and the groundwater modelling are summarised in the following sections.

4.9.1 Site investigations Site observations of the upper reaches of Trefoil Creek and Curl Curl Creek were undertaken in February 2015 to qualitatively assess flow conditions and the potential for reliance on groundwater inputs. Site investigations were undertaken following wet and dry conditions.

The wet event assessment was completed on 4 February 2015 following recorded rainfall of 16 mm on 2 February 2015 and 8.0 mm on 3 February 2015. The dry condition assessment was carried out on 18 February 2015 when there had been only 1 mm of rainfall over the preceding seven days, which occurred on 15 February 2015

Rainfall for both cases was recorded at the Australian Bureau of Meteorology (BOM) weather station 066182 at Frenches Forest.

Trefoil creek

The findings of the site investigations were:

y Under dry conditions the flows in the creek were observed to be very low and were qualitatively estimated to be less than 0.05 L/s. Given that these flows were observed after a short dry period, an extended dry period would result in further flow reductions and potentially no flow conditions y A comparison of flow conditions between wet and dry conditions suggested that surface water run-off contributes a significant portion of the overall flows in the Creek (ie after wet weather events flows are significantly higher) y No active groundwater seepages were observed flow along the channel. One very isolated wet patch was noted at outcropping bedrock around 30 to 50 m away from the channel at the interface between bedrock and overlying unconsolidated sediments

y The origin of the flows under dry conditions could not be identified as groundwater inputs and as such there is potential that the flows are due to discharge from up gradient residential or commercial water use. The observations suggest that, while groundwater inputs are likely to be very small compared to surface water inputs, they may support instream flows (albeit very small) during short term dry conditions.

Curl Curl Creek

The findings of the site investigation were:

y Under wet and dry conditions flows were observed to intermittently disappear beneath unconsolidated surficial sediment within the channel

46 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 y Groundwater seepage was observed to occur from the channel banks at two locations from overlying unconsolidated material and at the interface between unconsolidated sediments and bedrock y Under dry conditions the flows were qualitatively observed to be between 0.05 L/s and 0.1 L/s y A comparison of flow conditions between wet and dry conditions suggested that surface water run-off contributes a significant portion of the overall flows in the Creek (ie after wet weather events flows are significantly higher) y The origin of the flow up-gradient of identified seepage zones could not be identified as groundwater inputs. As such there is potential that some of the flow may be discharged from up gradient residential or commercial properties.

The observations suggest that, while groundwater inputs are likely to be relatively small compared to surface water inputs, they are likely to support instream flows during short term dry conditions and potentially for more extended dry periods.

4.9.2 Groundwater modelling The following summarises the results of groundwater modelling under existing conditions: y The red-crowned toadlet habitat located on Trefoil creek was modelled to have groundwater inflows under existing conditions between 15 m3/day and 97 m3/day. Based on field observations undertaken in this area, it is expected that the low end value is a more realistic representation of groundwater inputs under dry conditions. y The red-crowned toadlet habitat located on Curl Curl Creek was modelled to have groundwater inflows under existing conditions between 3 m3/day and 52 m3/day. Based on field observation undertaken in this area it is expected that the low end value is a more realistic representation of groundwater inputs under dry conditions. y The Middle Creek South West tributary was modelled to have groundwater inflows under existing conditions between 20 m3/day and 162 m3/day y The Bantry Bay tributary was modelled to have groundwater inflows under existing conditions between 121 m3/day and 971 m3/day.

The groundwater assessment estimated the effects of the Stage 2 Project on changes to the groundwater seepage flows and the effects on surface water are discussed further in Section 6.3.4.

It is proposed in the groundwater assessment that treated slot seepage water of up to 53 m3/day during construction and 42 m3/day during operation will be discharged back to the existing drainage lines. These discharge volumes will be similar to, and essentially returning, the amounts of seepage lost from the creeks due to the slot, although there may be an uneven redistribution between catchments compared with background conditions. Overall this means that the average groundwater flows in the creeks (due to groundwater seepage) will be similar to baseline conditions.

While essentially replacing lost groundwater seepage, the overall discharge volumes are negligible compared with flood flows which have been modelled as part of assessing surface water flooding, scour and erosion. As such, while these discharge volumes have not been assessed in the surface water modelling they are interpreted to have a negligible effect on the outcomes of the surface water assessment.

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 47 5. Proposed drainage description

The general works proposed for the Stage 2 Project are described in Section 1.4. The proposed changes affecting drainage from the Stage 2 Project site include slot drainage infrastructure, below-ground detention storages, cross-drainage upgrades and other network upgrades. These proposed works were identified during preliminary design in order to cater for changes in drainage flows and road profiles resulting from the proposed Stage 2 Project network enhancements. They serve to minimise any potential impacts on the existing environment and form part of the overall surface water management strategy for the project. These proposed works are presented in the design in Appendix F and may be subject to further refinement during detailed design.

5.1 Slot drainage The proposed four-lane, open, sub-surface slot on Warringah Road is a major alteration to the existing built environment within the Stage 2 Project site. Construction of the slot requires excavation below existing road levels, generally within the footprint of the existing road. The proposed road surface in the slot is up to eight metres below the existing surface near the intersection of Warringah Road and Hilmer Street. The proposed road surface in the slot generally drains away from the centre barrier, which separates the westbound and eastbound lanes. Drainage pipe alignments run parallel to the road on both side of the slot, which are connected to the road surface via grated pits. Nominal pipe diameters within the slot are generally 375 mm or 600 mm. Surface water in the slot drains to either Catchment 4 (Fitzpatrick Avenue East) or Catchment 6 (Aquatic Drive). The watershed boundary in the slot is about 160 metres west of Hilmer Street (between Forest Way and Wakehurst Parkway). Catchment 6 flows are drained from the slot via cross-drainage pipes to the west of Wakehurst Parkway, where existing culverts drain to Brick Pit Reserve. The slot to the east and west of Wakehurst Parkway is drained towards Brick Pit Reserve via two separate No. 675 mm diameter and two No. 525 mm diameter pipe culverts, respectively. Surface water on the westbound ramp, which connects with the proposed bridge on Wakehurst Parkway, is drained to the southern side of the road where pipe connections provide cross - drainage into the slot pipe system. Flow is then directed towards Aquatic Drive (Catchment 6).

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5.2 Cross-drainage upgrades Cross-drainage upgrades are described below and summarised in Table 10.

Warringah Road (SPI03578/ SPI03581) In the same location as the proposed slot drainage in Catchment 6 (see Section 5.1); existing culverts, which currently provide cross-drainage from areas north of Warringah Road to Brick Pit Reserve, are upgraded to three 600 mm diameter pipes. The culverts are also extended south by about 30 metres to fit with the proposed road widening on Warringah Road. Outflow from the Warringah Road culverts (ie. slot drainage and cross-drainage) are then piped to Aquatic Drive along the southern side of Warringah Road and western side of Wakehurst Parkway via one 1500 mm diameter pipe. By containing flows within the piped system, surface water is effectively diverted away from Brick Pit Reserve and bushland to the south east of the Warringah Road intersection with Wakehurst Parkway.

Warringah Road (SPI13463) The existing 900 mm diameter pipe culvert under Warringah Road, about 230 m east of Wakehurst Parkway, is realigned to fit with the proposed road widening and upgraded to a 1050 mm diameter pipe. The upstream and downstream connections remain as they are under existing conditions. No slot drainage is connected to the upgraded culvert.

Wakehurst Parkway (SPI00081) The existing 1050 mm diameter pipe culvert under Wakehurst Parkway, about 140 metres south of Warringah Road, is realigned to fit with the proposed road widening and upgraded to a box culvert (2.4 m x 1.2 m). Fauna crossings and downstream erosion protection are proposed for the culvert upgrade, which directs surface water into bushland to the south east. Surface water flows then drain overland toward culvert upgrades under Aquatic Drive as under existing conditions.

Aquatic Drive (SPI21177) The existing 1050 mm diameter pipe culvert under Aquatic Drive, about 80 metres east of Wakehurst Parkway, is realigned to fit with the proposed below ground detention storage and proposed works that connect Aquatic Drive to Wakehurst Parkway. The culvert is upgraded to a box culvert (2.4 m x 1.2 m). Downstream erosion protection is proposed for the culvert upgrade, which directs surface water south east as under existing conditions.

Wakehurst Parkway (SPI03303) The existing 375 mm diameter pipe culvert under Wakehurst Parkway, about 120 metres south of Aquatic Drive, will be reduced to a 300 mm pipe and extended about 20 metres to accommodate road upgrades. Upslope, a new 300 mm pipe and table drain will intercept surface water flows along the top-of-cut on the eastern side of the road, connecting with the extended pipe culvert. The culvert will no longer be connected to the surface of Wakehurst Parkway.

Wakehurst Parkway (SPI08537) The existing 450 mm diameter pipe culvert under Wakehurst Parkway, about 270 m south of Aquatic Drive, will be reduced to a 300 mm pipe and extended about 10 metres to accommodate road upgrades. Upslope, a new 300 mm pipe and table drain will intercept surface water flows along the top-of-cut on the eastern side of the road, connecting with the extended pipe culvert. The culvert will no longer be connected to the surface of Wakehurst Parkway.

GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 | 49 Table 10: Proposed culvert upgrades Existing pipe Catchment Location Existing size Proposed size ID description SPI03578/ 6 (Curl Curl Culvert draining 1 No. 375 mm 2 No. 675 mm SPI03581 Creek) south under 3 No. 600 mm Warringah Road, 2 No. 525 mm about 60 m west of Wakehurst Parkway SPI13463 6 (Curl Curl Culvert draining 1 No. 900 mm 1 No. 1050 mm Creek) south under Warringah Road, about 230 m east of Wakehurst Parkway SPI00081 6 (Curl Curl Culvert draining 1 No. 1050 mm 1 No. Box Culvert Creek) east under (2.4 m x 1.2 m) Wakehurst Parkway, about 140 m south of Warringah Road SPI21177 6 (Curl Curl Culvert draining 1 No. 1050 mm 1 No. Box Culvert Creek) south under (2.4 m x 1.2 m) Aquatic Drive, about 80 m east of Wakehurst Parkway SPI03303 6 (Curl Curl Culvert draining 1 No. 375 mm 1 No. 300 mm Creek) east under Wakehurst Parkway, about 20 m south of Aquatic Drive SPI08537 6 (Curl Curl Culvert draining 1 No. 450 mm 1 No. 300 mm Creek) east under Wakehurst Parkway, about 270 m south of Aquatic Drive

5.3 Detention storages The Stage 2 Project proposes two separate below ground detention storages as summarised in Table 11.

50 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 Table 11 Proposed below-ground detention storages as part of the Stage 2 Project Catchment Location Footprint Maximum Outlet Objective (m2) depth (m) control Catchment 4 South-eastern 800 1.5 1 No. 675 Limit flow rates to – Bantry Bay corner of the mm existing case for intersection of diameter 5-year, 20-year Fitzpatrick Avenue pipe and 100-year ARI and Warringah flow events Road Catchment 6 Below Aquatic Drive 1300 3.5 1 No. 750 Limit flow rates to – Curl Curl at the intersection mm existing case for Creek with Wakehurst diameter 5-year, 20-year Parkway pipe and 100-year ARI flow events

5.4 Other network upgrades Table 12 outlines other proposed changes to the existing stormwater drainage network under the Stage 2 Project. The pipe network and overland flow paths under existing and proposed conditions are shown in Map 4 and Map 6, respectively.

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Table 12 Summary of changes to existing drainage lines under the proposed Stage 2 Project Catchment Existing drainage Proposed changes under the Stage 2 Project Catchment 1 375 mm to 450 mm diameter pipes, which Removal of 375 mm diameter drain north to Holland Crescent pipes connected to Warringah Road surface 375 mm to 450 mm diameter pipes north of Warringah Road remain connected to Holland Crescent 375 mm to 450 mm diameter pipes, which No change drain north to Rabbett St, via Forest Way Catchment 2 The Stage 2 Project site does not contribute No change to runoff within Catchment 2 under proposed conditions Catchment 3 375 mm to 450 mm diameter pipes, which Additional 375 mm to 450 mm drain surface water runoff from Warringah pipe connections to Warringah Road and Rodborough Road Road, Rodborough Road and Allambie Grove

Catchment 4 225 mm to 450 mm diameter pipes, which Upgrade to 225 mm to 750 mm drains surface water runoff from Warringah diameter pipes Road 600 mm diameter pipe, connecting to Upgrade to 600 to 1050 mm Warringah Road drainage infrastructure and diameter pipes providing cross-drainage under Warringah Road to the south of Fitzpatrick Avenue 375 mm diameter pipe , connecting to Additional 300 to 1050 mm Warringah Road drainage infrastructure and diameter pipes, draining providing cross-drainage under Warringah Karingal Crescent and Road to drainage infrastructure within the Warringah Road and Pipeline Reserve, north of Parni Place connecting downstream to the Pipeline Reserve

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Catchment Existing drainage Proposed changes under the Stage 2 Project Catchment 5 Overland flow is generally conveyed Widening of Warringah Road and south along Hilmer Street via kerb corresponding 375 mm diameter pipe and channel to connect with connections capture and redirect stormwater infrastructure and stormwater generated by the Stage 2 overland flow paths draining south in Project site into Catchment 6. No Catchment 5 drainage infrastructure from the Stage 2 Project site contributes to runoff draining to Catchment 5 Catchment 6 375 mm diameter pipes, which drain Upgrade to 375 mm to 675 mm surface water runoff from Warringah diameter pipes, which drain surface Road, Wakehurst Parkway and water runoff from Warringah Road, Bantry Bay Road, into Brick Pit Wakehurst Parkway and Bantry Bay Reserve to the south to the south- Road, towards Brick Pit Reserve. west of the Warringah Road New 600 mm to 1500 mm diameter intersection with Wakehurst Parkway pipes, diverting pipe flows around Brick Pit Reserve towards Aquatic Drive 375 mm diameter pipes, which drain New 375 mm to 750 mm diameter surface water runoff from the pipes, draining road surfaces on the intersection of Warringah Road and south eastern side of the Warringah Wakehurst Parkway to bushland Road intersection of Wakehurst south east of the intersection Parkway, draining to the Aquatic Drive pipe network 375 mm to 1050 mm diameter pipes, Additional 375 mm to 1050 mm which drain surface water runoff diameter pipes draining the proposed southwards from Warringah Road road surface, including slot drainage (east of Wakehurst Parkway) and nearby properties north of Warringah Road Roadside channel draining south New 300 mm diameter pipes and table along Wakehurst Parkway, south of drains intercepting surface water flow Aquatic Drive, intercepting surface along the top-of-cut on the western side water flow from subareas to the west of Wakehurst Parkway, about 120 m of Wakehurst Parkway and road south of Aquatic Drive, and draining surface flows south to Curl Curl Creek via cross- drainage upgrades and existing Wakehurst Parkway overland flow paths

New 375 mm diameter pipes draining Wakehurst Parkway, south of the intersection with Aquatic Drive. The proposed pipes redirect stormwater flows (against surface gradient) to the north, connecting to the proposed Aquatic Drive below ground detention storage.

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6.1 Overview of changes to catchment hydrology The following hydrology overview is based on the proposed drainage summarised in Section 5. A summary of changes to catchment areas, relative to existing catchment conditions, including impervious fractions, is provided in Table 13. The Catchment 1 surface area is reduced by removing stormwater connections to the Warringah Road surface and redirecting flows to Catchment 4 via the slot drainage infrastructure. Changes to impervious areas connected to Catchment 1 are negligible. The Stage 2 Project does not affect stormwater runoff within Catchment 2. Whilst the Catchment 3 surface area remains the same, the Stage 2 Project increases the pipe network capacity and connectivity to Warringah Road and Allambie Grove surfaces. Impervious surfaces connected to Catchment 3 increase marginally (two per cent) caused by road works around the intersections of Rodborough Road and Allambie Road with Warringah Road. The drainage infrastructure proposed under the Stage 2 Project, including the slot and road surface design, marginally increases the Catchment 4 surface area (five per cent). The underground pipe network capacity and connectivity to Warringah Road surfaces is increased significantly. Impervious surfaces connected to Catchment 4 are also increased (10 per cent), mainly caused by road widening that encroaches on existing residential properties and vegetated reserves to the south of Warringah Road. The Stage 2 Project redirects stormwater on Hilmer Street to Catchment 4 and Catchment 6, such that the Stage 2 Project site no longer contributes to Catchment 5 runoff under proposed conditions. This marginally reduces the overall surface area of Catchment 5 (three per cent). Conversely, the proposed road design marginally increases the catchment area contributing to Catchment 6 runoff. The proposed road and drainage design also increases the Warringah Road surface area draining to Aquatic Drive via Wakehurst Parkway infrastructure. Stormwater connections to Brick Pit reserve and the bushland to the south-east of the Warringah Road intersection with Wakehurst Parkway are reduced. The pipe network capacity and connectivity to Warringah Road and Wakehurst Parkway surfaces is increased. Impervious surfaces connected to Catchment 6 are increased by about 10 per cent. Proposed pipe culvert upgrades, outlined in Table 10 increase the cross-drainage capacity under Warringah Road, Wakehurst Parkway and Aquatic Drive. Appendix D (table) summarises the peak flows for 1-year, 5-year, 10-year and 100-year events along each drainage line impacted by the Stage 2 Project. Modelling indicates that peak flows will increase at drainage outlets in Catchment 3 (Points 3.02, 3.03, 3.04 and 3.07 on Map 2) and Catchment 6 (Points 6.02, 6.08, 6.10, 6.11 and 6.12 on Map 2). Some increases in peak overland flows are also expected in Catchment 3 and Catchment 6. The specific impacts of these hydrological changes with regards to erosion, sediment and water quality (in accordance with the SEARs) are discussed in Section 6.2. Similarly, specific impacts of hydrological changes on water quantity are discussed in Section 6.3.

54 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 Table 13: Summary of catchment areas under existing and proposed conditions Catc hment Existing conditions Proposed conditions Percentage Total area Impervious Total area Impervious change in (ha) fraction (ha) fraction impervious area 1 93 55% 91 56% -1% 2 63 41% 63 41% 0% 3 61 56% 61 57% 2% 4 34 57% 36 60% 10% 5 27 51% 26 51% -3% 6 135 34% 136 38% 10% Total 412 - 412 - -

6.2 Erosion, sediment and water quality impacts

6.2.1 Erosion, scour and sedimentation

Construction phase Soil erosion and sedimentation are risks posed to surface water quality throughout the construction phase where earthworks, vehicle access and alterations to drainage lines and connections can lead to increased sediment loads entering downstream environments. Disturbed areas with surface grades greater than 2.5 per cent and vehicle access tracks where the surface is frequently disturbed by traffic are typically high risk areas during construction. The locations of disturbed areas with grades greater than 2.5 per cent could change throughout construction and would need to be identified in the construction and environmental management plan (CEMP) - see also section 7. Stockpiled soils can also erode during rainfall events and high winds and require careful on-site management. Other disturbed areas with exposed soils are also susceptible to erosion during rainfall events. Construction of the Stage 2 Project involves the removal of sections of road and drainage infrastructure in addition to the installation of new connections between proposed drains and existing pits. During the construction of these connections there is an increased risk that disturbed soil will enter drainage lines and waterways. Stormwater control and treatment measures, such as the systems located within the Aurora Business Park on Aquatic Drive in Catchment 6 (see Section 4.5.2), are located immediately downstream of the construction footprint. Whilst council drainage within the construction footprint is not directly connected to the Aurora Business Park drainage system, overland flow from the construction site may still enter the Aurora Business Park drainage catchment if not managed carefully. In the event that runoff from the site is permitted to leave the site in an uncontrolled manner and discharge into downstream waterways, localised scour could occur at the points of discharge.

Operational phase Post-construction, some areas within the Stage 2 Project footprint may take time to stabilise and remain susceptible to erosion well into the operational phase. In addition, changes to land cover and stormwater management have the potential to increase erosion and scour within waterways. Potential impacts include:

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y Increases to the frequency, magnitude and velocity of runoff as a result of increased impervious area and the hydraulic efficiency of drainage systems y Increases to the duration of runoff as a result of increased stormwater detention. Climate change would also impact upon scour and erosion risks, which is discussed in Section 6.6. In this study, rainfall-runoff modelling generated flows for 1-year ARI, 5-year ARI, 10-year ARI design storm events and 100-year ARI design storm events. Generally, an increase in peak flow will be accompanied by an increase in flow velocities, increasing the risk of erosion. As mentioned in Section 6.1, drainage lines in Catchment 3 and Catchment 6 are subject to increases in peak flows. Peak flow increases in Catchment 3 immediately downstream of the Stage 2 Project (ie. Points 3.02 and 3.03). At these points, flows are controlled by pipes and overland flow paths designed to convey stormwater (ie. sealed roads), which mitigates the risk of scour and erosion at these points. Further downstream (Points 3.04 and 3.07), increases in peak flows are marginal (less than 5 per cent) compared to the existing peak flow, which is within the margin of error within the model. Overall, the increased risk of erosion due to the Stage 2 Project is negligible within Catchment 3. The Stage 2 Project increases impervious areas in Catchment 4 and generates higher peak flows upstream of the below ground detention storage proposed at the intersection of Fitzpatrick Avenue and Warringah Road (see Table 11). Downstream, DRAINS modelling indicates the proposed detention storage reduces peak flows to existing levels. The risk of scour and erosion is therefore no worse under the Stage 2 Project. Peak overland flow increases in Catchment 6 at the following locations: y Aquatic Drive (Point 6.02) y Downslope of culvert upgrades on Wakehurst Parkway (Points 6.08 and 6.09) y Further downstream where impacted drainage lines enter Curl Curl Creek (Points 6.11 and 6.12). Increases in peak flows on Aquatic Drive (50-100%) are offset by reduced peak overland flows from Warringah Road cross-drainage through Point 6.03. The combined effect immediately downstream (modelled at Point 6.04 and 6.05) is an overall decrease in peak flows carried by downstream pipes, roads and waterways. The risk of scour and erosion is therefore no worse under the Stage 2 Project at these points. Increases in peak flows (10-100 per cent) immediately downslope of culvert upgrades on Wakehurst Parkway (see SPI03303 and SPI08537 in Table 10) impact existing waterways, which are susceptible to erosion and scour. These drainage lines also impact upon potential Red Crowned Toadlet habitat (refer to Section 7.1.1 for discussion of mitigation measures).The proposed upgrades include erosion protection immediately downstream of the culverts. However, increased flows propagate further downstream (Point 6.10) up to 80 per cent higher than existing overland flows. The impact is less prominent further downstream (less than 10 per cent) where flows enter Curl Curl Creek. The increase in peak flows poses a risk of erosion, which can be mitigated with alterations to the Wakehurst Parkway cross-drainage configuration. Mitigation is further discussed in Section 7.1.1.

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6.2.2 Chemical contamination

Construction phase The release of harmful chemicals is a risk to downstream environments during construction. Substances may include acids and chemicals from washing processes, fuels, oils, lubricants and hydraulic fluids. Release may occur due to: y Spills y Refuelling y Inappropriate storage, handling and use of the substances y Disturbance and handling of contaminated soils. These substances have the potential to be transported by surface water runoff to downstream waterways. The degrading influence of chemicals on downstream ecology is of particular concern. During construction, surface water can enter the slot from direct rainfall, groundwater seepage and overland flows. Surface water interacting with groundwater may be contaminated from the adjacent service station and pose a risk to downstream waterways and receiving environments. In addition, the Stage 2 Project alignment encroaches on existing dwellings, commercial buildings and infrastructure on the southern side of the site. Depending on the extent of excavation for Warringah Road surface lanes; there is potential to unearth contaminated land from the adjacent service station. Demolition and clearing activities also have the potential to increase pollutant loads entering downstream waterways, including: y Asbestos and other building materials y Toxic or pollutant laden soils y Heavy metals y Chemicals (including hydrocarbons) associated with demolition processes and machinery y Dust and airborne pollutants (which eventually come to rest and drain into existing waterways).

Operational phase There is an existing risk of chemical contamination to downstream waterways from traffic within the Stage 2 Project area. The Stage 2 Project allows for an increase in traffic and therefore a higher risk of waterway contamination in the occurrence of an accident or spill. Due to the sensitivity of the receiving environment, spill containment for major truck accidents is considered in Section 7.1.2.

6.2.3 Nutrient loads

Construction phase With provision of appropriate construction stage erosion and sediment controls, nutrient loads would be unlikely to increase significantly during the construction phase, however particulate phosphorous is likely to increase where erosion and scour is allowed to propagate.

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Operational phase MUSIC model estimates of pollutant loads along impacted drainage lines, under existing and proposed conditions, are summarised in Table 14. Detention storages were simulated in Catchment 4 and 6 in accordance with the Stage 2 Project stormwater design (Appendix F). All modelled locations (ie Points 1.05, 3.04, 4.01, 4.06, 5.05, 6.01, 6.02, 6.04, 6.06 and 6.13) are shown in Map 2 in Appendix C. Details of the model set-up are presented in Appendix A. The following key points are made in regard to the information presented in Table 14. y There are no significant increases associated with the Stage 2 Project for total suspended solids, total phosphorus and gross pollutants. This is expected to be due to the influence of the sub-surface detention tanks attenuating flow from these catchments y Increases (up to five per cent) in suspended solids, total phosphorus and gross pollutants simulated at Point 3.04 are considered to be within the margin of error for the modelling and as such are not considered to be significant y Nitrogen concentrations at Point 4.01 and 6.01 (which in the MUSIC model is simulated to be the exit point of the detention basin on Aquatic Drive) are simulated to have increases of up to 13%. With the inclusion of inflows from other sub-catchments, the loads are expected to fall to existing levels (within an acceptable margin of error) before entering tributaries of Bantry Bay Creek at Point 4.06 and tributaries of Curl Curl Creek at Point 6.06 and 6.13. As such, the results suggest that the increased nitrogen loads will be localised only. The localised increase in nitrogen between 4.01 and 4.06 is not an area of identified sensitive aquatic systems and is therefore not considered to represent a potential adverse impact. The impacted area down gradient of Point 6.01 and up gradient of 6.06 has been identified as Red Crowned Toadlet habitat. Figure 13 presents a plot of the total nitrogen loads from the MUSIC model for the catchment area between 6.01 and 6.06. Figure 13 suggests two key differences which are outlined below: – Peak nitrogen loads during large rainfall events are reduced which is considered to represent a positive outcome from the Stage 2 Project stormwater design. – The most common difference between existing nitrogen loads and nitrogen loads associated with the Stage 2 Project design are apparent immediately after rainfall. It is noted however, that the higher values are more consistent (less variable) and are within the normal range of total nitrogen discharging under lower flow conditions. Figure 15 presents a plot of the nitrogen concentrations from the MUSIC model for the catchment area between 6.01 and 6.06. The plot suggests that there is a more consistent response to nitrogen concentrations that on average appear to be higher than existing conditions, however, the change is within the existing variation that occurs within the discharge from this catchment. This is consistent with the information presented in Figure 13. Based on the outcomes of MUSIC modelling, t the average increases in nitrogen loads and concentrations are not considered to be significant. y The modelling results suggest the Stage 2 Project design will result in a net reduction in nutrient loads from the Stage 2 Project area as a whole. This is consistent with the Concept Proposal surface water strategy (Lyalls and Associates, 2014). In accordance with ANZECC Guidelines, the waterways affected by increased pollutant loads at 4.01 and 6.01 are classified as upland freshwater streams of high ecological and conservation value. This implies a precautionary approach should be undertaken. The precautionary approach is to consider applying a management measure for any apparent trend away from the

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baseline (existing conditions). Management options for water quality are discussed in Section 7.1.

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Table 14 MUSIC model results under existing and proposed conditions

1.05 3.04 4.01 5.05 4.06 Pollutant Existing Proposed % Existing Proposed % Existing Proposed % Existing Proposed % Existing Proposed % conditions conditions Change conditions conditions Change conditions conditions Change conditions conditions Change conditions conditions Change

Total Suspended 35,783 33,626 N.I. 59,013 1% 24,825 19,969 N.I. 29,015 28,332 N.I. 96,251 92,218 N.I. Solids (kg/yr) 59,679 Total Phosphorus (kg/yr) 60 57 N.I. 99 100 1% 43 41 N.I. 50 48 N.I. 166 163 N.I. Total Nitrogen 452 420 N.I. 748 0.30% 317 370 364 N.I. 1,247 1271 2% (kg/yr) 750 347 9% Gro ss P o l l u ta n ts 5,233 4,909 N.I. 8,643 1% 3,682 0 N.I. 4330 4,225 N.I. 143,776 10,607 N.I. (kg/yr) 8,687

6.01 (exit point of retention storage 6.04 6.06 6.13 on Aquatic Drive) Pollutant Existing Proposed % Existing Proposed % Existing Proposed % Existing Proposed % conditions conditions Change conditions conditions Change conditions conditions Change conditions conditions Change Total Suspended 11,418 7,945 N.I. 31,154 29,717 N.I. 58,013 42,778 N.I. 48,897 48,721 N.I. Solids (kg/yr) Total Phosphorus 21 19 N.I. 51 49 N.I. 102 81 N.I. 89 89 N.I. (kg/yr) Total Nitrogen 163 184 13% 377 373 N.I. 759 651 N.I. 678 670 N.I. (kg/yr) Gro ss P o l l u ta n ts 1,599 0 N.I. 4,401 4,295 N.I. 8,433 4,295 N.I. 7,355 7,171 N.I. (kg/yr)

Text in bold indicates pollutant and nutrient load increases over existing conditions outside of 5 per cent modelling error.

N.I. denotes no increase

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1 Reduced peak loads

0.1

Existing 0.01 Conditions

Proposed 0.001 Conditions

0.0001 Higher loads after rainfall

Nitrogen Load (kg/6min) (kg/6min) Load Nitrogen events but within range of existing conditions 0.00001

0.000001

0.0000001

Date

Figure 13: Nitrogen loads simulated by MUSIC for discharge from the detention storage on Aquatic Drive

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Existing Conditions Proposed Conditions Nitrogen concentration (mg/L) Nitrogen concentration

0.3

Date

Figure 14: Nitrogen concentrations simulated by MUSIC for discharge from the detention storage on Aquatic Drive

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6.3 Water quantity and stormwater impacts

6.3.1 Stage 2 Project drainage capacity DRAINS modelling indicated the proposed road drainage design for the Stage 2 Project, including the slot drainage design, generally has 100-year ARI system capacity (see Map 5). 100-year ARI flood depths on the road are generally below 50 mm, except at the junction of east and west draining pipes in the slot approximately 60 metres west of Wakehurst Parkway (Catchment 6) where surface water pools on the road up to 200 mm in depth and five metres wide. The proposed detention basins at Aquatic Drive and Fitzpatrick Avenue East are designed to reduce peak 5-year, 10-year and 100-year flows back to existing levels at the outlet.

6.3.2 Impacts on Council’s stormwater drainage system

Construction impacts As discussed in Section 6.2, there is an increased risk of sediment loads being transported from the Stage 2 Project site during construction. Sediment loads deposited in downstream Council drainage assets could lead to increased flooding due to decreased drainage capacity within the network. Exposed soil may also reduce the infiltration rates and lead to increases in surface flow. Changes to the flow regime can also be caused by temporary bunds, fencing and on-site stormwater detention used to control sediment loads. It is also possible that changes to the stormwater drainage system during construction, if not made in conjunction with any necessary temporary bypass or diversion works, have the potential to cause localised flooding during rainfall events.

Operation phase impacts The long-term impact to Council’s stormwater drainage network is related to: y Improved efficiency of the stormwater drainage system, reducing up-welling from pits, backwatering at cross-drainage points and local flooding y An associated increase in flows downstream resulting in greater flooding. DRAINS was used to estimate the likely impacts of Stage 2 Project drainage infrastructure on Council’s stormwater drainage system. Further detail of the DRAINS model development is provided in Appendix B. In general, the Stage 2 Project does not reduce the service level of Council’s stormwater drainage system (see Map 3 and Map 5). In Catchment 3, immediately downstream of the Stage 2 Project draining Allambie Road, Warringah Road and Frenchs Forest Road East (Point 3.02 and 3.03), increases in peak flows marginally reduce the hydraulic capacity of the pipe drainage system. This is likely to have implications for the Stage 1 Project at the intersection of Frenchs Forest Road East and Allambie Road, which is discussed further in Section 6.5. Pipe drainage on Aquatic Drive, near the intersection with Tilley Lane at the Aurora Business Park, is likely to receive a marginal increase in peak flows due to the Stage 2 Project. Any increase, however, is unlikely to cause additional flooding issues due to the existing pipe capacity providing enough capacity to carry 10-year ARI flows under proposed conditions.

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6.3.3 Impacts on stormwater flows into and surrounding the Forest High School and Frenchs Forest Public School The Forest High School lies within Catchment 1 on the northern side of Warringah Road and is not adversely affected by changes to catchment hydrology caused by the Stage 2 Project. The Frenchs Forest Public School lies partly within Catchment 4 to the west of the Warringah Road intersection with Forest Way and is not adversely affected by changes to catchment hydrology caused by the Stage 2 Project (refer to the results on Map 5 and Map 6, Sheet 2 of 14, Appendix C, which show no increase in flows near the school).

6.3.4 Impacts on downstream catchments and wetlands

Rainfall-runoff As outlined in Section 6.1, impervious areas directly draining to Catchments 3, 4 and 6 increase by 2 per cent, 10 per cent and 10 per cent respectively due to the Stage 2 Project. In this study, rainfall-runoff modelling generated design flows to estimate peak runoff, both existing and following construction of the Stage 2 Project. A comparison of peak flow at each point of interest provides an indication of hydrologic changes caused by the Stage 2 Project. Changes in peak flows entering receiving waterways at the boundary of the study area are outlined in Appendix D for 1-year, 5-year, 10-year and 100-year storm events, respectively. Modelling indicates that the Stage 2 Project will have minimal impact on peak flows entering downstream waterways. The greatest change was modelled in the Curl Curl Creek catchment (Point 6.13), where flows increased by less than 10 per cent for all modelled storm events. For all the other catchments impacted by the Stage 2 Project (Points 1.06, 3.07, 4.06), changes were within five per cent of existing peak flows. Where catchment impervious areas increase, the volume of stormwater reaching downstream waterways is also likely to increase. Given the highly developed nature of the existing catchment, and the relatively small project area relative to the overall catchment, the potential for these changes to adversely affect the existing ecological condition of downstream environments is considered to be limited. The Stage 2 Project is therefore expected to have little impact on the flow regime entering catchments or wetlands downstream of the study area.

Groundwater inflows As discussed in Section 4.9.2, the groundwater assessment estimated groundwater inflows to surface water under existing and post-developed conditions. The findings of the groundwater assessment are discussed in greater detail in the Northern Beaches Hospital Network Enhancement - Stage 2 Groundwater Assessment. A summary of the changes to groundwater seepage is provided in Table 15. The average creek flows generated from surface water runoff would be many times greater than the predicted changes in groundwater seepage resulting in negligible change to annual flow volumes . An assessment was carried out of the groundwater seepage likely to occur during periods of low or no surface water flows. The possibility of the groundwater flows supplying the red-crowned toadlet habitat was investigated. During dry periods, it is expected the groundwater seepage would be on the lower end of the range indicated in column A of Table 15.

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Table 15: Potential impacts of changes to groundwater seepage flows, dry periods Location A: Estimated B: Estimated C: Dry period D: Dry period groundwater change in minimum change maximum change in inflow to groundwater in groundwater groundwater inflow as surface water inflows as a inflow as a result a result of the Stage 2 (m3/day)+ result of the of the Stage 2 Project (m3/day) + Stage 2 Project (m3/day) Project, long- term (%) + Red-crowned 15 - 97 2.0 – 2.5 0.3 0.4 toadlet habitat on Trefoil Creek Red-crowned 3 - 51 30.0 – 50.0 0.9 1.5 toadlet habitat on Curl Curl Creek Middle Creek 20 - 162 3.0 – 6.0 0.6 1.2 south-west tributary Bantry Bay 121 - 971 0.2 – 1.5 0.2 1.8 tributary

+From groundwater assessment

During times of no surface water flow, changes to groundwater seepage are predicted to be of the order of that presented in Table 15. The Stage 2 Project biodiversity assessment (SMEC, 2015) found that potential breeding habitat for the red-crowned toadlet is likely to have a low dependence on groundwater and is therefore unlikely to be vulnerable to changes in groundwater resulting from the Stage 2 Project as documented in the above table.

6.4 Cumulative impacts on water sources Table 16 presents the catchment flows from each of the main surface water catchments intersecting the Stage 2 Project. The results presented in the text are summarised below: y The results suggest that there will be a small decrease in average flows from the Curl Curl Creek (4.6 %) and Middle Creek Catchments (1 %). y The small decrease in flows from the Middle Creek catchment (0.1 m3/day) is due to catchment changes associated with the Stage 2 Project and results in equal increases in flows in Bantry Bay catchment. Under the water sharing plan this represents a migration of water from the North Sydney Coastal management unit to the Middle Harbour management unit. When compared to allocated water sharing volumes for Northern Sydney Rivers water source (in which these management units are located) such as the share component for domestic and stock use (15 m3/day), it is considered to be negligible (i.e. < 1 %). y There is an overall reduction in average flows from all catchments intersecting the Stage 2 Project (0.4 m3/day), which approximates 1.5 per cent of total average flows from these catchments. The reduction is due to flow reductions in Curl Curl Creek catchment (Catchment 6). Figure 15 presents a plot of the existing and Stage 2 Project flows simulated by the music model at the confluence of all of the sub-catchments draining

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Catchment 6. The figure suggests that while there are small decreases in overall flows, relative to the overall flow regime the flow changes appear to relatively minor. y The flow changes in Curl Curl Creek are not expected to result in more than negligible changes in the water budget for Manly Dam. y Based on findings of the music modelling the contribution of the Stage 2 Project to cumulative impacts on water sources are considered to be minor.

66 | GHD | Report for SMEC - Northern Beaches Hospital Connectivity and Network Enhancements Stage 2, 21/24063 140000

120000

100000

80000

60000 Proposed Existing

40000

20000

Date

Figure 15: MUSIC model daily flows for Curl Curl Creek for an average year (1959)

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Table 16: Music Modelling Average Flow Outputs Water North Sydney Coastal Management Unit Middle Harbour Management Unit Total Flow Sharing Total Change Plan Manageme nt Unit Catchment Curl Curl Creek Middle Creek Bantry Bay Condition Existing Proposed Change Existing Proposed Change Existing Proposed Change Mean Flow 8.9 8.5 -0.4 (-4.6 %) 11.7 11.6 0.1 (-1 %) 4.6 4.7 0.1 (2.6 %) -0.4 (-1.5 %) (m3/day)

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6.5 Interaction with the Stage 1 Project and the Northern Beaches Hospital The potential impacts of the Stage 1 Project were assessed by L&A in the Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M (Lyall and Associates, 2014). The following sites for stormwater quality control measures were identified:

1. Catchment 1 - Cobb Street, intercepting stormwater from Catchment 1 2. Catchment 2 - Wakehurst Parkway, intercepting stormwater from Catchment 2 3. Catchment 3 - Patanga Road Surface water runoff interaction between the Stage 1 Project and Stage 2 Project sites was also assessed by L&A, though only at a conceptual level (Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M, Lyall &Associates, 2014). Interactions are most prominent in Catchment 3, with works to be completed in the vicinity of the Warringah Road intersection with Allambie Road under both the Stage 1 and Stage 2 Projects. The longitudinal connectivity of drainage infrastructure in Catchment 3, that receives stormwater from both the Stage 1 Project and the Stage 2 Project, presents a risk of cumulative impact. The impacts include potential for localised surface ponding or flooding where flow increases occur. Mitigation of these potential impacts is discussed in Section 7.3.

6.6 Climate change The New South Wales Office of Environment and Heritage (OEH) publishes information regarding the expected effects of future climate change on rainfall and sea levels. The document Metropolitan Sydney Climate change snapshot (OEH, November 2014) is the most recent information available and identifies predicted changes to rainfall seasonality and average rainfall. The findings of the document are that summer rainfall changes are predicted to be between a seven per cent decrease and a 28 per cent increase in the far future (from 2060 to 2079). The publication does not provide details regarding changes to flood-producing rainfall events other than to confirm that changes to rainfall intensity are expected. In 2007, CSIRO carried out climate change modelling for the NSW Government to assess the impacts of climate change on rainfall intensities. The results showed a trend for increased rainfall intensities for the 1 in 40-year ARI one-day rainfall event across New South Wales. The projected increase in rainfall totals in the Sydney Metropolitan Area is indicated in Table 17. Table 17: CSIRO indicative change in rainfall one-day totals (CSIRO, 2007) Location 40 Year 1 day rainfall total 40 Year 1 day rainfall total projected change by 2030 projected change by 2070 Sydney Metropolitan -3% to +12% -7% to +10% Catchment New South Wales average -2% to +15% -1% to +15%

The CSIRO modelling suggests catchments, which contribute to stormwater runoff carried by drainage infrastructure, may experience up to a 10 per cent increase in rainfall intensities as a result of climate change. However, it is important to note that results are only available for a single ARI and storm duration.

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As previously mentioned, DRAINS modelling indicates that the existing design standard of the underground stormwater drainage system, within the study area, is between the 5-year and 10- year ARI, and in some locations lower. The level of service may be reduced as a result of an increase in rainfall intensity of up to 10 per cent. 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 Stage 2 Project, though the capacity of the proposed network is generally improved over the existing case. The effects of climate change are not expected to be worse as a result of the Stage 2 Project. Note that the values in Table 17 are considered indicative only. OEH is currently working with the University of New South Wales on climate projections for impacts to flooding and it is expected that these figures will be revised. As the Stage 2 Project site is located on a ridgeline which lies more than 100 metre above mean sea level, projected rises in sea level linked to future climate change will not impact flooding behaviour along the road corridor.

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7. Surface water management strategy

The surface water management strategy recommended for the Stage 2 Project is as follows: y Implementation of the drainage works detailed in Chapter 5 of this report y Preparation and implementation of a soil and water management plan (SWMP) to cater for the construction activities. The primary focus of the SWMP is erosion and sediment control during the land disturbance phase of the project. SWMP should be prepared in accordance with the following guidelines: – Soils and Construction – Managing Urban Stormwater series (herein referred to as the “Blue Book”), comprising: Volume 1 (Landcom, 2004) 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). Important and additional considerations in surface water management and preparation of the SWMP are discussed below. This includes potential site constraints and considerations for both the construction and operational phases of the project. Overall implementation of the above measures is expected to minimise any adverse impacts on existing environment. The proposed strategy is discussed in relation to the key issues listed in the SEARs (as identified in Table 1). The strategy also incorporates the relevant elements of the Stage 1 and Concept Proposal surface water management strategy.

7.1 Measures to manage erosion, sediment and water quality impacts

7.1.1 Erosion and scour

Construction phase SWMP measures will be required to be implemented in accordance with the documents outlined above to manage scour and erosion during construction. These measures should also be documented with a construction and environmental management plan (CEMP) and be approved by Roads and Maritime and relevant regulatory authorities. 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 downstream of the Stage 2 Project, the installation of sediment retention basins is unlikely to be feasible. 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, is less than 150 m3. Erosion and sediment control measures, aimed at minimising the volume of sediment transported from disturbed areas, are required. Potential measures include: y Staging of works to minimise the extent of disturbance at any one time

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y Temporary stabilisation or revegetation/rehabilitation works to reduce the extent of disturbed surfaces y Application of temporary surface treatments or blanketing on exposed earth surfaces y Sediment barriers y Graded access tracks and speed restrictions for vehicles on site y Mandatory use of designated stations to wash all vehicles and machinery before exiting the site y Control of surface water runoff into slot during construction y Rainfall and wind forecasts to inform on-site managers of daily risks y Adaptable site activity scheduling to avoid periods of increased erosion risk due to wind, rain and runoff y Vegetative buffer strips y Stabilised drainage lines incorporating rock check dams at regular intervals. 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 from causing scour of disturbed surfaces. Where space allows, construction works should consider the potential for locating sediment retention basins along the southern side of Warringah Road, where widening of the existing westbound carriageway will require land clearing and excavation works, if possible. These basins would reduce the volume of sediment and turbidity levels in runoff potentially discharging to Catchments 4, 5 and 6.

Operation phase The impact of the Stage 2 Project on channel erosion and scour is assessed in Section 6.2.1. Of concern are the increased flows at Points 6.08, 6.09 and 6.10 downstream of the culvert upgrades on Wakehurst Parkway near the southern limit of works. The increase in peak flows will be mitigated by through minor modifications during detailed design, specifically to the Stage 2 Project drainage configuration (including cross-drainage upgrades) on Wakehurst Parkway south of Aquatic Drive. Stage 2 Project detailed design should aim to reduce peak flows entering waterways to the east of Wakehurst Parkway by redirecting flows further south via existing road drainage. This can be achieved without causing additional flooding on Wakehurst Parkway and therefore presents a low-risk and cost-neutral mitigation option. Increased flows are also expected in Catchment 3 and Catchment 4, though the assessed impact is minimal. Given the minimal impact a monitoring and maintenance based approach is recommended for these catchments. Drainage outlet conditions in Catchment 3 and Catchment 4, downstream of the Stage 2 Project, should be monitored for evidence of erosion and scour as part of regular maintenance. Where erosion and scour becomes apparent, suitable erosion control measures including rock armouring, gabion baskets, matting or concrete lining; depending on the specific site conditions, should be implemented.

7.1.2 Chemical contamination

Construction phase A SWMP and CEMP should be prepared and approved by Roads and Maritime and relevant regulatory authorities that include methods and procedures for reducing the risk of contamination.

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Reducing the risk of chemical spill during the construction phase requires the planning and implementation of the following on-site measures: y Mandatory use of fenced and impermeable areas for washing machinery and equipment with collection and treatment systems downslope y Mandatory use of fenced and impermeable refuelling stations (preferably off-site) y Storage of all chemicals and harmful waste products in secure designated areas (preferably off site) y Regular maintenance of machinery and vehicles, checking for oil, fuel or hydraulic leaks y Easily accessible chemical spill clean-up kits on site in case of emergency spills y Enforcement of incident reporting procedures to help treat serious spills y Testing of potential contaminated soils prior to excavation. In addition, reducing pollutant loads sourced from demolition works requires the planning and implementation of the following on-site control measures: y Scheduling of works to avoid strong winds and rainfall y Mandatory coverage of trucks carrying debris y Temporary barriers or dust screens, as appropriate, to suppress the effect of dust movement to uncontrolled sites y Dust suppression such as wetting measures y Fencing of temporary stockpiles on hardstands.

Operational phase Consideration should be given during detailed design to the incorporation of end-of-line and slot spill capture to account for the increased risk of spill due to increased traffic volumes. In the event that any spills do occur the spill would be directed to surface water capture systems. During the detail design stage, design of the surface water systems would include consideration of the following to mitigate the risk of spills: y Capacity to receive a spill, usually with a volume corresponding to that of a typical transport truck. y Potential for spillage control or containment y Capacity to treat first flush from the pavement surface and reduce the risk of spills discharging onto adjacent land or watercourses. y Design of structures to limit potential for infiltration to the underlying groundwater system (i.e. they would be appropriately lined). y Inclusion of temporary holding tanks in the discharge lines of suitable size to capture the expected volumes from cleaning activities and spills to prevent impacts from spills and cleaning activities in the slot the surface water collection infrastructure y Strategy for removal off-site of the captured water by a licensed waste contractor. Cleaning and spill response procedures would be documented in a site management plan to ensure that impacted water is contained, collected and managed appropriately.

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7.1.3 Nutrient and sediment loads

Construction phase During road construction, sediment loads need to be managed once erosion risks have been reduced. Stormwater control measures that reduce the amount of sediment leaving the site are to be included in the CEMP and implemented on site before, during and after construction until the site conditions have settled and operational measures are established. Reducing sedimentation from the Stage 2 Project construction requires the planning and implementation of the following on-site control measures: y Stormwater filters for existing stormwater drainage pits and inlets on-site or immediately downstream of the site y Bunds or fences downslope of designated machinery washing areas, depots and stock piles y Maintenance of clean zones along existing drainage lines, particularly at surface connections to the underground pipe network (e.g. sag and on-grade pits) y Site-specific soil and stormwater management plans for drainage lines draining to the bioretention systems within the Aurora Business Park on Aquatic Drive (Catchment 6) y Sedimentation basins where stormwater discharges from the Stage 2 Project site, sized to manage 10-year ARI flows in accordance with Soils and Construction Vol 1 (Landcom, 2004) (the “Blue Book”). 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 downstream of the Stage 2 Project, the installation of sediment retention basins is unlikely to be feasible. 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, is less than 150 m3.

Operational phase The MUSIC model results suggest that the Stage 2 Project design will maintain or improve existing water quality conditions during operation at most locations. Slightly higher nitrogen concentrations were simulated to be localised to a Red-crowned Toadlet habitat areas in catchment 6 (between points 6.01 and 6.06 in Sheet 1 of Map 5) are considered to be within the range of the existing conditions and are not considered to be significant. Based on the findings no additional measures are considered to be required for the Stage 2 Project design to manage discharge of nutrients and sediment.

7.2 Measures to mitigate water quantity and stormwater impacts

7.2.1 Council’s stormwater drainage system

Construction phase Construction activities such as temporary site facilities, bunds, fencing, access tracks and machinery should not alter existing flow paths. Stormwater should be managed on-site such that drainage is controlled to existing discharge points without altering the contributing catchment areas.

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Where the works involve decommissioning or upgrade of existing assets, it will be necessary to provide temporary stormwater drainage or diversion works. This would maintain system operation in the event of a large rainfall event during construction. Management of surface water runoff into the slot during construction will need to be managed through temporary bunding or stormwater drainage diversion to prevent flooding of the slot.

Operational phase Given the existing capacity of the drainage system is generally less than the peak 10-year ARI flow, any increase in flow is likely to reduce the drainage standard further. As discussed in Sections 6.3 and 6.5, the Stage 2 Project has potential to reduce the hydraulic standard of Council’s drainage system downstream of Points 3.01, 3.02 and 3.03 in Catchment 3. These impacts do not propagate further downstream and can be managed by upgrading local pipe infrastructure in conjunction with Stage 1 Project works as described in Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M (L&A, 2014). See also Section 7.3 for description of these works and discussion on cumulative impacts of Stage 1 and Stage 2 Project works. In addition, during development of the detailed design, consideration must be given to the potential for localised increases in ponding from stormwater pits or road flood widths and mitigated accordingly.

7.2.2 Downstream catchments and wetlands

Rainfall-runoff The Stage 2 Project is estimated to have limited impact on downstream catchments and wetlands, with the exception of Catchment 6. Measures to address localised impacts discussed in Sections 7.2.1 and 7.3 are unlikely to cause any regional impacts to the catchment downstream of Catchment 6, requiring no further intervention.

Groundwater flows Mitigation measures to address the potential for changes in groundwater flows are discussed in the groundwater assessment report.

7.3 Cumulative impact management Increases in peak flows draining north from the Stage 1 and Stage 2 Projects along Allambie Road (discussed in Section 6.5) are addressed in the Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M (L&A, 2014). 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 longitudinal connectivity of flow paths impacted by both the Stage 1 Project and the Stage 2 Project presents opportunities to combine or centralise the recommended stormwater control measures. For example, the Patanga Road site recommended by L&A (Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS Appendix M, 2014) for stormwater quality control may be designed to account for both Stage 1 and Stage 2 Project impacts. Alternatively, multiple and dispersed stormwater control measures could be considered. For other future development within the study area boundary, particularly works which lie along the upstream side of the Stage 2 Project (including the Northern Beaches Hospital

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development), measures will need to be implemented to maintain or improve the peak flow and quality of stormwater contributing to the downstream drainage system.

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8. References

ANZECC and ARMCANZ (2000), Australian and New Zealand Guidelines for Fresh and Marine Water Quality, National Water Quality Management Strategy, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand. DECC (2008). Managing Urban Stormwater – Soils and Construction. Volume 2D – Main Road Construction. Engineers Australia (2006). Australian Runoff Quality – A Guide to Water Sensitive Urban Design. Healthy Rivers Commission (2004). Gosford and Northern Beaches Lagoons, NSW Water Quality and River Flow Objectives. IEAust (1998). Australian Rainfall and Runoff. Volumes 1 and 2. Published by the Institution of Engineers, Australia. Lyall & Associates (2014). Northern Beaches Hospital Connectivity and Network Enhancement Project: Proposed construction and operational phase surface water management strategy concept proposal and Stage 1 project (Appendix M of the Northern Beaches Hospital Road Connectivity and Network Enhancements Project EIS). Montgomery Watson Harza (2004). Warringah Council Creek Management Study. O'Loughlin, G. (1993) The ILSAX Program for Urban Stormwater Drainage Design and Analysis (User's Manual for Microcomputer Version V2.13), Civil Engineering Monograph 93/1, University of Technology, Sydney. Patterson, Britton and Partners (PBP), (1999). Northern Beaches Stormwater Management Plan. RMS (2014a): Concept Design Geotechnical Interpretation Report, Northern Beaches Hospital Enabling Roadworks, Frenchs Forest. RMS (2014b): Concept Design Geotechnical Investigation Report, Northern Beaches Hospital Enabling Roadworks, Frenchs Forest. Scraggs, C. J., & Lemckert, C. J. (2004, December). Preliminary estimation methods for sizing detention basins in Queensland. In Fifteenth Australasian Fluid Mechanics Conference (AFMC). http://www. aeromech. usyd. edu. au/15afmc/proceedings/index. html. Walsh, C. J., Fletcher, T. D., & Ladson, A. R. (2005). Stream restoration in urban catchments through redesigning stormwater systems: looking to the catchment to save the stream. Journal of the North American Benthological Society, 24(3), 690-705. Walsh, C.J. and Kunapo, J. (2009). The importance of upland flow paths in determining urban effects on stream ecosystems. Journal of the North American Benthological Society, 28, 977- 990

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Appendices

Appendix A – MUSIC Model Development

Background to MUSIC Modelling MUSIC (Model for Urban Stormwater Improvement Conceptualisation) was developed by the CRC for Catchment Hydrology, as part of the Urban Stormwater Quality Program. It enables modelling of both urban stormwater pollution generation as well as treatment elements. In recent years it has developed into a standard tool for use in conceptual design for urban stormwater pollution. The choice of MUSIC parameters and modelling approach is described in further detail below.

Climate data and choice of timestep Climatic inputs to MUSIC include catchment-appropriate rainfall and potential evapotranspiration (PET) data. The choice of rainfall data should include minimum length of record of at least 5 years of continuous rainfall, with a minimum of data gaps As MUSIC is a continuous model, it is necessary to select a period of data that it representative of the typical climate experience in a region including incorporation of wet and dry years. The timestep selected should reflect either the time of concentration of the smallest subcatchment, or the shortest residence time of any treatment measure. In the majority of cases, a six minute timestep provides suitable model results. Given the steep grade and relative small catchment areas in the study area, a six minute timestep was adopted. The closest station to the Stage 2 Project site is BOM station 66182, Frenchs Forest (The Forest High School, Frenchs Forest Rd), which has a Mean Annual Rainfall (MAR) of 1376.8 mm. As no 6 minute rainfall data is available from the rainfall station at the Forest High School, historical climate data from Sydney’s Observatory Hill (1959) was adopted. The data has an annual rainfall of 1490 mm, which may overestimate rainfall runoff for Frenchs Forest but was considered a conservative representation for modelling purposes. The station Glen Street at Belrose was not used in this assessment due to data only being available for a one hour timestep.

Catchment area and impervious area The approach to deriving catchment areas and defining impervious and pervious fractions was adopted from the approach for source nodes greater than 10 ha documented in the draft MUSIC modelling guidelines for NSW (Sydney CMA,2010). Catchment areas were defined for each of the drainage paths 1 to 6. The site is located at the upstream of catchments draining north and south with minimal external catchments draining through draining through the site. Subcatchment areas were further defined based on the drainage network and Stage 2 Project site. Total impervious area (TIA) for each subcatchment was calculated on the basis outlined in Table 37.

Table 18: Fraction impervious assumptions by land use zone Land use Impervious fraction High density residential 0.6 Low density residential 0.2 Urban parks 0.1 Schools 0.7 Mixed used 0.7 Forest 0 Road 0.6 Warringah Road, existing conditions 0.7 Wakehurst Parkway, existing conditions 0.3 Warringah Road, proposed conditions 0.8 Wakehurst Parkway, proposed conditions 0.6 Intersection of Warringah Road and 0.9 Wakehurst Parkway, proposed conditions

The values in Table 18 were based on typical impervious and pervious areas defined in GIS. Effective impervious area (EIA) is required in MUSIC. However, in the absence of flow data local to the Stage 2 Project site for calibration, the TIA was adopted as a conservative estimate of EIA. In some cases, this may overestimate more frequent flows.

Urban node parameters The MUSIC pervious node parameters adopted in the modelling were as per Table 3-4 of the Draft MUSIC Modelling Guidelines for NSW (Sydney CMA, 2010).

Pollutant load parameters The pollutant generation parameters adopted were: x Baseflow concentration – Table 3-9 of the Draft MUSIC Modelling Guidelines for NSW (Sydney CMA, 2010). x Stormflow concentration – Table 3-9 of the Draft MUSIC Modelling Guidelines for NSW (Sydney CMA, 2010).

Network routing The ‘no routing’ option was adopted such that no attenuation or delay of peak flows was modelled between the source node and the receiving node.

Rainwater / detention basin assumed properties The sub-surface detention basins located at the western end of Aquatic Drive (Catchment 6) and Fitzpatrick Avenue East (Catchment 4) (as detailed in the stormwater design plans for the Stage 2 Project – drawing 150327 NBH Drainage Updates dated 1 April 2014) have been simulated in the model as a rainwater tank node. This was considered to be the most appropriate way to represent a sub-surface detention basin. The detention basins have been represented in the model using the available detention basin design information outlined in Section 5.3. The parameters used for detention basin in the model are presented in Table 19 below.

Table 19: Rainwater tank properties

Catchment 4 Catchment 6

Interpretation of modelling results The average load changes over the entire data set for total suspended solids, total nitrogen, total phosphorus and gross pollutants were used to assess impacts. This approach was considered to be the most suitable given the broad catchment approach adopted for the MUSIC modelling and the inherent ability of the model to accurately resolve the changes in sediment loads over the entire range in flow conditions. Further to the above, considering the high-level nature of MUSIC, a model accuracy of plus or minus five percent was ruled. Only changes in loads above a five percent margin were considered to be outside the margin of error of the modelling and therefore significant. It was noted during modelling that there was some variability in the modelled predictions. To reduce this variability, and provide more confidence in the model predictions, the average of five model runs was reported.

Appendix B – DRAINS model development

General DRAINS is a program used for designing and analysing various types of catchments and urban stormwater drainage systems. Its capabilities include modelling stormwater drainage systems of various scales using the ILSAX hydrology method, Rational Method and storage routing models. It simulates the conversion of rainfall patterns to stormwater runoff hydrographs and routes these through a one-dimensional network of pipes, channels and streams. The existing DRAINS model available for the site was updated with data provided by Warringah Council to prepare an existing case model. A post-developed model was prepared using the 2D design information available from the reference design.

Catchment delineation Catchment delineation was carried out using a Digital terrain model (DTM) based on 200 mm contours generated from the LiDAR data, with land use identified according to LPI lot boundary data. Impervious areas were estimated using the land use categorisation detailed in Table 18.

Hydrologic model A rainfall-runoff model converts rainfall precipitation inputs, in the form of a hyetograph (rainfall intensity against time), into runoff or streamflow values. The loss and routing methods used in ILSAX rainfall-runoff models (O’Loughlin, 1993) were adopted in this analysis. The rainfall-runoff model inputs were as follows: x Paved (impervious area depression storage (mm): 1 x Supplementary depression storage (mm): 0 x Grassed (pervious area depression storage (mm); 5 x Soil Type: 3 The infiltration model parameters for Antecedent Moisture Condition (AMC) and Antecedent Rainfall Depths are listed in Table 40 and Table 41 respectively. Table 20: DRAINS model overland flow paths Parameter Soil Type A (or 1) B (or 2) C (or 3) D (or 4) Initial Rate, f0 (mm/h) 250 200 125 75 Final Rate, fc (mm/h) 25 13 6 3 Shape Factor, k (h-1) 2 2 2 2 Antecedent Rainfall Depths (mm) for: AMC = 1 0 0 0 0 AMC = 2 50 38 25 18 AMC = 3 100 75 50 38 AMC = 4 150 100 75 50

Table 21: Antecedent Rainfall Depths Soil Type A (or 1) B (or 2) C (or 3) D (or 4) AMC = 1 250.0 200.0 125.0 75.0 AMC = 2 162.3 130.1 78.0 40.9 AMC = 3 83.6 66.3 33.7 7.4 AMC = 4 33.1 30.7 6.6 3.0

Rainfall data and design storms

Design storms were generated in accordance with Australian Rainfall and Runoff (1987). The rainfall Intensity-Frequency-Duration (IFD) data used in the modelling is shown in Figure 16

Source: Australian Government, Bureau of Meterology Figure 16: Intensity-Frequency-Duration table for the Northern Beaches Hospital Site

Hydrologic calculations DRAINS results change when it is run with different time steps. The minimum time step is 0.005 minutes or 0.3 seconds, and the maximum for pipe calculations is 1 minute. Generally, smaller time steps give more accurate and stable results, however hydraulic (DRAINS) modelling sometimes displays instabilities (rapidly fluctuating water surfaces) or spikes (sudden rises and falls in water level and flowrates). These are typically caused by overshooting of interpolation and extrapolation calculations, and can usually be ignored after checking the model. For this study, a time step of 0.005 minutes was chosen. Where instabilities were present in hydraulic modelling, explanations and interpretations are presented in this report and do not invalidate the results.

Drainage network cross-sections and pit inlets The following cross-sections were assumed for overland flow paths modelled in DRAINS: y Roads - “7.5 m roadway with 3 per cent crossfall and barrier kerb” y Natural gullies and unsealed overland flow paths - “Swale with 1:6 slopes”

These assumptions influence the velocities estimated in DRAINS modelling, however for the purpose of this assessment the broad adoption of cross-sections was considered appropriate.

Appendix C – Maps

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SMEC Australia Pty Ltd Paper Size A3 Stage 2 Regional Bantry Bay Job Number 21-24063 Boundary Northern Beaches Hospital Network Enhancement - Stage 2 Revision G 0200 400 800 1,200 1,600 Catchments Curl Curl Crek Date 20 May 2015 Metres Waterway Middle Creek Surface Water Model Map Projection: Transverse Mercator Sheet 1 / 1 Horizontal Datum: GDA 1994 Grid: GDA 1994 MGA Zone 56 Watercourse Regional Catchment Plan Map 1 N:\AU\Melbourne\Projects\21\24063\GIS\Maps\Deliverables\2124063_000_KBM_A3L.mxd [KBM: 19] 180 Lonsdale Street Melbourne VIC 3000 Australia T 61 3 8687 8000 F 61 3 8687 8111 E [email protected] W www.ghd.com © 2015. Whilst every care has been taken to prepare this map, GHD make no representations or warranties about its accuracy, reliability, completeness or suitability for any particular purpose and cannot accept liability and responsibility of any kind (whether in contract, tort or otherwise) for any expenses, losses, damages and/or costs (including indirect or consequential damage) which are or may be incurred by any party as a result of the map being inaccurate, incomplete or unsuitable in any way and for any reason. Data source: Aerial Image, Google Earth Pro, Extracted November 2014; Basemaps, LPI, 2012; Surface Water Model Layers, GHD, 2015 Created by: mcoker

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