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Mona Vale Road West Upgrade

Surface Water Strategy Hydrology and Hydraulics Report

Roads and Maritime

25 November 2016

Revision: 1

Reference: 250171

Project 250171 File 250171-WEST-RPT-HY-0001 MVR West Surface Water Strategy_Rev 1.docx 25 November 2016 Revision 1

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Document prepared by:

Aurecon Australasia Pty Ltd

ABN 54 005 139 873

Level 5, 116 Military Road Neutral Bay NSW 2089

PO Box 538 Neutral Bay NSW 2089 Australia

T F E W

+61 2 9465 5599+61 2 9465 [email protected]

A person using Aurecon documents or data accepts the risk of:

a) Using the documents or data in electronic form without requesting and checking them for accuracy against the original hardcopy version.

b) Using the documents or data for any purpose not agreed to in writing by Aurecon.

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Report title Surface Water Strategy Hydrology and Hydraulics Report

Document ID Project number 250171

File path pw:\\designshare.au.aurecon.info:PW_PROD_AU\Documents\Projects\25xxxx\250171 - A3 Mona Vale Rd\3 Project Delivery\Reports\MVR West Surface Water Strategy\MVR West Surface Water Strategy.docx

Client Roads and Maritime Client contact Matty Mathivanar

Rev

Date Revision details/status Author Reviewer Verifier (if required)

Approver

0 30 September 2016 DRAFT Ali Djozan Yannick Michel

Wendy Young

Tim Snape

1 25 November 2016 Final Ali Djozan/Ziyaad Hassan

Yannick Michel

Brian Sexton

Ian Wilcock

Current revision 1

Approval

Author signature Approver signature

Name Ali Djozan/Ziyaad Hassan Name Ian Wilcock

Title Senior Engineer Title Associate

Project 250171 File 250171-WEST-RPT-HY-0001 MVR West Surface Water Strategy_Rev 1.docx 25 November 2016 Revision 1

Mona Vale Road West Upgrade

Date 8 November 2016 Reference 250171 Revision 1

Aurecon Australasia Pty Ltd

ABN 54 005 139 873

Level 5, 116 Military Road Neutral Bay NSW 2089

PO Box 538 Neutral Bay NSW 2089 Australia

T F E W

+61 2 9465 5599+61 2 9465 [email protected]

Project 250171 File 250171-WEST-RPT-HY-0001 MVR West Surface Water Strategy_Rev 1.docx 25 November 2016 Revision 1 Page i

Contents Executive Summary 1

1 Introduction 2

1.1 Project Description 2

1.2 Study Scope 4

1.3 Study Objectives 4

2 Catchment Description 6

2.1 McCarrs Creek Catchment 6

2.2 Narrabeen Creek Catchment 6

3 Data Collection 7

3.1 Imagery 7

3.2 Topographical Data 7

3.3 Field Survey Data 7

3.4 Hydrological Models 7

3.5 Hydraulic Models 7

4 Hydrology 8

4.1 Review of Existing Studies 8

4.2 XP RAFTS Model Development 8

5 Drainage Strategy 10

5.1 Strategy Background 10

5.2 Harvey Road Extension 10

6 Hydraulic Modelling 11

6.1 Modelling Overview 11

6.2 Pre Upgrade Model Development 11

6.3 Assessment of Impacts 14

7 Water Quality 20

7.1 Operational Phase Water Quality 20

7.2 Construction Phase Water Quality 27

7.3 Design Criteria 27

7.4 Design Principles 28

7.5 RUSLE Rule Assessment 28

7.6 Summary of Results 31

8 Conclusion 32

Project 250171 File 250171-WEST-RPT-HY-0001 MVR West Surface Water Strategy_Rev 1.docx 25 November 2016 Revision 1 Page ii

Appendices Appendix A

Pre and Post Upgrade Catchment Plans

Appendix B

Rainfall IFD Data

Appendix C

Pavement Drainage Strategy and Reporting Locations

Appendix D

Hydraulic Model Schematization and Hydraulic Roughness Figure

Appendix E

Afflux Figures

Appendix F

Peak Flood Levels at Reporting Points

Appendix G

MUSIC Model Results

Appendix H

RUSLE Rule Calculations

Appendix I

Frog Assessment Hydrographs

Figures

Figure 1-1 Mona Vale Road West Upgrade 3 Figure 2-1 Study Area Catchment Plan 6 Figure 6-1 Pavement drainage flow application schematisation 13 Figure 7-1 Sydney Soil Landscape Map excerpt 29

Tables

Table 4-1: Comparison of the XP Rafts Results - Original vs Refined 9 Table 5-1: Harvey Road extension transverse drainage culvert configurations 10 Table 6-1: Manning’s n roughness coefficients for hydraulic modelling 12 Table 6-2: Peak Flow Impact Assessment 15 Table 6-3 Pre and post development flows (XP-RAFTS) at the affected frog habitats 18 Table 7-1: Sydney Monthly Evapotranspiration 20 Table 7-2: Catchment Land Use 21 Table 7-3: MUSIC Model Pollutant Signatures 21 Table 7-4: Pre Upgrade Annual Pollutant Loads 22 Table 7-5: Post Upgrade Annual Pollutant Loads (without treatment) 22 Table 7-6: Pre and Post Upgrade Annual Pollutant Loads discharging to Narrabeen Lagoon

(without treatment) 23

Project 250171 File 250171-WEST-RPT-HY-0001 MVR West Surface Water Strategy_Rev 1.docx 25 November 2016 Revision 1 Page iii

Table 7-7: Pre and Post Upgrade Annual Pollutant Loads discharging to McCarrs Creek

Catchment (without treatment) 24 Table 7-8: Pre and Post Upgrade Annual Pollutants Loads discharged to the Frog habitats

(without treatment) 24 Table 7-9: Pre and Post Upgrade Annual Pollutants Loads discharged to the Frog habitats

(with treatment) 25 Table 7-10: Pre and Post Upgrade Annual Pollutant Loads Narrabeen Lagoon - Mitigated 26 Table 7-11: Pre and Post Upgrade Annual Pollutant Loads McCarrs Creek - Mitigated 27 Table 7-12 Summary of RULSE Rule Calculations 31

Project 250171 File 250171-WEST-RPT-HY-0001 MVR West Surface Water Strategy_Rev 1.docx 25 November 2016 Revision 1 Page 1

Aurecon have undertaken this investigation to assess the requirements for stormwater runoff associated with the upgrade of Mona Vale Road West.

This report has been prepared based on the development of the concept design and expanded to consider the impact of the road upgrade on regional flooding.

This report will supersede the previous study ( Hydraulic Assessment Report-Final, Mona Vale Road Upgrade-Drainage and Utility Investigations and Concept Design prepared by Cardno dated October 2014) as assumptions made in that study and the scope of the assessment did not adequately assess the impact of the road upgrade on regional flooding, and downstream water quality.

The current study has been expanded to include an:

Assessment of the impact of additional road catchment on regional flooding.

Assessment of the impacts of proposed design upgrade pavement drainage on regional flooding.

Assessment of the road upgrade on downstream water quality during the operational andconstruction phase.

Each section of this report is summarised below for reference:

Section 1 contains a general description of the proposed upgrade.

Section 2 provides a description of the existing catchment characteristics for the upstream anddownstream catchment of the project area.

Section 3 outlines the data collected and adopted for this assessment.

Section 4 details the development of the hydrology model

Section 5 outlines the pavement drainage strategy developed to mitigate the impacts of peak flowchanges on sensitive receivers.

Section 6 describes the hydraulic model development used to quantify the impact of the change inpeak flows on flood levels.

Section 7 reviews the operational and construction phase water quality impacts and the mitigationmeasures that could be incorporated into the detailed design to mitigate the impacts of the upgrade.

Executive Summary


1.1 Project Description Mona Vale Road is the main east-west link between the Pacific Highway, Pymble and Pittwater Road at Mona Vale totalling about 20 kilometres in length and carrying about 36,900 vehicles per day in both directions. Roads and Maritime Services (Roads and Maritime) proposes to upgrade and widen about 3.4 kilometres of Mona Vale Road between McCarrs Creek Road, Terrey Hills and Powder Works Road, Ingleside, from a two lane (one in each direction) undivided road to a four lane (two lanes in each direction) divided road. The proposal includes the provision of a new local road connection between Bungendore Street and Powder Works Road/Mona Vale Road intersection. It would also involve construction of retaining walls, cuttings, adjustment of underground utilities as well as drainage and landscaping.

The proposal generally comprises:

Widening to provide an additional two traffic lanes (one in each direction) on Mona Vale Roadbetween McCarrs Creek Road and Powder Works Road.

Widening on the southern side of the existing carriageway between McCarrs Creek Road andKimbriki Road.

Deviation of the entire dual carriageway from the current road to the north of a rock outcropbetween Kimbriki Road and Tumburra Street.

Widening on the northern side of the existing carriageway from about 700 metres west of TumburraStreet to Addison Road.

Widening on both sides of the existing carriageway between Addison Road and Powder WorksRoad.

Provision of a new traffic signal intersection at Kimbriki Road including additional dedicated turninglanes and a truck climbing lane.

Restricting traffic movements at the intersection of Mona Vale Road and Tumburra Street to left-inand left-out only.

Deviating the Mona Vale Road and Tumburra Street intersection locally to the west by about 40metres to improve the existing steep grade on Tumburra Street.

Closing the existing intersection at Mona Vale Road and Addison Road to general traffic andmaking future access at this intersection restricted to emergency vehicles only.

Providing a new local road connection between Bungendore Street and Powder Works Roadutilising the existing Harvey Road corridor and extending the new local road east of Addison Roadto meet with the intersection of Mona Vale Road and Powder Works Road.

Removing the existing eastbound bus stop on Mona Vale Road at the intersection with KimbrikiRoad; and bus stops on either side of the intersection at Tumburra Street and re-directing busservices along the new local road connection and Tumburra Street to serve existing and future landuses.

Relocating the existing access to the National Baha’i Centre by about 120 metres west along thenew road.

Providing new and improved fauna connectivity between Mona Vale Road and the new local roadconnection by way of a fauna underpass on the new local road between Bungendore Street andAddison Road.

1 Introduction


Minor widening of Powder Works Road for a distance of about 80 metres east from the intersectionwith Mona Vale Road.

Providing a 40 metre wide fauna bridge over Mona Vale Road, east of Kimbriki Road, linking Ku-ring-gai and Garigal National Parks enabling new and improved fauna connectivity between theNational Parks by way of unimpeded passage.

Constructing retaining walls and/or sandstone cuttings at various locations along the alignment.

Constructing a shared use path on the northern side of Mona Vale Road for the full upgrade length.

Relocating and and/or adjusting underground and above ground utilities where required includingthe upgrade of street lighting for the full upgrade length.

Upgrading of the existing pavement and cross drainage structures including the construction,reconstruction and extension of pavement drainage lines.

Landscaping over the length of the proposal.

Installing traffic monitoring cameras at all signalised intersections to assist with traffic management.

The eastern portion of the proposal is located within the former Pittwater Council Local Government Area (LGA) and the western portion is located in the former Warringah Council LGA. Following the LGA amalgamation that occurred in early 2016 the entirety of the proposal area is now located within the Northern Beaches LGA and is within the Roads and Maritime Sydney Region. It would be funded by the NSW Government. Construction is expected to take about 30 months to complete, weather permitting. At the time of preparation of this report, funding for construction of the project had yet to be confirmed and identification of a specific commencement date is therefore not possible. However, construction is expected to follow that of the Mona Vale Road East Upgrade.

Figure 1-1 Mona Vale Road West Upgrade

The purpose of the current study is to:

Identify the existing flood regime within the study area.

Assess impacts of the road upgrade on downstream flooding.

Identify mitigation measures to offset the potential impact of the road upgrade on downstream floodpatterns.


Assess the impact of the road upgrade on operational water quality.

Assess the impact of the road upgrade on construction phase water quality and identify the erosionand sediment control strategy to mitigate this impact.

1.2 Study Scope

The scope of this study is to:

Review regional flood studies to identify an acceptable hydrology loss model consistent with thecurrent regional approach.

Develop a hydrology model to assess the impact that the changed road catchment will have on thepeak flow rates downstream of Mona Vale Road.

Prepare a hydraulic model to assess the impact of the changed flow conditions on downstreamflood levels when there is an increase in flow.

Develop a pavement drainage strategy.

Provide mitigation measures to mitigate the impacts of the road upgrade on downstream receivingenvirons.

Assess the impact of the road upgrade on downstream pollutant of concern.

Assess the requirements for temporary sediment basins during construction.

1.3 Study Objectives

The proposed road upgrade involves major changes to the road alignment and geometry. Changes to the catchment characteristics (fraction impervious for example) and flow regimes are expected as a result of the proposed road upgrade.

This surface water impact assessment has been undertaken with an overview to minimise the impacts on the downstream receiving water ways, however some impacts are anticipated as a result of the changes to the catchment characteristics and changes to the flow regime, and the limited space available within the project area to provide mitigation measures.

This study identifies and quantifies the impacts and recommend mitigation measures that will be incorporated in the detailed design to mitigate some of the impacts associated with the road upgrade.

Key Study objectives are identified and discussed below.

1.3.1 Operational Flood Immunity The design services brief requires that the proposed upgrade is to be designed and constructed to provide a flood immunity of 100 year ARI design event.

1.3.2 Hydraulic Impacts The proposed upgrade is expected to result in a change to catchment characteristics along the proposed upgrade. Some of the more significant of these changes to the catchment characteristics are outlined below:

Increase in the fraction impervious as a result of the proposed upgrade. This is expected to result inchanges in peak discharges as well as the flow regimes and consequently result in potentialadverse impacts upstream and downstream of the proposed road upgrade.


The proposed design road grade line is generally higher than the existing road grade-line to achievea higher flood immunity.

The current study has assessed the impacts of the road upgrade on downstream flood behaviour to the afflux criteria provided in the design services brief:

500 mm for open space, recreational rural land without buildings or sensitive structures

150 mm for urban, commercial or industrial land where buildings or sensitive structures are notinundated at the design flood levels.

50 mm for rural residential land where buildings or sensitive structure are inundated.

0 mm for urban residential, commercial or industrial land where buildings or sensitive structures areinundated.


This section of the report provides a description of the catchment which the Mona Vale Road West (MVRW) upgrade will ultimately discharge to. MVRW sits along the ridge line that bisects the two catchments of:

Deep Creek catchment to the south, which flows to Narrabeen Lagoon Catchment.

McCarrs Creek catchment to the north.

Refer to Figure 2-1 showing Mona Vale Road West (MVRW), McCarrs Creek catchment and Deep Creek catchment.

2.1 McCarrs Creek Catchment The MVRW predominantly drains north into the McCarrs Creek catchment, which ultimately discharges to Tasman Sea. Garigal National Parks covers most of the McCarrs Creek catchment.

2.2 Narrabeen Creek Catchment Part of the MVRW discharges to the south into Deep Creek catchment and ultimately to The Narrabeen Lagoon. Garigal National Park covers most of the Deep Creek catchment.

Figure 2-1 Study Area Catchment Plan

2 Catchment Description


3.1 Imagery Aerial imagery has been provided by RMS covering the proposed upgrade section of the Mona Vale Road and areas immediately upstream and downstream.

3.2 Topographical Data Topographical data was available in the form of LIDAR survey taken in 2011. The LIDAR data in the format of Digital Elevation Model (DEM) with a grid spacing of 1m was used for catchment delineation and development of the TUFLOW model.

3.3 Field Survey Data The field survey of the existing road was provided by RMS. This data was post-processed by creating a Triangular Irregular Network (TIN), which is a mass of interconnected triangles whose vertices correspond to the surveyed data points. The TIN was exported in the 12da format and was incorporated in the TUFLOW model for further refinement of the hydraulic model along the existing road.

3.4 Hydrological Models An XP-RAFTS model from the Narrabeen Lagoon Flood Study (2013) was supplied by Pittwater Council (Council) to assess the catchment runoff for application to the hydraulic model. The model was reviewed and refined for use in this assessment.

It is noted that Pittwater Council and Warringah Council have merged to form Northern Beaches Council on 12 May 2016. However, respective Local Environmental Plans and Strategic Planning Policies for former local government areas are still in effect.

3.5 Hydraulic Models Council’s TUFLOW model from the Narrabeen Lagoon Flood Study (2013) has been supplied for the analysis of the pre and post upgrade flood impacts. The supplied TUFLOW model was reviewed and refined as required for assessment on Mona Vale Road East. A separate TUFLOW model has been developed for Mona Vale Road West. However, the Mona Vale Road East model has been used as a guidance for setting up the general modelling structure and the hydraulic modelling parameters.

3 Data Collection


As noted in Section 2, the MVRW Project will discharge through two separate catchments, a review of Northern Beaches Council (formerly Pittwater and Warringah Council) database has identified that both catchments have a regional flood study developed in either a draft or final state.

At a meeting with Northern Beaches Council (Pittwater Council at the time) on the 26th April 2016 it was noted that as the McCarrs Creek Flood Study was still in a Draft format and not yet formally accepted by Council. The detailed hydrology and hydraulic models used in the development of the McCarrs Creek Flood Study was thus not made available to the project team.

Pittwater Council was able to provide the project team with the detailed XP-RAFTS hydrology model and TUFLOW hydraulic model used to inform the Narrabeen Lagoon Flood Study.

Aurecon have undertaken a review of the Narrabeen Lagoon Flood models to determine its suitability for adoption in informing this study.

4.1 Review of Existing Studies Narrabeen Lagoon Flood Study, BMT WBM, September 2013

Review of the Narrabeen Lagoon Flood study indicates that the XP-Rafts hydrology model developed was calibrated to the April 1998 historical event with validation to the March 2011 rainfall event. The calibration was reviewed and found to be reasonable to adopt for this flood impact assessment.

4.2 XP RAFTS Model Development Hydrology modelling has been developed in XP-RAFTS. XP-RAFTS is a non-linear runoff routing program which involves division of the catchment into sub-catchments, derivation of various physical properties of the sub catchments and assembly of the sub catchments by way of a nodal network. Hydrographs for design events were produced by routing excess design rainfall through a series of sub catchments and along channel links to enable flow determination.

4.2.1 Catchment Areas Catchment areas were delineated using a combination of detailed field survey along the Mona Vale Road corridor and Aerial Laser Survey (ALS) data obtained from the RMS, refer Figure A-1 (Appendix A) for a catchment layout and a detailed breakdown of each sub-catchment characteristics.

The entire catchment to the Narrabeen Lagoon was incorporated into the model. Sub-catchments were based on Council’s Narrabeen Lagoon XP-RAFTS model (2013).

4.2.2 Rainfall Data Rainfall intensities were adopted from the Narrabeen Lagoon Flood Study model. The IFD data was checked against that obtained from the Bureau of Meteorology website, which has been adopted for the pavement drainage design. A comparison showed a good correlation of data, therefore the data obtained from the council model is considered appropriate for this study, refer to Appendix B for both sets of data.

4.2.3 Catchment Characteristics Impervious areas applied to the XP Rafts model are based on a review of Pittwater Councils LEP and the fraction imperviousness applied to each land use consistent with those adopted in the Narrabeen

4 Hydrology


Lagoon Flood Study (2013). The following fraction imperviousness values were adopted for each land use:

National Park: 0%

Rural Urban: 30%

Road: 100%

4.2.4 Rainfall Loss Model The Initial Loss and Continuing Loss (IL/CL) values have been adopted as per Council’s Narrabeen Lagoon Flood Study. The loss model adopted is as follows:

Pervious Catchments:

Initial Loss: 10mm

Continuing Loss: 2.5mm/hr

Impervious catchments:

Initial loss: 2mm

Continuing Loss: 0mm/hr.

4.2.5 Model Validation Results from the MVRW XP-Rafts model, for 100 Year ARI design event, were compared against Council’s Narrabeen Lagoon model (2013) at three locations. The selected reporting locations vary from immediately downstream of the study area to the catchment outlet at Narrabeen Lagoon. Table 4-1 shows the comparison between peak flow results. The results show that the refined model has marginally lower peak flows with a difference of 3% to 8%. This is likely due to the refinement of the model where additional routing has been incorporated through the area of interest. The observed differences are minor with the MVRW model performing similarly to Council’s model and is therefore considered suitable for this assessment.

Table 4-1: Comparison of the XP Rafts Results - Original vs Refined

XP-Rafts Node Location Council Model-

Peak Discharges (m³/s)

Refined MVR Model-Peak

Discharges (m³/s) Difference

NL35 Downstream of

MVRW 124.85 114.35 -8%

NL48 Midway between

MVRW and Narrabeen Lagoon

149.46 141.78 -5%

Node1 Narrabeen Lagoon 161.5 156.11 -3%


As part of this investigation a pavement drainage strategy has been developed to mitigate the impacts of the road upgrade on peak flowrates within the downstream drainage systems.

Figure C-2 attached in Appendix C illustrates the recommended pavement drainage discharge points to be adopted in the detailed design to limit the impacts of the road upgrade on downstream drainage lines. The flood impacts of this strategy are investigated in the hydraulic model developed for this assessment. The development of the hydraulic model is discussed in detail in Section 6.

5.1 Strategy Background A key consideration in the development of the pavement strategy was to limit flood impacts on downstream drainage systems to the limits identified in Section 1.3.2

This drainage system has been developed around the following key principles:

Limit pavement drainage outlets into drainage systems that run through the rural residential areaaround Wirreanda Road and Tumburra Ave.

Divert pavement drainage discharge to locations within the Garigal and Ku-Ring-Gai National Parkswhere flow changes can be more readily accommodated.

Maintain existing flow regimes where possible around sensitive receivers such as:

Red Crown Toadlet and Giant Burrowing Frog habitats downstream of MVRW upgrade, refer toSection 0 for locations.

Onion Orchards North of the intersection of Kimbriki Road.

5.2 Harvey Road Extension As part of the MVRW upgrade, Harvey Road is proposed to be extended between the existing Harvey Road to the intersection of MVRW and Powder Works Road. The general layout of the Harvey Road extension is illustrated in the drainage strategy included in Appendix C.

The design of the Harvey Road extension and the associated transverse drainage has been undertaken with an overview to limit changes to the existing flow regime and minimise flood impacts. Two transverse drainage culverts are proposed through this section of road. The locations are shown in the drainage strategy figure provided in Appendix C.

Table 5-1 summarises the post upgrade culvert configurations for the Harvey Road extension and the associated hydrologic peak flows.

Table 5-1: Harvey Road extension transverse drainage culvert configurations

Culvert Descriptors 100year ARI Peak Flow

(m³/s) Configuration Comments

C125 4.4 2700x1200mm RCBC Headwall inlet

C640 1.0 2x1050mm RCP Inlet pit at upstream

5 Drainage Strategy


An assessment of the pre and post upgrade flooding at a number of locations to the south and north of the MVRW has been undertaken to identify the impact the project may have on the pre upgrade flood behaviour.

6.1 Modelling Overview Pre-upgrade and post-upgrade two-dimensional hydraulic TUFLOW models were developed for the hydraulic impact assessment for the MVRW.

The hydraulic model extent was defined based on the extent of the MVRW upgrade and the sensitive areas where the impacts were to be assessed.

The hydraulic parameters adopted were consistent with the TUFLOW model developed for Mona Vale Road East.

The TUFLOW model for Mona Vale Road East was developed based on the TUFLOW model prepared for Pittwater Council to inform the Narrabeen Lagoon Flood Study. This model has undergone calibration and validation to historical rainfall events.

The following sections further discuss the hydraulic model development.

6.2 Pre Upgrade Model Development A pre-upgrade TUFLOW model has been developed based on the existing MVRW conditions and to an extent where all the sensitive areas are included for the assessment of impacts.

No data was available on the existing longitudinal pavement drainage at the time of this study. Therefore, no pipe network was incorporated to the hydraulic model. The hydraulic model was developed as a two-dimensional model with no one-dimensional elements.

Accordingly, the discharges from the pavement drainage were modelled as 2d_SA (source area) inflows where the inflow hydrographs from the XP-RAFTS were applied to the hydraulic model as source points at the estimated discharge locations.

Information available on the existing pavement drainage was inadequate to identify the discharge outlet locations for the pre-upgrade scenario. The discharge points were identified for the purpose of this study using the road survey contours, LiDAR, and site inspections to identify high and low points along the existing road.

The methodology therefore, assumes that runoff from the pavement will sheet flow on the surface of the road to the low points and then will discharge at certain outlet points. No sheet flows off the shoulder has been allowed in this methodology.

6.2.1 Grid Size and timestep A grid size and timestep of 5 metres and 1 second were adopted respectively for this study. This is similar if not slightly higher resolution to the original Council study which adopted a grid size and timestep of 6m and 2 seconds respectively.

6.2.2 Topography The representation of the topography within the 2D domain of the model is based on a combination of LiDAR data and pre-upgrade field survey. For the representation of post-upgrade conditions, the road

6 Hydraulic Modelling


design TIN used for the pavement drainage design, was built into the model. This ensured consistency in the model representation of the post upgrade design.

6.2.3 Roughness factors The discretisation of roughness (Manning’s n value) was based on those adopted for the Narrabeen Lagoon Flood Study as the current study area covers a similar country in terms of the geography terrain, vegetation density and the developments characteristics. These values are generally within the typical range of Manning’s n values.

The adopted roughness values for each material type are shown in Table 6-1.

Table 6-1: Manning’s n roughness coefficients for hydraulic modelling

Material Type Manning’s n coefficient

National Park 0.1

Road 0.02

Rural Residential 0.07

A copy of the hydraulic roughness figure is presented as Figure D-1 in Appendix D.

6.2.4 Boundary Conditions The model was run with time varying flows. The inflows to the model were based on the sub-catchment hydrographs extracted from the pre- and post-upgrade XP-RAFTS model (Section 4.2) and applied to the pre- and post-upgrade hydraulic model respectively. The local hydrograph for each sub-catchment was applied to the TUFLOW model.

The outlet to the hydraulic model adopted a stage-discharge (HQ) outlet boundary condition using a ground gradient. This was selected based on the existing channel/topographic profiles along the main drainage line. The boundaries were also located far enough downstream so as not to affect water-levels in the project focus area.

A copy of the hydraulic model schematization including the outflow boundary conditions is provided as Figure D-2 in Appendix D.

6.2.5 Drainage Structures

6.2.5.1 MVRW upgrade

There are no cross drainage structures under the existing MVRW upgrade, as the road generally runs along a ridge.

As such, pavement drainage catchment hydrographs were generally directly applied to pavement low points or pavement discharge locations. An example of this is shown in . This methodology was also adopted for the post-upgrade pavement drainage points of discharge and is considered as a conservative modelling approach for smaller flood events.


Figure 6-1 Pavement drainage flow application schematisation

The exceptions to the above are noted in two instances:

Discharge locations P3-1 and P8-1 show an increases in flow immediately downstream of theMVRW upgrade and potential to impact adjacent properties. Therefore the discharges from theselocations, up to the 100 year ARI, have been designed to be piped downstream to a main drainageline (refer Figure C2, Appendix C). The proposed pipes and associated inlets were not modelled inthe hydraulic model at this stage. Pavement drainage catchment hydrographs were directly appliedto the proposed outlet locations. Pipe sizes were estimated using simplified calculations assumingfull pipe flow and acceptable pipe velocities. Approximately 2/1050mm RCPs will be required at P3-1 discharge location and 2/1200mm RCPs at P8-1 discharge location to convey the 100 year ARIpeak flow.

There are two existing outlets south of the intersection with Powder Works Road. The outlets havebeen hydraulically modelled and the associated catchments attributed at the pit inlet. Thismethodology has been adopted as not to overestimate the water discharging from the system forthe existing scenario.

For the proposed case, the southern outlet, discharge point P16, has been increased from a300mm to a 600mm pipe to alleviate impacts to the west of MVRW (refer Figure C2, Appendix C).The northern of the two outlets, located immediately south of the MVRW intersection with PowderWorks Road, associated catchment hydrograph has been applied directly to reporting point P12.This has been completed to simulate a formalisation of the downstream conveyance to bypass theadjacent property. This is anticipated to be either by a piped system or surface drain to the toe ofthe proposed embankment.


6.2.5.2 Harvey Road Extension

Pavement drainage catchment hydrographs were directly applied to pavement low points or pavement discharge locations downstream of the proposed culverts, in the same manner as shown above. The two culverts, C125 and C640, provided beneath Harvey Road extension have been incorporated into the hydraulic model. Refer to Table 5-1 above for details.

The exception to the above is for catchment MVRD24 which will be piped to outlet P10 to offset localised impacts (refer Figure C2, Appendix C). The proposed pipe and associated inlet were not modelled in the hydraulic model at this stage. The catchment hydrograph was directly applied to the proposed outlet location P10. Pipe sizes were estimated using simplified calculations assuming full pipe flow and acceptable pipe velocities. Approximately a 600mm RCP will be required.

6.3 Assessment of Impacts Flood levels downstream of the project area have the potential to be impacted by the following elements: Increase in post upgrade peak flowrates as a result of pavement drainage system upgrades

Changes to the drainage regime as a result of the upgraded pavement drainage (surface andunderground) and different discharge outlet locations.

Figures showing the change in flood levels for the 2, 10, 20, 50 and 100 Year ARI design events have been prepared and provided in Appendix E. These figures illustrate the impact that the road upgrade will have on the flood extents and peak flood levels for a range of design events.

The changes in peak flow which will result from the upgrade of MVRW are summarised in Table 6-2 at each outlet and various locations downstream as identified in Figure C-1 in Appendix C.


Table 6-2: Peak Flow Impact Assessment

Location Peak Flow (m3/s)

2 Year ARI 10 Year ARI 20 Year ARI 50 Year ARI 100 Year ARI

Pre-Upgrade

Post Upgrade

Change Pre-Upgrade

Post Upgrade

Change Pre-Upgrade

Post Upgrade

Change Pre-Upgrade

Post Upgrade

Change Pre-Upgrade

Post Upgrade

Change

P1 1.96 1.97 0.01 3.62 3.90 0.27 4.38 4.86 0.48 5.31 5.85 0.55 6.16 6.76 0.60

P2 0.13 0.26 0.13 0.20 0.37 0.17 0.23 0.43 0.20 0.25 0.48 0.23 0.28 0.54 0.25

P3 0.23 0.49 0.26 0.34 0.73 0.39 0.39 0.84 0.44 0.44 0.92 0.48 0.48 1.03 0.54

P3-1 9.39 8.87 -0.52 16.73 15.49 -1.24 21.05 18.69 -2.36 23.89 21.78 -2.11 28.47 24.95 -3.52

P3-1-1 11.69 12.05 0.36 21.21 26.82 5.61 28.96 32.92 3.96 37.46 40.16 2.71 43.18 46.52 3.33

P4 2.96 3.04 0.08 5.63 5.81 0.18 7.12 7.36 0.24 8.64 8.91 0.27 10.05 10.35 0.30

P4-1 3.01 3.13 0.12 5.64 5.74 0.10 7.08 7.29 0.22 8.54 8.79 0.25 9.87 10.16 0.29

P4-1-1 2.96 3.13 0.17 5.60 5.77 0.17 7.06 7.30 0.24 8.56 8.83 0.27 9.93 10.23 0.30

P5 0.28 0.00 -0.28 0.36 0.00 -0.36 0.40 0.00 -0.40 0.44 0.00 -0.44 0.50 0.00 -0.50

P6 6.52 6.41 -0.11 12.33 12.15 -0.18 15.23 14.99 -0.24 19.00 18.61 -0.38 22.18 21.74 -0.44

P6-1 6.54 6.42 -0.12 12.25 12.02 -0.23 15.17 14.89 -0.28 18.96 18.52 -0.43 22.06 21.66 -0.40

P6-1-1 6.50 6.37 -0.13 12.43 12.08 -0.34 15.09 14.99 -0.10 18.92 18.46 -0.46 22.11 21.72 -0.39

P7 1.59 1.19 -0.40 2.57 1.98 -0.59 3.12 2.50 -0.62 3.53 2.98 -0.55 4.03 3.46 -0.57

P8 2.66 2.31 -0.35 5.76 5.42 -0.34 7.55 7.01 -0.54 9.09 8.75 -0.34 10.38 9.95 -0.43

P8-1 2.33 3.64 1.30 6.05 7.69 1.64 8.00 9.34 1.34 9.64 11.58 1.94 10.45 13.72 3.27

P9 2.33 3.50 1.17 5.80 7.53 1.73 8.36 8.09 -0.27 10.48 9.20 -1.28 12.40 11.98 -0.42

P9-1 2.53 4.06 1.53 7.31 9.03 1.72 10.39 11.91 1.52 11.74 14.61 2.87 16.32 16.73 0.41

P9-1-1 2.07 4.13 2.06 6.72 10.18 3.46 9.71 12.14 2.43 13.51 16.33 2.82 14.86 18.61 3.75

P10 0.56 0.50 -0.06 1.15 0.98 -0.17 1.40 1.17 -0.23 1.61 1.35 -0.27 1.84 1.55 -0.29

P13 0.05 0.15 0.10 0.05 0.27 0.22 0.07 0.31 0.23 0.10 0.03 -0.07 0.13 0.36 0.23

P16 0.06 0.23 0.16 0.10 0.35 0.25 0.12 0.40 0.28 0.13 0.44 0.31 0.14 0.49 0.35


6.3.1 Discussion of the Impacts

6.3.1.1 Impacts on properties

The flood impact figures indicate that changes to the flow regimes are anticipated as a result of the proposed post upgrade MVRW compared to the pre upgrade MVRW. However, impacts on the properties to the north of MVRW are consistent with the afflux criteria outlined in the design services brief, and summarised in Section 1.3, with impacts on these properties less than 50mm. This is largely due to the proposed pavement drainage strategy, which has been prepared based on an objective of maintaining current flow distribution between the catchments north and south of MVRW.

Flood impact results are presented in Appendix E and show changes in the peak flood levels as a result of the proposed post upgrade compared to the pre upgrade scenario for the 2, 10, 20, 50 and 100 year ARI design events.

A summary of the peak flood levels for the pre upgrade and post upgrade scenarios and changes in the peak flood levels for 2, 10, 20, 50 and 100 year ARI design event are shown in Appendix F.

There are islolated impacts at outlet lcoations and at modle boudaries.These are artifual and a by product of the model behavior.

6.3.1.2 Impacts on Narrabeen Lagoon

The catchment area south of MVRW is National Park all the way down to Narrabeen Lagoon. Negligible increases in peak flows were observed south of MVRW. This is a direct result of the proposed drainage strategy pavement drainage strategy that maintains current flow distribution between the catchments north and south of MVRW. As such, there is no discernible impact on flooding to the southern side into Deep Creek, Narrabeen Creek and ultimately Narrabeen Lagoon.

6.3.1.3 Receiving Waterway Impacts

Flood level increases are observed along Wirreanda Creek in the order of 30mm to 50mm. This is a result of the piped pavement drainage flows from pavement outlets P3-1 and P8-1 (refer to Figure C2). The increase is less than the 500mm afflux limit specified in the design services brief and considered negligible.

6.3.2 Impacts on Sensitive Ecological Areas An assessment of the peak flows has indicated that the flows at Red Crowned Toadlet (RBT) and Giant Burrowing Frog (GBR) habitats in the vicinity of the study area will be changed due to the MVRW upgrade. The increase in the pavement area for the road upgrade will result an increase of peak flows at the new discharging points. As there is no kerb and piped drainage system throughout the majority of MVRW, the current pavement runoff generally sheet flows to natural watercourses.

A mitigation measure should be provided if the post development flows differ from the pre development flows by ±15%. The duration of inundation for the post development should be

The assessment shows that the impact at the following locations is greater or less than 15%.

FH2150.1

FH2150.2

FH2050.1

FH2050.2


FH2520.1

FH2520.2.

In order to minimise the impacts on frog habitats including peak flowrates and the duration of inundation, the following mitigation measures will need to be considered.

Bifurcation pit – to match pre-development condition peak flows to the frog habitats.

Multiple outlets to match the pre development condition catchment sizes.

The assessment demonstrates that the mitigation measures will offset the potential impacts by diverting the surface runoff to match the pre development flows. The hydrographs show that the duration of inundation for the post development condition for FH2150 is similar to the pre development as the amount of the existing pavement runoff has been maintained for the pre development condition.

The RCT/GBR habitats with relatively small catchments (FH2050.1 & 2 and FH2520. 1 & 2) where the existing road slopes away will have marginally longer duration of inundation due to new pavement outlets.

Frog habitats are mapped in Appendix A3 and A4 , the changes in the peak flows are tabulated in Table 5 and the hydrographs with mitigation measures are included in Appendix I.


Table 6-3 Pre and post development flows (XP-RAFTS) at the affected frog habitats

Location ID

Pre Post Post

Mitigation Change

(m3/s)

Change

(%) Mitigation Measure

Peak Q2

Flow (m3/s)

Peak Q2

Flow (m3/s)

Peak Q2

Flow (m3/s)

FH365.1 0.722 0.725 N/A 0.003 0.42%

FH365.2 5.804 5.724 N/A -0.08 -1.38%

FH830.1 0.038 0.038 N/A 0 0.00%

FH830.2 1.874 1.868 N/A -0.006 -0.32%

P3 0.325 0.327 N/A 0.002

FH1100.1 0.699 0.754 N/A 0.055 7.87%

FH1100.2 2.13 2.193 N/A 0.063 2.96%

P4 0 0.183 N/A 0.183

FH2150.1 0.797 0.7 0.792 -0.005 -0.63%

FH2150.2 3.154 2.85 3.121 -0.033 -1.05% Add pavement drainage outlets to match the peak flow.

FH2150B.1 0.777 0.54 0.761 -0.016 -2.06%

FH2050.1 0.098 0.066 0.097 -0.001 -1.02%Add pavement drainage outlets to match the peak flow.

FH2050.2 0.245 0.186 0.241 -0.004 -1.63%

FH2520.1 0.071 0.026 0.068 -0.003 -4.23%Add pavement drainage outlets to match the peak flow.

FH2520.2 0.193 0.135 0.197 0.004 2.07%

FH2500.1 0.217 0.217 N/A 0 0.00%

FH2500.2 2.684 2.669 N/A -0.015 -0.56%


Location ID

Pre Post Post

Mitigation Change

(m3/s)

Change

(%) Mitigation Measure

Peak Q2

Flow (m3/s)

Peak Q2

Flow (m3/s)

Peak Q2

Flow (m3/s)

FH2800.1 0.742 0.699 N/A -0.043 -5.80%

FH2800.2 2.521 2.465 N/A -0.056 -2.22%


This section details the strategy developed to mitigate the potential increase in downstream pollutant loads as a result of the road upgrade. This is based on the latest drainage design strategy and includes options to mitigate both the operation and construction phase water quality.

It is noted that from an operational point of view, MVRW will move from a diffuse pattern of water quality impacts to point location impacts.

7.1 Operational Phase Water Quality

Aurecon has undertaken an assessment of the impact of the proposed upgrade on the downstream water quality. This assessment takes into account the flow diversions proposed in section 6.3 to limit the impacts to the sensitive frog habitats.

The assessment has been undertaken using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC version 6), and has applied the total catchment draining to the outlets nominated as P3-1-1, P4-1-1, P6-1-1, P7 and P10 to P14 in the pavement drainage strategy.

The MUSIC model uses climate data and information about the pollutant profiles of the catchment or site of interest to create a continuous simulation model, to simulate the behaviour of stormwater runoff in catchments and quantify the pollutant loads found in the runoff.

MUSIC is a recognised tool for demonstrating the impact of changes in the catchment characteristics on the pollutant loads delivered to the downstream receiving environment. It must be noted that MUSIC is a conceptual model tool and the findings from this assessment will need to be revisited following any significant changes to the design.

MUSIC is also capable of assessing the performance of stormwater quality treatment systems within the catchment. The information presented in Appendix G and below outlines the method and assumptions made in estimating pollutant loads from the proposed MVRW upgrade for both the Pre and Post Upgrade conditions.

7.1.1 Model Inputs

The following section describes the model inputs adopted for the development of the MUSIC model.

7.1.1.1 Climate Data

Climate data ‘DUFFYS FOREST (NAMBA RD) plv066142’ used in the simulation has been obtained from the eWater Pluviograph rainfall data website. The Sydney mean monthly evapotranspiration values were adopted and are shown in.

Table 7-1: Sydney Monthly Evapotranspiration

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Evapotranspiration (mm) 180 135 128 85 58 43 43 58 88 127 152 163

7 Water Quality


7.1.1.2 Catchment Land use

Catchments within the study area have been reviewed and the land uses identified within the study are summarised in Table 7-2.

Table 7-2: Catchment Land Use

Catchment Land Use Impervious Area

Native 0%

Rural and Rural Residential 5%

Residential 55%

Special Purpose - Commercial Varies

Roads 100%

The pollutant concentrations for each land use have been adopted from the Draft NSW MUSIC Modelling Guidelines (August 2010) and are summarised in Table 7-3.

Table 7-3: MUSIC Model Pollutant Signatures

Catchment Land Use Base Flow Concentration

(mg/L-log10)

Storm Flow Concentration

(mg/L-log10)

Mean Std. Dev Mean Std. Dev

Total Suspended Solids (TSS)

Native 0.78 0.13 1.60 0.20

Rural and Rural Residential 1.15 0.17 1.95 0.32

Residential 1.20 0.17 2.15 0.32

Commercial 1.20 0.17 2.15 0.32

Roads 1.20 0.17 2.43 0.32

Total Phosphorous (TP)

Native -1.52 0.13 -1.10 0.22

Rural and Rural Residential -1.22 0.19 -0.66 0.25

Residential -0.85 0.19 -0.60 0.25

Commercial -0.85 0.19 -0.60 0.25

Roads -0.85 0.19 -0.30 0.25

Total Nitrogen (TN)

Native -0.52 0.13 -0.05 0.24

Rural and Rural Residential -0.05 0.12 0.30 0.19

Residential 0.11 0.12 0.30 0.19

Commercial 0.11 0.12 0.30 0.19

Roads 0.11 0.12 0.34 0.19

7.1.2 Pre Upgrade Pollutant Loads

Catchments were assessed for impact at locations P3-1-1, P4-1-1, P6-1-1, P7 and P10 to P14 as nominated in the pavement discharge point Figure C.2 in Appendix C.

A summary of the sub catchment land use details is summarised in Appendix G. The simulation noted the annual pollutant loading at each of the reported locations with results summarised in Table 7-4 .


Table 7-4: Pre Upgrade Annual Pollutant Loads

Reported Location Pollutant Load (kg/yr)

TSS TP TN

P10 3980.0 7.6 43.2

P4-1-1 2510.0 7.0 61.0

P6-1-1 6100.0 16.5 127.0

P12 174.0 0.8 8.2

P13 4460.0 10.4 86.5

P7-1 8050.0 15.9 104.0

P3-1-2 33200.0 79.4 668.0

P14 1810.0 5.6 57.2

P11 2890.0 10.7 89.0

7.1.3 Post Upgrade pollutant loads

Catchments were assessed for impact at locations P3-1-1, P4-1-1, P6-1-1, P7 and P10 to P14 as nominated in the pavement discharge point Figure C.2 attached in Appendix C. Where bifurcation pits are proposed for flow diversion, an additional node has been added to the MUSIC model to represent the diverted flows.

A summary of the sub catchment land use details is summarised in Appendix G. The simulation noted the annual pollutant loading at each of the reported locations with results summarised in Table 7-5.

Table 7-5: Post Upgrade Annual Pollutant Loads (without treatment)


TSS TP TN

P10 6560.0 11.8 58.5

P4-1-1 5710.0 12.6 85.3

P6-1-1 2410.0 10.7 27.4

P12 174.0 0.8 8.2

P13 5290.0 11.6 91.0

P7-1 7870.0 15.6 102.0

P3-1-2 - Post 57600.0 119.0 827.0

P14 1360.0 4.8 54.5

P11 2720.0 10.5 88.1

7.1.4 Assessment of Impact

The results of the MUSIC modelling predict that there is an increase in the pollutant loads as a result of the upgrade of Mona Vale Road. The increase in the pollutants are due to the increase in the road pavement area and not the increased traffic, as traffic loads do not change from the commencement of the operational phase of the project.

As highlighted in Section 6.3.2, the pollutant loads discharging at Red Crowned Toadlet and Giant Burrowing Frog habitats in the vicinity of the study area will increase due to the MVRW upgrade. In order to minimise the impacts on frog habitats, bifurcation pits are proposed to match pre-development peak flows to the frog habitats.


The results are summarised in Table 7-6 for Narrabeen Lagoon, and Table 7-7 for McCarrs Creek respectively. The impact at the vicinity of the frog habitats are summarised in Table 7-8.

Narrabeen Creek Catchment

The water quality results for Narrabeen Lagoon indicate that there is a marginal increase in the pollutant loads discharging to the creek from the catchments located south. Catchments P10, P4 and P13 report an increase in loads and this corresponds to the increase in pavement area. Further, the presence of bifurcation pits on catchment P6-1-1 results in diversion of the flows above the pre-development flows to the north.

Table 7-6: Pre and Post Upgrade Annual Pollutant Loads discharging to Narrabeen Lagoon (without treatment)

Reported Location Residual Pollutant Load (kg/yr)

TSS TP TN

P10 – Pre 3980.0 7.6 43.2

P10 - Post 6560.0 11.8 58.5

P10 – Change (%) 65% 54% 35%

P11 – Pre 2890.0 10.7 89.0

P11 - Post 2720.0 10.5 88.1

P11 – Change (%) -6% -2% -1%

P4-1-1 – Pre 2510.0 7.0 61.0

P4-1-1 - Post 5710.0 12.6 85.3

P4-1-1 – Change (%) 127% 80% 40%

P6-1-1 – Pre 6100.0 16.5 127.0

P6-1-1 - Post 2410.0 10.7 27.4

P6-1-1 – Change (%) -60% -35% -78%

P12 – Pre 174.0 0.8 8.2

P12 - Post 174.0 0.8 8.2

P12 – Change (%) 0% 0% 0%

P13 – Pre 4460.0 10.4 86.5

P13 - Post 5290.0 11.6 91.0

P13 – Change (%) 19% 12% 5%

TOTAL – Pre 20114.0 53.1 414.9

TOTAL - Post 22864.0 58.0 358.5

TOTAL – Change (%) 14% 9% -14%

McCarrs Creek Catchment

The MUSIC modelling results indicate that there is a net increase in pollutant loads discharging to McCarrs Creek. This is due to the increased flows being diverted from catchment P 6-1-1 bifurcation pits into P 3-1-1. For catchment P3-1-1, there is a net increase in the pollutant loads due to an increase in impervious area within P3-1-1 and additional flows from P6-1-1. It is noted that the discharges from this catchment drain via piped outlets and therefore options to provide water quality mitigation measures at the source is limited.

It is noted catchments P7-1 and P 14 report a decrease in pollutant loads in the post development conditions owing to a decrease in the impervious areas within these catchments.


Table 7-7: Pre and Post Upgrade Annual Pollutant Loads discharging to McCarrs Creek Catchment (without treatment)


TSS TP TN

P7-1 – Pre 8050.0 15.9 104.0

P7-1 - Post 7870.0 15.6 102.0

P7-1 – Change (%) -2% -2% -2%

P3-1-2 – Pre 33200.0 79.4 668.0

P3-1-2 - Post 57600.0 119.0 827.0

P3-1-2 – Change (%) 73% 50% 24%

P14 – Pre 1810.0 5.6 57.2

P14 - Post 1360.0 4.8 54.5

P14 – Change (%) -25% -14% -5%

TOTAL – Pre 43060.0 100.9 829.2

TOTAL - Post 66830.0 139.4 983.5

TOTAL – Change (%) 36% 28% 16%

Table 7-8: Pre and Post Upgrade Annual Pollutants Loads discharged to the Frog habitats (without treatment)

Catchment Habitat Reference Reported Location Pollutant Loads (kg/yr)

TSS TP TN

P14 FH365.1 P14 – Pre 455.0 0.8 3.1

FH365.2 P14 - Post 0.0 0.0 0.0

Change (%) -100% -100% -100%

P3-1-2 FH830.1 P3-1-2 – Pre 5620 9.46 38.6

FH830.2 P3-1-2 - Post 10300 16.8 69.2

Change (%) 83% 78% 79%

P4-1-1 FH1100.1 P4-1-1 – Pre 734.0 1.2 5.0

FH1100.2 P4-1-1 - Post 3680.0 6.2 25.5

Change (%) 401% 411% 411%

P6-1-1 FH2150.1 P6-1-1 – Pre 3350.0 5.7 22.8

FH2150.2 P6-1-1 - Post 3310.0 5.6 22.8

FH2150B.1

Change (%) -1% -2% 0%

P3-1-2 FH2050.1 P3-1-2 – Pre 1910 3.17 12.9

FH2050.2 P3-1-2 - Post 1870 3.14 12.8

Change (%) -2% -1% -1%

P3-1-2 FH2520.1 P3-1-2 – Pre 10400 17.5 71.4

FH2520.2 P3-1-2 - Post 10600 17.6 71

Change (%) 2% 1% -1%

P13 FH2800.1 P13 – Pre 2690.0 4.5 18.3

FH2800.2 P13 - Post 3480.0 5.9 24.1

Change (%) 29% 32% 32%


For catchments, where bifurcation pits have been proposed (P6), the pollutant loads entering the sensitive habitats match the pre-development levels.

MUSIC modelling results report an increase in the pollutants loads discharged at vicinity of the frog habitats due to increased pavement areas, specifically for catchments P3, P4 and P13. Mitigation measures for these catchments are discussed in the following section.

7.1.5 Mitigations Measures

To mitigate the impact of the road upgrade to Narrabeen Lagoon and McCarrs Creek, the design has been developed to include water quality improvement devices such as grass lined swales where possible. The addition of this swale into the design will significantly reduce the TSS and nutrient loading on the downstream catchment.

The details of proposed swales are listed below and the proposed locations are shown on the pavement drainage strategy Figure C.2 attached in Appendix C.

Outlet W2040: At this location it is feasible to provide a grassed drainage swale of approximately70m in length.



The three proposed swales have been aggregated into one treatment node in the MUSIC model.

In addition to the swales, the design also includes the provision of oil and grit separators (such as ROCLA Downstream Defender) at each outlet discharging upstream of the frog habitats to minimise the risk of oil spills as well as reduce the pollutant loads entering the habitats.

The location of the outlet points where the ROCLA devices are proposed is shown in Figure G.2. MUSIC model layout can be found in Appendix G. ROCLA device has been modelled as a GPT node with the following pollutant removal efficiencies: Gross Pollutant removal –5%, TSS removal – 88%, TP removal – 47% and TN removal – 0%.

The Music model was revised to assess the overall improvement that the inclusion of this water quality

strategy will have on the operational water quality. Table 7-9 summarises the change in the pollutant loads discharged at the vicinity of the frog habitats. The provision of ROCLA oil and grit separators and the swales result in the reduction of pollutant loads.

Table 7-9: Pre and Post Upgrade Annual Pollutants Loads discharged to the Frog habitats (with treatment)


TSS TP TN

P14 FH365.1 P14 – Pre 455.0 0.8 3.1

FH365.2 P14 - Post 0.0 0.0 0.0

Change (%) -100% -100% -100%

P3-1-2 FH830.1 P3-1-2 – Pre 5620 9.46 38.6

FH830.2 P3-1-2 - Post 2910 10.6 69.3

Change (%) -48% 12% 80%

P4-1-1 FH1100.1 P4-1-1 – Pre 734.0 1.2 5.0

FH1100.2 P4-1-1 - Post 632.0 3.5 25.7

Change (%) -14% 186% 415%



TSS TP TN

P6-1-1 FH2150.1 P6-1-1 – Pre 3350.0 5.7 22.8

FH2150.2 P6-1-1 - Post 527.0 3.1 22.7

FH2150B.1

Change (%) -84% -45% 0%

P3-1-2 FH2050.1 P3-1-2 – Pre 1910 3.17 12.9

FH2050.2 P3-1-2 - Post 244 1.7 12.9

Change (%) -87% -46% 0%

P3-1-2 FH2520.1 P3-1-2 – Pre 10400 17.5 71.4

FH2520.2 P3-1-2 - Post 3020 11 71.8

Change (%) -71% -37% 1%

P13 FH2800.1 P13 – Pre 2690.0 4.5 18.3

FH2800.2 P13 - Post 580.0 3.3 24.0

Change (%) -78% -27% 31%

Table 7-10 and Table 7-11 summarise the overall change in pollutant loads entering the Narrabeen and McCarrs Creek catchments.

Table 7-10: Pre and Post Upgrade Annual Pollutant Loads Narrabeen Lagoon - Mitigated


TSS TP TN

P10 – Pre 3980.0 7.6 43.2

P10 - Post 1720.0 7.6 59.1

P10 – Change (%) -57% 0% 37%

P11 – Pre 2890.0 10.7 89.0

P11 - Post 2720.0 10.5 88.1

P11 – Change (%) -6% -2% -1%

P4-1-1 – Pre 2510.0 7.0 61.0

P4-1-1 - Post 2620.0 9.8 85.1

P4-1-1 – Change (%) 4% 40% 40%

P6-1-1 – Pre 6100.0 16.5 127.0

P6-1-1 - Post 2260.0 10.5 109.0

P6-1-1 – Change (%) -63% -36% -14%

P12 – Pre 174.0 0.8 8.2

P12 - Post 174.0 0.8 8.2

P12 – Change (%) 0% 0% 0%

P13 – Pre 4460.0 10.4 86.5

P13 - Post 2330.0 9.1 91.1

P13 – Change (%) -48% -13% 5%

TOTAL – Pre 20114.0 53.1 414.9

TOTAL - Post 11824.0 48.3 440.6

TOTAL – Change (%) -41% -9% 6%


Table 7-11: Pre and Post Upgrade Annual Pollutant Loads McCarrs Creek - Mitigated


TSS TP TN

P7-1 – Pre 8050.0 15.9 104.0

P7-1 - Post 7870.0 15.6 102.0

P7 – Change (%) -2% -2% -2%

P3-1-2 – Pre 33200.0 79.4 668.0

P3-1-2 - Post 40900.0 105.0 826.0

P3-1-1 – Change (%) 23% 32% 24%

P14 – Pre 1810.0 5.6 57.2

P14 - Post 1360.0 4.8 54.5

P14 – Change (%) -25% -14% -5%

TOTAL – Pre 43060.0 100.9 829.2

TOTAL - Post 50130.0 125.4 982.5

TOTAL – Change (%) 14% 20% 16%

The assessment demonstrates that the proposed mitigation strategy using a combinations of ROCLA oil and grit separators and grassed swales will offset a significant portion of the downstream impacts. Minor increase in pollutant loads is noted for catchment P4-1-1, however the overall impact to Narrabeen Lagoon shows a decrease in loads due to the upgrade works.

Catchment P3-1-2 discharging to McCarrs creek reports an increase in overall pollutant loads, however this is considered acceptable as the increase in loads is downstream of the sensitive frog habitats. It is noted that the discharges from this catchment drain via piped outlets and therefore options to provide water quality mitigation measures at the source is limited.

7.1.6 Spill Containment

Risk Assessment to be undertaken, but at this stage likely need to place a spill containment structure at Tumburra Street and possibly an oil and grit separator on the outlets upstream of the frog breeding habita.

7.2 Construction Phase Water Quality Noting that the Mona Vale Road Corridor is constrained on both sides by the presence of the Ku-Ring Gai and Carrigal National Parks a key objective the erosion and sediment control strategy is to develop a strategy based on the control of erosion on the worksite as opposed to a strategy reliant dependent on end of line treatments such as temporary sediment basins.

7.3 Design Criteria The design of the principle erosion and sediment control elements has been undertaken in accordance with the requirements of “Managing Urban Stormwater: Soils and Construction”, and “Managing Urban Stormwater: Soils and Construction, Volume 2D Main Road Construction”, and the design brief.

For the purpose of assessment of construction phase water quality, the construction of the eastbound carriageway has been assessed as it would involve most of earthworks during this construction stage.


The assessment has shown that the temporary construction sedimentation basins will not be required when appropriate control measures are provided. The calculations and disturbed catchment areas are included in Appendix H.

7.4 Design Principles The design of the principle erosion and sediment control plans has incorporated the following general principles:

Provide off site diversion drains to direct upstream external flows around the work areas andthrough to the transverse drainage points. Where feasible, existing swale drains have beenmaintained during construction to control the external catchment runoff

Provide onsite diversion drains along the extents of works to direct sediment laden waters totreatment devices or control measures

Provide cut off drains across the works site at the end of each day to divert flow from the road to thestabilised on site water drains. Spacing of these cut offs has been determined for each subcatchment to limit the LS factor and as a result reduce the annual soil loss of each sub catchment.

Provide treatment measures such as batter drains, and check dams to reduce the erosive nature offlows as part of the conveyance system to outlet points.

7.5 RUSLE Rule Assessment An assessment has been undertaken of the annual soil loss using the RULSE rule. Locations where the RULSE rule identifies an annual soil loss less than 150m3/year, as nominated in Section 6.1 of “Managing Urban Stormwater: Soils and Construction, Volume 2D Main Road Construction”, the use of local controls will be adopted. If the annual soil loss is above 150m3/year, then a temporary sediment basin will be provided.

Due to the absence of large scale temporary erosion and sedimentation basin, effective local control measures are required to be implemented to minimise risk of erosion and sedimentation from disturbed catchment during construction. The local control measures include but not limited to the following procedures:

Provide off site diversion drains to direct upstream external flows around the work areas andthrough to the transverse drainage points. Where feasible, existing swale drains have beenmaintained during construction to control the external catchment runoff.


Provide cut off drains across the works site at the end of each day to divert flow from the road to thestabilised on site water drains. Spacing of these cut offs has been determined for each subcatchment to limit the LS factor and as a result reduce the annual soil loss of each sub catchment.

Provide treatment measures such as batter drains, and check dams to reduce the erosive nature offlows as part of the conveyance system to outlet points.

The parameters used in the RULSE rule assessment are summarised in the following section.

7.5.1 Soil Texture Group

The soil landscape map of Sydney 1:100 000 scale (Department of Land and Water Conservation, Soil Landscape Series Sheet 9130, Second Edition, 2002), in Figure 7-1, indicates Sombersby and Hawksbury soil landscapes within close proximity throughout the length of Mona Vale Road West.


Figure 7-1 Sydney Soil Landscape Map excerpt

Based on the Sydney Soil Landscape default parameters presented in Table C.19 of Managing Urban Stormwater the soil texture group for each of these landscapes are:

Somersby: Type C&D

Hawesbury: Type C, D & F

Tuggerah: Type C

7.5.2 Rainfall Erosivity Factor A rainfall erosivity factor of 4220 has been determined using the 2 year ARI 6 hour intensity of 13.9mm/h.

7.5.3 Soil Erodability Factor Soil erodability factors have been adopted from Table C20, Appendix C of “Managing Urban Stormwater: Soils and Construction”, for the soil landscapes mentioned in Section 10.1 for this area. The adopted K-factor is 0.027.

Legend:

Tuggerah

Oxford Falls

Lambert landscape

Erina landscape

Hawkesbury

Watagan landscape

Somersby

Mona Vale Road West


7.5.4 Length Slope Factor The length slope factor for each basin has been adopted based on Table A1 in Appendix A of Managing Urban Stormwater. To limit the factor and reduce the size of the sediment basins, batter drains will be provided in areas of fill to intercept runoff and discharge it to on-site diversion drains, thereby reducing the level of erosion expected. For the estimation of the annual soil loss using the RULSE rule a length suitable to adequately reduce the annual soil loss.

7.5.5 Erosion Control Practice Factor An erosion control practice factor of 1.3 has been adopted based on a construction site where the soil will be hard and compact.

7.5.6 Ground Cover Factor A ground cover factor 1.0 has been adopted based on a construction site where the topsoil has been stripped.

7.5.7 Rainfall Data The design rainfall data adopted for the design is the 5 day 85th percentile. This was adopted based on the recommendation of Table 6.1 from Volume 2D of Managing Urban stormwater assuming a 6-12 month operation within a sensitive receiving area. The adoption of sensitive receiving area is based on the close proximity various identified sensitive environmental areas such as:

Frog breeding habitat.

Onion Orchard.

Ku-Ring Gai National Park.

Garigal National Park.


7.6 Summary of Results

Table 7-12 Summary of RULSE Rule Calculations

Outlet Location LS Factor Adopted Annual Soil Loss

(m3/yr) Basin Required

STN 0 0.19 4 N

STN 130 0.19 16 N

STN 220 0.19 1 N

STN 550 1.19 77 N

STN 760 2.81 56 N

STN 880 2.81 134.4 N

STN 940 1.19 16 N

STN 950 1.19 68 N

STN 1200 1.19 120 N

STN 1450 1.31 147 N

STN 1640 (Divided for varying slopes)

0.53 30 N

STN 1980 (Divided for varying slopes)

0.3 37 N

STN 2140 0.42 83 + 30 (STN 1640) + 37

(STN 1980) = 150 N

STN 2780 1.75 136 N

STN 3150 1.19 61 N

Local Road (STN 2300) 1.98 113 N

Due to absence of large scale temporary erosion and sedimentation basin, effective local control measures are required to be implemented to minimise risk of erosion and sedimentation from disturbed catchment during construction. The local control measures include the following procedures but not limited to:

Provide off site diversion drains to direct upstream external flows around the work areas andthrough to the transverse drainage points. Where feasible, existing swale drains have beenmaintained during construction to control the external catchment runoff


Provide cut off drains across the works site at the end of each day to divert flow from the roadto the stabilised on site water drains. Spacing of these cut offs has been determined for eachsub catchment to limit the LS factor and as a result reduce the annual soil loss of each subcatchment.

Provide treatment measures such as batter drains, and check dams to reduce the erosivenature of flows as part of the conveyance system to outlet points.


The findings of this study can be summarised as follows:

The pavement drainage strategy has been prepared to achieve a 100 year ARI flood immunity forMVRW with an overview to minimize the adverse impacts on the surrounding properties.

The pavement drainage strategy was proposed to match existing flow regimes and minimisechanges to flow distribution to receiving catchments north and south of MVRW.

No properties to the north of the MVRW are expected to be adversely impacted as a result of theproposed MVRW upgrade. Predicted impacts are within the design criteria limits specified in theproject brief.

The proposed increase of flow at discharge point P16, towards the Golf Club south of MVRW,reduces impacts to the north of MVRW. There has been minor increase in flood extent, howeverreported afflux is within the afflux criteria.

To reduce impacts downstream of the MVRW pavement runoff at discharge locations P3-1 and P8-1 will be piped downstream to a main drainage line. Approximately 2/1050mm RCPs will berequired at P3-1 discharge location and 2/1200mm RCP’s at P8-1 discharge location to convey the100 year ARI flows. Modelling of the pipes will be required at the next design stage.

To reduce the impacts observed at the intersection of Powder Works Road and Mona Vale Road aformalisation of the downstream conveyance route is required. This is anticipated to be either apiped system or surface drain to the toe of the proposed embankment.

Discharges to ecological areas will generally be maintained to existing 2 year ARI discharges.Where flows are increased mitigation measures, such as bifurcation structures, will be provided todivert excess flows.

Water quality impact assessment for McCarrs Creek Catchment shows a minor increase in pollutantloading, particularly from catchment P3-1-2. However the discharge to the frog habitats within thiscatchment are mitigated by the use of oil and grit separators at each outlet location. Therefore, thenet increase in pollutants loads downstream of the sensitive corridors is considered acceptable.Opportunities to provide additional treatment at the source for P3-1-1 was assessed and consideredunviable as the discharge from this catchment is noted as piped discharge. Therefore no additionalmitigation measures have been proposed at this stage of design development.

The water quality impact assessment for Narrabeen Lagoon catchment shows a net overalldecrease in pollutant loads. Treatment measures particularly for the catchments P10, P4-1-1 andP6-1-1 discharging to the frog habitats include oil and grit separators at each outlet location. Forcatchment P6-1-1, additional treatment in the form of grassed swales have been incorporated intothe design.

The construction water quality assessment has shown that temporary construction sedimentationbasins are not required when appropriate local control measures are provided.

8 Conclusion

Appendix A Pre and Post Upgrade Catchment Plans

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FIGURE A3-1: Post Development Catchments for Assessment on Sensitive Ecological Areas

Mona Vale Road West Hydraulic Impact Assessment

Source: Aurecon, LPI, RMS

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