Water Cycle Management Study...Lowes Creek Maryland Precinct Water Cycle Management Study 80215038 |...

Lowes Creek Maryland Precinct Water Cycle Management Study

80215038 | 26 September 2018 Cardno i

Water Cycle Management Study

Lowes Creek Maryland Precinct 80215038

26 September 2018


80215038 | 26 September 2018 Cardno ii

Contact Information Cardno (NSW/ACT) Pty Ltd Trading as Cardno (NSW/ACT) ABN 95 001 145 035 Level 9, The Forum, 203 Pacific Highway St Leonards NSW 2065 PO Box 19 St Leonards NSW 1590 Telephone: 02 9496 7700 Facsimile: 02 9439 5170 International: +61 2 9496 7700 [email protected] www.cardno.com Author(s): Edmund Han Engineer Pak Lau Engineer Approved By: David Stone Principal Water Engineer

Document Information Prepared for Department of Planning

and Environment Project Name Lowes Creek Maryland

Precinct File Reference 80215038 Lowes Creek

Maryland Draft WCMS RevF.docx

Job Reference 80215038 Date 26 September 2018 Version Number Rev F Effective Date 26/9/2018 Date Approved: 26/9/2018

Document History Version Effective

Date Description of Revision Prepared by: Reviewed by:

Rev A 21/09/2016 Preliminary Finding for Stakeholder Workshop

Martin Griffin Venus Jofreh Sabina Lohani

Emma Maratea

Rev B 20/01/2016 Draft Report for Client Review Martin Griffin Andrew Simon Jenifer Hammond

Emma Maratea

Rev C 26/07/2018 Draft Report for Client Review Edmund Han Pak Lau David Stone

Rev D 10/08/2018 Draft Report for Client Review Edmund Han Pak Lau David Stone

Rev E 7/09/2018 Draft Report for Client Review Edmund Han Pak Lau David Stone

Rev F 26/09/2018 Draft Report for Client Review Edmund Han Pak Lau David Stone

© Cardno. Copyright in the whole and every part of this document belongs to Cardno and may not be used, sold, transferred, copied or reproduced in whole or in part in any manner or form or in or on any media to any person other than by agreement with Cardno. This document is produced by Cardno solely for the benefit and use by the client in accordance with the terms of the engagement. Cardno does not and shall not assume any responsibility or liability whatsoever to any third party arising out of any use or reliance by any third party on the content of this document.


80215038 | 26 September 2018 Cardno iii

Executive Summary This report by Cardno details the procedures used and results obtained from analyses of the water cycle management strategy for the Lowes Creek Maryland Precinct. This supports the master planning by providing engineering input to assist in the development of an Indicative Layout Plan (ILP). The strategy has been developed using an integrated approach to flood risk management and water sensitive urban design and streambank management principals, meeting relevant standards.

The objective of the Water Cycle Management Strategy (WCMS) is to:

Identify the stormwater and flood management issues to be taken into account in the future development of the Precinct;

Identify flood impacts and an appropriate flood evacuation strategy for the Precinct;

Assess the existing streambank conditions, likely development impacts on streambank condition and appropriate management techniques; and

Identify appropriate options and locations for the control of the quantity and quality of stormwater leaving the site.

This document outlines the methodology adopted in assessing flooding, water quality and streambank erosion; the Water Cycle Management Strategy ensures the proposed development meets requirements with regards to the management of impacts on waterways, the receiving environment and public safety relating to flooding.

Flood modelling has been completed to assess the effectiveness of the Precinct’s water quantity management strategies. The flood assessment shows that post-development 1% Annual Exceedance Probability (AEP) flows are controlled and contained within the proposed detention basins and riparian corridors of the Precinct. The strategy provides a balance between maintaining riparian corridor functions, providing floodplain management and meeting development outcomes.

The water quality strategy developed for the Precinct will also ensure that the quality of stormwater discharging from the Precinct meets the requirements of Camden Council and will ensure potential stormwater pollutant impacts are mitigated.

A comprehensive assessment of streambank stability has been undertaken so that Lowes Creek has low to moderate bank erosion under existing flow and channel conditions. Under post-development conditions, erosion would increase if no mitigation measures were adopted. An analysis of the modelling results showed that increased erosion during post-development conditions was not a result of increasing peaks but of increasing flow durations where excess shear stresses occur. These modelling results emphasise the notion that just looking at peak flows is not sufficient, as erosion is the product of the magnitude of the excess stress, times the duration of that excess stress.

Proposed bank erosion mitigation measures include small drop structures along Lowes Creek and a portion of the Western Tributary. These measures would ensure that erosion is not increased as a result of the proposed Precinct development, while also minimising the disturbance to the existing riparian corridor vegetation.

This document will be placed on public exhibition for review and comment by interested stakeholders. Comments and input received as an outcome of this process will be considered and addressed, as appropriate.


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Table of Contents 1 Introduction 1

1.1 Objective of the Report 1 1.2 Scope of Work 1 1.3 Lowes Creek Maryland Precinct 2

1.3.1 Location 2 1.3.2 Precinct Planning Context 2

1.4 Water Cycle Management Strategy 5 1.5 Draft Indicative Layout Plan 6

2 Available Data 7 2.1 Development Guidelines 7

2.1.1 Growth Centres Development Code 7 2.1.2 Camden Growth Centre Precincts Development Control Plan 7 2.1.3 Guidelines for Controlled Activities on Waterfront Land 9 2.1.4 Camden Councils Engineering Design Specification 10

2.2 Upper South Creek Floodplain Risk Management Study and Plan 10 2.2.1 FRMS&P Review and Update 10

2.3 Survey Data 10 3 Consultation 12

3.1 Riparian Corridors 12 3.2 Hydrologic, Hydraulic and Water Quality Modelling 12 3.3 Australian Rainfall and Runoff 2016 14 3.4 Online Basins 15

4 Flood Assessment 16 4.1 Study Area 16 4.2 Data Review 18

4.2.1 Digital Elevation Model 18 4.2.2 Existing Farm Dams 19

4.3 Hydrologic Model Setup 20 4.3.1 IFD Data 20 4.3.2 Temporal Patterns 21 4.3.3 Perviousness, Storm Losses & Surface Roughness 22 4.3.4 Pre-Burst Rainfall and Burst Losses 22 4.3.5 Pre-Development Catchment Delineation 23 4.3.6 Post-Development Catchment Delineation 25 4.3.7 Flow Routing 26 4.3.8 Critical Duration & Median Temporal Patterns 26

4.4 Pre-Development Hydraulic Model Setup 26 4.4.1 Terrain 26 4.4.2 Inflow Hydrographs 26 4.4.3 Downstream Boundary Conditions 27 4.4.4 Surface Material Types 27 4.4.5 Existing Hydraulic Structures 27

4.5 Model Validation 29 4.6 Pre-Development Hydraulic Modelling Results 29

4.6.1 Lowes Creek 29 4.6.2 Culvert Crossing at The Northern Road 29


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4.6.3 Western Tributary 29 4.6.4 Central Tributary 29 4.6.5 Northern Western Tributary 30

4.7 Post-Development Hydraulic Model Setup 30 4.7.1 Terrain 30 4.7.2 The Northern Road Upgrade 33 4.7.3 Riparian Corridor Works 33 4.7.4 Proposed Hydraulic Structures 34

4.8 Post-Development Hydraulic Modelling Results 36 4.8.1 Lowes Creek 36 4.8.2 Culvert Crossing at The Northern Road 36 4.8.3 Western Tributary 36 4.8.4 Central Tributary 36 4.8.5 Northern Western Tributary 36

5 Flood Emergency Response Strategy 37 6 Stormwater Quantity Management Strategy 39

6.1 Modelling Approach 39 6.1.1 Post-Development Scenario Model Set-Up 39 6.1.2 Modelling of Detention Basins 39

6.2 Basin Strategy 39 6.2.2 Basin Design Principles 40

6.3 Proposed Online Detention Basins 41 6.3.1 Western Online Basin 42 6.3.2 Central Online Basin 42

6.4 Proposed Offline Detention Basins 43 6.4.2 Offline Basin Performance 43

6.5 Comparison of Peak Discharge Flows 44 7 Water Quality Management 45

7.1 Background 45 7.1.1 Pollutant Reduction Targets 45 7.1.2 Water Quality Treatment Approach 45

7.2 Water Quality Modelling 47 7.2.1 Rainfall Data 48 7.2.2 Evapo-Transpiration Data 48 7.2.3 Rainfall Runoff Parameters 48 7.2.4 Pollutant Generation 49 7.2.5 Model Layout 50 7.2.6 Catchment Details 50 7.2.7 Modelling of Gross Pollutants Traps 51 7.2.8 Modelling of Bio-retention Basins 51 7.2.9 Overall Precinct Treatment Performance 52

7.3 Summary 53 8 Streambank Condition Management 54

8.1 Approach to Streambank Condition Assessment 54 8.2 Field Investigations 55

8.2.1 Rapid Geomorphic Assessments 55 8.2.2 Detailed Site Data Collection 56

8.3 Bank-Erosion Modelling 60


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8.3.1 Flow Data for BSTEM Simulations 60 8.3.2 Existing Conditions 61 8.3.3 Proposed Conditions 61 8.3.4 Erosion Mitigation Measures 62

8.4 Summary of Results 65 9 Operations and Maintenance 70 10 Conclusions 72 11 Glossary 73 12 References 74

Appendices Appendix A Draft Indicative Layout Plan Appendix B XP-RAFTS Catchment Details Appendix C Flood Behaviour Figures Appendix D ARR2016 Data

Tables Table 2-1 Stormwater Detention Upper (PSD) and Lower (SSR) Limits 9 Table 2-2 Water Quality and Environmental Flow Targets 9 Table 3-1 Feedback on Hydrologic and Hydraulic Modelling Approach 12 Table 3-2 Feedback on Proposed Online Dams 15 Table 4-1 Area Analysis of Waterway Catchments for Lowes Creek Maryland Study Area 16 Table 4-2 ARR2016 IFD Data for Lowes Creek Maryland 21 Table 4-3 Storm Losses & Surface Roughness 22 Table 4-4 Median Pre-Burst Rainfall Depths & Burst Initial Losses for the 50% AEP Event 22 Table 4-5 Median Pre-Burst Rainfall Depths & Burst Initial Losses for the 10% AEP Event 23 Table 4-6 Median Pre-Burst Rainfall Depths & Burst Initial Losses for the 1% AEP Event 23 Table 4-7 Critical Duration and Upper Median Temporal Patterns 26 Table 4-8 Adopted Roughness Values in XP-RAFTS Model 27 Table 4-9 Adopted Hydraulic Structure Parameters 27 Table 6-1 Details of the Western Online Basin 42 Table 6-2 Details of the Central Online Basin 42 Table 6-3 Lowes Creek Offline Basin Sizes and Modelling Results 43 Table 6-4 Stormwater Detention Checks 43 Table 6-5 Peak Flow Comparison Downstream of Lowes Creek Maryland 44 Table 7-1 Pollutant Reduction Targets 45 Table 7-2 Water Cycle Management Measures for Lowes Creek Maryland Precinct 45 Table 7-3 Selected Rainfall Gauge Data 48 Table 7-4 Average Daily Evapo-Transpiration by Month (mm) 48 Table 7-5 Rainfall Runoff Parameters 49 Table 7-6 Base Flow Pollutant Concentration Parameters 49


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Table 7-7 Catchment land use and characteristics 50 Table 7-8 GPT Input Parameters 51 Table 7-9 Bio-retention Basin Input Parameters 51 Table 7-10 Bio-retention Basin Filter Surface Areas and Pollutant Loads and Reduction Performance 52 Table 7-11 Lowes Creek Pollutant Loads, Reduction Percentages and Targets 53 Table 7-12 Northern Tributary Discharge - Pollutant Loads, Reduction Percentages and Targets 53 Table 8-1 Geotechnical Parameter Values Measured with the Borehole Shear Tester 58 Table 8-2 Lowes Creek Jet Test Results 60 Table 8-3 BSTEM Results – Existing Conditions 61 Table 8-4 BSTEM Results – Proposed Conditions 62 Table 8-5 BSTEM Results – Effectiveness of Mitigation Measures 62 Table 8-6 Proposed Mitigation Strategies 63 Table 9-1 WSUD Maintenance Schedule 71

Figures Figure 1-1 Location of Lowes Creek Maryland Precinct (NearMap, 2018) 2 Figure 1-2 Precinct Status in South West Growth Area (Source: NSW DPE, 2018) 3 Figure 1-3 Lowes Creek Maryland Precinct 4 Figure 1-4 Draft Indicative Layout Plan (dated 22/08/2018) for Lowes Creek Maryland Precinct 6 Figure 2-1 Surface Differences – Detailed Site Survey Less Upper South Creek FRMS&P LiDAR 11 Figure 4-1 Study Area, Waterway Catchments and Existing Irrigation Dams 17 Figure 4-2 Pre-development Scenario DEM Model 18 Figure 4-3 Dam Embankment Cross Sections for West and Central Farm Dams 20 Figure 4-4 Temporal Pattern Regions (extracted from ARR2016 Book 2 Section 5.3.3) 21 Figure 4-5 XP-RAFTS Catchment Delineation – Pre-Development Scenario 24 Figure 4-6 XP-RAFTS Catchment Delineation – Post-Development Scenario 25 Figure 4-7 Surface Material Types and Culverts in Pre-Development TUFLOW Model 28 Figure 4-8 Post-Development Terrain showing the Preliminary Grading Design 32 Figure 4-9 Surface Material Types and Culverts in Post-Development TUFLOW Model 35 Figure 5-1 General Flood Hazard Curves (extracted from ARR2016, Book 6, Section 7.2.7) 38 Figure 6-1 Online and Offline Detention Basin Layout 40 Figure 7-1 Post-development Bio-retention Basin Locations 47 Figure 7-2 MUSIC Model Layout 50 Figure 8-1 Six Stages of Channel Evolution (from Simon and Hupp, 1986 and Simon, 1989) 55 Figure 8-2 Location of Detailed Field Sites 56 Figure 8-3 Schematic of Borehole Shear Tester (BST) (modified from Thorne et al., 1981) 57 Figure 8-4 Scaled Down “Mini-Jet” 59 Figure 8-5 Example Scour Plot from Mini-Jet Test 59 Figure 8-6 Lowes Creek Bed Elevation, Channel Slope, 25% Slope Reduction Thresholds and Drop

Structure Requirements 64 Figure 8-7 LC-01 Site Photo and BSTEM Bank Erosion Results 66 Figure 8-8 LC-02 Site Photos and BSTEM Bank Erosion Results 67


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Figure 8-9 LC-03 Site Photo and BSTEM Bank Erosion Results 68 Figure 8-10 WT-01 BSTEM Bank Erosion Results 69


80215038 | 26 September 2018 Cardno 1

1 Introduction

1.1 Objective of the Report This report by Cardno details the procedures used and results obtained from analyses of the water cycle management strategy for the Lowes Creek Maryland Precinct, and to support the master planning by providing engineering input to the development of an Indicative Layout Plan (ILP). The strategy has been developed using an integrated approach to flood risk management and, water sensitive urban design and streambank management principals, meeting relevant standards.

1.2 Scope of Work The purpose of the analyses was to:

establish a water cycle management strategy based on water sensitive urban design principles;

provide input into the development of the riparian corridors assessment;

provide input into the development of the riparian land management and planning controls;

undertake a hydrologic, hydraulic and water quality assessment of the Precinct as an integrated approach to flood risk and water cycle management;

develop a flood evacuation strategy to assist the State Emergency Services in directing residents in the Precinct during large storm events; and

The analyses have taken into consideration the economic, engineering, environmental and social aspects of the planning proposal as documented in the draft ILP. Particular emphasis has been placed on protecting the environment and enhancing the biodiversity in the receiving water bodies and surrounding environment by implementing water sensitive urban design and best management practices.

The following methodology has been adopted in order to assess the above scope of work:

Collation of available data and previous studies;

Detailed site inspections to gain an appreciation of the site context, existing conditions, waterways and catchment;

Liaison with specialists and government agencies (including Camden Council) to establish waterway and riparian corridor requirements and an appropriate flood modelling approach;

Undertake hydrologic catchment analysis to compare existing site flows to proposed flows and determine preliminary stormwater detention strategies;

Undertake hydraulic modelling to finalise the stormwater detention strategies and assess the impact the proposed development has on surrounding environs;

Assess the impact the proposed development has on regional water quality and develop water quality treatment strategies; and

Assess the impact the proposed developed condition flows have on streambank erosion and develop streambank management strategies.



1.3 Lowes Creek Maryland Precinct

1.3.1 Location The Lowes Creek Maryland Precinct lies within the Camden Council Local Government Area (LGA). The Precinct is located between the suburb of Bringelly to the north, and the Oran Park to the south. The Precinct lies on the western side of The Northern Road (Highway A9), approximately 3 km south of its intersection with Bringelly Road.

The Precinct is within the upper South Creek catchment, a major waterway that drains a large portion of western Sydney. South Creek discharges to the Hawkesbury River to the north, near Windsor.

The location of the Precinct is shown in Figure 1-1.

Figure 1-1 Location of Lowes Creek Maryland Precinct (NearMap, 2018)

1.3.2 Precinct Planning Context Lowes Creek Maryland (Parts) Precinct lies within the South West Priority Growth Area. The Precinct has been released, but not yet rezoned. The location of the Precinct in the context of the growth area is shown in Figure 1-2.

Lowes Creek



Figure 1-2 Precinct Status in South West Growth Area (Source: NSW DPE, 2018)

Lowes Creek



On 25 September 2015, the NSW Department of Planning and Environment (DPE) executed a Voluntary Planning Agreement (VPA) with Macarthur Developments Pty Ltd relating to the Lowes Creek Maryland Precinct. The VPA relates to the release of the Precinct under the NSW Government’s Precinct Acceleration Protocol (PAP). The PAP has been developed by Government to allow the early release of Precincts within the Growth Centres for development.

The Precincts covers an area of approximately 531 ha.

The boundary of the Lowes Creek Maryland Precinct is shown in Figure 1-3.

Figure 1-3 Lowes Creek Maryland Precinct



1.4 Water Cycle Management Strategy In accordance with the provisions of the PAP agreement, all necessary technical studies must be forward funded by the developer to ensure the planning for the Precinct is comprehensive with consideration of all social, environmental and financial factors.

One of the key technical studies required is a water cycle management study, involving assessment of a number of waterway considerations such as flooding, water quality and geomorphology management. Cardno has been engaged to prepare a Water Cycle Management Strategy (WCMS) for the Lowes Creek Maryland Precinct.

The objective of the WCMS is to:

Identify the stormwater and flood management issues to be taken into account in the future development of the Precinct;

Identify flood impacts and an appropriate flood evacuation strategy for the Precinct;

Assess the existing streambank conditions, likely development impacts on streambank condition and appropriate management techniques; and

Identify appropriate options and locations for the control of the quantity and quality of stormwater leaving the site.

The WCMS addresses engineering considerations whilst placing a strong focus on conserving and enhancing the bio-diversity, ecological health and positive water quality benefits within the existing riparian corridors to provide an integrated natural resource for the incoming residents.

The investigations currently being undertaken to develop the WCMS include the following specific tasks:

Liaise with the Department of Planning and Environment, Camden Council, Department of Primary Industries (Water) and the Precinct’s Master Planner to determine their specific requirements for development of the Precinct;

Investigate a range of stormwater management and water sensitive urban design measures suitable for the site.

Undertake a hydrologic analysis to determine the peak flows for 50%, 10%, and 1% Annual Exceedance Probability (AEP) events and the Probable Maximum Flood (PMF) under existing conditions;

Develop a two-dimensional hydraulic flood model for the Precinct and assess the above mentioned storm events under existing conditions;

Determine the required fill levels for developable areas such that they are above the required Flood Planning Level (1% AEP + 0.5 m freeboard);

Determine the minimum detention storage requirements to restrict post development flows to pre-development conditions;

Undertake an assessment of different development scenarios within the hydraulic model to determine the impact of the development on the flood regime and the impacts of the flood on the development;

Identify appropriate flood evacuation routes within the Precinct;

Undertake water quality modelling to determine the minimum treatment device areas required to achieve Camden Council’s water quality targets;

Undertake geomorphological field investigations and streambank modelling to assess existing streambank condition and mitigate any potential impacts of changes to post development flow conditions;

Prepare typical preliminary engineering concept designs for any measures required to achieve the water quality and quantity and streambank objectives;

Prepare a Water Cycle Management Concept Plan; and

Prepare a Water Cycle Management Strategy Report to support the rezoning for the Precinct, detailing the investigations, findings, calculations and design details.



1.5 Draft Indicative Layout Plan As part of the Precinct Planning process, a draft Indicative Layout Plan (ILP) was prepared. It identifies the various land uses proposed for the Precinct. The Draft ILP (dated 22 August 2018) is shown in Figure 1-4 below, and in Appendix A.

Figure 1-4 Draft Indicative Layout Plan (dated 22/08/2018) for Lowes Creek Maryland Precinct

The Draft ILP sets aside land for stormwater management measures (both quantity and quality) which are required to manage the runoff associated with urbanising the Precinct. Many of the active open space areas will be used for the co-location of stormwater management measures.



2 Available Data

2.1 Development Guidelines The following guidelines are being considered in the Water Cycle Management assessments.

2.1.1 Growth Centres Development Code The Growth Centres Development Code (2006) identifies the following matters for consideration with regard to Water Sensitive Urban Design and stormwater management:

Stormwater management strategies should be based on the objectives and principles of Water Sensitive Urban Design. They should promote water reuse and maximize potable water conservation;

Existing waterways and riparian zones should be conserved and enhanced where possible;

Stormwater management strategies should be developed and implemented in a manner which considers and addresses potential salinity hazards;

Stormwater management strategies should be adopted by the ILP to maximize efficient use of land and facilitate adequate allocation of land for stormwater management purposes;

The ILP should be planned, designed and undertaken in a manner which allows for appropriate control of erosion and sediment pollution;

A treatment train approach should be used, incorporating structural stormwater treatment measures at the primary, secondary and tertiary levels as necessary to comply with the stormwater management targets;

Treatment of stormwater should be considered on a sub-catchment or Precinct basis and not be heavily dependent on “end of pipe‟ treatment systems;

The design of stormwater management systems should be integrated with the planning of road layout and design, given the potential benefits of incorporating suitable WSUD elements into road corridors;

Stormwater reuse, retention and detention strategies should be used to minimize changes to the hydrologic (or flow) regime of receiving waterways;

Urban stormwater should not be discharged to areas of native bushland unless such discharge cannot be avoided. High levels of stormwater treatment and flow retention or detention should be implemented where such a discharge occurs to limit soil erosion and weed growth within areas of native vegetation;

Management of stormwater should be considered on a sub-catchment basis to employ source control techniques in preference to highly centralized “end-of-pipe‟ treatment measures wherever practicable;

WSUD drainage systems may be incorporated into other roads and streets, where practicable and compatible with other design issues, including safety requirements of the relevant Road Authority;

Any development within the 1% AEP flood level and the PMF should be designed to provide for emergency access;

Critical infrastructure, such as major roads and rail, are to be located above the 1% AEP flood level wherever possible; and

Evacuation routes that continually rise from residential properties to higher land should be provided.

2.1.2 Camden Growth Centre Precincts Development Control Plan This Development Control Plan (DCP) was originally adopted by the Deputy Director General Planning Strategies, Housing and Infrastructure (under delegation from the Director-General) of the Department of Planning & Environment on 21 March 2013 and came into force on 3 April 2013. The DCP applies to the South West Growth Centre Precincts, or parts of the Precincts, within Camden Local Government Area where precinct planning has been completed. It is expected that this DCP will apply to Lowes Creek Maryland when precinct planning is complete.



The water cycle management objectives of the DCP are: To ensure that the quality of stormwater discharged from urban areas into the environment complies with

appropriate standards;

To minimise potable water consumption and maximise re-use of stormwater within urban areas;

To ensure that the water cycle management infrastructure is cost effective and maintainable; and,

To maintain and enhance the quality of natural water bodies.

The water cycle management controls included in the DCP are:

1. Management of ‘minor’ flows and ‘major’ flows within subdivisions and development sites is to be in accordance with Council’s Engineering Specification.

2. Stormwater within new subdivisions is to be managed primarily through a gravity network of pipes and overland flows generally following streets where flow volumes exceed the capacity of pipes in accordance with Council’s Engineering Specification.

3. All new development is to be connected, via the network described in control 1 above, to the Council’s trunk drainage system shown on the Key elements of the water cycle management and ecology strategy figure, in the relevant Precinct Schedule.

4. The acquisition of drainage easements over downstream properties, or inclusion of drainage easements on subdivision plans, will be required where direct access to Council’s drainage system or discharge of stormwater to a creek via the street network is not possible (i.e. street kerb and gutter, piped system or open channels and watercourses). However, the design of subdivisions is to generally comply with controls above and management of stormwater through easements will only be permitted by Council in exceptional circumstances where no other practical solution is available.

5. Roads on primary drainage lines shown on the Key elements of the water cycle management and ecology strategy figure, in the relevant Precinct Schedule, are to be constructed in the locations shown (subject to detailed survey and subdivision design), and are to be designed in accordance with Council’s Engineering Specifications.

6. The developed 1%, 20% and 50% AEP peak flows are to be maintained at pre-development flows through the incorporation of stormwater detention and management devices. Where subdivision works occur prior to the completion of required trunk drainage works, temporary on site facilities need to be provided in order to limit drainage volume and velocity to that experienced prior to development. It is noted that for this Study the 10% AEP has been selected as the mid-range flood event as opposed to the 20% AEP event.

7. Where development includes the construction of water quality treatment infrastructure, the infrastructure is to be constructed in accordance with the Precinct Water Cycle Management Strategy (ie. this WCMS when adopted) and Council’s Engineering Specification. The applicant must demonstrate that the proposed infrastructure will achieve the water quality targets in Table 2-2.

8. Trunk drainage channels are to be designed and constructed as naturalised channels.

9. Council may consider amendments to the Precinct water cycle management strategy if a revised strategy is submitted that can demonstrate to Council’s satisfaction:

a. compliance with the targets in Table 2-2;

b. any costs associated with construction (including the cost of land) will be met by the applicant; and,

c. a maintenance framework addressing maintenance strategies and life-cycle maintenance costs.

10. Where development includes land within a Riparian Protection Area (refer to the Riparian Protection Areas Map that is part of the Growth Centres State Environmental Planning Policy (SEPP)) applicants are to refer to the Guidelines for riparian corridors on waterfront land prepared by the NSW Office of Water. The guidelines contain the outcomes and requirements for development on land containing a riparian protection area within the Growth Centres.



11. The primary stormwater quantity objective as noted in one of the above points is to ensure post-development flows do not exceed existing flows. The Site Storage Requirement (SSR) and Permissible Site Discharge (PSD) values included in Table 2-1 represent lower and upper limits respectively.

Table 2-1 Stormwater Detention Upper (PSD) and Lower (SSR) Limits 50% AEP 1% AEP

Site Storage Requirement (SSR) Target (m3/ha) 300 594

Permissible Site Discharge (PSD) Target (l/s/ha) 30 170

The water quality and environmental flow targets in the DCP are included in Table 2-2.

Table 2-2 Water Quality and Environmental Flow Targets

Water Quality (% Reduction in Pollutant Loads)

Environmental Flows

(Stream Erosion Control Ratio)

Gross

Pollutants (>5mm)

Total Suspended

Solids

Total Phosphorous Total Nitrogen

Stormwater Management Objective

90 85 65 45 3.5 – 5.0 : 1

‘Ideal’ Stormwater Outcome 100 95 95 85 1 : 1

2.1.3 Guidelines for Controlled Activities on Waterfront Land Controlled activities carried out in, on or under waterfront land are regulated by the Water Management Act 2000 (WM Act). NSW Department of Industry - Water (DoI Water), formerly NSW Office of Water (NOW), administers the WM Act and is required to assess the impact of any proposed controlled activity to ensure that no more than minimal harm will be done to waterfront land as a consequence of carrying out the controlled activity.

DoI Water has released three guidelines for four types of works within waterfront lands that are relevant for this WCMS; riparian corridors, outlet structures, watercourse crossings and instream works (NOW, July 2012). The key recommendations relevant to the formation of the WCMS are:

Offline detention basins are permitted within the outer 50% of the corridor for all stream orders.

Online detention basins are permitted within first and second order stream orders if they adhere to the following requirements:

- be dry and vegetated and be only for temporary flood detention only with no permanent water holding;

- have an equivalent Vegetated Riparian Zone (VRZ) for the corresponding watercourse order; and,

- not be used for water quality treatment purposes.

Water quality basins are not permitted within the VRZ;

Stormwater outlets are permitted within the riparian corridor but must consider scour protection and bed stabilisation through such things as reinforced turf, rip rap, and rock armouring;

Streams may be realigned if the hydrologic behaviour is maintained and scour protection provided.

Eco Logical Australia has prepared a Draft Riparian Corridor Management Report (Eco Logical Australia, 21 September 2016) that has been used to inform the Water Cycle Management Strategy. Within this report stream ordering, riparian corridor allowances, vegetation plans and other riparian corridor works are assessed with the DoI Water (NOW) riparian corridor guidelines.



2.1.4 Camden Councils Engineering Design Specification In 2009 Camden Council released an Engineering Design Specification. The objective of the specification was for the guidance of owners, applicants, superintendents, consultants, contractors and representatives thereof to outline Council's engineering specifications for the design of subdivisions and the development of land within the Camden LGA.

While the objective of the specifications is focussed on more detailed design stages such as the development application stage, they provide a useful insight into Councils preferred standard of development. Therefore, the WCMS has been developed and assessed in accordance with Council’s specifications. The specifications not only provide guidance on the design of stormwater infrastructure but also outlines model set-up criteria and other guidance relevant to this WCMS.

2.2 Upper South Creek Floodplain Risk Management Study and Plan The Upper South Creek Floodplain Risk Management Study and Plan (FRMS&P) was prepared by Cardno in 2014 on behalf of Camden Council, with a final draft report issued in June 2014. The study area for the FRMS&P included the catchment for South Creek upstream of Bringelly Road. The Lowes Creek Maryland Precinct lies within this study area as Lowes Creek is a western tributary to South Creek.

The modelling of the catchment was completed using TUFLOW 1D/2D ‘rainfall on grid’ model with 10m x 10m grid cell size. The pre-development scenario was re-established from that of the preceding Flood Study due to the development of a number of Precincts within the study area including Oran Park, Catherine Fields, Turner Road, and Leppington. The modelling concluded that the development of these Precincts resulted in increased flows further downstream in the South Creek floodplain.

2.2.1 FRMS&P Review and Update Concurrent to the preparation of this Water Cycle Management Study, Cardno was engaged by Camden Council to review the findings of the Upper South Creek FRMS&P particularly relating to several large farm dams located in the upper South Creek catchment. The recommendation from the original FRMS&P was that these large farm dams, seven in total in the catchment, be retained in the future to retain the hydraulic behaviour of the existing Precinct. Of the seven existing large dams, two are located within the Lowes Creek Maryland Precinct with an additional one located within the hydraulic model study area (refer to Section 4.2.2 for further details).

In September 2015, Cardno provided Camden Council a discussion paper that suggested that based on preliminary review the Upper South Creek flood model had misrepresented the flood storage provided by the large farm dams.

In mid-2016, Camden Council commissioned Cardno to update hydraulic modelling to assess the impact of removing these large farm dams and returning future landform to a more natural landscape without these man made water bodies. As the Lowes Creek Maryland WCMS initiated prior to this work being completed, it was agreed that the WCMS would take a conservative approach and assume the retention of the farm dams as the “base case”. In order to ensure that there was no perception of conflict of interest on the part of Cardno, the outcomes of this ongoing work were not made available to the project team involved in this Water Cycle Management Study.

2.3 Survey Data A range of survey data was acquired to inform this Water Cycle Management Study:

For the Lowes Creek Maryland Precinct site, detailed ground survey was recorded by Craig and Rhodes Surveyors in late 2015. This data included not only surface levels for the area but also recording of dam invert levels for all farm irrigation dams including the large west and central dams.

Two sets of LiDAR data were acquired for the area:

- The data used as the basis for the Upper South Creek FRMS&P hydraulic model. This data was sourced from Camden Council in order to inform this study.

- LiDAR data recorded by NSW Land and Property Information (L&PI) in 2011. This data was compared to the LiDAR data set from the Upper South Creek FRMS&P and was found to be in good



agreement. Therefore, the Upper South Creek FRMS&P LiDAR data set has been adopted in preference to this 2011 L&PI data.

The method of data recording for LiDAR data means that it is not possible for survey points to penetrate standing water bodies to record their inverts. Therefore, LiDAR represents the surface level of water bodies as the standing water level at the time of recording.

A comparison of the surface levels of the ground survey and the LiDAR data is shown in Figure 2-1. It shows that there is generally good agreement between the elevation data of the two models. The key area of difference is the farm dams throughout the Precinct where, as discussed above, the ground survey has recorded the dam inverts while the LiDAR has surveyed the standing water level of the dam at the time of recording. This is the reason that the ground survey is significantly lower than the LiDAR for the farm dams.

Figure 2-1 Surface Differences – Detailed Site Survey Less Upper South Creek FRMS&P LiDAR



3 Consultation

In preparing this WCMS, consultation has been undertaken with various agency and organisational stakeholders. The issues requiring input from stakeholders included the stream classification and delineation of riparian corridor requirements, the approach to assessing and designing the online onsite detention basins, and the hydrologic and hydraulic assessment approaches.

A workshop was undertaken on 4 October 2016 to present the technical studies for the rezoning of the Precinct to Council staff and other government agencies.

Direct consultation has also been undertaken with Department of Planning and Environment, Camden Council and DoI Water on specific issues.

3.1 Riparian Corridors Eco Logical Australia undertook field and desktop assessment associated with stream ordering, riparian corridor allowances and vegetation plans. Cardno and Eco Logical consulted closely on these issues and sought input from DoI Water regarding riparian corridors.

The riparian corridor details are provided in Draft Riparian Corridor Management Report (Eco Logical Australia, 21 September 2016).

3.2 Hydrologic, Hydraulic and Water Quality Modelling A preliminary draft WCMS report was presented to Camden Council and other government agencies at a workshop held on 4 October 2016. The preliminary report outlined the proposed approach to hydrologic and hydraulic modelling as well as the geomorphological assessment of streambank erosion.

The key issues raised at the workshop and through follow up correspondence are summarised in Table 3-1. Where comments related to model results that have subsequently been superseded, these comments have not been included.

Table 3-1 Feedback on Hydrologic and Hydraulic Modelling Approach Agency Comment Response

Camden Council

The pre-developed scenario is regional farm dams at full supply level. However, the starting water level in farm dams (west dam level 88.0m AHD and central dam level 78.1m AHD) indicates the Aerial Laser Survey (ALS) starting water levels as adopted in Upper South Creek FRMS&P. The starting water levels lower than the full supply level of farm dams underestimate the pre-developed scenario. Council is considering adopting the regional farm dams at full supply level after completion of current assessment of ‘Flooding Impact of Regional Farm Dams in the Upper South Creek Catchment’.

The ground survey adopted for the Lowes Creek Maryland Precinct suggests that the low point of the dam weirs is approximately equal to the ALS levels in the Upper South Creek FRMS. Therefore the adopted levels should be approximately equivalent to the full capacity level.

Camden Council

The capacity of culverts at the crossing of Lowes Creek and The Northern Road should be assessed to ensure no adverse impacts on the development. As discussed with Cardno, Council will provide RMS drawings for The Northern Road upgrade works at this location.

The post-development flood modelling results for the Lowes Creek Maryland Precinct have included the proposed The Northern Road culverts and raised The Northern Road embankment to be located above the 1% AEP flood level. The fill design for the Precinct has incorporated the impacts of The Northern Road design, anticipating the impacts this road upgrade will have on future flood levels. .



Agency Comment Response

Camden Council

It is understood that off-line basins are provided for addressing the impact of increased impervious areas and filling associated with the development. The basin volumes required based on the guidelines provided in the Growth Centre DCP is much higher than the total volume of (8) eight basins provided in Table 3-6 of the WCM. This is only a preliminary check. The detailed flood modelling is required to be provided for justification of basin volumes.

The two online basins also provide storage to allow for increased development run-off, however it is difficult to distinguish which portion is addressing previous dam storage and which is addressing additional run-off. Assessment of detention basin volumes has been compared with the SSR and PSD recommendations of the Growth Centres DCP. As noted by Council if flows are matched in the model than this is assumed a better check of sufficient basin volume.

Camden Council

As discussed with Cardno, Upper South Creek Catchment is sensitive for filling in floodplain, and the impacts of filling the developable area in floodplain up to Flood Planning Level should be assessed and addressed. It is required to indicate the area to be filled and provide the volume of filling.

The loss of flood storage is translated to water level increases within the riparian corridors of the site. To ensure the loss of flood storage does not adversely affect areas outside of the development area, water level impacts have been assessed for 50%, 10% and 1% AEP events.

Camden Council

The preliminary draft WCM Report states that the XP-RAFTS parameters adopted for WCM is in accordance with Council’s Design Specifications. A number of flood studies exist in this catchment and the XP-RAFTS models used in these studies have already been calibrated. Therefore it is more appropriate to adopt the parameters that are associated with the existing calibrated models.

XP-RAFTS parameters have been checked against the Upper South Creek FRMS model. Checks against other WCMS in the Upper South Creek catchment have also been conducted.

Camden Council

The maintenance of bio-retention devices is expensive. Is there a potential for stormwater reuse strategy at regional level in this Precinct?

Regional stormwater reuse schemes may be considered at later more detailed stages of the process. However, for the purpose of Precinct planning stormwater reuse has not been assumed so that water quality treatment basins will be conservatively sized in the WCMS and Section 7.11 Costings ensuring provisions for the future basins in the Precinct will be adequate (i.e. allowing scope for reduction in size but not increase).

Camden Council

Council will expect that water quality outcomes are consistent with the pollution retention criteria contained within the Oran Park DCP and this is to be demonstrated for the proposed rezoning with respect to protecting the water environment of Lowes Creek, South Creek and the Nepean River.

The WCMS ensures that the development achieves the water quality criteria included in the Camden Growth Centre Precincts DCP.

OEH It would be preferable that the hydrologic model parameters are verified against existing calibrated models.

The hydrologic model has been validated using results from the Upper South Creek Floodplain Risk Management Study and Plan, with comparison of flow hydrographs at two locations; 270 m upstream of The Northern Road crossing of Lowes Creek and 480 m downstream of The Northern Road crossing at the downstream boundary of the WCMS model. The verification is provided in Section 4.5.

OEH In 2016, Camden Council and DPE with OEH coordination engaged Cardno to assess the impact of the regional dams on the flooding behaviour within the Upper South Creek catchment. Cardno's The Flooding Impact of Regional Farm Dams in the Upper South Creek Catchment Discussion Paper dated 12 September 2016 (dams discussion paper, Cardno, 2016) provides information on the active storage of these regional dams that is required to minimise the impacts on the 1% flood event

The hydraulic modelling undertaken as part of the WCMS has assumed a full supply level as the starting level for the farm dams contained within the site.



Agency Comment Response within the Camden LGA and at the downstream boundary with Liverpool LGA (i.e. at Bringelly Road). Based on the dams discussion paper, Camden Council is currently considering the adoption of the Full Supply Level (FSL) for all regional dams as the base case scenario. It is noted that two of these farm dams are within the Precinct.

OEH OEH support the adoption of the FSL as the base case as proposed by the Water Cycle and Flooding Report. However, it is noted that Section 3.2.2 identifies the FSL for the West and Central dams as 88.0m AHO and 78.1m AHO. These levels represent the ALS levels adopted by the FRMSP Cardno 2014. The dam discussion paper estimates the FSL for the West and Central dams as 88.6m AHO and 78.2m AHO respectively.

Within the WCMS model, the adopted survey data for the site was ground survey recorded specifically for this site, as it was assumed this data provided additional accuracy than ALS data. This survey data showed that the minimum embankment levels of the two dams in question was lower than presented in the ALS data adopted within the Upper South Creek FRMS model. It is the differences in survey data which result in the recent assessment for Council proposing FSL which are 0.1 m and 0.6 m higher than those adopted in the WCMS model. The FSL currently adopted within the WCMS are assumed to be appropriate based on the following:

- As discussed above the ground survey has minimum levels in the embankment equivalent to the ALS levels which is why they were adopted. Therefore, water in the dams is free to overtop from time 0 and the FSL is appropriate and therefore no misrepresented additional detention storage is provided in the model.

- The currently adopted FSLs in the Lowes Creek Maryland WCMS assessments are lower than the 2016 paper prepared by Cardno for Council and OEH so the risk is that the WCMS assessment actually overestimates the available active storage of the dams (i.e. airspace available to detain flows during a storm event).

- The current calibration results suggest that the dam detention is appropriate as flows are well matched with the Upper South Creek FRMS model.

Further discussion regarding this comparison is provided in Section 4.2.2.

3.3 Australian Rainfall and Runoff 2016 Following issue of the previous draft WCMS reports, Australian Rainfall and Runoff 2016 (ARR2016) was released. A sensitivity analysis, particularly in relation to hydrology, was undertaken and the results discussed with Camden Council and DPE. Subsequent to this discussion, it was agreed to adopt the methodologies contained in ARR2016 for this study.



3.4 Online Basins The use of online basins and the approach to modelling these basins was identified as a key issue. Consultation was undertaken with DoI Water on this matter.

Online basins have been proposed in the location of the existing farm dams. Initial consultation was undertaken with DoI Water in July 2016. The guidance and comments received from DoI Water are summarised in Table 3-2.

Table 3-2 Feedback on Proposed Online Dams Agency Comment Response

DoI Water Online basins should typically be located offline for 3rd order and greater watercourses in accordance with the DoI Water CAA Guidelines for riparian corridors on waterfront land. DoI Water will consider all online basins on larger order streams based on merit.

As discussed further in Section 6.3, the merits of the proposed online basins are that they are replacing two large existing farm dams. Neither of these existing farm dams have low flow outlets meaning that existing environmental flows for the west and central tributaries are in a very poor condition. The proposed online basins will in fact improve environmental flows as they are dry basins with low flow outlets at the invert of the basin ensuring all low flows are detained and not retained as in the existing farm dams.

DoI Water Online basins should provide for a fully functioning Vegetated Riparian Zone in accordance with the CAA Guidelines.

A vegetated riparian zone of 80m has been proposed in the ILP for the online basins.

DoI Water Online basins should provide a naturally functioning stream channel with a range of geomorphic features.

The inverts of the online basins have been designed to allow for 1% bench slopes on either side concentrating flows in the centre of the riparian corridor. The dry basins have been designed to have a longitudinal slope of 0.5%. This design promotes low flows to be concentrated with a steady velocity. A design channel has not been proposed within the online basins as detention occurs for events as frequent as the 50% AEP, meaning further concentrating flows in the riparian corridor was not a major priority. If requested these types of elements can be further developed within the online basins in further detailed design stages.

DoI Water Online basins must be located within watercourse areas of low riparian and geomorphic value.

The online basins have been proposed in the location of the existing farm dams.

DoI Water All suitable offline areas should be reviewed before presenting online basins on 3rd order streams.

A number of offline basins have been proposed, the online basins are required to ensure that the development does not adversely impact flooding downstream of the Precinct. Offline basins were not feasible for either the central or west tributaries as the basins need to account for increased developed run-off, but also compensate for the loss of flood storage from the removal of the farm dams. The required detention could not be achieved without online basins.



4 Flood Assessment

4.1 Study Area The most significant waterway through the Lowes Creek Maryland Precinct is Lowes Creek. Lowes Creek originates to the west of the Precinct and conveys flow through the site in an easterly direction before discharging through culverts on the eastern Precinct boundary under The Northern Road. The Lowes Creek crossing of The Northern Road is the primary discharge point for the Precinct.

The flood assessment study area is the extent of the Precinct and surrounding area that are accounted for in both the hydrologic and hydraulic model. To ensure that the flood related assessment confidently assess the potential impacts of development on flood behaviour downstream, the extent of the study area includes both the upstream catchment and also extends downstream of the Precinct boundary. In order to achieve this, the study area for the Lowes Creek Maryland Precinct has been extended approximately 550 metres downstream of the Precinct boundary to the east.

The major waterways within the study area are:

Waterway A – Lowes Creek: As discussed above, Lowes Creek is the main waterway through the site, generally flowing in a west to east direction.

Waterway B – West Tributary: This tributary of Lowes Creek originates from the south-west of the Lowes Creek Maryland Precinct and generally flows in a northerly direction before converging with Lowes Creek close to the centre of the Lowes Creek Maryland Precinct.

Waterway C – Central Tributary: This tributary originates from the south of the Lowes Creek Maryland Precinct and generally flows in a northerly direction before converging with Lowes Creek immediately upstream of the Lowes Creek Maryland Precinct discharge point.

Waterway D – East Tributary: This catchment originates from the south-east of the Precinct conveying flow on the eastern side of The Northern Road, converging with Lowes Creek downstream of the Precinct.

Waterway E – North Tributary: In addition to Lowes Creek and its tributaries, there is a minor catchment on the northern side of the Precinct which does not discharge to Lowes Creek. This catchment flows in a north-east direction converging with South Creek immediately downstream of the Bringelly Road crossing, meaning it is still part of the upper South Creek catchment.

The flood assessment study area is approximately 1,359 ha in size and is shown in Figure 4-1. A summary of the catchment areas of the various waterways and the catchment area with the Lowes Creek Maryland Precinct is summarised in Table 4-1.

Within the study area there are three existing large farm irrigation dams. Each dam has a large volume of retained water retained behind large embankment walls with each being located on the downstream end of the three tributaries of Lowes Creek; west, central and east tributaries. The surface areas of the permanent water bodies are approximately 18.1 ha, 19.4 ha and 6.2 ha for the west, central and east dams respectively. The locations of the dams are shown in Figure 4-1.

Table 4-1 Area Analysis of Waterway Catchments for Lowes Creek Maryland Study Area

Catchment Total Catchment Area (ha) Catchment Area within Precinct (ha)

Waterway A – Lowes Creek 449 251

Waterway B – West Tributary 259 74

Waterway C – Central Tributary 426 189

Waterway D – East Tributary 177 0

Waterway E – North Tributary 48 17

Total 1,359 531



Figure 4-1 Study Area, Waterway Catchments and Existing Irrigation Dams



4.2 Data Review

4.2.1 Digital Elevation Model A Digital Elevation Model (DEM) was established for the study area using a number of survey data sources (background on survey data can be found in Section 2.3):

The base of the data is the LiDAR data used to inform the Upper South Creek FRMS&P (Cardno, 2014);

Preferentially detailed ground survey from Craig and Rhodes has been adopted where available. This was approximately limited to the Lowes Creek Maryland Precinct.

The pre-development scenario DEM used for the hydrology and hydraulic model is shown in Figure 4-2.

Figure 4-2 Pre-development Scenario DEM Model



The DEM surface was established for a 3m x 3m grid cell size. The Upper South Creek FRMS&P (Cardno, 2014) model grid was 10m x 10m, which was not considered appropriate for the level of detail required for this Water Cycle Management Study. The reason for this is that some of the waterways in the catchment have channel widths that are less than 10 metres wide meaning the channel would not be accounted for appropriately in the model. Our review of the site, survey data and model set up concluded that a 3m x 3m cell size will account for channel geometry to a sufficient level of detail for the purposes of this study.

As the study area for this Precinct assessment is only a portion of the wider upper South Creek study area it is possible to reduce the grid cell size to 3 metres while still keeping model run times to a sub-daily manageable level.

4.2.2 Existing Farm Dams As discussed in Section 2.3, a comparison of ground survey and LiDAR data showed that the key differences between the data sets relates to ground survey recording dam inverts while LiDAR records water surface levels.

To model the farm dams conservatively, it has been assumed that the dams are full at the time of flooding. This is considered a reasonable assumption as it is likely that the design storms to be modelled would in reality be imbedded in a wider rainfall pattern with preceding rainfall filling the dams to full capacity prior to the design storm.

It is noted that determining the full capacity level of these large farm dams has been the cause for significant review in the Upper South Creek FRMS&P. In the original FRMS&P model, the DEM level of the dams matched the recorded LiDAR level, an elevation of 88.0m AHD and 78.1m AHD for the west and central dams respectively. However as mentioned in Section 2.2.1, a preliminary review prepared by Cardno on behalf of Camden Council in September 2015 suggested these water levels were below the minimum level of the embankments, meaning they did not represent the full dam capacity.

To confirm the suitability of the previously adopted initial water levels, cross-sections of the two dam embankments were assessed as part of this WCMS. The assessment compared DEM levels across the embankments for the following:

Upper South Creek FRMS&P model: A 10m x 10m model grid based on previous LiDAR data;

Lowes Creek Maryland WCMS model: A 3m x 3m model grid based on detailed ground survey data.

Cross-sections for the dam embankments of the west and central dams are included in Figure 4-3. The cross-sections show that the minimum embankment levels in the WCMS DEM are approximately equal to the standing water levels from the Upper South Creek FRMS&P. As a result, these previously assumed standing water levels have been adopted within the flood model for this study as they are theoretically equal to the dams at full capacity.

As noted, this conclusion differs from that prepared by Cardno in September 2015 where it was concluded that these levels do not represent full capacity. This is a result of differences between the DEM embankment levels of the two studies. While there is generally good agreement between the two DEM grids, critically the WCMS DEM has recorded minimum embankment levels significantly lower than those recorded in the previous Upper South Creek FRMS&P model. As shown in Figure 4-3, the low points are for a short section of the respective embankments, suggesting that the additional detail of the WCMS DEM detected them while the FRMS&P model has not.

While the Upper South Creek FRMS&P appeared to have adopted an underestimate of the standing water level based on the survey available at the time, the initial water level used in the farm dams have been updated according to the detailed survey of the dams. The west dam and central dam have been modelled with initial water levels of 88.21 mAHD and 78.14 mAHD, respectively.



Figure 4-3 Dam Embankment Cross Sections for West and Central Farm Dams

4.3 Hydrologic Model Setup Hydrologic modelling of the Precinct was undertaken using XP-RAFTS modelling software, which is an acceptable hydrologic modelling suite in accordance with Table 3.9 of the Council’s Design Specifications (Camden Council, 2009).

The models were set up with data extracted from the ARR Data Hub, in accordance with Australian Rainfall & Runoff 2016 (ARR2016).

4.3.1 IFD Data The ARR2016 Intensity-Frequency-Duration (IFD) data for Lowes Creek Maryland was extracted from the ARR Data Hub, and is shown in Table 4-2.



Table 4-2 ARR2016 IFD Data for Lowes Creek Maryland

Storm Duration ARR2016 Rainfall Depth (mm)

50% AEP Event 10% AEP Event 1% AEP Event

1-hour 25.1 41.4 65.7

2-hour 31.1 50.7 80.4

3-hour 35.5 57.7 91.8

6-hour 45.9 74.9 120

9-hour 54.3 89.4 145

12-hour 61.5 102 167

18-hour 73.4 125 204

24-hour 83.1 143 236

4.3.2 Temporal Patterns

Lowes Creek Maryland is located within the East Coast South temporal pattern region, as shown in Figure 4-4. The corresponding temporal patterns were extracted from the ARR2016 Data Hub for the 1% AEP event, for a range of storm durations (refer Appendix D).

Figure 4-4 Temporal Pattern Regions (extracted from ARR2016 Book 2 Section 5.3.3)



4.3.3 Perviousness, Storm Losses & Surface Roughness

Rural initial and continuing storm losses for Lowes Creek Maryland were extracted from the ARR Data Hub. The adopted rainfall losses for each of the three categories defined in ARR2016 adopted in the XP-RAFTS model is shown in Table 4-3.

In the pre-development model, the entire catchment was designated as Pervious Area.

In the post-development model, the urbanised areas were separated into Effective Impervious Areas (EIA) and Indirectly Connected Areas (ICA), in accordance with ARR2016. This was undertaken by firstly assigning all open space areas and riparian corridors shown in the ILP as Pervious Areas. For urbanised areas in the ILP, 40% was designated as EIA and 60% was designated as ICA, as agreed with Council.

Table 4-3 Storm Losses & Surface Roughness Rainfall Loss Category Storm Initial Loss (mm) Storm Continuing Loss (mm/hr) Manning’s ‘n’

Effective Impervious Areas 1.5 0 0.025

Indirectly Connected Areas 26.6 2.5 0.025

Rural Areas 38 3.7 0.04

As described in the following section, ARR2016 storm losses must account for pre-burst rainfall before being simulated in the hydrologic model.

4.3.4 Pre-Burst Rainfall and Burst Losses

As agreed with Council, the median pre-burst rainfall has been adopted for the hydrologic model of Lowes Creek Maryland. The adopted pre-burst rainfall and burst initial losses for each event and duration are shown in Table 4-4 to Table 4-6. Burst initial losses are calculated by subtracting pre-burst rainfall depths from the storm initial losses that are shown in Table 4-3.

Table 4-4 Median Pre-Burst Rainfall Depths & Burst Initial Losses for the 50% AEP Event

Storm Duration Median Pre-Burst Depth for the 50% AEP Event (mm)

Rural Burst Initial Losses for the 50% AEP event (mm)

1-hour 1.0 37.0

2-hour 0.0 38.0

3-hour 1.5 36.5

6-hour 2.0 36.0

9-hour 1.4# 36.6

12-hour 0.8 37.2

18-hour 1.0 37.0

24-hour 0.0 38.0 # Interpolated from the 6-hour and 12-hour pre-burst rainfall depths






1-hour 1.4 36.6

2-hour 1.1 36.9

3-hour 1.9 36.1

6-hour 11.3 26.7

9-hour 9.4# 28.6

12-hour 7.4 30.6

18-hour 8.4 29.6





1-hour 1.3 36.7

2-hour 1.5 36.5

3-hour 2.7 35.3

6-hour 24.0 14.0

9-hour 22.5# 15.5

12-hour 21.0 17.0

18-hour 16.9 21.1


4.3.5 Pre-Development Catchment Delineation The pre-development scenario hydrologic sub-catchments have been delineated based on the existing landform. The layout of the existing hydrologic catchments is shown in Figure 4-5.

There are a total of 64 sub-catchments within the pre-development scenario XP-RAFTS model, with an average size of 21 ha each. This average catchment size is considered appropriate for the purposes of this study as it provides a suitable level of detail in the hydrologic model. Each sub-catchment has a representative catchment slope value and were delineated to ensure a relatively consistent surface slope within the individual sub-catchment.

Catchment details including areas, impervious percentages, and slope for the pre-development scenario XP-RAFTS model have been included in Appendix B.



Figure 4-5 XP-RAFTS Catchment Delineation – Pre-Development Scenario



4.3.6 Post-Development Catchment Delineation The post-development scenario hydrologic sub-catchments have been delineated based on consideration of existing landform, proposed land-use from the ILP, and assumed stormwater drainage networks and future diversion to stormwater basins. The proposed road network has been used as a guide to the orientation of stormwater networks throughout the post-development Precinct. Hydrology catchments that lie outside of the Lowes Creek Maryland Precinct have been assumed to remain unchanged.

The post-development hydrology catchment layout for the Lowes Creek Maryland Precinct is shown in Figure 4-6. With catchment details including area, impervious percentage and slope included in Appendix B.

Figure 4-6 XP-RAFTS Catchment Delineation – Post-Development Scenario



4.3.7 Flow Routing The hydrologic model has adopted a constant lag from modelling results of TUFLOW models that were developed for previous iterations of the Indicative Layout Plan. The flow velocities for the 1% AEP event were used to estimate link lags between sub-catchments.

4.3.8 Critical Duration & Median Temporal Patterns The hydrologic model was used to undertake ensemble modelling of ten temporal patterns for a range of storm durations. The critical duration and upper median temporal pattern for each storm event, at the downstream boundary of the proposed development, is shown in Table 4-7 for the pre-development and post-development scenarios.

Table 4-7 Critical Duration and Upper Median Temporal Patterns 50% AEP Event 10% AEP Event 1% AEP Event

Pre-Development Critical Duration 24-hour 9-hour 6-hour

Pre-Development Upper Median Temporal Pattern Pattern #8 Pattern #8 Pattern #1

Post-Development Critical Duration 24-hour 6-hour 12-hour

Post-Development Upper Median Temporal Pattern Pattern #4 Pattern #5 Pattern #6

4.4 Pre-Development Hydraulic Model Setup A two-dimensional TUFLOW hydraulic model of the catchment was developed for the pre-development and post-development conditions at Lowes Creek Maryland, to assess the potential flood impacts caused by the proposed development.

4.4.1 Terrain The terrain used in the pre-development hydraulic model is shown in Figure 4-2.

4.4.2 Inflow Hydrographs The TUFLOW model inflow hydrographs have been extracted from the XP-RAFTS hydrologic model for sub-catchments and applied to the hydraulic model at their corresponding locations.

As noted in Section 4.3.6, the hydrologic catchment delineation varies between existing and developed scenarios. Therefore, different flow hydrographs were applied to the model at different locations for each scenario.

This could ultimately lead to local differences in flood levels across the Precinct purely through differences in modelling and may not always be associated with the development. Based on our experience in modelling similar scenarios, we have assumed that in general these localised water level differences are only minor and localised to the vicinity of model inflow locations, and therefore this approach is appropriate. The results have been reviewed in consideration of this.

4.4.2.1 Model Inflow Locations

XP-RAFTS model hydrographs are inserted into the TUFLOW model as point flows, inserting local hydrographs for each sub-catchment. The exception is for catchments located a significant distance from the mainstream; in these cases, tributaries have been inserted into the model as a total hydrograph directly near the tributary waterway.

These flows have not been inserted into the model at these intermediate points within the Precinct, as it would result in overland flow-paths being formed away from the mainstream flow areas. These overland flows are not necessarily representative, and the objectives of the WCMS do not relate to the management of locally generated overland flow. The conveyance of such minor overland flows is assumed to be addressed in future stages of the Precinct.



4.4.3 Downstream Boundary Conditions A review of the water level results from the Upper South Creek FRMS&P model show that there is a consistent water level gradient in the vicinity of the downstream model boundary. Therefore, the application of a normal-depth downstream boundary condition in the hydraulic model is appropriate.

The location of the downstream boundary condition has been applied such that there are negligible tailwater effects on the flood behaviour within the study boundary. The normal-depth assumption downstream boundary condition was adopted using a slope of 0.5%, and has been applied to both pre-development and post-development models.

4.4.4 Surface Material Types Surface material types were delineated in the hydraulic model to represent the roughness of surfaces. Different surface roughnesses have been accounted for using Manning’s ‘n’ values based on observed site conditions for the pre-development scenarios, and these are shown in Figure 4-7.

A summary of adopted roughness values for representative land surfaces is summarised in Table 4-8.

Table 4-8 Adopted Roughness Values in XP-RAFTS Model Land Use Adopted Roughness Value

Floodplain / Rural Areas 0.04

Road/Asphaltic Concrete Surfaces 0.02

Permanent Water Body 0.015

Unsealed Roads 0.03

Heavy Vegetation 0.08

Buildings 1.0

The adopted roughness values in the TUFLOW hydraulic model are consistent with those adopted within the Upper South Creek FRMS&P (Cardno, 2014). However, in the FRMS&P model for the pre-development scenario the entire area was assumed to have a roughness value of 0.04, whereas for this study the existing site can be divided into more specific surface types. The majority of the catchment remains as rural land use with a Manning’s “n” value of 0.04, however for example in the model the water bodies of the large farm dams have been assigned a Manning’s “n” value of 0.015.

4.4.5 Existing Hydraulic Structures Culverts were modelled in TUFLOW using parameters shown in Table 4-9. Based on industry common practice the entry and exit losses adopted are appropriate for the culverts within the Precinct and the roughness value is within commonly accepted roughness for reinforced concrete pipes.

It is noted that the design blockage percentage for structures of 50% is in accordance with requirements of Council’s Engineering Design Specifications (Camden Council, 2009). This approach is also consistent with the 50% blockage that was also applied to one-dimensional elements used in the Upper South Creek FRMS&P (Cardno, 2014).

Table 4-9 Adopted Hydraulic Structure Parameters Parameter Adopted Value

Manning’s ‘n’ 0.015

Entry Loss 0.5

Exit Loss 1.0

Design Blockage Percentage 50%

The pipe and culvert elements outside the Precinct remain unchanged for the post-development scenario, with a series of detention basin outlets added to the model, which are discussed further in Section 6.2. The model parameters for one-dimensional elements of the post-development model are the same as those in Table 4-9, the only exception is that proposed detention basin outlets have been modelled with no blockage



factor applied. The objective of the detention basins is to detain flows meaning that potential blockage of these pipes will further detain flows and improve detention performance. In this way, not applying blockage factors to basin outlets is a conservative assessment of the basins performance as the maximum possible flow is discharged from the basin.

A number of existing culverts are modelled as one-dimensional elements in the TUFLOW model. The culverts and surface material types adopted in the pre-development model is shown in Figure 4-7

Figure 4-7 Surface Material Types and Culverts in Pre-Development TUFLOW Model



4.5 Model Validation The pre-development hydrology and hydraulic models of Lowes Creek Maryland were previously validated using modelling results from the Upper South Creek FRMS&P (Cardno, 2014). The original hydrologic and hydraulic models of Lowes Creek Maryland were set up using Australian Rainfall and Runoff 1987 (ARR87) to ensure that there was an appropriate comparison with the results from The Upper South Creek FRMS&P, which also adopted ARR87 modelling methods.

However, since the release and adoption of ARR2016, this comparison is no longer valid as there are significant differences between ARR87 and ARR2016 methodologies.

4.6 Pre-Development Hydraulic Modelling Results The pre-development hydraulic model was used to simulate the 50%, 10% and 1% AEP events. Peak flood level, depth and velocity mapping for these modelling results are presented in Appendix C.

The pre-development flood behaviour of the Lowes Creek Maryland Precinct has been summarised into the following main areas:

4.6.1 Lowes Creek The Upper Lowes Creek floodplain is generally confined to the creek channel with minimal overbank flooding for all events up to the 1% AEP. The upper reaches of Lowes Creek have a steep grade which results in the flood extents being well confined; however, this is also associated in relatively high velocities (exceeding 1 m/s in the 50% AEP event).

Further downstream along the eastern portion of Lowes Creek the channel grade flattens, and the additional flows of the various tributaries contribute to a widening of the floodplain. Flood depths within the floodplain of Lowes Creek in the 10% and 1% AEP events is limited to less than 300mm. The flood velocities within the Lowes Creek channel progressively decrease downstream as an increasingly higher proportion of floodwaters are conveyed within overbank areas.

4.6.2 Culvert Crossing at The Northern Road The existing culverts under The Northern Road, to the north-western side of the proposed development, are able to convey flows in the 50% AEP event without overtopping the road. However, flows overtop The Northern Road at a low point to the north of the culverts in the 10% and 1% AEP events.

It is expected that during the 1% AEP event, the culverts beneath The Northern Road will convey a peak flow of approximately 25 m3/s, and the flow overtopping the road is expected to have a peak flow of 29 m3/s, in the pre-development scenario.

4.6.3 Western Tributary Flows from the western tributary are well contained within the channel in all events up to and including the 1% AEP event. The existing culvert at the south-western edge of the study boundary is expected to convey the majority of 1% AEP flows (peak of 13 m3/s through culvert), with minor overtopping of the road (peak of 0.6 m3/s flow).

As the western farm dam significantly widens and deepens, flow velocity is reduced. Flows are discharged from the farm dam spillway, located at the eastern side of its embankment. The confluence of the Western Tributary and Lowes Creek is approximately 400 metres from the western farm dam spillway.

4.6.4 Central Tributary Several farm dams are located along the Central Tributary, upstream of study boundary. During the 1% AEP, overbank flooding is expected along the Central Tributary. In addition, eastern channel flows not conveyed through The Northern Road crossing form a secondary flowpath, which diverts flows to the central channel at the upstream boundary of the Lowes Creek Maryland Precinct. This diversion of flows from the eastern channel to the central channel was not reflected in the coarser modelling of the Upper South Creek FRMS&P (Cardno, 2014).



Two culvert systems convey flows under the existing access road for the central channel with approximately half of the 1% AEP flow overtopping the access road surface. Central channel flows cross a secondary, less formal access road through the site immediately upstream of the large central dam which widens, and slows flows to match the dam footprint. Discharge from the central dam is via two low points on the eastern and western edges of the dam embankment. These flows converge downstream of the central dam and are conveyed through a low-lying, heavily disturbed floodplain with no recognisable channel and a series of smaller low lying dam structures. Flows from the central channel converge with Lowes Creek flows immediately upstream of The Northern Road culvert crossing.

4.6.5 Northern Western Tributary Two minor tributaries of Lowes Creek have been modelled on the northern side of the study area, which both convey flows from the elevated sides of the catchment via steep channels. Flows from the heavily vegetated north-west tributary converge on an upstream farm dam, with flows being diverted in two directions:

The majority of the 1% AEP from the north-west tributary (3.4 m3/s) is conveyed as overland flow east to a larger farm dam on the upstream end of the secondary tributary. This secondary tributary as several streams of sheet flow, eventually converging with Lowes Creek near the centre of the Lowes Creek Maryland Precinct.

The remaining flow from the north-west tributary (1.5 m3/s in the 1% AEP) is conveyed directly to the south, through a heavily vegetated and well defined channel to a downstream farm dam before converging with upper Lowes Creek on the western side of the Lowes Creek Maryland Precinct.

The upper reaches of a northern tributary that does not discharge to Lowes Creek collects flow in a farm dam at the northern boundary of the Lowes Creek Maryland Precinct. Flows overtop the dam embankment and is conveyed as overland flow through rural lands to the north with no defined channel downstream of the Precinct.

4.7 Post-Development Hydraulic Model Setup The pre-development hydraulic model was modified in order to develop the post-development hydraulic model. The post-development hydraulic model was required to account for a number of proposed changes to the site, which are discussed in the following sections:

Increased stormwater run-off from the developed catchments of the proposed development (refer to Section 4.3.6 for further details);

Impact of proposed online and offline detention basins. The model methodology, design and performance of these basins is discussed in further detail in Section 6;

Filling of developable areas on the fringes of the floodplain – Section 4.7.1;

Proposed road crossings of the various waterways in the Precinct;

Though not part of the Lowes Creek Maryland Precinct development, the proposed box culvert upgrade to The Northern Road has not been included in the model. Only the road raising has been included; and

Proposed works within the riparian corridor including re-aligning of channels and vegetation management works.

4.7.1 Terrain The assumed approach to flood risk management for the future Lowes Creek Maryland Precinct is that all developable areas (residential areas, commercial areas and proposed road crossings) will be filled to an elevation at or above the Flood Planning Level. The Flood Planning Level is defined as the 1% AEP peak flood level plus 500mm freeboard. This approach is in accordance with the provisions of the engineering specification (Camden Council, 2009) and the Growth Centres Development Code (NSW Government, 2006).

This approach provides several advantages:

It ensures that flood risk to future properties in the Precinct is suitably low;



It reduces potential risk to life in all flood events up to and including the 1% AEP flood. It is noted that flood risk to life is not completely removed as there is a risk of flooding from events greater than the 1% AEP event, such as the PMF. To address this residual risk, an assessment of evacuation potential has been provided in Section 5 to ensure there are suitable evacuation routes with rising grade away from the floodplain;

It ensures that future subdivided lots in the Precinct will not have 1% AEP flood notifications associated with the properties, and it ensures building floor levels do not need to be elevated above future ground levels as a result of FPL requirements.

In preliminary discussions with Camden Council, it has been requested that future sports fields which are proposed to adjoin the riparian corridors in the Precinct be made flood-free up to the 1% AEP event. The intention is to maximise the utility of the sports fields and limit flood damage to surfaces in the future. Therefore, the only flood affected portions of the proposed Lowes Creek Maryland Precinct are those assigned as riparian corridor or drainage land for basins.

Areas within the development that are filled above the existing terrain are matched to the terrain within the riparian corridor using batter slopes of 1V:4H (that is, a 1 metre vertical slope occurs over a 4 metre horizontal length). The edge of the riparian corridor has been set at the fill level with battered slopes into the riparian corridor. This battering into the riparian corridor is a suitable use within the corridor as defined within the Guidelines for Controlled Activities on Waterfront Land (NOW, 2012).

The preliminary grading design used in the post-development hydraulic model is shown in Figure 4-8. It should be noted that while the preliminary grading design is subject to further design in later stages, it is appropriate for the preliminary assessment of the post-development flood behaviour at Lowes Creek Maryland.



Figure 4-8 Post-Development Terrain showing the Preliminary Grading Design



4.7.2 The Northern Road Upgrade In response to a preliminary summary report for this WCMS, Camden Council requested that the development of the Lowes Creek Maryland Precinct consider the upgrade of The Northern Road crossing of Lowes Creek proposed by NSW Roads and Maritime Services (RMS).

In order to assist this Study, Camden Council provided 80% design drawings for the road upgrade on 30 October 2016. The design drawings (drawing number DS 2015/000783) provide details on the proposed culvert design, with 13 cells, all 2.4m W x 2.1m H box culverts, with details shown in .

However, the existing culverts have been modelled for both pre-development and post-development scenarios, for the purposes of this study. The existing culvert design consists of triple-cell 3.6m W x 2.0 m H box culverts.

It is envisaged that by modelling the upgraded culvert design will lower peak flood levels in both pre-development and post-development scenarios. This would potentially provide more freeboard to roadways and habitable floor levels in the vicinity of The Northern Road crossing at Lowes Creek.

It has been assumed that the proposed road surface will be raised above the 1% AEP level, whereas in the pre-development scenario a significant portion of the 1% AEP flow overtops the road to the north. Therefore, even though the proposed culvert system is significantly greater than in the pre-development scenario, the removal of road overtopping restricts overall discharge. Based on comparison of post-development modelling with and without the road upgrade, results show peak water level increases upstream of The Northern Road of approximately 0.65 metres for the 1% AEP. These flood level increases have been considered in the fill design.

4.7.3 Riparian Corridor Works While the approach adopted within the Lowes Creek Maryland Precinct has been to maintain existing riparian corridor conditions as much as possible, some minor changes have been proposed. These changes have been proposed in order to improve the hydraulic behaviour within the riparian corridor, with environmental impacts of these works to be offset by revegetation plans and geomorphic stabilisation works.

The modelled changes to the post-development riparian corridor include:

Downstream of both the proposed West and Central basins there are segments of floodplain that are highly disturbed with poorly defined channels. In order to improve hydraulic efficiency and reduce erosion at these locations, natural-style channels are proposed to be constructed to connect the outlets of the online basins with the confluences with Lowes Creek.

For the downstream portion of the North Tributary in the pre-development scenario, flows were diverted into a farm dam prior to discharge to Lowes Creek. The footprint of this farm dam is proposed to be the location of the offline basin NW2 (refer to Section 6.4). With the conversion of this dam to an offline basin, a design channel has been proposed for the tributary diverting flows to the east and discharging them to Lowes Creek at a confluence further downstream than in the pre-development scenario. Parts of this channel reach are well vegetated in the pre-development scenario so management plans will need to ensure the channel works are undertaken in a manner to minimise impacts on existing vegetation.

As part of the riparian corridor management plan prepared by Eco Logical Australia, a vegetation management plan has been proposed that involves the connection of existing vegetation in the upper reaches of the North Tributary with the existing vegetation of Lowes Creek downstream. The impacts of these areas of new vegetation on hydraulic behaviour have been incorporated into the model through adopting a manning’s roughness value of 0.08 replacing a lower roughness of 0.04 corresponding to open space in the pre-development scenario.



4.7.4 Proposed Hydraulic Structures There are seven proposed road crossings of waterways within the proposed Lowes Creek Maryland site:

Three crossings of Lowes Creek, in the upper, mid, and lower reaches of the Precinct;

One crossing of North-west Tributary, in between proposed Basins NW1 and NW2;

One crossing of West Tributary, upstream of the proposed online basin; and,

Two crossings of Central Tributary, upstream of the proposed online basin.

These crossings have all been nominally modelled as box culverts. However, in accordance with the Guidelines for Controlled Activities on Waterfront Land (NOW, 2012), the three Lowes Creek crossings and the crossing for the North-west Tributary are expected to ultimately be constructed as bridges. This is because both waterways are nominated as key environmental protection areas, with bridge crossings having the least impact on a range of factors including maintaining geomorphic, hydrologic and provide better fish passage and environmental connection than culverts.

For the proposed crossing of West Tributary and two crossings of Central Tributary, the environmental concerns for the waterways are not significant as they are located immediately upstream of the online basins, the online basins are discussed further in Section 6.3. As a result, pipes have been proposed for these three crossings, in order to provide additional detention for the online basins downstream. The details of the culverts under road crossings are provided in Figure 4-9.



Figure 4-9 Surface Material Types and Culverts in Post-Development TUFLOW Model



4.8 Post-Development Hydraulic Modelling Results The post-development hydraulic model was used to simulate the 50%, 10% and 1% AEP events. Peak flood level, depth and velocity mapping for these modelling results are presented in Appendix C. Flood level difference mapping is also included

The post-development flood behaviour of the Lowes Creek Maryland Precinct has been summarised into the following main areas:

4.8.1 Lowes Creek The flood levels along the Lowes Creek channel have mainly remain unchanged. The road crossings along the Lowes Creek confluence causes minor flood level decreases of approximately 130 mm in the 10% and 1% AEP events. Localised increases in flood extent and levels are expected at these road crossings during the 50% AEP event; however, it should be noted that the modelled culverts at these crossings have been nominally sized, and are subject to further design in later stages.

4.8.2 Culvert Crossing at The Northern Road The existing culverts under The Northern Road, to the north-western side of the proposed development, is able to convey flows in the 50% AEP event without overtopping the road. However, flows overtop The Northern Road at a low point to the north of the culverts in the 10% and 1% AEP events.

It is expected that during the 1% AEP event, the triple-cell box culverts beneath The Northern Road will convey a peak flow of 16 m3/s, and the flow overtopping the road is expected to have a peak flow of 25 m3/s, in the post-development scenario. When compared to the pre-development scenario, the reduced peak flow is likely caused by the change in critical duration, and the proposed road crossings may cause some flow retardation.

4.8.3 Western Tributary Flows from the western tributary are well contained within the channel in all events up to and including the 1% AEP event. The existing culvert at the south-western edge of the study boundary is expected to convey the majority of 1% AEP flows (peak of 10 m3/s through culvert). While the roadway was overtopped in the 6-hour storm in the pre-development scenario, the 12-hour storm modelled for the post-development scenario shows that this roadway is not overtopped.

4.8.4 Central Tributary The flood behaviour within the Central Tributary has remained unchanged in areas upstream of the proposed development. However, changes in flood behaviour at the southern boundary of the study area are expected due to the proposed filling and inclusion of water quality basins. These impacts are contained within the study area and therefore do not affect the flood behaviour in adjacent properties.

Again, since the post-development model was simulated for the 12-hour 1% AEP duration storm, flows do not spill from the Eastern Tributary into the Central Tributary.

4.8.5 Northern Western Tributary In the post-development scenario, the proposed development involves filling over one of the channels that convey flow from the North-Western Tributary to Lowes Creek. As such, all flow that overtops the farm dam at the North-Western Tributary flow directly south toward Lowes Creek.

In addition, proposed development involves filling of a portion of the floodplain at this channel. Accordingly, flow along this tributary is expected to have areas where flood velocities of 1-2 m/s in the 1% AEP event. This flood velocity differences are expected to be up to ±0.7 m/s when compared to the pre-development scenario within this channel.



5 Flood Emergency Response Strategy

The proposed development at Lowes Creek Maryland has been designed such that areas zoned for residential or commercial use are filled to an elevation above the 1% AEP peak flood level plus 500 mm freeboard. All access roadways and roadways within the proposed development are not expected to be inundated during the 1% AEP event. Accordingly, no flood emergency response is required for events up to and including the 1% AEP event.

However, some proposed lots and roadways are expected to be inundated during the PMF (refer Appendix C, Figure 25). Key areas that are predicted to be inundated or isolated include:

The roadway at the southern side of the proposed development is expected to be inundated at two locations, at the Western Tributary crossing and at the Central Tributary crossing. The road is expected to have a H5 flood hazard, and is therefore not trafficable, causing all proposed lots between the Western and Central Tributaries to become isolated. The duration of inundation at these roadway crossings is between 1.5 to 2 hours, before the flood hazard is reduced to H1 and becomes trafficable.

The Northern Road (in its existing condition) is also expected to be inundated at two locations: north-east of the study area, and south-east of the study area. As such, the proposed lots situated between the Central Tributary and The Northern Road would become isolated during a PMF. The Northern Road is expected to be untrafficable for a duration of greater than 6 hours north-east of the study area and for a duration of approximately 1 hour south-east of the study area. After this, The Northern Road should have a flood hazard that reduces to H1 and therefore becomes trafficable.

The proposed lots located immediately to the east of the Central Online Basin are expected to be inundated to a depth of approximately 800mm. The inundation at these lots is associated with a H3 hazard, which is unsafe for vehicles, children and the elderly (refer Figure 5-1). The duration of inundation at these lots is expected to be approximately 1.5 hours before the flood hazard is reduced to H1, and therefore becomes generally safe for people, vehicles and buildings. Some adjustment to the proposed terrain along the eastern bank of the Central Tributary would likely reduce the depth of inundation and associated flood hazard in this area.

As noted above, some areas within the proposed development at Lowes Creek Maryland are likely to become isolated during the PMF. The access roads leading to these isolated areas are untrafficable for approximately 2 hours. This short duration of isolation is generally considered to be acceptable.

The only access route into the proposed development that remains untrafficable for longer than 2 hours (approximately 6 hours) is at the Lowes Creek crossing of The Northern Road. However, The Northern Road was modelled at its current condition, and does not represent any future proposed upgrades to the road. It is assumed that any future upgrades of this road will provide sufficient flood-free access to the proposed development at Lowes Creek Maryland in the event of a PMF.

It is expected that further consultation will be undertaken with Council and NSW SES to develop a flood emergency response plan following rezoning.



Figure 5-1 General Flood Hazard Curves (extracted from ARR2016, Book 6, Section 7.2.7)



6 Stormwater Quantity Management Strategy

6.1 Modelling Approach

6.1.1 Post-Development Scenario Model Set-Up The stormwater quantity and flood management measures have been designed with the flood models that were detailed in Section 4.

It is noted that the adopted post-development scenario in the analysis of detention basins includes The Northern Road in its current configuration. NSW Roads and Maritime Services (RMS) are planning an upgrade of the road however this road upgrade is not related to the proposed development of the Lowes Creek Maryland Precinct. Therefore, in order to ensure the flows are compared to the same scenario as the existing base case and only account for the development of the Lowes Creek Maryland Precinct and the raising of the terrain at The Northern Road upgrade.

The proposed upgraded culverts have not been modelled in this iteration of the hydraulic model. However, it is envisaged that the upgraded culverts will cause an overall increase in flows through The Northern Road crossing in both pre-development and post-development scenarios.

6.1.2 Modelling of Detention Basins Detention basins have been sized through an iterative process to ensure that discharges from the Lowes Creek Maryland site do not exceed those of the pre-development scenario TUFLOW model.

The modelling of detention basin storage has been conducted within the two-dimensional domain of the TUFLOW model with unmitigated developed catchment flows being inserted into the TUFLOW model within the basin footprint and basin embankments included in the 2D domain. The 3 metre grid cell size of the TUFLOW model is able to provide a suitable level of detail to accurately model the detention storage of the basins.

Basin outlets, including overflow weirs, have been modelled within the one-dimensional domain of the TUFLOW model. As noted previously, the basin outlets have been modelled with no design blockage applied as unblocked outlets is a conservative assumption when assessing the flood attenuation performance for the basins.

6.2 Basin Strategy A total of 6 offline stormwater detention basins and 2 online stormwater detention basins are proposed for the Lowes Creek Maryland Precinct. The basin design approach maximised the use of offline basins in preference of online basins, thereby reducing the volume of the online basins as much as practicable.

The layout of the proposed detention basin network is shown in Figure 6-1.



Figure 6-1 Online and Offline Detention Basin Layout

6.2.2 Basin Design Principles The design of the detention basins has been guided by a number of recommendations from the Camden Council Engineering Design Specification (2009), as well as the (Guidelines for Controlled Activities on Waterfront Land (NOW, 2012)). The basic design principles applied to the detention basins are as follows:



All basins have been designed to have side slopes of no steeper than 1V:4H, with side slopes to be lined with normal planting. The exception is for Basin NT1 where space restrictions have resulted in retaining walls to be proposed on all four sides of the basin allowing for a far steeper side slope.

Top of bank widths of embankments have been assumed to be 3 metres to allow vehicular access if needed in the future;

Basin embankments have been designed to the Flood Planning Level (1% AEP plus 0.5 metre freeboard);

Bio-retention basins have been co-located in all offline basins. In accordance with provisions from NOW (2012) all online basins have not had water quality treatment co-located within them;

Basin invert surfaces for all online basins have a slope of approximately 1% grade; 0.5% grade in the downstream direction of the basin, and 0.5% cross grade perpendicular to the flow direction from the sides of the basin to a low point at the centre. Offline basin inverts have been applied at a constant elevation, however it is recommended that further detailed design stages apply a grade of 1% on the basin invert and consider the relative elevation of any co-located bio-retention basins to ensure low flows drain to the water quality treatment measures in the basin;

Offline basin inverts are generally elevated above the adjacent 1% AEP mainstream flood level. This approach will ensure that tailwater effects shall not affect the detention performance of the basin in all events up to the 1% AEP event;

Low flow pipe outlets, designed to attenuate the 50% AEP flows have been proposed at the following:

- 0.3 metres above basin inverts for offline basins to ensure extended detention depth is maintained for co-located bio-retention basins;

- At the low point of the basin invert for all online basins to ensure online basins are dry in accordance with recommendations from NOW (2012).

High flow outlets have been designed to discharge the 1% AEP flows mostly as overflow weirs, however two basins have secondary pipe outlets (NT1 and CT2). All high flow outlets are located above the 50% AEP level in the basin.

6.3 Proposed Online Detention Basins As discussed in Section 4.2.2 there are two major farm dams within the Lowes Creek Maryland Precinct; referred to in this study as west dam and central dam. The intention for the post-development Precinct is for the removal of these farm dams, with the permanent water bodies to be completely drained. The outer portions of the dam footprints are proposed to be converted to developable land, and the inner portions to be retained as an 80 metre wide riparian corridor.

The removal of these dams has the potential to have a significant impact on the post-development hydraulic behaviour within and downstream of the Precinct. The Pre-development Model results suggest the basins provide a significant volume of flood storage for the west and central tributaries in the pre-development scenario.

Therefore, it is proposed that two large online basin systems be developed for these two tributaries in the approximate location of the existing dam footprints. The objective of these two proposed online basin systems would be:

To supplement the flood storage provided by the existing farm dams, allowing flows to be detained in order to match the existing discharges from the west and central dams in the post-development scenario.

To reduce the increased stormwater run-off from the portions of the proposed Precinct which drain to the west and central tributaries. The increased stormwater run-off from these catchments within the Precinct is created by the increased impervious surfaces for the post-development site.

It is understood that for a number of relevant State Government agencies and Council, online basins are not seen as the preferred type of detention basins for development Precincts. This is primarily due to the



disturbance of environmental flows for waterways and the potential impacts this may have on other environmental factors such as fish passage.

However, in the instance of these two proposed basins this is not seen as such a key concern, due to the fact that the proposed online basins will be located on the footprints of existing large farm dams. In effect, the existing environmental flows for these two tributaries have been severely compromised for several decades. In this way, the proposed online basins would represent an improvement to the environmental flow regime compared to the pre-development scenario: The online basins are proposed to include low flow outlets, allowing discharge to Lowes Creek from the

tributaries even during smaller rainfall events. While review of site survey suggests the existing dam embankments had low points equating to low flow weirs (see Section 4.2.2 for further discussion), pipe outlets at the basin invert are a preferential way to guarantee environmental flows are maintained;

Details of the two online basins and their performance are discussed in the following sections.

6.3.1 Western Online Basin The proposed Western Online Basin has been designed to fit within the existing western farm. The details of the Western Online Basin are summarised in Table 6-1.

Table 6-1 Details of the Western Online Basin

Parameter Western Online Basin

Basin Footprint Area 98,300 m2

Downstream Basin Invert 82.1 mAHD

Embankment Crest Elevation 85.3 mAHD

Low Flow Outlet Details* 450 mm diameter pipe culvert

Modelled Spillway Weir Width 10 m rectangular weir

Spillway Elevation 84.4 mAHD

50% AEP Peak Water Level 82.0 mAHD


50% AEP Volume 4,100 m3


*All invert levels for online basin low flow outlets are equal to the downstream basin invert.

6.3.2 Central Online Basin The proposed Central Online Basin has been designed to fit within the existing central farm dam. The details of the Central Online Basin are summarised in Table 6-2.

Table 6-2 Details of the Central Online Basin

Details Central Online Basin

Basin Footprint Area 85,700 m2

Downstream Basin Invert 73.5 mAHD

Embankment Crest Elevation 77.7 mAHD



Low Flow Outlet Details* 450 mm diameter pipe culvert

Modelled Spillway Weir Width 10 m rectangular weir

Spillway Elevation 76.5 mAHD





*All invert levels for online basin low flow outlets are equal to the downstream basin invert.

6.4 Proposed Offline Detention Basins A network of offline basins is proposed for developed catchments discharging directly to Lowes Creek and two other northern tributaries.

There are 6 offline basins proposed adjacent to Lowes Creek. The details of the basins, including peak 50% AEP and 1% AEP flood levels and volumes are summarised in Table 6-3.

Table 6-3 Lowes Creek Offline Basin Sizes and Modelling Results

Details LC1 LC3 NW2 WQ4 WQ5 NT1

Basin Area (m2) 19,500 18,750 6,900 54,900 33,500 9,950

Basin Invert (mAHD) 86.4 73.7 86.5 76.0 69.0 87.7

Embankment Crest (mAHD) 88.1 75.2 87.7 78 71.7 89.9

Low Flow Outlet Pipe Diameter 525 mm 600 mm 375 mm 750 mm 450 mm 225 mm

Rectangular Spillway Width (m) 10 15 5 5 5 1

Spillway Invert Elevation (mAHD) 87.4 74.5 87.4 77.3 71.0 88.0

50% AEP Water Level (mAHD) 87.0 74.2 86.7 76.7 69.7 88.8

1% AEP Water Level (mAHD) 87.6 74.7 87.2 77.5 71.2 89.4

50% AEP Volume (m3) 3,550 4,200 400 13,250 5,900 4,000

1% AEP Volume (m3) 9,550 8,850 1,800 36,100 31,400 7,550

6.4.2 Offline Basin Performance The most robust method of assessing the effectiveness of OSD basins is the detailed 2D modelling provided in Section 4. However, to provide additional review of the proposed basins, the basin performance has been reviewed against the SSR and PSD requirements provided in the Growth Centres DCP and included in Table 6-4.

Table 6-4 Stormwater Detention Checks 50% AEP 1% AEP



Site Storage Requirement (SSR) Target (m3/ha) 300 594

Permissible Site Discharge (PSD) Target (m3/s/ha) 0.03 0.17

The total storage volume of all proposed online and offline basins have been included in Appendix B and are compliant with the SSD and PSD requirements outlined above.

6.5 Comparison of Peak Discharge Flows The peak discharge flows from the furthest downstream point of the proposed development was extracted from the post-development hydraulic model and compared with the pre-development modelling results. The peak outflows were compared for the 50%, 10% and 1% AEP events, and are shown in Table 6-5.

Table 6-5 Peak Flow Comparison Downstream of Lowes Creek Maryland

Event Pre-Development Scenario

Post-Development Scenario

Percentage Difference

50% AEP 5.3 m3/s 5.3 m3/s 0%

10% AEP 17.9 m3/s 13.6 m3/s –24%

1% AEP 55.1 m3/s 30.8 m3/s –44%

With the current online and offline detention basin low-flow pipes and spillway configurations, there is expected to be minimal change in peak flows for the 50% AEP event and decreases in the 10% AEP and 1% AEP events. Given there is a decrease in flows for the 10% AEP and 1% AEP events, this means the basin outlets could be further optimised to reduce storage volumes and/or allow for future design changes or unforeseen site constraints.



7 Water Quality Management

7.1 Background

7.1.1 Pollutant Reduction Targets The aims of the water quality modelling were to assess the impacts of the proposed development on stormwater quality and estimate the sizes of the WCM measures required to meet the water quality objectives for the Precinct as set out in Table 7-1. An ideal stormwater outcome scenario has also been identified in Council’s Growth Centres DCP; however, this is an ideal outcome and not a requirement of the treatment strategy.

These reduction scenarios align with those specified within the Camden Growth Centre Precincts Development Control Plan (refer to Section 2.1.2 for further details).

The critical pollutants modelled are Gross Pollutants, Total Nitrogen (TN), Total Phosphorous (TP) and Total Suspended Solids (TSS).

Table 7-1 Pollutant Reduction Targets

Pollutant % Reduction Target Ideal Stormwater Outcome

TSS 85% 90%

TP 65% 85%

TN 45% 85%

Gross Pollutants 90% 100%

Reduction percentages are based on comparison of developed conditions with and without water quality treatment measures.

7.1.2 Water Quality Treatment Approach Gross Pollutant Traps (GPTs) and bio-retention basins are the proposed water quality treatment measures to achieve the target reductions of pollutants for the post-development Precinct. The majority of the catchment will discharge to the water quality elements prior to discharge to Lowes Creek and tributaries.

Water quality treatment measures proposed for the Precinct are outlined in Table 7-2.

Table 7-2 Water Cycle Management Measures for Lowes Creek Maryland Precinct Management Measure

Description

Gross Pollutant Traps (GPT)

Neighbourhood scale control of gross pollutants, suspended solids and phosphorous in purpose designed devices. Proprietary products are most appropriate for underground drainage systems and trash racks/deflectors are most appropriate for the inlets to detention basins.

Bio-retention Basins

The basins will incorporate a GPT at the inlet and a bio-filter area at the low point to provide biological treatment of low flows from frequent storms. The bio-retention system will be sized to meet targets.

Monitoring A water quality monitoring plan is to be developed both with baseline data and additional on-site sampling for water quality in the nearest riparian watercourse. Water quality monitoring probes for automated water quality sampling are recommended to establish baseline water quality data prior to urban development. The probes should remain in place and continue to monitor water quality both during and following construction. Additional on-site sampling is to be undertaken upstream and downstream of the development input to the water course along with sampling from the development itself. Reporting of the testing results is to be included throughout all stages of the planning process. Auditing and corrective action should be outlined in a Soil & Water Management Plan.

Previous designs of water quality treatment measures for the Precinct attempted to utilise a scheme of ‘over-treating’ some developed catchments to compensate for other catchments allowing them to discharge to waterways without any water quality treatment. However, upon review this approach was found to be an



inefficient approach to water quality treatment requiring very large water quality basins to allow for the compensation ensuring overall water quality reductions were met.

As a result, the adopted approach to water quality treatment has been revised so that almost all development areas of the Precinct drain to a bio-retention basin for treatment prior to discharge to waterways.

There are some minor developable areas proposed to discharge without treatment, located along Lowes Creek at the centre of the Precinct. Due to both topography and configuration of the Precinct layout it was assumed to be too difficult to direct stormwater from these areas to a proposed bio-retention basin. However, as these untreated development areas are relatively small in size, the additional basin area required to compensate is only minor.

There are 21 bio-retention basins strategically located throughout the Precinct, each at the downstream boundary of their respective catchments as shown in Figure 7-1. The bio-retention basins are proposed in the following locations:

A total of six (6) bio-retention basins are co-located within offline detention basins (WQ1, WQ2, WQ3, WQ4, WQ5, WQ10);

Two bio-retention basins or bio-retention swales are proposed to be located within the two large sporting field areas of the proposed Precinct (LC6 and LC7);

Thirteen bio-retention basins are located within open space areas adjacent to the riparian corridor, three adjacent to the western basin corridor (WT1, WT2, and WT3), four located adjacent to the eastern basin corridor (CT1, CT2, CT3, and CT4). Two basins are located on the western end of the precinct near the junction of the two tributary (NW1 and LC2), there are four basins are located at the downstream of the eastern basin corridor (CT5, CT6, CT7 and CT8); and

The total bio-retention basin filter area across all 21 basins is 48,300 m2. Compared to the total developable area within the Lowes Creek Precinct, excluding riparian corridors, heritage sites and sports fields, of 447.9 ha, the basin filter media area is equivalent to approximately 1.1%.



Figure 7-1 Post-development Bio-retention Basin Locations

7.2 Water Quality Modelling The water quality analysis for this study has been undertaken using MUSIC (Model for Urban Stormwater Improvement Conceptualisation) Version 6.3. The model provides a number of features relevant for the development:

It is able to model the potential pollutant reduction benefits of gross pollutant traps, constructed wetlands, grass swales, bio-retention systems, sedimentation basins, infiltration systems and it incorporates mechanisms to model stormwater re-use as a treatment technique;

It provides mechanisms to evaluate the attainment of water quality objectives;

The MUSIC modelling was undertaken to demonstrate that the water cycle management system proposed for the Precinct would result in a reduction in overall post-development pollutant loads. MUSIC modelling has been undertaken for the post-development scenario based on the proposed Lowes Creek Maryland Precinct Indicative Layout Plan, with water quality treatment measures designed and sized to treat stormwater from the developed catchments.

The following sections detail the model methodology and parameters adopted.



7.2.1 Rainfall Data Pluviograph rainfall data was downloaded from the eWater website for the nearest daily rainfall station at North Parramatta (Gauge Station 066124). The adopted gauge site is located approximately 30 km to the north-east of the Lowes Creek Maryland Precinct. This gauge was selected for the following reasons:

The gauge was one of the closest gauges available on the eWater website with pluvio data available;

The length of data and quality of recorded data (the least amount of gaps in records) for this gauge was preferential to that of the other pluvio gauges available;

While the gauge is located a significant distance from the Lowes Creek Maryland Precinct, the two sites are both located on the western side of the Sydney basin with comparable elevations. Therefore, it is assumed that the two sites have similar climactic data meaning that the selected rainfall records are appropriate for adoption in this study.

Details of the rainfall gauge are summarised in Table 7-3.

Table 7-3 Selected Rainfall Gauge Data Data Station 066124

Location North Parramatta

Data Period 02/02/1985 - 22/05/2010

Data period used in MUSIC Model 05/03/1996 - 05/03/2008 (12 years)

Data Type Pluvio

Total Rainfall for Period (mm) 9618

Average Annual Rainfall (mm) 801

7.2.2 Evapo-Transpiration Data Evapo-transpiration data has been included as monthly average values extracted from the North Parramatta gauge. The monthly average evapotranspiration (ET) rates adopted in the MUSIC model are listed in Table 7-4.

Table 7-4 Average Daily Evapo-Transpiration by Month (mm) Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec

5.57 4.58 3.74 2.52 1.62 1.28 1.24 1.79 2.5 3.87 4.85 4.97

7.2.3 Rainfall Runoff Parameters A land capability study has been prepared by Douglas Partners (September 2016) has identified the soil profile within the site is generally silty clay, the adopted rainfall-runoff parameters for the post-development catchments are provided in Table 7-5. The adopted rainfall runoff parameters have been based on the NSW MUSIC Modelling Guidelines (BMT WBM, 2015).



Table 7-5 Rainfall Runoff Parameters Parameter Values

Rainfall Threshold (mm/day) 1

Soil Storage Capacity (mm) 54

Soil Initial Storage (% of Capacity) 25

Field Capacity (mm) 51

Infiltration Capacity coefficient - a 180

Infiltration Capacity exponent - b 3

Initial Depth (mm) 10

Daily Recharge Rate (%) 25

Daily Baseflow Rate (%) 25

Daily Deep Seepage Rate (%) -

7.2.4 Pollutant Generation In MUSIC software, stormwater quality is characterised by event mean concentrations (EMC) for storm flow and base flow conditions. In this study, the default EMC parameters were adopted from MUSIC. Base flow and storm flow parameters are provided in Table 7-6. It is noted that the pollutant generation parameters listed below were adopted for all post-development catchments.

Table 7-6 Base Flow Pollutant Concentration Parameters

Parameter Adopted Value

Baseflow TSS Mean (log mg/L) 1.200

Baseflow TSS Standard Deviation (log mg/L) 0.170

Baseflow TSS Estimation Method Stochastic

Stormflow TSS Mean (log mg/L) 2.15

Stormflow TSS Standard Deviation (log mg/L) 0.32

Stormflow TSS Estimation Method Stochastic

Baseflow TP Mean (log mg/L) -0.850

Baseflow TP Standard Deviation (log mg/L) 0.19

Baseflow TP Estimation Method Stochastic

Stormflow TP Mean (log mg/L) -0.60

Stormflow TP Standard Deviation (log mg/L) 0.25

Stormflow TP Estimation Method Stochastic

Baseflow TN Mean (log mg/L) 0.11

Baseflow TN Standard Deviation (log mg/L) 0.12

Baseflow TN Estimation Method Stochastic

Stormflow TN Mean (log mg/L) 0.30

Stormflow TN Standard Deviation (log mg/L) 0.19

Stormflow TN Estimation Method Stochastic



7.2.5 Model Layout The MUSIC model layout showing the post-development scenario is provided in Figure 7-2.

Figure 7-2 MUSIC Model Layout

7.2.6 Catchment Details The proposed development site has been divided into a number of sub-catchments based on the proposed grading and the land use. The site is divided into 4 categories:

Low density residential area;

Medium/High density residential area;

Commercial area; and

Park.

The effective impervious area of the catchment has been calculated based on the NSW MUSIC Modelling Guidelines (BMT WBM, 2015) and is summarised in the Table 7-7.

Table 7-7 Catchment land use and characteristics

Basin ID

Total Impervious Area (%) EIA Factor Adopted EIA (%)

Low density residential

75 0.6 45

Medium/High density residential area

80 0.6 48

Commercial area 90 0.8 72

Park 30 0.05 10*

*10% is adopted to assume future amenity buildings, footpaths and hard surface (netball courts) are directly connected to future drainage network

All sports fields and open space areas within the Precinct have been assumed to also drain to water quality treatment and therefore have been included in the model. However, riparian corridor catchments are not proposed to be treated and have not been included in the MUSIC model.



7.2.7 Modelling of Gross Pollutants Traps Gross Pollutant Traps (GPTs) have been provided to filter stormwater prior to discharge into the bio-retention basins. The expected pollutant removal rates adopted within the model is provided in Table 7-8. For the purposes of MUSIC modelling it was assumed that the GPTs will be located upstream of bio-retention basins. Additionally, it was assumed that GPTS will be located at all other outflows into the waterways.

Table 7-8 GPT Input Parameters Pollutant Input Output

TSS (mg/L)

0 0

75 75

1000 350

TP (mg/L)

0 0

0.5 0.5

1 0.85

TN (mg/L)

0 0

0.5 0.5

5 4.3

GP (mg/L) 0 0

15 1.5

7.2.8 Modelling of Bio-retention Basins The design parameters adopted for the bio-retention systems are shown in Table 7-9. Filter media depths are proposed to be 0.4m.

Extended detention depth of all bio-retention basins has been modelled as 0.3m. For bio-retention basins co-located within detention basins, a weir should be constructed to be 0.3m above the base of the bio-retention basin, ensuring at least an extended detention depth of 0.3 m.

Within the MUSIC model, the basin surface area (the surface area at the extended detention depth) has been set equal to the filter media area (basin invert area). This is considered a conservative approach as in reality all basins are likely to have side slopes of at least 1V:4H meaning the surface area will be greater than the filter media area. However, this simplified approach is considered appropriate at this stage as it allows for optimisation of bio-retention design in later detailed design stages.

Table 7-9 Bio-retention Basin Input Parameters Parameters Value

Saturated Hydraulic Conductivity (mm/hr) 100

Filter Depth (m) 0.4

Extended Detention (m) 0.3

TN Content (mg/kg) 400

Orthophosphate Content (mg/kg) 40

Exfiltration Rate (mm/hr) 0.0

Base Lined Yes

The approximate location of bio-retention basins and the layout of catchments that drain to the basins are provided in Figure 7-1. The proposed surface areas of the bio-retention basins is included in Table 7-10. In addition, included in the table is a summary of the source pollutant loads for each basin and a summary of individual basin performance in removal of TSS, TP, and TN. The basin performances include consideration of the GPTs proposed upstream of the respective basins.



Table 7-10 Bio-retention Basin Filter Surface Areas and Pollutant Loads and Reduction Performance Basin ID Bio-retention

Area (m2) Total Suspended Solids Total Phosphorous Total Nitrogen

Residual Load (kg/yr)

Reduction %


Reduction %


Reduction %

WQ1^ 2700 2350 87 9.8 67 99 55

WQ2 500 479 87 2.0 66 19.6 54

WQ3 1400 1370 85 5.3 65 51.3 54

WQ4 5800 3310 89 15.6 69 152 59

WQ5^ 8000 7260 82 30.6 67 294 57

WQ10 2800 2560 87 10.6 66 105 55

LC2 1500 1150 88 5.3 67 54 56

LC6 1000 1480 71 3.8 55 34 46

LC7 1000 1410 71 3.7 55 31 47

NW1 1000 627 88 2.8 68 27.8 58

WT1 3500 3520 86 14.7 65 147 52

WT2 700 763 85 3.2 65 31.3 51

WT3 1500 1310 87 5.4 66 53 54

CT1 2400 2390 86 9.9 65 98 54

CT2 1800 1490 87 6.4 67 63 56

CT3 2000 1730 88 8.2 66 83 54

CT4^ 2200 2030 88 9.6 66 95 55

CT5 5200 4010 86 16 65 158 55

CT6* 800 794 87 3.5 65 35.2 54

CT7 1400 1030 89 4.7 68 47 56

CT8 1100 679 90 3.5 69 36 57

^ These basins have been modelled and specified as single basins, but have been indicatively split in Figure 7-1 to suit the proposed development catchments and site constraints. Future design development may choose to combine or split these basins, providing water quality targets are still met.

* CT6 is in a commercial precinct and the bio-retention basin may be replaced by proprietary products at this location or other forms of bio-retention (eg; tree pits) incorporated within the commercial precinct design.

The results presented in Table 7-10 show that the majority of bio-retention basins achieve, or are close to achieving the reduction targets for their individual catchments. This has several implications:

It shows that within the Precinct all development catchments are having a significant proportion of pollutants removed prior to discharge, distributing the discharge of pollutants throughout the Lowes Creek riparian corridor;

The basins LC6 and LC7 have lower pollutant reduction as they had been assumed to be smaller scale measures such as tree pits or bio-retention swales. GPTs were not included in the treatment train for these basins; and

It shows that from a water quality treatment stand point each catchment can be developed in isolation with the development of each water quality basin, assisting in future development staging.

7.2.9 Overall Precinct Treatment Performance As shown in the model layout in Figure 7-2, there are two modelled discharge points in the MUSIC model; northern tributary discharge point, and Lowes Creek discharge point. These are the key points of interest in



considering the Precinct’s overall performance as they represent the total pollutant loads being discharged to the two respective receiving waterways. Therefore it is at these two points that the pollutant reduction targets (refer to Section 7.1.1) need to be met.

Results from the MUSIC analysis at the discharge point from the Lowes Creek Maryland Precinct for Lowes Creek and Northern Tributary are presented in Table 7-11 and Table 7-12 respectively.

Table 7-11 Lowes Creek Pollutant Loads, Reduction Percentages and Targets

Parameter Lowes Creek Maryland Precinct Model

Target Ideal Stormwater Outcome Source Loads

(kg/yr) Residual Loads

(kg/yr) Reduction

TSS (kg/yr) 319,000 43,200 86% 85% 90%

TP (kg/yr) 511 177 65% 65% 85%

TN (kg/yr) 3,800 1,730 54% 45% 85%

Gross Pollutants (kg/yr) 45,600 1,130 98% 90% 100%

Table 7-12 Northern Tributary Discharge - Pollutant Loads, Reduction Percentages and Targets

Parameter Lowes Creek Maryland Precinct Model

Target Ideal Stormwater Outcome Source Loads

(kg/yr) Residual Loads

(kg/yr) Reduction

TSS (kg/yr) 9,450 1,370 85% 85% 90%

TP (kg/yr) 15.2 5.3 65% 65% 85%

TN (kg/yr) 113 51 54% 45% 85%

Gross Pollutants (kg/yr) 1,410 47 96% 90% 100%

As can be seen in the results presented in Table 7-11, the proposed treatment measures achieve pollutant reduction percentages that exceed the targets at both Precinct discharge points.

7.3 Summary A water quality treatment strategy including 21 bio-retention basins with associated GPT’s has been designed for the Lowes Creek Maryland Precinct, with the majority of developable areas in the Precinct flowing to a bio-retention basin prior to discharge from the Precinct. The system has been designed to ensure each individual bio-retention basin removes a significant portion of pollutants from the stormwater discharge of each catchment, ensuring pollutants are not concentrated at any particular stormwater outlet.

A MUSIC model was established for the development areas of Lowes Creek Maryland with the performance of the treatment strategy assessed for removal targets for four key pollutants: Total Suspended Solids; Total Phosphorous, Total Nitrogen, and Gross Pollutants. The MUSIC model results show that the proposed bio-retention basin strategy achieves overall pollutant reduction targets.



8 Streambank Condition Management

Streambank erosion can be an important source of sediment in catchments, particularly in streams that have been subject to changes in runoff conditions that created hydraulic conditions of enhanced flow energy. Resulting channel incision can cause taller and steeper banks that become unstable leading to heightened sediment loads. Bank erosion results in loss of adjacent land, can threaten infrastructure, deteriorate instream habitat and cause sedimentation in receiving water bodies such as reservoirs and estuaries.

If not managed properly, urbanisation results in a decrease in infiltration by creating impervious surfaces. This typically results in shorter times to peaks and increased peak flows. These greater discharges provide for greater erosive power in the channels that can cause channel incision and bank instability depending on the resistance of the channel boundary to those hydraulic forces.

To quantify and address these issues Cardno is conducting an investigation into the channel and bank erosion issues along Lowes Creek and its tributaries. This work was undertaken to determine current rates of erosion and the effectiveness of potential mitigation measures to reduce bank erosion.

8.1 Approach to Streambank Condition Assessment To formulate informed management strategies to assess the streambank condition, manage sediment loadings under developed conditions and protect riparian areas and proposed local infrastructure, Cardno developed a research approach to address the hydraulic and geotechnical aspects of the bank instability and associated sediment delivery downstream. Streambank erosion is controlled by the magnitude of the driving forces provided by the flow in the channel, and the gravitational forces acting on the banks that act against the resisting forces provided by the channel boundary. The approach addresses these issues in detail by relying on the fundamental principles of streambank erosion included within the Bank-Stability and Toe-Erosion Model (BSTEM; Simon et al., 2000; 2011).

Specifically, the adopted fluvial geomorphological and stream restoration approach was developed to meet the following objectives:

Provide analysis of geomorphic stability in the study reach of Lowes Creek, from the downstream end of Lowes Creek at the eastern boundary of the site to the upstream end at the western site boundary;

Quantify boundary resistances and bank-erosion rates under existing and developed conditions, and under a range of mitigation alternatives;

Quantify potential bank-derived sediment loadings at each site under existing, developed and mitigated conditions;

Determine the cost-effectiveness for a range of options aimed at reducing bank erosion and associated downstream sediment loadings; and

Develop a conceptual design for managing bank erosion within the study reach.

To understand the scope of the channel instability, Rapid Geomorphic Assessments (RGAs) were conducted along selected reaches in the Lowes Creek Catchment. The outcomes of the RGAs are provided in Section 8.2.1.

Resistance to hydraulic and geotechnical forces was quantified in situ at the representative sites. The field data-collection techniques and results are detailed in Section 8.2.2.

Having completed the field investigations and following analysis of site data, streambank stability modelling has been carried out using BSTEM Dynamic (Ver. 2.3) to provide analysis of rates of bank erosion under existing, developed and mitigated conditions. The methodology and results are presented in Sections 8.3 and 1.1.



8.2 Field Investigations

8.2.1 Rapid Geomorphic Assessments An understanding of the longitudinal (upstream-downstream) variation in channel-stability conditions provides valuable insights into determining why certain reaches of a river behave differently to a given hydrologic event than others. Placing a rapidly-eroding reach in the context of the broader spatial trends of channel stability allow for river managers to address the overall problems and not just local symptoms.

Analysis of current geomorphic conditions and dominant channel processes along the study reaches were conducted by Cardno staff, in part through the use of Rapid Geomorphic Assessments (RGAs). RGAs utilise diagnostic criteria of channel form to infer dominant channel processes and the magnitude of channel instabilities through a series of nine channel criteria. Inclusion of each criterion in the ranking scheme is founded on 30 years of field experience in research on the controlling forces and processes in unstable channels (Simon and Hupp, 1986; Simon, 1989; Simon and Downs 1995)

For each RGA, the dominant processes occurring along a reach are recorded using a Channel-Stability Ranking Scheme. Scoring for each criterion is such that a higher value indicates greater potential for erosion and instability. A maximum value of four (4) can be assigned to each, preventing subjective assumptions on the relative importance of each criterion. The nine criteria are directed at determining trends of recent channel adjustments through identification of the stage of channel evolution as impaired streams undergo a systematic adjustment (stages of channel evolution) as processes migrate through a channel network with time (Figure 8-1). Values less than 10 generally indicate a very stable channel with minimal erosion. Values greater than 20 are indicative of a very unstable channel often characterized with actively failing banks and a degrading bed. Index values between 11 and 19 generally represent moderate instability.

Figure 8-1 Six Stages of Channel Evolution (from Simon and Hupp, 1986 and Simon, 1989)

In general, most sections of the main stem of Lowes Creek, can be considered somewhat incised and in stage III of the channel evolution model. Some reaches, however, are incised to the extent that with fluvial erosion and undercutting, banks are actively failing. Bank heights are in the range of 1 to 2.5 m. Locations on outside of bends or adjacent to roads crossings are particularly prone to bank failures due to exacerbated hydraulic forces. In most places though banks remain generally stable. Some fluvial erosion occurs along the low-bank faces and bank toes while established trees provide stability to bank tops and overbank areas. The current geomorphic conditions of the channel indicate, however, that additional vertical cutting (incision) due to an increase in the magnitude of peak flows could shift Lowes Creek from a Stage III channel characterized by incision and steep banks to Stage IV dominated by channel widening due to a further increase in bank heights.

There is very little erosion protection such as rock placed within the channel. The exception to this is immediately adjacent to road crossings. Significant rock had been placed downstream of the road crossing on one of the southern tributaries and appears to be fulfilling its intended role of reducing bed erosion.

t o p b a n k d i r e c t i o n o f f l o w

aggraded material

h

h

Stage I. Sinuous, Premodified h<h c

h h

direction of bank or bed movement

critical bank height h c = =

h

slumped material

slumped material

aggraded material

oversteepened reach

precursor knickpoint

plunge pool

secondary knickpoint

primary knickpoint

aggraded material

terrace h bankfull

bank

floodplain terrace

terrace

Stage V. Aggradation and W idening h>h c

Stage III. Degradation h<h c

Stage VI Stage V

Stage IV Stage III

Stages I, II

Stage II. Constructed h<h c

Stage IV. Degradation and W idening h>h c

Stage VI. Quasi Equilibrium h<h c

aggradation zone


aggraded material

h

h

Stage I. Sinuous, Premodified h<h c

h h

direction of bank or bed movement

critical bank height h c = =

h

slumped material

slumped material

aggraded material

oversteepened reach

precursor knickpoint

plunge pool

secondary knickpoint

primary knickpoint

aggraded material

terrace h bankfull

bank

floodplain terrace

terrace

Stage V. Aggradation and Widening h>h c

Stage III. Degradation h<h c

Stage VI Stage V

Stage IV Stage III

Stages I, II

Stage II. Constructed h<h c

Stage IV. Degradation and Widening h>h c

Stage VI. Quasi Equilibrium h<h c

aggradation zone


a g g ra d e d m a te ria l

h

h

S ta g e I. S in u o u s , P re m o d ifie d h < h c

h h

d ire c tio n o f b a n k o r b e d m o ve m e n t

c ritica l b a n k h e ig h t h c = =

h

s lu m p e d m a te ria l

s lu m p e d m a te ria l


o ve rs te e p e n e d re a ch

p re cu rso r kn ickp o in t

p lu n g e p o o l

se co n d a ry kn ickp o in t

p rim a ry kn ickp o in t


te rra ce h b a n k fu ll

b a n k

flo o d p la in te rra ce

te rra ce

S ta g e V . A g g ra d a tio n a n d W id e n in g h > h c

S ta g e III. D e g ra d a tio n h < h c

S ta g e V I S ta g e V

S ta g e IV S ta g e III

S ta g e s I, II

S ta g e II. C o n s tru c te d h < h c

S ta g e IV . D e g ra d a tio n a n d W id e n in g h > h c

S ta g e V I. Q u a s i E q u ilib riu m h < h c

a g g ra d a tio n zo n e



8.2.2 Detailed Site Data Collection Based on a review of aerial photographs, the location of tributaries and the proposed development information three field sites were selected along Lowes Creek for detailed data collection and bank-stability modelling (Figure 8-2). At all three sites measurements of bank-material properties were undertaken for use in bank-stability modelling with BSTEM Dynamic (Ver. 2.3). The goal of the field data collection was to provide the geotechnical and hydraulic resistance parameter inputs required by BSTEM Dynamic. These data, in combination with the surveyed cross section data and design flood-flow hydrographs were then used to determine erosion rates under existing, developed and mitigated conditions at the three sites.

Figure 8-2 Location of Detailed Field Sites



8.2.2.2 Bank Materials

Bank materials are typically fine-grained, composed predominantly of silts and clays with varying amounts of fine sand. The only observations of gravel and cobble-sized materials were in the vicinity of river-crossing structures. Field observations disclosed that materials were of low-to moderate cohesion. At the three detailed study sites, clay contents and cohesion seemed to increase with depth, particularly below 0.8m.

8.2.2.3 Geotechnical Data Collection: Erosion Cohesion and Friction Angle

To properly determine the resistance of bank materials to erosion by mass movement, data must be acquired on those characteristics that control shear strength; that is cohesion, angle of internal friction, pore-water pressure, and bulk unit weight. Cohesion and friction angle data can be obtained from standard laboratory testing (triaxial shear or unconfined compression tests), or by in-situ testing with a borehole shear-test (BST) device (Lohnes and Handy 1968; Thorne et al. 1981; Lutenegger and Hallberg 1981; Little et al. 1982). To gather data on the internal shear-strength properties of the banks, in-situ tests with the Iowa Borehole Shear Tester (BST) was used (Figure 8-3).

Geotechnical data (cohesion and friction angle) obtained in situ with the BST are the fundamental measures of bank strength used to simulate and predict bank stability under a range of moisture conditions. Results of the individual tests show that apparent cohesion (ca) of the bank materials along the study reach are variable, ranging from 0.0 to 11.9 kPa, (Table 8-1). This cohesion represents the sum of the effective cohesion (c’) due solely to the electro-chemical bonds of the soil skeleton and matric suction (negative pore-water pressure). Because moisture contents and, therefore matric suction will vary widely over time, effective cohesion needs to be calculated by subtracting values of matric suction from ca. This is accomplished using measurements of matric suction obtained from cores at each test depth, resulting in the effective cohesion (c’) values shown in Table 8-1. These are the values used in BSTEM and range from 0.0 to 9.3 kPa.

Figure 8-3 Schematic of Borehole Shear Tester (BST) (modified from Thorne et al., 1981)



Table 8-1 Geotechnical Parameter Values Measured with the Borehole Shear Tester Site Test Depth (m) ca (kPa) c’ (kPa) ’ (degrees)

LC-1 1 0.8 7.89 7.19 39.8

2 1.55 9.61 7.86 32.6

LC-2 1 0.5 5.3 1.97 35.3

2 0.4 0.00 0.00 32.5

LC-3 1 0.95 11.9 8.45 33.0

2 1.55 10.2 9.26 26.7

8.2.2.4 Bulk Density / Unit Weight of In-Situ Bank Sediments

Bulk unit weight is one of the required parameters to calculate both the driving and resisting forces responsible for bank stability. In addition, bulk density is used in calculations to convert predicted erosion volumes (in m3) to mass (in tonnes). However, bulk density tends not to be highly variable, particularly in alluvial settings and can often be estimated from default values provided within BSTEM. Still, a 5 cm by 5 cm diameter core was extracted from each borehole at the depth of geotechnical testing with the borehole shear test device (BST) to be used to measure negative pore-water pressure and potentially, bulk unit weight.

8.2.2.5 Hydraulic Resistance Data Collection: Critical Shear Stress and Erodibility

The hydraulic resistance of the bank-toe and bank-face are important for predicting scour and undercutting of the channel banks within BSTEM. Where materials are non-cohesive, resistance is due to particle size and weight, therefore, a bulk particle-size or particle count is sufficient to describe resistance properties. However, entrainment of cohesive materials into the water column are not predicted as a function of particle size and weight, but as a function of the strength of the electro-chemical bonds between particles. To test entrainment thresholds and erodibility of these fine-grained, cohesive materials in situ, a submerged jet-test device was developed by the USDA, Agricultural Research Service (Hanson, 1990; ASTM, 1995). This device was developed based on knowledge of the hydraulic characteristics of a submerged jet and the characteristics of soil-material erodibility.

The Mini-Jet used throughout this project is a scaled-down version of the original instrument (Figure 8-4). Side-by-side testing of the mini-jet and the standard submerged jet are reported in Simon et al., 2010; 2011 and Al-Madhhachi et al., 2013. The method provided by Al-Madhhachi et al., (2013) to scale mini-jet results to the full-size jet was adopted in this work.

Depth-of-scour is measured manually using a point gauge at known increments over time. As the scour depth increases with time, the applied shear stress decreases, due to increasing dissipation of jet energy within the plunge pool. Detachment rate is initially high and asymptotically approaches zero as applied shear stress approaches the critical shear stress of the material (Figure 8-5). This represents the critical shear stress of the material.



Figure 8-4 Scaled Down “Mini-Jet”

Figure 8-5 Example Scour Plot from Mini-Jet Test

The susceptibility of surficial bank-face and bank-toe materials to erosion by hydraulic forces is important to modelling and predicting bank-erosion rates because it is the hydraulic processes (during peak flows) that can cause steepening and potential undercutting of the bank making it more susceptible to collapse. Results of the in situ tests with the submerged jet-test device are shown in Table 8-2.

As with the geotechnical data discussed in the previous section, average or median values only provide a general sense of the relative resistance and erodibility of the bank sediments comprising the channel boundary. It is the individual values of the different surfaces at the three sites that are specifically used to characterize the resistance of the bank-toe and bank-face to hydraulic forces.

Overall, the resistance of bank materials is equivalent to non-cohesive materials in the sand- and gravel-size ranges. Average values for a given location at a given site ranged from 0.85 Pa (equivalent to sand-sized materials) for the bank toe at site LC-2 to 44.5 Pa (coarse grave-sized materials) for the bank face at site LC-3. The median value for all tests is 8.5 Pa.

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 35 40

DEP

TH O

F SC

OU

R, I

N M

ILLI

MET

ERS

TIME, IN MINUTES



Table 8-2 Lowes Creek Jet Test Results Site τc (Pa) Median τc (Pa) k (cm3/N-s) Median k (cm3/N-s)

LC-1 Bank Toe 67.6 40.1

0.024 0.04

LC-1 Bank Toe 12.6 0.056

LC-1 Bank Face 0.223 5.93

0.424 0.24


LC-1 Bed 4.58 5.04

0.093 0.09

LC-1 Bed 5.51 0.085

LC-2 Bank Toe 0.073 0.85

0.738 0.45

LC-2 Bank Toe 1.62 0.157

LC-3 Bank Toe 2.59 17.8

0.124 0.08

LC-3 Bank Toe 33.0 0.035


0.034 0.03


8.3 Bank-Erosion Modelling Previous sections of this chapter focus on the geomorphic conditions encountered along the study reach and the characteristics of its boundary sediments. While the former provides overall geomorphic context and some important information on the extent of bank instabilities, the latter provides the required quantitative information on the resistance of the bank materials to erosion by hydraulic and gravitational forces. Bank erosion modelling has been undertaken to bring together these data sets along with data representing the primary driving force, the flow in the channel.

As described in previous sections, the Bank-Stability and Toe-Erosion Model (BSTEM) is a deterministic model that simulates the hydraulic and geotechnical processes responsible for bank erosion. Including the effects of vegetation, pore-water pressure and confining forces due to flow in the channel. Both the static and dynamic versions of BSTEM have been used worldwide to predict and address issues involving bank erosion. The version used in this study represents the latest version of BSTEM-Dynamic (Ver. 2.4). The dynamic model allows the inclusion of the entire flow series, rather than a single discharge value.

Application of BSTEM gives the user the ability to quantify the amount of erosion under different flow regimes and channel conditions for the purpose of predicting bank retreat and sediment loads from bank erosion. Having this ability allows for quantitative comparisons between existing and proposed conditions, and for testing of alternative mitigation measures if predicted erosion exceeds acceptable levels. Ultimately this provides for robust and flexible channel design by being able to evaluate threshold conditions for erosion as well as changes to flow magnitudes and durations.

8.3.1 Flow Data for BSTEM Simulations To develop a data set for each site, the event based flood hydrographs from the TUFLOW model (Section 4.4) were utilised. Hydrographs were extracted from the 2D hydraulic model for the 50%, 10% and 1% AEP flood events for the 9-hour duration storms. The complete flow series provided a hydrograph over 12 hours that included the entire recessional limb of the hydrograph. This is important for bank-stability modelling as it represent the most critical condition, when bank may be saturated yet lose the support of the water in the channel. The hydraulic and geotechnical calculations were made for every 1-minute time step over the period of the hydrograph.

Using the flow series data and the surveyed cross sections, a normal-depth analysis has been performed for each of the sites to convert discharge values to stage data for each time step. This is a required for input to BSTEM. Calculations have been made for a range of instream Manning’s roughness coefficients with the appropriate n-value selected based on the conditions at each site.

Stage data was then input to BSTEM along with bank geometry, layering information, and the geotechnical and hydraulic parameters measured in the field. A reasonable amount of lateral retreat as interpreted from



field conditions will be used as the metric to compare with the amount of retreat predicted over the flood events.

8.3.2 Existing Conditions The modelled flood events flows for existing conditions for the 50%, 10% and 1% AEP events have used as inputs to BSTEM for the following purposes:

To calculate the extent of bank erosion as a distance from the existing top of bank;

To calculate unit bank-derived sediment loads in (m3/m of channel length);

To use as a baseline condition to compare simulations results under developed conditions and from the various mitigation strategies to determine effectiveness.

Bank-resistance data collected at the three main-stem sites on Lowes Creek (Figure 8-2) were used with the hydraulic data from the hydrologic model as inputs into BSTEM. Results of the BSTEM simulations showed that under existing flow and channel conditions, bank-erosion rates along Lowes Creek can be considered low to moderate. Erosion rates ranged from 0.06 m3/m for the 2-year flow at LC-03 to 0.35 m3/m for the 10-year flow at LC-02. Erosion along the left bank at LC-02 is exacerbated by the fact it is located on a sharp bend. Significantly higher erosion rates are predicted for a site on the western tributary (WT-01), with values ranging from 1.56 to 1.63 m3/m (Table 8-3).

Table 8-3 BSTEM Results – Existing Conditions

Site ARI TOTAL Hydraulic Geotechnical

Unit Erosion (m3/m)

LC-01 2 0.06 0.06 0.00

LC-01 10 0.17 0.17 0.00

LC-01 100 0.33 0.33 0.00

LC-02 2 0.30 0.06 0.23

LC-02 10 0.35 0.09 0.26

LC-02 100 0.16 0.15 0.02

LC-03 2 0.06 0.06 0.00

LC-03 10 0.12 0.12 0.00

LC-03 100 0.19 0.19 0.00

WT-01 2 - - -

WT-01 10 - - -

WT-01 100 - - -

8.3.3 Proposed Conditions The erosion rates for existing flow and channel conditions thereby represent a baseline by which to evaluate proposed development condition erosion rates. This is summarized in Table 8-4 where differences in erosion rates for proposed hydraulic conditions are compared to those under existing hydraulic conditions. As can be seen, erosion rates are generally higher for the developed condition by an average of 27% (median of 21%). This can be attributed to the extended duration of flows where the shear stress imparted by the flow exceeds the critical value (resistance) of the channel boundary.

Whilst an increase in erosion is observed at all sites under the developed conditions, the erosion rates are still relatively low at all sites along Lowes Creek (all less than 0.5 m/m3). Resulting in a low risk to the riparian corridor and nearby infrastructure and buildings. The exception to this is the site on the Western Tributary, where erosion rates are significantly higher (approximately 1.6 m/m3). This could pose a risk to the riparian corridor vegetation at this site and any infrastructure proposed adjacent to this site (e.g. roads).



Table 8-4 BSTEM Results – Proposed Conditions

Site ARI TOTAL Hydraulic Geotechnical Erosion

Change from Existing (%) Unit Erosion (m3/m)

LC-01 2 0.09 0.09 0.00 39.3

LC-01 10 0.20 0.20 0.00 15.6

LC-01 100 0.42 0.42 0.00 28.0

LC-02 2 0.36 0.09 0.27 20.6

LC-02 10 0.39 0.11 0.28 12.1

LC-02 100 0.15 0.14 0.00 -8.2

LC-03 2 0.12 0.12 0.00 107

LC-03 10 0.15 0.15 0.00 26.8

LC-03 100 0.22 0.22 0.00 0.0

WT-01 2 1.56 0.01 1.55 -

WT-01 10 1.63 0.02 1.60 -

WT-01 100 1.56 0.04 1.53 -

8.3.4 Erosion Mitigation Measures Reducing channel erosion is a matter of either increasing the resistance of the channel boundary and/or reducing the forces acting on that boundary. For Lowes Creek, a general erosion-mitigation strategy was adopted based on the concept of maintaining the established riparian corridor. Thus, options to reduce boundary shear stresses by widening the channel or flattening of the banks, or increasing stream length were not considered appropriate because of the potential damage to the riparian corridor by earth-moving machines. A strategy of reducing channel slope using a series of drop structures was considered. To test the effectiveness of this alternative, model runs were conducted that assumed a reduction in bed slope of 25%. These results, relative to both existing and post-development conditions are also shown in Table 8-5. As expected, for all sites and modelled flows, erosion rates decreased for the post-development, slope-reduction scenario, with an average reduction of 31.6%; an 11.7% reduction from existing conditions.

Mitigation work beyond the 25% slope reduction should be considered for LC-02 where access by heavy machinery is possible. Here, in an effort to limit further bank erosion and meander migration, additional BSTEM simulations were conducted that assumed a battered bank of 3:1 with placement of rock at the bank toe. Adopting this approach at this site resulted in bank erosion being completely halted.

Table 8-5 BSTEM Results – Effectiveness of Mitigation Measures

Site ARI Total Unit Erosion (m/m3)

Erosion Change from Existing (%)

Erosion Change from Proposed (%)

Erosion Change from All Conditions (%)

25% Slope Reduction Battered 3:1 with Rock Toe

LC-01 2 0.06 -6.3 -32.8 -

LC-01 10 0.14 -19.9 -30.7 -

LC-01 100 0.30 -9.4 -29.2 -

LC-02 2 0.20 -33.4 -44.8 -100

LC-02 10 0.22 -36.7 -43.6 -100

LC-02 100 0.12 -28.1 -21.7 -100

LC-03 2 0.09 50.7 -27.3 -

LC-03 10 0.11 -7.7 -27.2 -

LC-03 100 0.16 -14.3 -26.9 -



Site ARI Total Unit Erosion (m/m3)

Erosion Change from Existing (%)

Erosion Change from Proposed (%)

Erosion Change from All Conditions (%)

WT-01 2 1.22 - -21.7 -

WT-01 10 1.25 - -23.0 -

WT-01 100 1.22 - -22.0 -

Because in situ field data was not collected at WT-01 average values from the other sites were used to predict erosion under post-developed conditions. This signifies that there is reasonable uncertainty in the modelled results at this site. That said, although a 25% slope reduction decreases erosion rates to less than the no-action alternative, these rates are still higher than the existing conditions at the other sites. Thus, consideration should be given to altering the proposed channel geometry to reduce imposed hydraulic shear stresses (i.e. by widening or bank-slope flattening). The exact nature of these works will be dependent on opportunity (e.g. construction works proposed in this area) and sensitivity of the adjacent riparian corridor to erosion (i.e. if the erosion of this location is considered low risk and recoverable based on land use then it may be accepted that significant flooding may result in erosion at this location).

8.3.4.2 Concept Design of Mitigation Measures

Each site and reach was assessed to determine the number of drop structures needed to reduce slope by 25% within 50 meters of channel length. To help determine locations within each sub-reach that require slope modification, the channel slope was calculated along the entire reach. When plotted with the slope thresholds (25% slope-reduction goals) the length of reach that need modification can be seen as slope areas above the threshold line. Because each individual site was modelled with its local slope, a single recommendation for the number of drop structures per 50 meters could not be applied along the entire sub-reach. Therefore, ranges of slopes in each sub-reach were calculated to prescribe the number of drops per 50 meters. Figure 8-6 shows channel bed elevation with the channel slope and the 25% slope thresholds for each of the Lowes Creek sites. The lengths of channel that need a slope modification can be seen when the plotted slope climbs above the threshold line. Indicative requirements for drop structures (per 50m length of channel) have also been provided in Figure 8-6 for reaches of similar characteristics. Given the importance of protecting the riparian corridor along Lowes Creek, the use of heavy equipment is limited to locations of existing or planned bridges. Based on the results of the BSTEM modelling that tested mitigation alternatives, a summary of the suggested mitigation strategies is shown in Table 8-6.

Table 8-6 Proposed Mitigation Strategies Site Suggested Mitigation

LC-01 25% slope reduction using drop structures LC-02 3:1 bank battering with minimum 125 mm, rock toe LC-03 25% slope reduction using drop structures WT-01 25% slope reduction using drop structures and channel re-shaping



Figure 8-6 Lowes Creek Bed Elevation, Channel Slope, 25% Slope Reduction Thresholds and Drop Structure Requirements



8.4 Summary of Results Overall the erosion evaluation for Lowes Creek showed low to moderate bank erosion was occurring under existing flow and channel conditions. Under post-development conditions, erosion would increase if no mitigation measures were adopted. The increased erosion under developed conditions was not a result of increasing peaks but of increasing flow durations where excess shear stresses exist. These results stress the notion that just looking at peak flows is not sufficient, as erosion is the product of the magnitude of the excess stress, times the duration of that excess stress. In addition:

Post-development has altered hydrographs to increase the duration of flows that have excess shear stresses and, therefore, greater amounts of channel erosion.

To mitigate, we are limited to reducing slope and shear stress for the purpose of reducing erosion. Reducing slope by 25% does not stop erosion but for the most part reduces it to below existing rates. Another option would be to control flows with better retention since post-development has greater durations of excess stress. Under proposed development there will be 2 online detention basins and 10 offline basins (i.e. same volume of water over the hydrograph but probably need to retain more and release it over a longer duration to reduce depths and shear stresses).

To accomplish the 25% slope reduction, we are considering a series of small (10 cm) drops. These may be composed of tree trunks, keyed back into the banks. If required, greater drops can be taken at bridge locations if they these larger drops are in concert with ecological requirements.

Attention should be given to the conditions along the reach represented by site WT-01. Here, even with the 25% slope reduction, erosion rates are still moderate, thereby indicating that additional controls would be required. This might include widening the channel to provide shallower flow depths and thus lower shear stresses.



LC-01 (Figure 8-7) is the downstream-most site with the second lowest amount of predicted erosion. On average, at LC-01 there is a 28% increase in erosion under the new developed conditions. To reduce erosion to existing conditions, a reduction in channel slope is proposed. A 25% reduction in slope will achieve these results. In order to remove this amount of slope, one small drop structure of 0.1m in height can be added every 50 meters of channel length. Figure 6 shows the results of the three modelling scenarios, including existing conditions, the proposed development conditions and for the 25% slope reduction.

Figure 8-7 LC-01 Site Photo and BSTEM Bank Erosion Results

69.5

69.7

69.9

70.1

70.3

70.5

70.7

70.9

71.1

71.3

71.5

80 81 81 82 82 83 83 84 84 85 85

ELEV

AT

ION

, IN

MET

ERS

STATION, IN METERS

LC-01

Start

2 year

10 year

100 year

2 year proposed

10 year proposed

100 year proposed

2yr Slope Reduction

10yr Slope Reduction




LC-02 (Figure 7) is the upstream-most site with the largest amount of erosion on Lowes Creek. LC-02 has an average slope about 3 times greater than LC-01 and 8 times greater than LC-03. LC-02 also contains some of the weakest bank materials. Under the new developed conditions, LC-02 saw an average increase in erosion of about 8%. As with site LC-01 a 25% slope reduction will reduce erosion back to existing conditions. Because LC-02 saw a moderate amount of top-bank retreat even under existing conditions, an additional scenario was considered. The banks at LC-02 were battered at a 3:1 ratio and 125 mm rock was added at the toe to a height of 1 meter. Under these battered bank conditions, the erosion was reduced to zero for all three ARI’s. Figure 8-8 shows the results of the four modelling scenarios, including the existing conditions, the proposed development conditions and the 25% slope reduction conditions.

Figure 8-8 LC-02 Site Photos and BSTEM Bank Erosion Results

81.2

81.4

81.6

81.8

82.0

82.2

82.4

82.6

31.5 31.7 31.9 32.1 32.3 32.5 32.7 32.9

ELEV

AT

ION

, IN

MET

ERS

STATION, IN METERS

LC-02

Start

2 year

10 year

100 year

2 year proposed

10 year proposed

100 year proposed

2yr Slope Reduction





LC-03 is the middle site with the lowest amount of predicted erosion. Figure 8-9 shows the results of the three modelling scenarios, including existing conditions, proposed development conditions and with the 25% slope reduction.

Figure 8-9 LC-03 Site Photo and BSTEM Bank Erosion Results

72.0

72.5

73.0

73.5

74.0

74.5

75.0

27.0 27.5 28.0 28.5 29.0 29.5 30.0

ELEV

AT

ION

, IN

MET

ERS

STATION, IN METERS

LC-03

Start

2 year

10 year

100 year

2 year proposed

10 year proposed

100 year proposed

2yr Slope Reduction





An additional site on the West Tributary (WT-01) was added to the modelling and analysis based on observations of significant erosion. West Tributary has the steepest slope of the four sites. WT-01 was simulated for the proposed development conditions and a 25% slope reduction. WT-01 experiences the largest amount of erosion compared to the three sites on Lowes Creek. Even under the 25% reduction scenario, WT-01 still shows a large amount of bank-top retreat. It is suggested that at the minimum slope should be reduced in the West Tributary. To reduce slope by 25% on West Tributary, two small drop structures of 0.1m in height can be added every 50 meters of channel length. Figure 8-10 shows the results of the two modelling scenarios, including the proposed development conditions and the 25% slope reduction.

Figure 8-10 WT-01 BSTEM Bank Erosion Results

89.00

89.50

90.00

90.50

91.00

91.50

92.00

92.50

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Elev

atio

n

Station

WT-01

Start

2 year proposed

10 year proposed

100 year proposed

2yr Slope Reduction





9 Operations and Maintenance

The operation of WSUD measures is reliant on periodic maintenance to ensure that various elements of the measure are in good working order. WSUD measures comprise, for the most part, natural materials which can be quickly degraded by high volumes of stormwater. Stormwater can contain gross pollutants and sediment that can smother filtration media, plants and drainage structures. In addition, stormwater can also reach high velocities that can cause scour and erosion.

Gross Pollutant Traps (GPTs) need to be regularly maintained to remove captured pollutants. Often these devices are located underground and can be neglected if maintenance routines are not observed. Failure to maintain GPTs can exacerbate stormwater pollution by potentially releasing nutrients that are bound to sediments captured in GPTs.

In light of these issues it is recommended that the WSUD measures be included in the public domain so that they are visible to the public and are accepted as part of the landscape. Segregation of WSUD measures with fencing and dense peripheral vegetation can lead to the measure becoming isolated and neglected.

Integration of the WSUD measures and the open spaces should promote regular maintenance to ensure that the amenity of the public open space. Local land care groups can also be encouraged to take responsibility for local assets and to share maintenance duties with Councils.

The construction period is one of the main threats to fouling of WSUD measures if the construction is not staged in a way that will protect the measures. Release of sediments into stormwater during construction is common and although soil and water management controls are put in place, they are often neglected and fail during storms. The following recommendations are made to protect the measures from fouling during construction:

Locate the WSUD measure off-line until the commissioning phase of the development. This will ensure that any stormwater generated during construction is routed around the WSUD measures;

Delay landscaping of the WSUD measures to the final stages of construction to reduce the risk of surface degradations and plant loss; and

Temporarily create a small inlet zone to retarding basins and bio-filters that will accept small amounts of local stormwater during construction. This will allow plants to establish in the greater area of the basin/filter without risk of fouling.

The design life of the WSUD measures is highly dependent on the maintenance regime. If a maintenance regime is followed, then the life of the WSUD elements will be maximised and a reliable level of pollutant capture will be achieved. Note that an establishment period will be required to ensure that any vegetation included in the WSUD measure is healthy and robust. A vegetation management plan should be provided with the detailed design of measures such as retarding basins and bio-retention systems that includes full details on the procurement and establishment of plants.

A maintenance schedule for proposed WSUD measures is outlined in Table 9-1.



Table 9-1 WSUD Maintenance Schedule

WSUD Measure

Maintenance Actions Frequency Waste Management Measure Responsible Party

Gross Pollutant

Traps (GPTs)

Remove collected pollutants

Quarterly or after each storm event of 20mm in rainfall depth or more

Dispose of in-organic material to waste disposal facility.

Council Check inlet and outlet structures for signs of blockage

Annually Dispose of in-organic material to waste disposal facility.

Replace filter mesh (if included in device)

Every 5 years Nearest waste disposal facility

Bio-retentions

Basins

Remove pollutants collected on the surface


Dispose of in-organic material to waste disposal facility. Use organic material as mulch.

Council / Landcare

Group

Flush stand pipes of bio-filter

Half yearly or after each storm event of 20mm in rainfall depth or more

Collect material flushed into stormwater pits and re-use as mulch.

Check surfaces for any signs of erosion or displacement of scour protection/soil/mulch

Quarterly or after each storm event of 20mm in rainfall depth or more for the first 24 months and annually thereafter.

No waste – collect dislodged materials and re-use.

Replace damaged plants

Annually Re-use organic material in separate gardens or landscaped areas

Replace filtration media

Every 5 years as a minimum or up to 20 years as a maximum depending on pollutant load from the catchment.

Dispose of in-organic material to waste disposal facility. Use organic material as mulch.

Rainwater Tanks (if proposed

in the future)

Clean out first flush device of any sediment and debris build up.


Dispose of in-organic material to waste disposal facility.

Property manager Drain tank and clean

sediment/organic matter from tank base

Bi-annually Re-use organic material in separate gardens or landscaped areas



10 Conclusions

This document outlines the methodology adopted in assessing flooding, water quality and streambank erosion; the outcomes of this assessment and the proposed Water Cycle Management Strategy to ensure the proposed development meets its requirements with regards to the management of impacts on waterways, the receiving environment and public safety relating to flooding.

Flood modelling has been completed to assess the effectiveness of the Precinct’s water quantity management strategies. The flood assessment shows that post-development 1% AEP flows are controlled and contained within the proposed detention basins and riparian corridors of the Precinct. The strategy provides a balance between the riparian corridor functions, floodplain management, and development outcomes.

The water quality strategy developed for the Precinct will also ensure that the quality of stormwater discharging from the Precinct meets the requirements of Council and will ensure stormwater pollutant impacts of urban development are mitigated.

A comprehensive assessment of streambank stability has been undertaken that identifies that overall the erosion evaluation for Lowes Creek showed low to moderate bank erosion was occurring under existing flow and channel conditions. Under post-development conditions, erosion would increase if no mitigation measures were adopted. The increased erosion under developed conditions was not a result of increasing peaks but of increasing flow durations where excess shear stresses exist. These results stress the notion that just looking at peak flows is not sufficient, as erosion is the product of the magnitude of the excess stress, times the duration of that excess stress. Proposed mitigation measures involve the inclusion of small drop structures along Lowes Creek and a portion of the Western Tributary. This strategy ensures that erosion is not increased as a result of the proposed Precinct development, while also minimising the disturbance to the existing riparian corridor vegetation.

This document will be placed on public exhibition for review and comment by interested stakeholders. Comments and input received as an outcome of this process will be considered and addressed, as appropriate.



11 Glossary

AEP – Annual Exceedance Probability AHD – Australian Height Datum ALS – Aerial Laser Survey ARI – Average Recurrence Interval ARR – Australian Rainfall & Runoff CAA – Controlled Activity Approval BST – Borehole Shear Test/Tester BSTEM – Bank-Stability and Toe-Erosion Model DCP – Development Control Plan DEM – Digital Elevation Model DoI – Department of Industry EIA – Effective Impervious Area EMC – Event Mean Concentration ET – Evapotranspiration FRMS&P – Floodplain Risk Management Study & Plan GPT – Gross Pollutant Trap ICA – Indirectly Connected Area IFD – Intensity-Frequency-Duration ILP – Indicative Layout Plan LGA – Local Government Area LiDAR – Light Detection and Ranging L&PI – Land and Property Information MUSIC – Model for Urban Stormwater Improvement Conceptualisation NOW – New South Wales Office of Water OEH – Office of Environment & Heritage PAP – Precinct Acceleration Protocol PSD – Permissible Site Discharge RGA – Rapid Geomorphic Assessment RMS – Roads and Maritime Services TN – Total Nitrogen TP – Total Phosphorus TSS – Total Suspended Solids SEPP – State Environment Planning Policy SES – State Emergency Services SSR – Site Storage Requirement VPA – Voluntary Planning Agreement VRZ – Vegetated Riparian Zone WCMS – Water Cycle Management Strategy WM Act – Water Management Act WSUD – Water Sensitive Urban Design



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Simon, A., and Hupp, C. R., 1986. Channel evolution in modified Tennessee channels, Proceedings of the Fourth Federal Interagency Sedimentation Conference, March 1986, Las Vegas, Nevada, v. 2, Section 5, 5-71 to 5-82.

Simon, A.and Downs, P.W. 1995. An interdisciplinary approach to evaluation of potential instability in alluvial channels, Geomorphology, 12(3): 215-232,

Simon A, Curini A, Darby S.E, Langendoen E.J., 2000. Bank and near-bank processes in an incised channel, Geomorphology 35: 183-217.

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Thorne, C. R., Murphey, J. B. and Little, W. C., 1981. Stream Channel Stability, Appendix D, Bank Stability and Bank Material Properties in the Bluffline Streams of Northwest Mississippi. U.S. Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory. Oxford, MS. 227 p.

WMAwater (2012) Upper South Creek Flood Study, Final, May

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