Castle Hill, NSW 2155 Detailed Design Report

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Our Reference: 2220 – Bidjigal Creek - DDR_REV C

Bidjigal Creek Rehabilitation Design Timothy Avenue Reserve

Castle Hill, NSW 2155 Detailed Design Report

Final

For Hills Shire Council

Prepared by:

Storm Consulting Suite 7.01 Level 7 3 Rider Boulevarde Rhodes NSW 2138

P: 02 9869 1855 F: 02 9869 2341

E: reception@crhodes.com.au W: stormconsulting.com.au

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INDEX

1 Introduction ... 2

1.1 Objective and Scope ... 3

1.2 Description of Proposed Works ... 3

2 Available Data ... 4

3 Existing Conditions ... 5

3.1 Existing Creek Hydrology ... 5

3.2 Existing Creek Hydraulics ... 5

4 Detailed Design ... 7

4.1 Detailed Design Drawings ... 7

4.2 Creek Hydrology and Hydraulics under Proposed Design Conditions ... 8

4.3 Review of Environmental Factors (REF) ... 10

4.4 Vegetation Management Plan (VMP) and Construction Environmental Management Plan (CEMP) ... 10

4.5 Geotechnical Investigation ... 10

4.6 Bill of Quantities and Cost Estimates ... 10

4.7 Geotechnical Investigation ... 10

Appendix A – Detailed Design Drawings ... 11

Appendix B – HEC-RAS Results ... 12

Appendix C – Review of Environmental Factors (REF) ... 13

Appendix D – Vegetation Management Plan (VMP) ... 14

Appendix E – Construction and Environmental Management Plan (CEMP) ... 15

Appendix F – Geotechnical Report... 16

Appendix G – Bill of Quantities and Cost Estimates ... 17

Appendix H – Project Specific Specifications ... 18

DOCUMENT CONTROL

Revision Date Details Author Reviewed

A 11.03.21 Detailed Design Report RK/KL RK

B 05.07.21 Detailed Design Report RK/KL

C 23.07.21 Detailed Design Report RK/KL

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1 Introduction

Storm has been engaged by the Hills Shire Council to undertake detailed design of the Bidjigal Creek Waterway Rehabilitation works located in Timothy Avenue Reserve, Castle Hill.

The existing Bidjigal Creek water quality is poor, with highly incised and unstable banks currently threatening the backyards of adjacent properties, and extensive invasive weed species present within the riparian corridor.

The Bidjigal Creek Rehabilitation works aims to improve the natural function of the waterway and to restore the natural and recreational benefits for the community.

The project extents are bounded to the north-west by Timothy Avenue and to the south-west by residential properties that front Aubrey Close and Walsh Avenue (Figure 1).

Figure 1 Project Extent

This report summarises the work undertaken for the detailed design phase of the project. The report will be updated and refined as the design is finalised.

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1.1 Objective and Scope

For the purposes of this detailed design, the following scope of work was undertaken:

• Review of available data including collation of Council data, DBYD services data, existing DTM, existing catchment characteristics and existing flood reports/studies/constraints;

• Strategic Assessment including the understanding of creek catchments and existing creek flow behaviour (Existing Creek HEC-RAS modelling), development of design criteria table and risk assessment;

• Detailed design of preferred option in consultation with Council, incorporating appropriate creek rehabilitation and bank protection techniques;

• Geotechnical Investigation;

• Review of Utilities (DBYD);

• Design hydraulic analysis (using HEC-RAS) of proposed works;

• Construction Environmental Management Plan (CEMP)

• Vegetation Management Plan (VMP)

• Review of Environmental Factors (REF)

• Preparation of Detailed Design Drawings

1.2 Description of Proposed Works

HEC-RAS modelling of the existing creek identified five ‘high risk’ areas susceptible to bank erosion and unstable creek flow regimes. As such, treatment measures are proposed in these locations to mitigate further damage to the creek and restore creek stability. The treatment measures have been designed to minimise any adverse hydraulic impacts on upstream and downstream reaches of the creek.

Pool and riffle systems are proposed to provide a more stable creek morphology by dissipating energy that otherwise may have further eroded the banks. The pools and riffles are also designed to enhance the creek ecology and biodiversity. Bank treatment using rock and riprap are proposed to stabilise the eroded and unstable sections of the creek banks. This includes protection of the tributaries discharging into the main creek near their confluences.

Geotechnical investigations indicated the presence of rock along most sections of the creek bed. Consequently, the extent of excavation works proposed for the design is only what is required to create the pool and riffle system at the above five locations, and in order to anchor and stabilise the banks. Bank profiles considered stable from the results of the geotechnical investigation are retained where possible.

Council has advised that the design storm event for the pool and riffles system be the 2-year or 5-year storm event. On this basis, the 5-year event has been adopted for the purposes of the detailed design. It is expected that some maintenance and restoration work may be required after passage of a larger flood, particularly where there is excessive movement of the rock riprap.

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2 Available Data

Available guidelines and data adopted for the detailed design included the following:

• Australian Rainfall & Runoff, Engineers Australia (1987);

• Floodplain Development Manual – The Management of Flood Liable Land, (NSW Government, April 2005);

• Australian Rainfall & Runoff Project 15 Report – Two Dimensional Modelling in Urban and Rural Floodplain (Engineers Australia, 2012);

• Floodplain Risk Management Guidelines (Office of Environment & Heritage);

• Guidelines for Controlled Activities on Waterfront Land (NSW DPI, 2018);

• The Hills Shire Council Design Guidelines Subdivision/Development (2011) and standard drawings;

• Waterways Drainage Design Requirements (The Hills Shire Council);

• Site Survey data (Hills Shire Council, dated 12 April 2019, survey reference:

MT1678.dwg);

• Utilities data (DBYD and sewer data from Metro Water);

• Council flood data (Hills Shire, 14 October 2019);

• Proposed Bidjigal Creek Waterway Rehabilitation (Geotechnical Investigation), Timothy Avenue Reserve, Castle Hill NSW (AssetGeo, 4 February 2021) and

• Concept Design Options (Storm, April 2020) and Preferred Concept Design (December 2020).

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3 Existing Conditions

3.1 Existing Creek Hydrology

The flood data provided by Council for Bidjigal Creek (14/10/19) is presented in Figure 2.

Figure 2 Flood Information Plan supplied by Council

3.2 Existing Creek Hydraulics

Council’s flood data was used to build and verify a HEC-RAS model for Bidjigal Creek under existing conditions. HEC-RAS modelling was undertaken to determine the areas of high erosion potential. Parameters including bed shear force, creek velocity and Froude number were used as key metrics to determine areas of high erosion potential.

The HEC-RAS layout of the Bidjigal Creek under existing conditions is presented in Figure 3.

The flood extent for the 5-year and 100-year events for Bidjigal Creek under existing conditions is shown in Figure 4. Additional details are included in Appendix B.

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Figure 3 Existing Conditions HEC-RAS Layout

Figure 4: 5-year and 100-year Flood Extents under Existing Conditions

Inflow location from council

5-year Flood Extent 100-year Flood Extent LEGEND

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4 Detailed Design

The detailed design was undertaken in consultation with Council and based on the preferred concept design (December 2020). The detailed design components included hydrological modelling and analysis, pool and riffle design, vegetation management planning, environmental assessment, and 3D modelling of the proposed earthworks.

4.1 Detailed Design Drawings

The detailed design drawings for the proposed works (Section 1.2) and associated drawings are listed below and are included in Appendix A.

Table 1 Detailed Design Drawing Set and Associated Drawings

DRAWING No. DRAWING TITLE REV.

2220-DD-0001 COVER SHEET, LOCALITY PLAN AND DRAWING INDEX C

2220-DD-0002 GENERAL NOTES B

2220-DD-0003 LEGEND B

2220-DD-0004 KEY PLAN B

2220-DD-0051 BULK EARTHWORK PLAN B

2220-DD-0101 GENERAL ARRANGEMENT PLAN OVERALL B

2220-DD-0111 GENERAL ARRANGEMENT PLAN SHEET 1 OF 3 B

2220-DD-0201 CREEK LONGITUDINAL PROFILE SHEET 1 OF 2 B

2220-DD-0202 CREEK LONGITUDINAL PROFILE SHEET 2 OF 2 B

2220-DD-0203 STORMWATER OUTLET 02 & 03 AND SIDE CHANNEL LONGITUDINALPROFILE B

2220-DD-0301 STRUCTURE 01 CROSS SECTION SHEET 1 OF 1 B

2220-DD-0309 STORMWATER OUTLET 02 CROSS SECTION SHEET 1 OF 1 B

2220-DD-0313 SIDE CHANNEL 01 CROSS SECTION SHEET 1 OF 1 B

2220-DD-0314 STORMWATER OUTLET 03 CROSS SECTION SHEET 1 OF 1 B

2220-DD-0401 DETAILED PLAN AND SECTION STRUCTURE 01 C

2220-DD-0404 DETAILED PLAN AND SECTION STRUCTURE 04 & SW OUTLET 02 C

2220-DD-0406 DETAILED PLAN AND SECTION SW OUTLET 03 & SIDE CHANNEL 01 C

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2220-DD-0511 SEWER PROTECTION PLAN SHEET 1 OF 3 B

2220-DD-0521 SEWER DETAILED SURVEY PLAN B

2220-DD-0601 SETOUT PLAN AND TABLE STRUCTURE 01 B

2220-DD-0604 SETOUT PLAN AND TABLE STRUCTURE 04 & SW OUTLET 02 B 2220-DD-0605 SETOUT PLAN AND TABLE STRUCTURE 05, SW OUTLET 03 SIDE CHANNEL 01 -

SHEET 1 OF 2 B

2220-DD-0606 SETOUT PLAN AND TABLE STRUCTURE 05, SW OUTLET 03 SIDE CHANNEL 01 -

SHEET 2 OF 2 B

2220-DD-0701 TREE REMOVAL PLAN B

2220-DD-0801 PLANTING PLAN B

2220-DD-0901 SEDIMENT AND EROSION CONTROL PLAN B

2220-DD-0902 SEDIMENT AND EROSION CONTROL DETAIL AND NOTES B ASSOCIATED DRAWINGS

DRAWING No. DRAWING TITLE REV.

PAGE 1 PLANTING NOTES AND SPECIFICATIONS 3

PAGE 2 PLANTING PLAN AND MATRIX 1 AND 2 FOR STRUCTURE 01 AND 02 3 PAGE 3 PLANTING PLAN AND MATRIX 1 AND 2 FOR STRUCTURE 03 AND 04 3 PAGE 4 PLANTING PLAN AND MATRIX 1 AND 2 FOR STRUCTURE 05 3

PAGE 5 PLANTING PLAN AND MATRIX 3 FOR OVERALL SITE 3

4.2 Creek Hydrology and Hydraulics under Proposed Design Conditions

The creek hydrology under proposed conditions remains the same as that under existing conditions.

The HEC-RAS layout of Bidjigal Creek under proposed conditions is presented in Figure 5.

The flood extents for the 5-year and 100-year events under proposed conditions is shown in Figure 6. Detailed HEC-RAS result tables for the proposed condition are provided in Appendix B.

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Figure 5: HECRAS layout under Proposed Conditions

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4.3 Review of Environmental Factors (REF)

REF has been prepared for the site. The REF indicated that, as the proposed works are within an existing modified and degraded area, the potential for environmental impacts is low. It is anticipated that there will be typical works impacts, such as minor clearing of vegetation, additional traffic and noise but that these should not be significant and can be effectively managed as part of the Construction Environmental Management Plan. Once operational, adverse environmental or community impacts are not expected.

The REF report is included in Appendix C.

4.4 Vegetation Management Plan (VMP) and Construction Environmental Management Plan (CEMP)

A Vegetation Management Plan (VMP) was prepared to assist in managing, rehabilitation and revegetation of any disturbed areas affected by the proposed works. The VMP report is included in Appendix D and the CEMP in Appendix E.

4.5 Geotechnical Investigation

The geotechnical investigation and findings are documented in Appendix F.

4.6 Bill of Quantities and Cost Estimates

The Bill of Quantities and Cost Estimates are presented in Appendix G. The proposed works are estimated to cost about $700,000.

4.7 Geotechnical Investigation

The Project Specific Specifications are documented in Appendix H.