Terara Village Floodplain Management Study
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TERARA VILLAGE FLOODPLAIN MANAGEMENT STUDY Shoalhaven River, 1978 Flood FEBRUARY 2002 SHOALHAVEN CITY COUNCIL TERARA VILLAGE FLOODPLAIN MANAGEMENT STUDY FEBRUARY 2002 Webb, McKeown & Associates Pty Ltd Prepared by: ___________________________ Level 2, 160 Clarence Street, SYDNEY 2000 Telephone: (02) 9299 2855 Facsimile: (02) 9262 6208 Verified by: ____________________________ 98066:TeraraFMS PDF Version.wpd TERARA VILLAGE FLOODPLAIN MANAGEMENT STUDY TABLE OF CONTENTS PAGE FOREWORD SUMMARY 1. INTRODUCTION ..................................................... 1 1.1 The Flood Problem ............................................ 1 1.2 Floodplain Management Process ................................. 3 1.3 Council’s Interim Flood Policy .................................... 4 2. BACKGROUND ..................................................... 5 2.1 Catchment Description ......................................... 5 2.2 Terara Village ................................................ 5 2.2.1 Description ........................................... 5 2.2.2 Zoning .............................................. 6 2.2.3 Heritage ............................................. 7 2.2.4 Environmental ........................................ 8 2.3 Lower Shoalhaven River Flood Study ............................. 8 2.3.1 Review .............................................. 8 2.3.2 Design Flood Levels ................................... 9 2.4 Stream Morphology .......................................... 10 2.5 Public Consultation Program ................................... 11 2.5.1 Components ......................................... 11 2.5.2 Terara Flood Awareness Survey (1993) ................... 11 2.5.3 Terara Questionnaire (July 1998) ........................ 12 2.5.4 Public Meeting (October 1998) .......................... 13 3. EXISTING FLOOD PROBLEM ......................................... 15 3.1 Flooding Mechanism ......................................... 15 3.2 Hydraulic Classification ........................................ 15 3.3 Flood Hazard Classification .................................... 16 3.4 Flood Damages ............................................. 18 4. FLOODPLAIN MANAGEMENT MEASURES .............................. 20 4.1 Introduction ................................................ 20 4.2 Measures Not Considered Further ............................... 21 4.2.1 Flood Mitigation Dams, Retarding Basins, On-Site Detention ... 21 4.2.2 Floodways .......................................... 22 4.2.3 Catchment Treatment ................................. 22 4.2.4 Flood Insurance ...................................... 22 4.3 Flood Modification Measures ................................... 23 4.3.1 River Improvement Works .............................. 23 4.3.2 Shoalhaven Heads Entrance ............................ 25 4.3.3 Local Drainage ....................................... 26 4.3.4 Levees ............................................. 27 4.4 Property Modification Measures ................................. 30 4.4.1 Voluntary Purchase ................................... 30 4.4.2 House Raising ....................................... 31 4.4.3 Flood Proofing ....................................... 31 4.4.4 Planning Considerations ............................... 32 4.5 Response Modification Measures ................................ 36 4.5.1 Flood Warning ....................................... 36 4.5.2 Evacuation Planning .................................. 37 4.5.3 Flood Awareness and Preparedness ...................... 39 4.6 Other Issues ................................................ 42 4.6.1 Bank Erosion ........................................ 42 4.6.2 Raising Terara Road .................................. 44 5. DEVELOPMENT MEASURES .......................................... 45 5.1 Control of Development Outside the Study Area .................... 45 5.2 Deposition of Silt on Pig Island .................................. 45 5.3 The Greenhouse Effect ....................................... 46 6. ACKNOWLEDGMENTS .............................................. 48 7. REFERENCES ..................................................... 49 LIST OF APPENDICES APPENDIX A: DESCRIPTION AND ASSESSMENT OF FLOOD DAMAGES APPENDIX B: PUBLIC CONSULTATION PROGRAM APPENDIX C: BANK EROSION AND FAILURE APPENDIX D: FLOOR LEVEL DATABASE APPENDIX E: POST FLOOD EVALUATION AND REVIEW LIST OF TABLES Table i): Summary of Floodplain Management Measures ...................... ii Table 1: Flood Events ................................................ 1 Table 2: Peak Levels of Major Floods (mAHD) .............................. 3 Table 3: Design Flood Levels (mAHD) .................................... 9 Table 4: Depths and Velocities ......................................... 16 Table 5: Hazard Classification ......................................... 17 Table 6: Terara Village - Damage to Residential Buildings ................... 18 Table 7: Floodplain Management Measures Not Considered Further ........... 21 Table 8: Dredging - Reduction in Flood Level (m) .......................... 24 Table 9: Impacts of Entrance Conditions at Shoalhaven Heads (mAHD) ......... 26 Table 10: Flood Related Issues Considered in Planning ...................... 33 Table 11: Proposed Changes to Council Flood Policy ........................ 35 Table 12: Rates of Rise - 1% AEP Design Flood ............................ 38 Table 13: Flood Education Methods ...................................... 41 Table A1: Flood Damages Categories ....................................A3 Table A2: Residential Flood Damage Surveys ..............................A8 Table A3: Summary of Post Flood Damage Surveys .........................A9 Table A4: Residential Stage-Damage for Actual Direct Damage to Structure and Contents .................................................A11 Table A5: Adopted Residential Depth/Damage Data ........................A12 Table E1: Design Flood Levels (mAHD) ..........................................................E2 LIST OF FIGURES Figure 1: Catchment Map Figure 2: Locality Plan Figure 3: Cell Model Layout Figure 4: Design Flood Profiles Figure 5: Building Inundated Terara Village Floodplain Management Study The State Government’s Flood Prone Land Policy is directed at providing solutions to existing flooding problems in developed areas and to ensuring that new development is compatible with the flood hazard and does not create additional flooding problems in other areas. Under the Policy, the management of flood liable land remains the responsibility of local government. The State Government subsidises flood mitigation works to alleviate existing problems and provides specialist technical advice to assist Councils in the discharge of their floodplain management responsibilities. The Policy provides for technical and financial support by the Government through the following four sequential stages: 1. Flood Study • determines the nature and extent of the flood problem. 2. Floodplain Management Study • evaluates management options for the floodplain in respect of both existing and proposed development. 3. Floodplain Management Plan • involves formal adoption by Council of a plan of management for the floodplain. 4. Implementation of the Plan • construction or implementation of floodplain management measures to protect existing development, • use of Local Environmental Plans to ensure new development is compatible with the flood hazard. The Terara Village Floodplain Management Study constitutes the second stage of the management process and has been developed by the Shoalhaven Floodplain Management Committee. It was prepared for the Committee by Webb, McKeown & Associates and provides the basis for the future management of flood liable lands at Terara. The terminology used in this report is in accordance with the NSW Government’s Floodplain Development Manual (1986 edition) and draft Floodplain Manual (1999 edition). Subsequently the final Floodplain Management Manual was released in January 2001. This latter document provided several changes in terminology which have not been included in this report. Webb, McKeown & Associates Pty Ltd 98066:TeraraFMS PDF Version.wpd:26 February, 2002 Terara Village Floodplain Management Study This Floodplain Management Study examines flooding issues relating to the village of Terara (Figures 1 and 2) and adjoining properties which are located on the south bank of the Shoalhaven River approximately 2.5 kilometres east of Nowra Bridge. Terara has a population of approximately 150 and has experienced major flooding in 1860, 1870, 1974 and 1978. The study was initiated by Shoalhaven City Council to address the management of the flood problem of Terara. The primary objectives of the Study were to define the nature and extent of the hazard; to identify, assess and optimise measures aimed at reducing the impact of flooding on both existing and future development; and to make recommendations for the future management of the study area. This Floodplain Management Study builds on the Lower Shoalhaven River Flood Study, which was completed in April 1990 and defines design flood levels within the Lower Shoalhaven floodplain, including Terara. Once a preferred scheme is adopted, an overall Floodplain Management Plan can be prepared. A summary of the measures considered in the course of the study is provided in Table i). Webb, McKeown & Associates Pty Ltd 98066:TeraraFMS PDF Version.wpd:26 February, 2002 i Terara Village Floodplain Management Study Table i): Summary