ST GEORGES BASIN FLOODPLAIN RISK MANAGEMENT STUDY SUSSEX INLET - JUNE 1991 FLOOD DECEMBER 2006 NEXUS ENVIRONMENTAL PLANNING PTY LTD SHOALHAVEN CITY COUNCIL ST GEORGES BASIN FLOODPLAIN RISK MANAGEMENT STUDY DECEMBER 2006 Webb, McKeown & Associates Pty Ltd Prepared by: ___________________________ Level 2, 160 Clarence Street, SYDNEY 2000 Telephone: (02) 9299 2855 Facsimile: (02) 9262 6208 Verified by: ____________________________ 20034:StGeorgesFPMS.wpd ST GEORGES BASIN FLOODPLAIN RISK MANAGEMENT STUDY TABLE OF CONTENTS PAGE FOREWORD SUMMARY GLOSSARY 1. INTRODUCTION .................................................. 1 1.1 The Flood Problem ......................................... 1 1.2 Historical Flood Data ........................................ 2 1.3 Floodplain Risk Management Process .......................... 3 2. BACKGROUND .................................................. 5 2.1 Catchment Description ...................................... 5 2.2 Description of Study Area.................................... 6 2.2.1 Climate ........................................... 6 2.2.2 Geology and Soils ................................... 6 2.2.3 Land Uses ......................................... 8 2.2.4 Ecology........................................... 9 2.2.5 Waterways ....................................... 11 2.2.6 Water Quality ..................................... 12 2.2.7 Social Characteristics ............................... 12 2.3 Flood Study Review ........................................ 13 2.4 Review of Existing Policies and Strategies ...................... 14 2.4.1 Council’s Interim Flood Policy ......................... 14 2.4.2 Council’s Existing Planning Instruments and Related Documents ................................................ 15 3. EXISTING FLOOD PROBLEM ...................................... 17 3.1 Flooding Mechanism ....................................... 17 3.2 Hydraulic Classification..................................... 17 3.3 Flood Hazard Classification .................................. 19 3.4 Flood Damages ........................................... 27 4. COMMUNITY CONSULTATION ..................................... 29 4.1 Components ............................................. 29 4.2 Questionnaire - December 2000 .............................. 29 4.3 Stakeholder Options Workshop ............................... 32 4.4 Open Shop Days .......................................... 32 4.5 Public Meetings........................................... 33 5. STUDY AREA ISSUES ............................................ 35 5.1 General Issues for the Whole Study Area ....................... 35 5.2 Discussion of Specific Issues ................................ 35 5.2.1 St Georges Basin By-Pass (The Wool Road) ............. 35 5.2.2 Sussex Inlet Channel ................................ 36 5.2.3 Jervis Bay Settlement Strategy........................ 36 5.2.4 Nebraska Estate ................................... 37 5.2.5 Flood Warning System .............................. 37 5.2.6 Filling of Land ..................................... 38 5.2.7 Local Flood Plan ................................... 39 5.2.8 Council’s Interim Flood Policy ......................... 40 5.2.9 Local Overland Flooding ............................. 41 6. FLOODPLAIN RISK MANAGEMENT MEASURES ...................... 43 6.1 Introduction .............................................. 43 6.2 Discussion of Possible Management Measures Not Considered Further 44 6.2.1 Flood Mitigation Dams, Retarding Basins, On-Site Detention . 45 6.2.2 Floodways ........................................ 45 6.2.3 Catchment Treatment............................... 45 6.2.4 Rezoning......................................... 46 6.2.5 Flood Insurance .................................... 46 6.3 Assessment of Flood Modification Measures .................... 47 6.3.1 The Wool Road - Sanctuary Point Bypass ................ 47 6.3.2 General Channel and Creek Improvement Works .......... 50 6.3.3 Sussex Inlet Channel Works .......................... 51 6.3.4 Levees General .................................... 53 6.3.5 Fairview Crescent Levee............................. 53 6.4 Assessment of Property Modification Measures .................. 55 6.4.1 Voluntary Purchase................................. 55 6.4.2 House Raising..................................... 57 6.4.3 Flood Proofing ..................................... 58 6.5 Assessment of Response Modification Measures ................. 59 6.5.1 Flood Warning ..................................... 59 6.5.2 Evacuation Planning ................................ 62 6.5.3 Evacuation Access ................................. 64 6.5.4 Flood Awareness and Readiness ...................... 66 6.6 Planning and Future Development Control Measures .............. 70 6.6.1 Review and Formalise the Current Interim Flood Policy ..... 70 6.6.2 Flood Planning Levels ............................... 73 6.6.3 Review and Update Section 149 Certificates .............. 80 6.6.4 Review and Update Local Environmental Plans and Development Control Plans...................................... 80 6.6.5 Planning Regulations - Caravan Parks .................. 82 6.6.6 Filling of the Floodplain .............................. 87 6.6.7 The Greenhouse Effect .............................. 89 7. ACKNOWLEDGMENTS ........................................... 91 8. REFERENCES .................................................. 93 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 APPENDIX F: REVIEW OF RELATED PLANNING DOCUMENTS APPENDIX G: STAKEHOLDER OPTIONS WORKSHOP - SUMMARY OF MEASURES APPENDIX H: UPDATES TO STUDY SINCE COMMENCEMENT APPENDIX I: ST GEORGES BASIN FLOOD ALERT SYSTEM - UPGRADE LIST OF TABLES Table i): Summary of Damages to Property ............................. ii Table ii): Summary of Proposed Floodplain Risk Management Measures ....... v Table 1: Catchment Area Breakdown.................................. 5 Table 2: St Georges Basin - Coastal Lake Classification Data Summary ...... 12 Table 3: Design Flood Level Ranges ................................. 19 Table 4: Comparison of Modelled Historical and Design Flood Levels ........ 21 Table 5: Comparison of Modelled Historical and Design Flood Velocities ...... 22 Table 6: Comparison of Catchment Response/Waiting Times .............. 23 Table 7: Critical Storm Durations for St Georges Basin and Tributaries ....... 24 Table 8: Summary of Damages to Property ............................ 28 Table 9: Respondents Perception of Potential Flood Affectation ............. 30 Table 10: Floodplain Risk Management Measures ........................ 44 Table 11: 1% AEP Flood Level Results ................................. 48 Table 12: 1% AEP Flood Velocities (m/s) ............................... 49 Table 13: PMF Flood Level Results ................................... 49 Table 14: Relative Impacts of Enlarging Sussex Inlet Channel (m) ............ 52 Table 15: Summary of Damages to Property in Fairview Crescent, Sussex Inlet . 54 Table 16: Summary of Properties Inundated above Floor Level in a 10% AEP Event ....................................................... 56 Table 17: Flood Education Methods ................................... 69 Table 18: Review of Current Flood Policies and Related Issues .............. 71 Table 19: Flood Related Issues to be Considered in the Selection of Flood Planning Levels.................................................. 75 Table 20: Flood Related Development Controls - General Development ....... 78 Table 21: Graded Development Controls for Caravan Parks in Flood Prone Areas 85 Table 22: Relative Impacts of Cumulative Filling (m) ....................... 88 LIST OF DIAGRAMS Diagram 1: Flood Experiences ......................................... 30 Diagram 2: Possible Flood Mitigation Options ............................. 31 LIST OF PHOTOGRAPHS Photograph 1: St Georges Basin Foreshore - June 1991 Flood ................... 2 Photograph 2: River Road, Sussex Inlet - June 1991 Flood ..................... 26 LIST OF FIGURES Figure 1: Locality Map and Study Area Figure 2: Environmental Issues Figure 3: Land Use Figure 4: Hydraulic and Hazard Categorisation - St Georges Basin Floodplain Figure 4a: Hydraulic and Hazard Categorisation of the Floodplain - Sussex Inlet Figure 4b: Hydraulic and Hazard Categorisation of the Floodplain - Cow, Tullarwalla and Wandandian Creeks Figure 4c: Hydraulic and Hazard Categorisation of the Floodplain - Pats and Home Creeks Figure 4d: Hydraulic and Hazard Categorisation of the Floodplain - Tomerong and Erowal Creeks and Worrowing Waterway Figure 5: Stage Hydrographs - 1% AEP Design Flood - Critical Durations Figure 6: Stage Hydrographs - 1% AEP Design Flood - 9h Storm Duration Figure 7: Stage Hydrographs - 1% AEP Design Flood - 48h Storm Duration Figure 8: Stage Hydrographs - 1971 Flood Event Figure 9: The Wool Road Bypass Figure 10: Sussex Inlet Channel Cross-Sections Figure 11: Fairview Crescent - Proposed Levee Location Figure 12: Possible Flood Warning Gauge Locations Figure 13: Caravan Parks St Georges Basin Floodplain Risk 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 so that it does not create additional flooding