EUROBODALLA SHIRE COUNCIL TOMAKIN, MOSSY POINT, BROULEE AND MOGO FLOOD STUDY FINAL REPORT FEBRUARY 2017 Level 2, 160 Clarence Street Sydney, NSW, 2000 Tel: 9299 2855 Fax: 9262 6208 Email: [email protected] Web: www.wmawater.com.au TOMAKIN/MOSSY POINT/BROULEE/MOGO FLOOD STUDY DRAFT REPORT FEBRUARY 2017 Project Project Number Tomakin/Mossy Point/Broulee/Mogo Flood Study 114088 Client Client’s Representative Eurobodalla Shire Council Norman Lenehan Authors Prepared by Dr. Philip Conway Erika Taylor Date Verified by 21 February 2017 Revision Description Distribution Date 6 Final Report ESC, OEH FEB 2017 5 Draft Report ESC, OEH MAY 2016 4 Draft Report ESC, OEH APR 2016 3 Draft Report ESC, OEH MAR 2016 2 Draft Report ESC, OEH DEC 2015 1 Calibration and Validation Report ESC, OEH JUN 2015 TOMAKIN/MOSSY POINT/BROULEE/MOGO FLOOD STUDY TABLE OF CONTENTS PAGE FOREWORD................................................................................................................................ i 1. INTRODUCTION ........................................................................................................ 1 1.1. Background ................................................................................................ 1 1.2. Objectives ................................................................................................... 2 2. AVAILABLE DATA .................................................................................................... 3 2.1. Topographic Data ....................................................................................... 3 2.1.1. LiDAR Survey ............................................................................................. 3 2.1.2. Bathymetric Survey ..................................................................................... 4 2.1.3. Ground Survey ........................................................................................... 4 2.1.4. Cross-sectional Levels ................................................................................ 5 2.2. Culvert and Bridge Data.............................................................................. 5 2.3. Pit and Pipe Data ........................................................................................ 6 2.4. Historic Water Level Data (Continuous) ...................................................... 6 2.5. Historic Ocean Tide Datum (Continuous) .................................................... 6 2.5.1. NSW Tidal Planes Analysis ........................................................................ 7 2.6. Historic Rainfall Data .................................................................................. 7 2.6.1. Analysis of Pluviometer Data ...................................................................... 9 2.6.2. Analysis of Daily Read Data ..................................................................... 10 2.7. Design Rainfall Data ................................................................................. 13 2.8. Previous Reports ...................................................................................... 14 2.8.1. Report on Mogo Flood Study .................................................................... 14 2.8.2. Mogo Floodplain Management Study........................................................ 14 2.8.3. Mogo Commercial Area Drainage Study ................................................... 14 3. COMMUNITY CONSULTATION ............................................................................... 16 3.1. Online Media ............................................................................................ 16 3.2. Community Questionnaire and Information Sheet ..................................... 16 3.3. Drop-in Session ........................................................................................ 16 3.4. Consultation – Public Exhibition ................................................................ 17 4. STUDY METHODOLOGY ........................................................................................ 18 5. HYDROLOGIC MODEL DEVELOPMENT ................................................................ 20 5.1. Introduction ............................................................................................... 20 5.2. Sub-catchment Delineation ....................................................................... 20 5.3. Model Parameters .................................................................................... 20 5.3.1. Lag Parameter .......................................................................................... 21 5.3.2. Stream-flow Routing Parameter ................................................................ 21 5.3.3. Rainfall Losses ......................................................................................... 21 5.3.4. Impervious Areas ...................................................................................... 22 5.3.5. Summary of Model Parameters ................................................................ 22 6. HYDRAULIC MODEL DEVELOPMENT ................................................................... 23 6.1. Introduction ............................................................................................... 23 6.2. Model Extent ............................................................................................. 23 6.3. Digital Elevation Model ............................................................................. 24 6.4. Roughness Coefficient .............................................................................. 24 6.5. Hydraulic Structures ................................................................................. 25 6.6. Blockage Assumptions ............................................................................. 26 7. HISTORIC FLOOD MODELLING ............................................................................. 28 7.1. Introduction ............................................................................................... 28 7.2. Tidal Calibration ........................................................................................ 30 7.2.1. Description................................................................................................ 30 7.2.2. Methodology ............................................................................................. 30 7.2.3. Calibration Results.................................................................................... 30 7.3. Rainfall Calibration – February 2010 Event ............................................... 31 7.3.1. Description................................................................................................ 31 7.3.2. Methodology ............................................................................................. 31 7.3.3. Calibration Results.................................................................................... 32 7.4. Flow Validation – Regional Flood Frequency Estimates............................ 33 7.4.1. Description................................................................................................ 33 7.4.2. Methodology ............................................................................................. 33 7.4.3. Validation Results ..................................................................................... 33 7.5. Flood Extent Verification –Reported Flooding ........................................... 34 7.5.1. Description................................................................................................ 34 7.5.2. Verification Results ................................................................................... 34 8. DESIGN FLOOD MODELLING ................................................................................ 36 8.1. Introduction ............................................................................................... 36 8.2. Oceanic Coincidence ................................................................................ 36 8.3. Rainfall Critical Duration ........................................................................... 39 8.4. Analysis .................................................................................................... 40 8.4.1. Provisional Hydraulic Hazard .................................................................... 40 8.4.2. Provisional Hydraulic Categorisation......................................................... 41 8.4.3. Preliminary Flood Emergency Response Classification of Communities ... 42 8.5. Results ..................................................................................................... 42 8.5.1. Peak Flood Depths and Levels ................................................................. 42 8.5.2. Peak Flow ................................................................................................. 43 8.5.3. Provisional Hydraulic Hazard .................................................................... 44 8.5.4. Provisional Hydraulic Categorisation......................................................... 44 8.5.5. Preliminary Flood Emergency Response Classification of Communities ... 44 9. DESIGN FLOOD MODELLING – SENSITIVITY ANALYSIS .................................... 45 9.1. Introduction ............................................................................................... 45 9.2. Background to Sea Level Rise .................................................................. 45 9.3. Background to Increased Rainfall ............................................................