Shoalhaven City Council Broughton Creek Flood Study Final Draft Report April 2007 Prepared by SMEC Australia Pty Ltd Project Number: 31455 SHOALHAVEN CITY COUNCIL BROUGHTON CREEK FLOOD STUDY - FINAL DRAFT REPORT - April 2007 Prepared by: SMEC Australia Pty Ltd ABN 47 065 475 149 Project Number: 31455 DOCUMENT RELEASE INFORMATION Client Shoalhaven City Council Project Name Broughton Creek and Kangaroo Valley Flood Studies Document Number 3001049 Document Title Broughton Creek Flood Study – Final Draft Report Revision Status April 2007 Document prepared by: SMEC AUSTRALIA PTY LTD ACN 065 475 149 Level 6, 76 Berry Street, North Sydney 2060 Telephone (02) 9925 5555 Facsimile (02) 9925 5566 Prepared by: Reviewed by: Chris Koutsellis Jerzy Zastawny Amir Deen Water Resources Manager, Water Services © COPYRIGHT OF SMEC AUSTRALIA 2007 Any use of this material except in accordance with a written agreement with SMEC Australia is prohibited. TABLE OF CONTENTS 1 FOREWORD .....................................................................................................................5 2 EXECUTIVE SUMMARY ..................................................................................................6 3 INTRODUCTION...............................................................................................................7 4 BACKGROUND ................................................................................................................8 4.1 General......................................................................................................................................8 4.2 The Flood Problem...................................................................................................................8 5 DATA COLLECTION ........................................................................................................9 5.1 Reports ......................................................................................................................................9 5.1.1 General ..................................................................................................................................9 5.1.2 Broughton Creek ................................................................................................................. 10 5.2 Rainfall and Streamflow Data............................................................................................... 11 5.2.1 Rainfall Stations .................................................................................................................. 11 5.2.2 Streamflow Data.................................................................................................................. 12 5.2.3 Zoning and Contours ........................................................................................................... 12 5.2.4 Survey.................................................................................................................................. 12 5.2.5 Bridge and Culvert Drawings.............................................................................................. 12 5.2.6 Flood information – Newspapers, Plans and Questionnaires .............................................. 12 5.2.7 Aerial Photographs .............................................................................................................. 12 5.3 Community Consultation ...................................................................................................... 12 5.3.1 General Information ............................................................................................................ 13 5.3.2 Flood History....................................................................................................................... 13 5.4 Data Collection Summary ..................................................................................................... 15 6 HYDROLOGY .................................................................................................................16 6.1 General.................................................................................................................................... 16 6.2 Model Selection....................................................................................................................... 16 6.3 Broughton Creek Hydrologic Model .................................................................................... 16 6.3.1 Design Floods...................................................................................................................... 18 6.4 Hydrology Summary.............................................................................................................. 20 7 HYDRAULICS.................................................................................................................21 7.1 Model Selection....................................................................................................................... 21 7.1.1 Broughton Creek Hydraulic Model ..................................................................................... 21 7.2 Model Setup ............................................................................................................................ 21 7.2.1 General ................................................................................................................................ 21 7.2.2 Cross Sections ..................................................................................................................... 22 7.2.3 Boundary Conditions........................................................................................................... 22 7.3 Model Calibration & Verification......................................................................................... 23 7.3.1 General ................................................................................................................................ 23 7.3.2 Results ................................................................................................................................. 24 7.3.3 Historical Events ................................................................................................................. 24 7.4 Design Events.......................................................................................................................... 25 7.4.1 Sensitivity Analysis............................................................................................................. 27 7.5 Severity of the June/July 2005 Event.................................................................................... 28 7.5.1 Introduction ......................................................................................................................... 28 7.5.2 Methodology ....................................................................................................................... 28 7.5.3 Summary ............................................................................................................................. 32 7.6 Local Drainage ....................................................................................................................... 32 7.7 Flood Behaviour ..................................................................................................................... 35 7.8 Summary and Conclusions.................................................................................................... 39 8 HYDRAULIC CATEGORISATION & PROVISIONAL HAZARD MAPPING..................40 9 FINDINGS .......................................................................................................................41 10 REFERENCES................................................................................................................42 11 APPENDICES .................................................................................................................43 Figures Figure 1 – Study Area: Broughton Creek – Berry Town Map Figure 2 – RAFTS-XP Sub-Catchment Layout – Broughton Creek Figures 3a-d – Isohyetal Maps Broughton Creek 1974, 1988, 2002, 2005 Figures 4a-b – Mike 11 Model Layout Cross Sections - Broughton Creek Figures 5a-b – Flood Extent Map 2005 Figures 6a-b – Flood Extent Map PMF Figures 7a-b – Flood Extent Map 1% AEP event Figures 8a-b – Flood Extent Map 5% AEP event Figures 9-12 – Flow & Velocity Peaks 2005, PMF, 1%, 5% AEP events Figures 13-15 – Flood Contour Plans PMF, 1%, 5% AEP events Figures 16-18 – Flood Hazard Maps PMF, 1%, 5% AEP events Figures30-33 –Historical Flood RAFTS Results Broughton Creek 1974, 1988, 2002, 2005 Charts Chart 6.1 – Albert Street Drainage Catchment Map Chart 6.2 – DRAINS Plan Layout Albert Street Drainage Chart 6.3 – Albert Street Drainage 100% AEP Flood Profile 1 FOREWORD The township of Berry has experienced flooding in the past resulting in damage to private property, goods and roads. In response to these flood hazards and a desire to prepare a long-term management plan for the town, Shoalhaven City Council (SCC) are preparing a Floodplain Risk Management Plan to manage the existing and future flood risk to the town. This plan will be developed in accordance with the NSW Flood Prone Land Policy and the principles and guidelines in the Floodplain Development Manual 2005 (The Manual). The process of developing a Floodplain Risk Management Plan is illustrated in Schematic 1 below. Schematic 1: Process of Developing Floodplain Risk Management Plan Floodplain Risk Management Committee Established