Dorrigo Flood Study
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BELLINGEN SHIRE COUNCIL DORRIGO FLOOD STUDY FINAL REPORT December 2007 BELLINGEN SHIRE COUNCIL DORRIGO FLOOD STUDY FINAL REPORT December 2007 Prepared By: PATERSON CONSULTANTS PTY LIMITED 1/45 PRINCE STREET GRAFTON NSW 2460 P O BOX 596 GRAFTON NSW 2460 Tel: (02) 6643 1588 Fax: (02) 6642 7566 E-mail: [email protected] Version 6 Authorised for Release Copy of . DOCUMENT CONTROL SHEET Paterson Consultants Pty Ltd Suite 1, 45 Prince Street P O Box 596 Grafton NSW 2460 Grafton NSW 2460 Telephone: (02) 6643 1588 Job Name: Dorrigo Flood Study Facsimile: (02) 6642 7566 Job No.: 05-025 Email: [email protected] Original Date of Issue: October 2006 DOCUMENT DETAILS Title: Dorrigo Flood Study Principal Author: K W Paterson Client: Bellingen Shire Council Client Address: 33-39 Hyde Street, Bellingen NSW 2454 Client Contact: Ms Denique Littler REVISION / CHECKING HISTORY Version Number Version Name Date Issued By 1 Draft Report – Version 1 October 2006 2 Draft Report – Version 2 November 2006 3 Draft Report – Version 3 February 2007 4 Exhibition Report September 2007 5 Final Report October 2007 6 Final Report December 2007 KWP 7 8 DISTRIBUTION RECORD Version Number Destination 1 2 3 4 5 6 7 8 9 Client (bound) 3 15 Client (electronic) 1 1 1 1 3 Department of Natural Resources 1 1 Dept of Environment and Climate 1 1 Change (electronic) File Copy 1 1 1 Paterson Consultants Library 1 1 Paterson Consultants Pty Limited 5. TABLE OF CONTENTS PAGE NUMBER ACKNOWLEDGEMENTS 8 FOREWORD 9 SUMMARY 10 1. INTRODUCTION 14 2. STUDY APPROACH 15 3. AVAILABLE DATA 16 3.1 Overview 16 3.2 Previous Studies, Documents 16 3.3 Water Level and Rainfall Gauges 20 3.4 Resident Interviews 22 3.5 Local Newspapers 23 3.6 Waterway Cross-sections 24 4. HYDROLOGICAL INVESTIGATION 25 4.1 Overview 25 4.2 Hydrology Model - Bielsdown River 26 4.3 Flood Frequency Analysis 27 4.4 Initial Analysis - Design Hydrographs - Bielsdown River 29 4.5 Refinement of Design Hydrographs 29 4.6 Flood Inflows - Un-named Tributary 32 4.7 PMF Estimates 35 4.8 Summary of Hydrological Analysis 36 5. HYDRODYNAMIC INVESTIGATIONS 38 5.1 Overview 38 5.2 MIKE-11 Model of the Bielsdown River 38 5.3 MIKE-11 Model Calibration and Verification 40 5.4 HEC-RAS Model - Un-named Tributary 42 5.5 Design Flood Profiles 43 5.6 Sensitivity Analysis 44 5.7 Provisional Flood Hazard and Hydraulic Analysis 48 6. CONCLUSIONS 50 REFERENCES 51 GLOSSARY 52 Bellingen Shire Council Dorrigo Flood Study Final Report - December 2007 R90\05025.V6 6 Paterson Consultants Pty Limited TABLE OF CONTENTS (Cont) PAGE NUMBER TABLES 1. Significant Historical Floods (Station 204017, "Bielsdown River @ Dorrigo") 18 2. Historical Flood Levels - PWD Sewage Compilation (1954 flood) 19 3. Historical Flood Levels - WRC Inundation Map 19 4. Significant Historical Floods vs Available Records 21 5. Historical Flood Levels - Resident Interviews 23 6. Comparison of Modelled and Recorded Floods 27 7. Peak Flood Discharges - "Bielsdown @ Dorrigo No. 2 and No. 3" 28 8. Peak Flow - Bielsdown River 29 9. Comparison of Peak 1% AEP Flows 30 10. Adopted Peak Flows - Bielsdown River 32 11. Peak Flows - Un-named Tributary (design 1% AEP flood) 34 12. Design Peak Flows - Un-named Tributary 35 13. PMP Rainfalls - Bielsdown River @ Dorrigo 36 14. PMF Flow Estimates 36 15. Adopted Design Peak Flows 37 16. Cross-section River Distances 39 17. Adopted Mannings `n' Values 41 18. Comparison of Flood Levels - 2001 Flood 41 19. Comparison of Flood Levels - 1954 Flood 42 20. Comparison of Flood Levels – Sensitivity Runs 46 FIGURES 1. Study Locality 2. Historical Flood Levels 3. Location of Rainfall Stations 4. Ground Survey 5. Study Catchment 6. RORB Model Layout 7. Comparison - 1973 & 1974 Floods 8. Comparison - 1977 & 1985 Floods 9. Comparison - 1989 & 2001 Floods 10. Rainfalls for Model Calibration 11. Flood Frequency - Bielsdown River 12. Frequency - Design vs Historical Floods 13. Design Flood Hydrographs, Bielsdown River 14. Un-Named Tributary - DRAINS Model 15. Riverine Models Layout 16. Historical Flood Profiles - Bielsdown River 17. Design Flood Profiles - Bielsdown River 18. Design Flood Profiles - Un-named Tributary 19. 5% AEP Flood Contours 20. 1% AEP Flood Contours Bellingen Shire Council Dorrigo Flood Study Final Report - December 2007 R90\05025.V6 Paterson Consultants Pty Limited 7. TABLE OF CONTENTS (Cont) PAGE NUMBER FIGURES (Cont) 21. PMF Flood Contours 22. Provisional Flood Hazard – 5% AEP 23. Provisional Flood Hazard – 1% AEP 24. Provisional PMF Flood Hazard 25. Flood Hazard Diagram 26. Provisional Hydraulic Categories – 5% AEP 27. Provisional Hydraulic Categories – 1% AEP APPENDICES A. Questionnaire B. Adopted Rating Curves - Station 204017, Bielsdown @ Dorrigo No. 2 & 3" C. Annual Flood Series, Station 204017, "Bielsdown @ Dorrigo No. 2 & 3" D. Bureau of Meteorology Stations E. Design Rainfalls - Intensity-Frequency-Duration F. Design Flood Levels - Bielsdown River G. Design Flood Flows - Bielsdown River H. Design Flood Velocities - Bielsdown River I. Design Flood Levels - Un-named Tributary J. Design Flood Velocities - Un-named Tributary K. Sensitivity Testing L. Cross-section Plot Positions Bellingen Shire Council Dorrigo Flood Study Final Report - December 2007 R90\05025.V6 8 Paterson Consultants Pty Limited ACKNOWLEDGEMENTS This study has been undertaken by Paterson Consultants under the auspices of the Bellingen Shire's Floodplain Risk Management Committee. The Committee comprises representatives from: - Elected representatives, Bellingen Shire Council; - Council officers, Bellingen Shire Council; - Nominated officers of State Emergency Services, Department of Natural Resources; - Nominated community representatives. The study has been funded by Federal Government, State Government and Bellingen Shire Council on a 1:1:1 contribution basis from each body. Bellingen Shire Council Dorrigo Flood Study Final Report - December 2007 R90\05025.V6 Paterson Consultants Pty Limited 9. FOREWORD The New South Wales Government's Flood Prone Land Policy is directed at providing solutions to existing flooding problems in developed areas as well as ensuring that new development is compatible with the flood hazard and that it does not create additional flooding problems in other areas. Under the policy, the management of flood-prone land remains the responsibility of local government. The State Government provides specialist technical advice and financial subsidies for studies and capital works to assist councils in the discharge of their floodplain management responsibilities. The flood policy provides for technical and financial support by the government through the following four sequential stages: * Stage 1 - Flood study: Determines the nature and extent of the flood problem. * Stage 2 - Floodplain Risk Management Study: Evaluates management options for the floodplain in respect of both existing and proposed development. * Stage 3 - Floodplain Risk Management Plan: Involves formal adoption by council of a plan of management for the floodplain. * Stage 4 - Implementation of the plan: Involves construction of flood mitigation works to protect existing development and includes use of local environmental plans to ensure new development is compatible with the flood hazard. The Dorrigo Flood Study constitutes part of the first stage of the management process for the Bielsdown River through Dorrigo and an unnamed tributary to the Bielsdown River, through Dorrigo. The unnamed tributary falls within a "greenfields" area likely for future development activity. The Dorrigo Flood Study has been prepared for Bellingen Shire Council to determine an appropriate floodplain risk management strategy. Bellingen Shire Council Dorrigo Flood Study Final Report - December 2007 R90\05025.V6 10 Paterson Consultants Pty Limited SUMMARY The Dorrigo Flood Study has been undertaken to provide flood information for establishment of a floodplain risk management plan for Dorrigo. The end use of this study will most likely be for the setting of development controls and addressing flood access issues. The study area covers: - the Bielsdown River from the Dorrigo Golf Course to Dangar Falls; - an un-named tributary, which runs along the eastern side of the developed area of Dorrigo and joins the Bielsdown River some 800 metres upstream of Dangar Falls. The study approach adopted involves: - collection and assessment of flood data from government resources and resident interviews; - definition of waterway areas along the Bielsdown River and the un-named tributary by ground survey; - establishment and calibration of an hydrologic model and a riverine hydraulic model to predict design flood levels for the 0.5, 1, 2, 5, 10 and 20% AEP floods and the Probable Maximum Flood (PMF); - presentation of design flood information as: - flood profiles for each of the design events; - tabulated values for each of the flood contours; - flood contours over cadastral maps for three of the seven design floods investigated. Available data on flooding comprises: - recorded flood flows; - recorded rainfalls; - recorded flood levels. The nearest (and only relevant) stream gauging station is located on the Bielsdown River at the disused Glenreagh - Dorrigo Railway. The Station, identified as Station 204017, "Bielsdown River at Dorrigo No. 2 and No. 3", has stream flow and flood level records from 1947 to current period. Initial analysis of the flood records indicated a number of anomalies. Further review indicated that the water level gauges had been moved several times and that the official record did not include any adjustments for the shift in gauges. Bellingen Shire Council Dorrigo Flood Study Final Report - December 2007 R90\05025.V6 Paterson Consultants Pty Limited 11. The adjusted flood record shows the 1954 event as the largest flood since 1947. The March 2001 flood is ranked sixth largest and appears to be smaller than floods occurring in 1973 and 1974. Planning for sewerage in Dorrigo commenced in the late 1960's. The "Sewage Compilation Map" for the sewerage scheme notes three 1954 flood levels near the Coramba Road crossing of the Bielsdown River and near the sewage treatment plant.