Merrigum Flood Study Final Report Level 5, 99 King Street Project Manager: Michael Turnley MELBOURNE VIC 3000 Australia Author: Michael Turnley, Ingrid Nolle
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MERRIGUMMERRIGUM FLOODFLOOD STUDYSTUDY GREATERGREATER SHEPPARTONSHEPPARTON FINAL REPORT September 2005 Merrigum Flood Study Final Report Offices Prepared For: Greater Shepparton City Council Brisbane Denver Karratha Melbourne Morwell Prepared By: WBM Oceanics Australia Newcastle Perth Sydney Vancouver T:\M6359.LH.MERRIGUM\09-REPORTING\R.M6359.001.03.STUDYREPORT.DOC 16/9/05 11:09 DOCUMENT CONTROL SHEET WBM Oceanics Australia Document: R.M6359.001.03.StudyReport.doc Melbourne Office: Title: Merrigum Flood Study Final Report Level 5, 99 King Street Project Manager: Michael Turnley MELBOURNE VIC 3000 Australia Author: Michael Turnley, Ingrid Nolle PO Box 604 Client: Greater Shepparton City Council Collins Street West VIC 8007 Client Contact: Greg McKenzie Telephone (03) 8620 6100 Facsimile (03) 8620 6105 Client Reference: Contract No. 876 www.wbmpl.com.au Synopsis: This report documents hydrologic and hydraulic modelling, results of a ACN 010 830 421 community flood questionnaire, damage assessment and inundation mapping for the township of Merrigum, located adjacent to the Mosquito Depression, 24 km west of Shepparton. The report focuses on the determination of flood levels and flood extents within the township and associated floodplain areas for a range of flood events. REVISION/CHECKING HISTORY REVISION DATE CHECKED BY ISSUED BY NUMBER 0 10/05/2004 MT IN 3 16/09/2005 MJ MT DISTRIBUTION DESTINATION REVISION 0 1 2 3 4 5 6 7 8 9 10 Greater Shepparton City Council 1 Not Issued 7 Greater Shepparton City Council (CD) 0 1 Goulbourn Broken CMA 1 0 WBM File (M6359) 1 1 WBM Library 1 1 T:\M6359.LH.MERRIGUM\09-REPORTING\R.M6359.001.03.STUDYREPORT.DOC 16/9/05 11:09 CONTENTS I CONTENTS Contents i List of Figures iv List of Tables v 1 INTRODUCTION 1-1 1.1 Background 1-1 1.2 Catchment Description 1-1 1.3 Study Area 1-1 1.4 History of Flooding 1-2 1.5 Study Objectives 1-3 1.6 Study Approach 1-3 1.7 Technical Steering Committee 1-5 2 DATA COLLECTION AND REVIEW 2-1 2.1 Existing Data 2-1 2.2 Additional Data 2-3 2.2.1 Photogrammetry and DEM 2-3 2.2.2 Site Inspection of Floodplain Obstructions 2-4 2.2.3 Survey 2-4 2.3 Data Gaps 2-5 2.4 Community Consultation 2-5 2.4.1 Results of Community Questionnaires 2-6 3 HYDROLOGY 3-1 3.1 Objective 3-1 3.2 Review of Available Data 3-1 3.3 Data Gaps 3-2 3.4 Hydrologic Modelling Approach 3-2 3.4.1 RAFTS Simulation of Design Storm Events 3-3 3.4.2 Rainfall Intensity and Losses 3-3 3.5 Verification of Hydrologic Model 3-4 3.5.1 Empirical Catchment Multiplier Factor 3-4 T:\M6359.LH.MERRIGUM\09-REPORTING\R.M6359.001.03.STUDYREPORT.DOC 16/9/05 11:09 CONTENTS II 3.5.2 Catchment Characteristics and Catchment Multiplier 3-5 3.5.3 Catchment Multiplier Translated Flows 3-5 3.6 Results of Hydrologic Modelling 3-8 3.7 The Probable Maximum Flood 3-9 4 HYDRAULIC ANALYSIS 4-1 4.1 Objective 4-1 4.2 Hydraulic Modelling Approach 4-1 4.3 Hydraulic Model Development 4-1 4.3.1 Model Resolution 4-1 4.3.2 Model Layout 4-2 4.3.3 Model Boundary Conditions 4-3 4.4 Structures 4-4 4.5 Design Event Modelling 4-5 5 FLOOD DATA CORRELATION 5-1 5.1 Comparison of Modelled and Recorded Flows 5-1 5.2 Comparison of Modelled and Recorded Flood Levels 5-1 6 FLOOD DAMAGE ASSESSMENT 6-1 6.1 Damage Curves 6-1 6.1.1 Above Floor Level Flooding 6-3 6.1.2 Below Floor Level Flooding 6-3 6.1.3 Estimated Damages 6-6 7 MAPPING 7-1 7.1 Inundation Mapping 7-1 7.2 Planning Map 7-1 8 FLOOD MITIGATION OPTION OVERVIEW 8-1 8.1 Option Assessment 8-1 8.1.1 Levees 8-2 8.1.2 Flood Warning System 8-2 8.1.3 Floodplain Education Program 8-3 8.1.4 Planning Scheme Amendments 8-3 8.1.5 Individual Property Flood-proofing 8-3 8.1.6 Flood Insurance 8-4 T:\M6359.LH.MERRIGUM\09-REPORTING\R.M6359.001.03.STUDYREPORT.DOC 16/9/05 11:09 CONTENTS III 8.1.7 Purchase and Relocation 8-4 8.1.8 Floodways 8-4 8.1.9 Floodwalls 8-4 8.1.10 Floodplain Modification 8-4 8.1.11 Removal of Obstructions 8-4 8.1.12 Data Collection - Hydrographic Gauge 8-4 9 CONCLUSION AND RECOMMENDATIONS 9-1 10 REFERENCES 10-1 APPENDIX A: COMMUNITY CONSULTATION DOCUMENTATION A-1 APPENDIX B: FLOODPLAIN OBSTRUCTIONS B-1 APPENDIX C: FLOOD MARK SURVEY MAP C-1 APPENDIX D: FLOOD INUNDATION MAPS D-1 APPENDIX E: FLOOD LEVEL AND PROPOSED PLANNING MAP E-1 T:\M6359.LH.MERRIGUM\09-REPORTING\R.M6359.001.03.STUDYREPORT.DOC 16/9/05 11:09 LIST OF FIGURES IV LIST OF FIGURES Figure 1-1 Study Area 1-2 Figure 1-2 Project Task Structure 1-5 Figure 2-1 Merrigum Study Area DEM 2-4 Figure 3-1 Mosquito Depression RAFTS Model Network 3-3 Figure 3-2 Catchment Multiplier Method 3-6 Figure 3-3 10-year ARI Hydrographs at Merrigum 3-8 Figure 3-4 100-year ARI Hydrographs at Merrigum 3-9 Figure 3-5 500-year ARI Hydrographs at Merrigum 3-9 Figure 4-1 2D Hydraulic Model Layout 4-2 Figure 4-2 Manning’s ‘n’ Distribution 4-3 Figure 4-3 Boundary Condition Configuration 4-4 Figure 5-1 Comparison of Modelled 10-year ARI Peak Flood Levels and October 1993 Flood Marks* 5-3 Figure 5-2 Comparison of Modelled 10-year ARI Peak Flood Levels and May 1974 Flood Marks* 5-4 Figure 6-1 Residential Stage Damage Curve for Merrigum 6-2 Figure 6-2 Distribution of Above and Below Floor Inundation - 100-year Event 6-4 Figure 6-3 Distribution of Above and Below Floor Inundation - 10-year Event 6-5 Figure 6-4 ANUFLOOD Damages Curve 6-6 T:\M6359.LH.MERRIGUM\09-REPORTING\R.M6359.001.03.STUDYREPORT.DOC 16/9/05 11:09 LIST OF TABLES V LIST OF TABLES Table 1-1 Technical Steering Committee Members 1-6 Table 2-1 Background Data and Survey Information 2-2 Table 2-2 Additional Background Data Acquired after TSC Meeting 2 2-3 Table 2-3 Results of Flooding Questionnaire 2-6 Table 3-1 Rainfall Depth and Losses for Merrigum Catchment 3-4 Table 3-2 Comparison of Modelled 100-year ARI Peak Flows at Tatura 3-4 Table 3-3 Summary of Translated Flows using Catchment Multiplier Method 3-7 Table 3-4 RAFTS Design Peak Flows and Time to Peak 3-8 Table 4-1 TUFLOW Manning’s ‘n’ Coefficients 4-3 Table 5-1 Comparison of Modelled and Recorded Flow at Merrigum 5-1 Table 5-2 Flood Mark and Level Comparison - October 1993 and modelled 10-year ARI Peak Flood Levels 5-2 Table 5-3 Flood Mark and Level Comparison - May 1974 and modelled 10-year ARI Peak Flood Levels 5-3 Table 6-1 Summary of Factors applied to ANUFLOOD Curves for Merrigum 6-1 Table 6-2 ANUFLOOD Potential Residential Damages at Merrigum 6-2 Table 6-3 Adjusted ANUFLOOD Actual Residential Damages at Merrigum 6-3 Table 6-4 Number of Properties Inundated Above Floor Level 6-3 Table 6-5 Above Floor Level Inundation by Depth Increment 6-6 T:\M6359.LH.MERRIGUM\09-REPORTING\R.M6359.001.03.STUDYREPORT.DOC 16/9/05 11:09 INTRODUCTION 1-1 1 INTRODUCTION 1.1 Background Greater Shepparton City Council (GSCC) commissioned WBM Oceanics Australia (WBM) to undertake a flood study for the township of Merrigum, with a population of 468 (2001 census), located adjacent to the Mosquito Depression in central Victoria. The objectives of the study were to identify, analyse and document flooding and flood risk at Merrigum. This report details: • The project objectives; • The adopted approach and methodology, including key assumptions; and • Results of key elements of the investigation, including flood mapping (extent and height), damage assessment and a preliminary assessment of risk treatments. Commencing June 2004, a draft copy of this report, Merrigum Flood Study (Document R.M6359.001.02.StudyReport.doc), was put on public exhibition. The community were invited to submit comments on the report and associated mapping. Council did not receive any submissions. 1.2 Catchment Description The township of Merrigum is situated approximately 25 km West of Shepparton, adjacent to the Mosquito Depression. The upstream catchment consists of approximately 228 km2 of a mix of irrigated and non-irrigated crops, orchards and pastoral land. The Mosquito Depression winds through the catchment as a series of defined ephemeral flow. Generally, drainage of the catchment occurs via the Mosquito Depression Drain. The drain is an earth lined, open trapezoidal channel cut through the depression in the mid 1980’s and extended in the early 1990’s. Mosquito Drain Extension Schedule of Works plans at Merrigum (Stage 5) state a design flow of 150 ML/d or 1.8 m3/s for the drain. This is the flow estimated to result from a 2-year average recurrence interval (ARI) summer design storm of 50mm in 24 hours. Embankments on one or both sides of the drain provide additional flow capacity through the urban areas of Merrigum. The design storm used to determine embankment elevation is stated as the 10-year ARI summer design storm of 75mm in 24 hours.