Climate Futures for Tasmania Flood Inundation Mapping 25 February 2011 Climate Futures for Tasmania - Flood Inundation Mapping 25 February 2011 Executive summary Changes to design flood hydrographs were assessed using projected changes to the magnitude of extreme rainfall events as a result of climate change. The Climate Futures for Tasmania (CFT) project has produced dynamically downscaled high-resolution climate change projections for Tasmania using the CSIRO Conformal Cubic Atmospheric Model (CCAM). The IPCC emissions scenario (high (A2)) was used alongside six Global Climate Models to simulate the Tasmanian climate to 2100. The A2 scenario better represents observed emissions since 2000 (Allison et al, 2009) and is used for the analyses presented in this report. The impact of climate change on flood levels was assessed for selected Tasmanian catchments, namely the lower reaches of the Mersey River, Forth River, Huon River and Derwent River. A series of design flood hydrographs for the 1 in 10, 1 in 50, 1 in 100 and 1 in 200 annual exceedance probability (AEP) events have been produced for the observed 1961-1990 period, then scaled for three future periods (2010-2039, 2040-2069 and 2070-2099) using the CFT projections. MIKE 11 and where available MIKE 21 hydraulic models were run for each of the selected catchments and each of the design floods. Results from MIKE 11 and MIKE 21 were used to generate flood inundation maps for use by the Tasmanian State Emergency Services (SES). Hydrographs were developed for the four catchments of interest using a systematic stepwise process. Catchment areas and sub areas for each of the catchments were obtained from previous studies. Rainfall/runoff routing models, the rainfall/runoff routing network, non linearity exponent and the loss parameters for each of the models were sourced from previous studies. Design rainfalls have been obtained from either FORGE (Focussed Rainfall Growth Estimate Technique) or Australian Rainfall and Runoff. Climate Futures for Tasmania gridded rainfall data has been used to derive catchment wide information pertaining to the percentage change in design rainfall depth for the 1 in 10, 1 in 50, 1 in 100 and 1 in 200 AEP storm event across a series of durations. Through a critical duration analysis for each of the catchments it was found that catchments which have a critical duration of greater than or equal to 72 hours are unlikely to see any significant increase in flood extent as the design rainfall is not projected to increase as a result of climate change. Conversely, catchments with a critical duration less than 72 hours will see proportional increases in rainfall and as such the critical duration will be more likely to shorten. It was found that short duration events are projected to become more intense as a result of climate change, and as such it is likely that those catchments with fast response times are likely to be the most impacted by climate change. The peak discharge in the Mersey, Forth and Huon Rivers was found to increase significantly through to the end of the 21st century. The peak discharge in the Derwent River was not predicted to increase significantly due to the long duration of the critical storm events and significant upstream storages. Pre-existing hydraulic models (Mike 11 and Mike Flood where available) were run with the updated hydrographs for the baseline period. Further adjustments were made to the hydraulic models to replicate future conditions more fully. In this analysis, sea level rise for each of the future periods was assumed to be 0.8m as is described in full in the IPCC (2007) and has been used by both the Victorian Coastal Council (2008) and the Queensland Department of Environment and Resource Management (2009) for planning purposes in the absence of national benchmarks. Water levels were found to increase across all catchments for all off the design floods investigated, with the exception of the Derwent, which showed only a minimal change in water level, well within the error bounds of the modelling. i Error! Main Document Only. - Flood Inundation Mapping 25 February 2011 Flood inundation maps were developed from the analysis, showing the difference in flood extents between the baseline period and the start, middle and end of the 21st century. ii Climate Futures for Tasmania - Flood Inundation Mapping 25 February 2011 Contents Common Abbreviations 1 Glossary 2 1. Introduction 3 2. Limitations of this Study 5 3. Overview of Method 7 4. Hydrological Modelling 9 4.1 Modelling Approach 9 4.2 Previous Projects 9 4.3 Design Rainfalls 10 4.3.1 Design Rainfalls generated by CFT 10 4.3.2 Calculating Percentage Change in Rainfall from CFT Design Rainfall Inputs 10 4.3.3 Australian Rainfall and Runoff and CRC-FORGE Rainfalls 15 4.3.4 Adopted Design Rainfalls 15 4.3.5 Design Temporal Patterns 17 4.3.6 Spatial Patterns 17 4.4 Catchment Modelling 17 4.4.1 The Catchment Model 17 4.4.2 Dam and Storage Characteristics 18 4.4.3 Variation in losses with AEP for the design analysis 18 4.4.4 Validation of Catchment Model 19 4.4.5 The Design Model 19 4.5 Results for Design Flood Events 20 4.5.1 Critical Duration of the Outflow Flood 20 4.6 Comparison against Previous Studies 23 4.7 Flood Frequency Analysis 23 4.8 Hydrographs 24 5. Hydraulic Modelling 29 5.1 Previous Studies 29 5.2 Storm Surge 29 5.3 Sea level 29 5.3.1 Baseline 29 5.4 Sea Level Rise 30 5.5 Tributary Effects 30 5.6 Hydraulic Model 31 5.6.1 Mersey River Model 31 5.6.2 Forth River Model 32 5.6.3 Huon River Model 32 iii Climate Futures for Tasmania - Flood Inundation Mapping 25 February 2011 5.6.4 Derwent River Model 33 5.7 Flood Modelling Results 34 6. Flood Mapping 41 7. Conclusion 43 8. Acknowledgements 45 9. Disclaimer 47 10. References 49 Appendices A Climate Futures for Tasmania Rainfall Surface Map B Flood Frequency Curves C Hydrographs D Flood Maps E Mike11 Hydraulic Model Results Annexure List of figures Figure 3-1 Summary of method used ..................................................................................................................... 8 Figure 4-1 Annual rainfall over Tasmania simulated by a) A global climate model, b) Interpolated observations (AWAP, Jones et al. 2009), and c) 0.1 degree CCAM dynamically downscaled GCM (from Corney et al. 2010) .. 11 Figure 4-2 Mean rainfall depth for the 24 hour duration storm event between the baseline period and the end of the 21st century................................................................................................................................................. 13 Figure 4-3 Percentage change in rainfall depth between the baseline period (1961-1990) and the end of the 21st century (2070-2099)...................................................................................................................................... 14 Figure 4-4 Percentage change in Outflow Flood between 2070-2099 and the baseline period for the Mersey . 21 Figure 4-5 Percentage change in Outflow Flood between 2070-2099 and the baseline period for the Forth..... 22 Figure 4-6 Percentage change in runoff versus percentage change in design rainfall for the Mersey and the Forth catchments. ................................................................................................................................................. 27 Figure 5-1 Modelled Scenarios ............................................................................................................................. 31 Figure 5-2 Modelled Scenarios ............................................................................................................................. 32 iv Climate Futures for Tasmania - Flood Inundation Mapping 25 February 2011 Figure 5-3 Modelled Scenarios ............................................................................................................................. 33 Figure 5-4 Modelled Scenarios ............................................................................................................................. 34 Figure 5-5 Change in water level between the baseline period and the three future periods for the Mersey River downstream of Parangana Dam .................................................................................................................. 36 Figure 5-6 Change in water level between the baseline period and the three future periods for the Forth River downstream of Paloona Dam ............................................................................................................................... 37 Figure 5-7 Change in water level between the baseline period and the three future periods for the Huon River downstream of Judbury ........................................................................................................................................ 38 Figure 5-8 Change in water levels between the baseline period and the three future periods for the Derwent River downstream of Meadowbank Dam ............................................................................................................. 39 List of tables Table 4-1 Magnitude of the 24-hr duration 1 in 200 year AEP for 1961-1990 estimated from AWAP observations (5th/95th confidence intervals in brackets), with projected multi-GCM ensemble change for 2010-2039, 2040- 2069, 2070-2099, at five representative locations across Tasmania. AEPs estimated using a Generalised Pareto distribution. Multi-GCM ensemble AEPs estimated using the six downscaled GCMs for SRES A2. AEPs are expressed in mm, and