Investigating detailed impacts of climate change on flood inundation: a case-study from Tasmania

The impacts of climate change on flooding in Tasmania have not previously been subject to detailed research. In this study, design rainfall outputs generated from an ensemble of six fine-scale regional climate models by the Climate Futures for Tasmania project were used to assess impacts on flood extent for four large Tasmanian rivers for the SRES A2 emissions scenario.

Current design rainfalls were obtained using either focused rainfall growth estimate technique (FORGE) or Australian Rainfall and Runoff (ARR). Climate Futures for Tasmania gridded rainfall data was used to derive catchment wide percentage change in design rainfall depth for the 1 in 10, 1 in 50, 1 in 100 and 1 in 200 Annual Exceedance Probability (AEP) storm events for durations of 24, 48 and 72 hours. Changes in design rainfalls for these durations were used to extrapolate design rainfalls for durations of 12 and 18 hours.

Hydrographs were developed for four catchments: the Derwent River, the Forth River, the Huon River and the Mersey River. Catchment areas and sub areas for each of the rivers were obtained from previous studies undertaken for Hydro Tasmania. Rainfall-runoff routing models, the routing network, routing parameters and the loss parameters for each of the models were sourced from previous flood studies for each of the four rivers. Preexisting hydrological models developed for Hydro Tasmania to model dam break floods (Smythe (1995, 2001a, 2001b), Knight (2007)). were modified for use in modelling the 1 in 10, 1 in 50, 1 in 100 and 1 in 200 AEP events for the baseline period (1961-1990) and future periods (2010-2039, 2040-2069 and 2070-2099). The critical durations of the floods were determined for each AEP.

Projected future flood hydrographs were used as inputs to hydraulic models of the river downstream of the damsites. Existing flood hydraulic models were run for each of the selected catchments and each of the design floods under baseline and future climate periods.

Flood extents increased under climate change for all rivers with the exception of the Derwent River. The critical duration of design rainfall for the Derwent River at the point of interest was 72 hours. This study found that climate change will substantially increase the intensity of shorter duration rainfall events, but will only have a limited impact on the intensity of rainfall events with durations in excess of 24 hours.