Full Technical Report: Climate Change and Flash Floods
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Copernicus Climate Change Service Full Technical Report: Climate Change and Flash Floods Author name: Günter Humer 1, Stefan Achleitner 2, Simon Lumassegger 2, Andreas Reithofer 1 Author organization: 1: Dipl.-Ing. Günter Humer GmbH, Austria 2: Unit of Hydraulic Engineering, University of Innsbruck, Austria Full Technical Report: Climate Change and Flash-Floods Copernicus Climate Change Service Summary The government of Upper Austria prepares a tender for a state-wide hazard map of flash flood endangered areas away from permanent water bodies. Areas prone to inundation shall be delineated using computer simulations. The boundary conditions for these simulations (design rain under climate change, initial soil moisture conditions) must be derived. The investigation of flash-floods and the triggering preconditions showed that rain exceeding a certain amount can trigger a flash flood independent of preconditions, although the necessary rain must be higher if soil is less saturated following less antecedent rain. As the first results show a big influence of preconditions on the discharge of a flash-flood, we recommend using a highly saturated soil. The IDF curves from this study showed lower intensities that the corresponding Austrian design rains. The climate impact indicators show an increase in rain intensities for all emission scenarios. Therefore, it can be assumed that design-rains should increase accounting for future climate conditions. The client will set the rain intensities for his project of a hazard-map considering these insights. Full Technical Report: Climate Change and Flash-Floods Copernicus Climate Change Service Contents Introduction ............................................................................................................................................. 1 Step 1: Assessment of relative changes of design rainfall based on pan European indicator (provided by SWICCA) ......................................................................................................................... 2 Description: ..................................................................................................................................... 2 Results: ............................................................................................................................................ 3 Step 2: Relative changes to design rainfalls (varying durations and intensities) from pan European data are applied to local design rainfalls. ............................................................................................ 5 Description: ..................................................................................................................................... 5 Results: ............................................................................................................................................ 5 Step 3: Simulation of flash flooding considering varying inputs (durations and intensities) .............. 6 Description: ..................................................................................................................................... 6 3.1 Project region Poneggen ........................................................................................................... 6 3.2 Project region Wallern ............................................................................................................... 6 Results: ............................................................................................................................................ 7 3.1 Project region Poneggen ........................................................................................................... 7 3.2 Project region Wallern ............................................................................................................. 13 Step 4: Assess preconditions given at past flash flood events. Prior rainfall intensity and/or sums may be derived from pan European or local rainfall datasets. ......................................................... 15 Description: ................................................................................................................................... 15 Results: .......................................................................................................................................... 17 Step 5: Simulation of flash flooding (hydrology and hydraulic) using different preconditions. ....... 21 Description: ................................................................................................................................... 21 Results: .......................................................................................................................................... 22 Step 6: Derive flood extent maps (local data, e.g. DEM) .................................................................. 26 Description: ................................................................................................................................... 26 Results: .......................................................................................................................................... 26 Step 7: Derive flood inundation maps based on local data (e.g. DEM) ............................................. 27 Description: ................................................................................................................................... 27 Results: .......................................................................................................................................... 27 Step 8: Comparison of flood extent and inundation maps due to original and altered design rainfall input. Present the results to the client and describe the effects of preconditions and altered design rains on flash floods. Derive flood inundation maps based on local data (e.g. DEM) ...................... 31 Description: ................................................................................................................................... 31 Full Technical Report: Climate Change and Flash-Floods Copernicus Climate Change Service Results: .......................................................................................................................................... 31 Step 9: Derivation of the sensitivity of flood extents / inundations onto initial loss variations ....... 32 Description: ................................................................................................................................... 32 Results: .......................................................................................................................................... 32 Conclusion of Full Technical Report ...................................................................................................... 33 References ............................................................................................................................................. 34 Full Technical Report: Climate Change and Flash-Floods Copernicus Climate Change Service Introduction The government of Upper Austria prepares a tender for a state-wide hazard map of flash flood endangered areas away from permanent water bodies. Areas prone to inundation shall be delineated using computer simulations. The boundary conditions for these simulations (design rain under climate change, initial soil moisture conditions) must be derived. For the client, basically two aspects are of interest: • What assumption can be made for the initial loss in a design process? • How do design rainfalls alter under climate change conditions? The first aspect can be categorized as an indicator regarding the proneness to flash flooding with regard to initial conditions of a catchment. The second question is answered by providing (a) the relative or (b) the absolute change of rainfall input. The case study deals with flash floods occurring away from permanent water bodies. Storm event of high intensity introduce an overland flow and flood situation before the water reaches a river or creek. Today, the federal government seeks to prepare state wide maps, indicating zones prone to flash flood. Instead of deriving classical flood hazard maps indicating recurrence intervals and flood inundations for certain recurrence intervals, zones being save from flash flooding are intended to be mapped. Classical inundation mapping is planned in a second step, considering a more detailed simulation approach. For the flash flood simulations, standardized boundary conditions, initial conditions and inputs under current and future climate situation are to be derived. Whereas current design rainfalls are available in Austria, altered design rainfalls (or relative changes) are not. Secondly, the preconditions in the rainfall-runoff situation are to be defined. Change in design rains and soil moisture data in combination with daily rain-sums for the reference period and future period give the bandwidth for the selection of initial conditions and allow accounting for the future situation. Change of Relative changes Simulating in design flash floods Comparison of design rain discharges with varying maps inputs Flood Flood extent inundation maps Preconditions: Correlation Simulating flash maps Derivation of events from between rain floods with the sensitivity firebrigade, prior and flashfloods different of flood extents rainfall-sums preconditions Full Technical Report: Climate Change and Flash-Floods 1 Copernicus Climate Change Service Step 1: Assessment