A downward counterfactual analysis of flash floods in Germany
Abstract. In this study, we present the results of a counterfactual search for flash flood events in Germany. We used DWD’s RADKLIM product to identify the ten most extreme precipitation events in Germany from 2001 to 2022, and then assumed that any of these top 10 events could have happened anywhere in Germany. In other words, the events were shifted around all over Germany. For all resulting positions of the precipitation fields, we simulated the corresponding peak discharge for any affected catchment smaller than 750 km2. From all the realisations of this simulation experiment, the maximum peak discharge was identified for each catchment.
In a case study, we first focused on the devastating flood event in July 2021 in western Germany. We found that a moderate shifting of the event in space could change the event peak flow at the gauge Altenahr by a factor of two. Compared to the peak flow of 1004 m3/s caused by the event in its original position, the worst case counterfactual of that event led to a peak flow of 1311 m3/s. Shifting another event that had occurred just one month earlier in eastern Germany over the Ahr river valley even effectuated a simulated peak flow of 1651 m3/s.
For all analysed subbasins in Germany, we found that, on average, the highest counterfactual peak exceeded the maximum original peak (between 2001 and 2022) by a factor of 5.3. For 98 % of the basins, the factor was higher than 2.
We discuss various limitations of our analysis, which are important to be aware of: with regard to the quantification and selection of candidate rainfall events, the hydrological model, and the design of the counterfactual search experiment. Still, we think that these results might help to expand the view on what could happen in case certain extreme events occurred elsewhere, and thereby reduce the element of surprise in disaster risk management.
Status: final response (author comments only)
Interactive computing environment
Examplary model code and notebooks https://github.com/plvoit/counterfactual_flash_flood_analysis
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