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Abstract

In July 2021, extreme weather conditions caused flooding in multiple tributaries of the Meuse and Rhine rivers. Particularly the Ahr Valley and the Erft Valley in Germany were greatly affected, where exceptional damage and severe human loss was registered. Since then, several studies have been conducted to understand the extremity of this event, the major driving forces, and the particular mechanisms of this flood. Furthermore, the possible impacts that climate change and human intervention may have contributed to the generation of such an event are discusses widely. In this study, we focus on the hydrological analysis of the July 2021 flood in the Ahr catchment, looking at the dominant hydrological processes that lead to rapid raising, extreme runoff rates and high water levels. For this purpose, we analyze the spatio-temporal distribution of the runoff coefficient using a semi-distributed hydrological catchment model, calibrated for this event based on the observed discharge measurements. The uncertainty in the rainfall input and the calibrated parameter set are as well incorporated in order to understand their influence on the spatio-temporal runoff yield patterns. Observed traces of erosion, surface water and flow paths are employed to shed light on the hydrological processes that took place. Furthermore, to understand how the dominant processes may have changed from previous events, the results of this analysis are compared to the 2016 flooding in Ahr Catchment. Lastly, the hydrologic response of this event is put on a broader perspective and compared with other extreme flood events in Europe.