Flood Risk Management Using Representative Hillslopes: Insights From a Historical Flood in Southwest Germany

ABSTRACT Understanding and preparing for extreme events in a warming climate remains challenging, particularly for modelling flash floods in small‐ to mesoscale catchments. While top‐down modelling approaches that describe fluxes at the system scale are often effective for riverine floods driven by saturation‐excess runoff, bottom‐up approaches are better suited to capturing intensity‐controlled runoff generation and associated preferential flow processes. Based on the gradient‐conserving simplification of representative hillslopes, a meso‐catchment scale spatially distributed, process‐based model was applied to simulate a severe summer flood event that occurred in 1994 in southwest Germany. Our approach provides a balance between the complexity required to represent coupled flow processes at the hillslope scale and the practical constraints of scaling these to the mesoscale. Following evaluation against available observations, the model is used to reconstruct flood magnitudes in poorly gauged but severely affected headwater regions in the catchment. The results highlight the influence of spatial variability in gradients and land use on runoff generation in these areas. To further explore these findings, we conducted additional simulations across a range of precipitation return periods to examine the sensitivity of flood response under different scenarios. The results suggest that uncertainties are more pronounced at smaller spatial scales, likely due to data limitations. Finally, simplified nature‐based solution (NbS) scenarios were implemented at the hillslope and headwater scales to explore their potential influence on downstream flood response. This study contributes to improved understanding of overland flow responses over mesoscale catchments, a critical scale for flood management, particularly under increasing convective extremes as a result of anthropogenic climate change.