An Index-Based Approach for Flash Flood Hazard Assessment in Germany Using Freely Available Geospatial Data

Flash floods are among the most destructive and unpredictable hydrometeorological hazards, frequently causing severe economic damage and loss of life. In Germany, the intensity and frequency of such events are projected to increase under ongoing climate change, underscoring the need for robust flood risk management supported by comprehensive spatial data and hazard information. However, a simple, homogeneous, and nationwide model for flash flood hazard assessment is still lacking.

This study presents the development and testing of an uncalibrated, index-based approach for flash flood hazard assessment in Germany, utilizing exclusively freely available and nationwide homogeneous geospatial datasets. Based on an extensive literature and data review, the Flash Flood Potential Index (FFPI) was identified as a suitable indicator for estimating flash flood susceptibility. A Python-based model was developed to calculate the FFPI using four key parameters—slope, land use, tree density, and soil type—derived from open national geodata. The relative weighting of these parameters was determined using the Analytic Hierarchy Process (AHP) method. The model was applied to three study areas in Germany representing diverse topographic and land cover conditions, and tested with varying parameter weightings and digital elevation model (DEM) resolutions.

In addition, a novel, supplementary module was implemented to compute FFPI-weighted flow accumulation, enabling the identification of downstream areas potentially affected by flash flood propagation. Test results indicate that the proposed modelling framework and additional module are suitable for flash flood hazard assessment across Germany, with four out of five predefined model expectations satisfactorily fulfilled. With further calibration and refinement, the model is expected to provide a cost-effective, transferable, and operationally simple tool for nationwide flash flood hazard estimation, contributing to improved risk management and early warning capacities under changing climatic conditions.