Hazard assessment of compound pluvial and fluvial (CPFF) flooding: a case study in the Ahr valley

Current flood hazard mapping and risk management practices typically address pluvial and fluvial flooding separately. In many regions, however, compound pluvial and fluvial flooding (CPFF) are a significant challenge.  We develop a methodological approach to explore the relevance of CPFFs for the reconstruction processes and flood risk management in the Ahr valley in western Germany, devastated by the July 2021 flood. A non-stationary regional weather generator is applied to generate 100 realizations of synthetic precipitation and air temperature daily time series over a 72-year historical period (1950-2021). The method of fragments is used to disaggregate daily precipitation into hourly scale. The mHM hydrological model is used for rainfall-runoff simulation, producing the hourly discharge at the gauge Altenahr as fluvial boundary conditions for the downstream area. A total of 208 CPFF events are identified from 100 model realizations. The inundation depth and extent of these CPFF events are subsequently simulated using the RIM2D hydrodynamic model. We present a comparison of resulting CPFF flooding versus fluvial flooding alone. Our analysis reveals that fluvial flooding dominates maximum inundation depths, while pluvial rainfall expands the flood extent. Furthermore, CPFF events demonstrate substantially more severe hazards compared to fluvial flooding alone, which is typically the baseline for flood protection practices. These findings underscore the urgency of integrating CPFF into risk assessments and planning, offering policymakers critical insights to improve resilience against compound flood hazards.