Amplification of Flood Hazard and Damage by Compounding Pluvial and Fluvial Flooding

Abstract

Flood risk management has traditionally treated fluvial and pluvial flooding in isolation, for example, by producing inundation maps separately for each flood type. Here, we develop an integrated modeling framework capable of capturing the interplay of both flood types when they co‐occur. The framework consists of the non‐stationary weather generator nsRWG, a disaggregation method, the hydrological model mHM, the hydrodynamic model RIM2D and the damage model FLEMO _flash . It is applied to the Ahr Valley in western Germany, which was severely affected by the July 2021 flood disaster. Using this model chain, we generate 7,200 years of spatially consistent hourly climate and streamflow data (100 realizations of the period 1950–2021) that are translated into inundation and flood damage time series. Within this data set, spatially and temporally consistent 208 compound pluvial and fluvial flood (CPFF) events are identified. Their inundation characteristics and damage values are compared to those obtained under the conventional approach when both flood types are simulated in isolation. Our results demonstrate that CPFF events consistently produce greater hazard and damage than isolated flood types that contribute to a specific CPFF event. Pluvial contributions primarily expand inundation extents, while fluvial processes increase inundation depths, together amplifying overall flood impact. Compared to fluvial‐only flooding, CPFF events increase damages by up to 1,000% for frequent river flooding (return periods less than 20 years) and up to 20% for rare events (with return period exceeding 500 years) for the study area. Relative to pluvial‐only flooding, CPFF events increase damages by up to 300% for frequent events and up to 100% for events with return periods exceeding 50 years. Compound events involving moderate return periods (e.g., a 10‐year fluvial flood with a 7‐year pluvial flood) can cause damages comparable to rare single‐driver events (e.g., 50‐year fluvial or pluvial floods). However, the sum of single‐type flood hazard indicators (maximum inundation depths and areas) and corresponding damages are systematically higher than hazard and damage of the resulting CPFF events due to temporal asynchrony and spatial overlapping effects. These findings highlight the limitations of conventional flood hazard and risk assessments that analyze pluvial and fluvial events separately. The proposed framework provides a robust, integrated methodology for flood risk assessment that accounts for compounding mechanisms and offers valuable insights for improving flood risk management.