Statistical and hydraulic analysis of flood hazard in the Ahr valley, Germany considering historical floods

The flood event in July 2021 caused more than 180 death tolls in Germany and several billion Euro damage. In particular, the Ahr valley was severely hit by the flood, which lead to unprecedented inundation, houses and bridge destruction. Flood response was based on the official flood hazard maps, which were designed in the course of the EU Flood Directive. We revisit flood hazard in the Ahr valley by considering historical flood events in the extreme value statistics. A state-of-the-art bootstrapping approach is applied to estimate flood quantiles considering uncertainties in estimating historical discharges. Inclusion of historical floods dramatically changes estimation of flood quantiles. Nevertheless, we observe significant deviations between empirical and theoretical flood distributions due to singular outstanding extremes and discuss the suitability of the GEV model. We further drive a 2D hydraulic model with the estimated quantiles to compute inundation areas, depths and flow velocities. Human stability indicator is additionally computed. We conclude that the consideration of historical events is paramount for assessing flood hazard. We recommend to revisit flood hazard maps and include historical floods either into extreme value statistics or as indication of potential extreme inundation.