Impacts of heavy and persistent precipitation on railway infrastructure – insights from the Ahr valley, Germany July 2021

Heavy precipitation events are an important trigger for flash floods and landslides on local and regional scale. In contrast to river floods, the enormous erosion potential in catchment areas contributes significantly to the extent of damage of infrastructure located in the valley floors. Due to the mobilisation of large amounts of sediment and deadwood, entrapment occurs at narrow points, e.g. bridges, which can subsequently lead to high flash flood waves, leading to destruction of transport infrastructure located close to rivers and streams. Considering climate change, such events are supposed to increase in the future. Hence, there is an urgent need to increase traffic resilience to this hazard.

This contribution examines the impact of the heavy precipitation event from 14./15. July 2021 on the railway in the Ahr valley in Rhineland-Palatinate, Germany. Large parts of the railway infrastructure were completely destroyed by the flood event. In a first step, a detailed overview of the climatological and hydrological drivers, by means of spatially high-resoluted distribution of precipitation and peak discharges modelling, is given. The event is than compared to past flash flood events of 1910 and 2016 along the Ahr valley. The second step presents a detailed mapping of the damage that occurred along the railway line based on aerial photographs, and addresses the question of the extent to which the railway infrastructure, especially bridges, contributed to the increase in the flood wave and the erosion potential. Based on the analysis, recommendations for action to foster the resilience of railway infrastructure to flash floods are presented, especially the question what magnitudes and return periods of events future dimensioning of railway infrastructure should be based on.