Hydrodynamic simulations of the flash flood event in July 2021 in the Wesselbach catchment in Germany, and the effects of land use changes

Flash floods are among the most dangerous natural hazards and the associated risks are likely to increase due to climate change and increased urbanization. Observations of the last decades and projections of the future climate show an increase in the frequency and intensity of heavy rainfalls for many land surfaces. In July 2021, many European countries have been severely affected by large-scale heavy rainfalls. In Germany, the federal states of North Rhine-Westphalia and Rhineland-Palatinate have been particularly affected with at least 180 fatalities, hundreds of injuries, lots of heavily damaged buildings, and extensive infrastructural damages. The modeling of flash floods is essential for effective risk management to produce hazard and risk maps, investigate the effects of land use changes, and plan mitigation measures.

This works aims to investigate the flash flood event in the Wesselbach catchment in North Rhine-Westphalia (Germany), which was generated by an extreme, short rainfall event of 118 mm within less than two hours in the late evening of 13^th July 2021. The catchment is part of the city of Hagen, and the considered model domain of approximately 3 km² is characterized by steep slopes, a main soil type of silty loam, and a main land use type of forest, with settlements along the main watercourse in the downstream half of the domain. Large portions of coniferous areas in the catchment have exhibited decreasing vitality since 2018, up to complete dead or cleared areas. The in-house robust shallow water model hms⁺⁺ is used to simulate the flash flood event using the measured rainfall data of a nearby rainfall gauge as input. Spatially distributed Manning’s roughness coefficients are used to account for the different land use types. Infiltration is neglected as the soils in that area show limited infiltration capacity, and the worst-case is considered that the soils are already saturated. Building heights have been included in the digital elevation model.

The results include the temporal development of flooding areas, spatial distributions of maximum water depths, and flow velocities in the Wesselbach catchment as well as hydrographs at different cross-sections of the main water course. Furthermore, the effects of forest damage on the discharge behavior and flooding areas will be investigated. Later on, structural mitigation measures will be included in the model to study their effectiveness for different heavy rainfall events.