Studies on the active use of urban forest areas as pluvial flood prevention

In recent years, heavy rainfall events have caused significant damage in urban areas across Germany. Experiences in coping with pluvial floods show that single measures alone cannot reduce the risk, but the combination of different measures is required. Economic aspects and limited land availability in cities emphasize the demand for multifunctional and sustainable retention areas. In the ongoing research project WaldAktiv^*, we investigate the integration of existing urban forest areas into municipal flood prevention. The idea is to direct parts of the surface stormwater run-off into urban forest areas for storage and infiltration to reduce flooding in built-up areas. As study area, we use the district of Siegen-Wittgenstein, which has a high vulnerability to pluvial flooding due to its low mountain range topography. At the same time, with an area share of 71%, it is the most densely forested district in Germany and thus particularly well suited to determine corresponding potentials. However, this aim and other positive synergy effects are countered by (ecological) risks, such as the possible entry of pollutants into the forest areas, which must be taken into account during the studies.

First, potential flow paths and terrain depressions are identified based on a digital elevation model using a topographic analysis. While flow paths are used to delineate the individual catchments, terrain depressions in the urban forest areas represent potential retention basins for stormwater run-off. Although many terrain depressions are found, the analyses show that they are rarely located in suitable areas, so that artificial retention basins may have to be created in certain forest areas. Using hydrological modelling, the capability of the forest soil in terms of infiltration is estimated based on various soil geodata sets. In order to model the measures and assess their effectiveness, hydrodynamic numerical modelling is performed for different rainfall scenarios. In this contribution, we will present methods and current findings of the research project.

^* WaldAktiv is a research project, funded by the German Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV) and the district of Siegen-Wittgenstein through the project management of Zukunft – Umwelt – Gesellschaft (ZUG) gGmbH under the grant number 67DAS179.