A comparative assessment of nature-based flood mitigation measures in a small catchment

Flash floods pose significant hazards for settlements and infrastructure in small rural catchments, especially when steep terrain, erodible sediments, and intensive land use come together. Wagenhausen, a village in Lower Franconia (Germany), is located within a 3 km² catchment, dominated by mixed forest and agricultural land. It lies along a small creek, a tributary of the Main River. In response to repeated flooding during intense rainfall events in the past, a series of cascading retention basins was constructed upstream of the settlement in the summer of 2024. In addition, a land-use change is being implemented by reforesting an agricultural field, a potential sediment source, upstream.

This study presents a work-in-progress, comparative assessment of the effects of these hydrological and sediment-related mitigation measures. Event-based simulations are conducted using the SIMWE hydrological model (r.sim.water). We analyse changes in runoff generation and flood intensity under different scenarios, including retention basins, reforestation, and their combined implementation. The modelling approach focuses on relative differences between scenarios rather than absolute discharge values, on account of the data-scarce nature of the catchment and the absence of gauge measurements.

Field investigations include soil analyses, infiltration measurements, and UAV-based surveys, conducted before and after the implementation of the retention basins. This provides information on soil properties, topography, and potential sediment source areas.

The study aims to improve process understanding of flash flood generation and sediment mobilisation in small catchments and to evaluate the role of land-use-based and structural nature-based solutions. The work is part of the EFRE-funded MainPro project, which investigates future hazards and ecosystem-based adaptation strategies for geo-ecosystems, settlements, infrastructure, and agriculture.