Retention potential analysis of river restoration and floodplain measures in different catchments of Bavaria, Germany

Nature-based solutions are an important component of integrated flood mitigation strategies for improving both the protection against hazardous flood events and the ecological conditions of river-floodplain systems. In order to be able to take these types of measures into account in upcoming flood management decisions, it must be possible to reliably estimate their effects on flood events. Therefore, this study focuses on a more general view on the catchment dependent contribution of combined river and floodplain restoration measures to the strengthening of river retention and flood protection. Furthermore, the importance of considering site-specific circumstances (e.g., the superposition of the flood waves of the main river and its tributaries), is evaluated.

The study is based on five investigation areas in Bavaria (Germany) with various topographic properties and different spatial scales (~ 90 – 560 km²). For each catchment, a physically based hydrological model (WaSiM) was coupled with the two-dimensional hydrodynamic model HYDRO_AS-2D by means of direct and diffuse inflow boundary conditions. Five flood events with various rainfall characteristics (advective/convective) and different return periods (5, 20 and 100 years) were generated with WaSiM. The holistic restoration scenarios are implemented by catchment dependent modifications of river channels and floodplains. As the aim of this study is to analyze the maximum possible efficiency of the restoration scenarios, it is assumed that almost the entire floodplain is available for the implementation of these measures. Highly restricted areas (e.g., settlement & industrial areas, important infrastructure) are excluded from this assumption. First results show that the peak discharge attenuations resulting from the restoration measures are exemplarily dependent on the characteristics of the floodplains (e.g., slope and extent) and the volumes of the flood events. It could be shown that the largest peak discharge attenuations (up to 28 %) and retardation (up to 8 h) occur in catchments with relatively flat and wide floodplains in combination with comparatively small flood volumes. Furthermore, the effectiveness of these measures can be considerably affected by local superposition effects with incoming tributaries. These effects can have site and event specific positive or negative impacts on the peak discharges and may not be neglected when planning restoration measures.

Based on these investigations, it is possible to evaluate if catchments are likely to be suitable for river and floodplain restoration in the course of flood management decisions. However, the effectiveness of the measures is always influenced by a combination of many area-specific factors that can only be predicted to a limited extent and therefore requires the modelling of an area.