There is a general agreement on the impact of beaver dams regarding the increasing diversity of habitats and the improvement of the water quality, whereas the retention effect during flood events is still being discussed. In this study, we modeled 12 beaver dam cascade scenarios in two catchments for eight flood events with a two-dimensional (2D) hydrodynamic model. The implementation of the potential cascades in the model is based on the developed three-stage model for predicting location-dependent dam cascades in Bavaria. A Bavaria-wide questionnaire regarding dam occurrences and characteristics in combination with a detailed survey of 51 dams was used to set up a prediction scheme. It was observed that beaver dams are most likely built in rivers with riparian forest, with widths from 2 to 11 m and depths smaller than 1 m. The hydraulic model results showed larger inundation areas (>+300%) for the beaver dam scenarios. There is a noticeable peak attenuation and translation for elevated peak discharges (five times the annual mean discharge: up to ≤13.1% and 2.75 h), but no remarkable effect could be observed for flood events with return periods of more than 2 years. We conclude from the results that beaver dam cascades can have an impact on runoff characteristics, but do not lead to relevant peak reductions during flood events and therefore cannot be counted as flood mitigation measure.

Nature-based retention measures are an essential part of a sustainable and integrated flood protection strategy and can contribute to a holistic flood mitigation approach. Thereby river restoration measures such as channel widening, or flow path extension to increase the channel meandering represent successfully used components. Coupled with flood plain measures, retarding and retention effects of flood events are possible. These effects are commonly computed applying two-dimensional hydrodynamic modelling approaches. However, these developments rely on high spatial and temporal resolutions which are generally characterized by a high computational demand and are hence time and cost expensive. Thus, the evaluation and derivation of flood routing parameters to reproduce the resulting hydrodynamical processes in hydrological models can provide an effective and fast computation of river restoration scenarios. The objective in the present study is the derivation and application of flood routing parameters which can account for the effects of river restoration and flood plain measures in hydrological models. Further, this study aims to determine if the catchment and scale specific outcomes and parameter sets are also applicable to a broader range of catchments.For this purpose, commonly applied flood routing approaches and the associated parameters used in hydrological models (e.g. the kinematic wave approach in the WaSiM model) are investigated for catchments of different scales in Bavaria (Germany) and for flood events of varying characteristics (e.g. return period, flood volume). To determine the effects of channel restoration and flood plain measures, two-dimensional hydrodynamic models (HYDRO_AS-2D) are set up to simulate the current state as well as restoration scenarios. Based on the simulation results of the hydrodynamic models, the parameters of the flood routing approaches are calibrated to match the catchment specific restoration effects for a first set of river sections. Catchment and scale dependent parameter sets (dominating valley type, flood plain slopes) are then derived to reproduce the specific river restoration. First results of the calibration of the parameter sets show a satisfying fit of the hydrological model to different restoration scenarios of the hydrodynamic model. For the validation of the derived parameter sets of the flood routing methods in the hydrological model additional river sections of the hydrodynamic models are subsequently investigated.The implementation of the new flood routing parametrization of the hydrological models is finally examined as an alternative resource efficient way of calculating the effects of river restoration scenarios. Moreover, the applicability of the outcomes as a cost-efficient alternative compared to hydrodynamic models in land use planning and risk assessment is assessed and discussed within the frame of river restorations as flood mitigation measures.

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.