Abstract
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.
Highlights
The occurrence of severe flood events in the last decades, especially in 1999, 2005, and 2013, led to a new integrated flood mitigation concept for Bavaria
The majority of beaver territories are located in ponds, lakes, rivers, or streams that already meet beaver habitat requirements, and no beaver dam construction activity is observed
The evaluation of 114 beaver dam cascades and the detailed survey of 51 dams enabled the development of a three-stage prediction model for developing likely beaver dam cascade locations and configurations
Summary
The occurrence of severe flood events in the last decades, especially in 1999, 2005, and 2013, led to a new integrated flood mitigation concept for Bavaria. The concept is a combination of technical measures and nature-based solutions, which include, for example, land use changes and river restorations [1], and is based on German guidelines [2]. In the forthcoming Bavarian water management strategy, nature-based solutions and the associated combination of retention effects and ecological effects are of even greater importance [3]. This development toward natural flood management strategies can be noted in other countries, e.g., the UK, and is accompanied by several studies quantifying their potential retention effect [4,5,6,7,8]. In the UK, beavers started to be released in the last years and were even discussed in terms of flood peak reductions [12]
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