Hydraulic Modeling of Beaver Dams and Evaluation of Their Impacts on Flood Events

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.

Dam busy: beavers and their influence on the structure and function of river corridor hydrology, geomorphology, biogeochemistry and ecosystems

“…These mechanisms of loss may also vary with dam age and level of maintenance by beaver populations (Woo and Waddington, 1990). A survey of 51 beaver dams of varying age in Germany found gapflow was by far the dominant mechanism of water release (Neumayer et al, 2020). Crucially, these observations suggest that the quantification of the hydraulics of beaver dams is difficult when based upon analogies with human-engineered instream structures (e.g.…”

“…This is not to say that once floodplain diversion is included, all river systems with beaver dams will have significant attenuation. Neumayer et al (2020) conducted a comprehensive 2D hydrodynamic model experiment by numerically inserting beaver dam cascades into two sites in southern Germany for a wide range of flood event conditions. Interestingly, they found flood volume attenuation and the delay in flood peak timing was only significant for smaller discharge events and were much more pronounced at the site with lower slope and higher floodplain connectivity.…”

“…However, it is unclear if beavers with multiple habitate selection options prefer already forested sites. In a study across 51 dam locations in southeastern Germany, 60% were constructed in areas of uniform riparian forest and only 2% in areas with no riparian forest (Neumayer et al, 2020), in Lithuania they preferred forested drainage canals (Ulevičius et al, 2011), however deciduous tree abundance was only of marginal importance in site selection in Sweden (Hartman, 1996). In terms of initial impacts, when permanently inundated, most deciduous canopy trees will die within a year, and smaller sub-canopy species even earlier (Härkönen, 1999;Müller-Schwarze, 2011), but given more variable surface inundation or a slowly rising groundwater table from below, trees at the margins or at slightly higher elevations may die a slower death or even survive, albeit potentially under sub-optimal growing conditions and thus with stunted growth (Härkönen, 1999;Reddoch and Reddoch, 2005).…”

“…Potential attenuation of smaller floods, unclear for larger floods, highly context dependent (e.g. floodplain diversion capacity); Unclear impacts on low flows (baseflow) but may increase in some cases, Neumayer et al, 2020;Nyssen et al, 2011;Puttock et al, 2017;Stabler,1985 3.3, 3.4 Groundwater-surface water interactions…”

“…valleys width > 4 stream widths) (Suzuki andMcComb, 1998, Pollock et al 2003) where beaver meadows can also develop (Figure 2 b). In addition, a single dam may not be built in isolation, with multiple dams over a reach termed a beaver dam cascade, and in this case lower peak discharges and higher river valley slope appear to be more important in allowing higher dam numbers to be constructed per cascade (Neumayer et al, 2020) (Figure 2a). This is not to say beavers do not construct dams outside these ranges (Pinto et al, 2009), or that other habitat factors such as vegetation (see section 6.5) are not important, only that they appear to be the preferred conditions for dam construction within a wide distribution of activity.…”