Abstract
Abstract. In avalanche-prone areas, deflecting dams are widely used to divert avalanches away from endangered objects. In recent years, their effectiveness has been questioned when several large and multiple avalanches have overrun such dams. In 2008, we were able to observe a large wet-snow avalanche, characterized by an high water content, that interacted with a deflecting dam and overflowed it at its lower end. To evaluate the dam's performance, we carried out an airborne laser scanning campaign immediately after the avalanche. This data, together with a video sequence made during the avalanche descent, provided a unique data set to study the dynamics of a wet dense snow avalanche and its flow behavior along a deflecting dam. To evaluate the effect of the complex flow field of the avalanche along the dam and to provide a basis for discussion of the residual risk, we performed numerical simulations using a two-dimensional dense snow avalanche dynamics model with entrainment. In comparison to dry dense snow avalanches, we found that wet-snow avalanches, with high water content, seem to be differently influenced by the local small-scale topography roughness. Rough terrain close to the dam deflected the flow to produce abrupt impacts with the dam. At the impact sites, instability waves were generated and increased the already large flow depths. The complex flow dynamics around the dam may produce large, local snow deposits. Furthermore, the high water content in the snow may decrease the avalanche internal friction angle, inducing wet-snow avalanches to spread further laterally than dry-snow avalanches. Based on our analysis, we made recommendations for designing deflecting dams and for residual risk analysis to take into account the effects of wet-snow avalanche flow.
Highlights
Avalanche deflecting dams are often considered to reduce the vulnerability of endangered objects in the track and run-out zone of avalanches
In comparison to dry dense snow avalanches, we found that wet-snow avalanches, with high water content, seem to be differently influenced by the local small-scale topography roughness
Information about wet-snow avalanche flow and its interaction with a deflecting dam was obtained from laser scanning measurements, video analysis and numerical calculations
Summary
Avalanche deflecting dams are often considered to reduce the vulnerability of endangered objects in the track and run-out zone of avalanches. Dam design and the evaluation of the residual risk after a dam have been completed has been subject of discussion after the catastrophic winter 1999 in the Alps, when multiple avalanches overran dams, drawing attention to the limits of such protection measures (SLF, 2000) In recent years, both laboratory experiments and theoretical analysis have improved our understanding of the interaction of the flow of snow avalanches with dams and, new design criteria have been proposed on the basis of jumps or shocks theories for fast dry-snow avalanches (Hakonardottir et al, 2003; Faug et al, 2007; Cui et al, 2007; Johannesson et al, 2009; Baillifard, 2007).
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