Avalanche Run-Out on Counter-Slopes

Abstract

Counter-slopes in avalanche paths influence avalanche flow in similar ways as defence structures. Measurements and calculations of run-out on such slopes will improve our understanding of retaining dam design. We have studied three different, well described avalanche events; for each case we have calculated the run-out distance using the Noren, Irgens and Schieldrops continuum model (NIS), adjusting the parameters to fit the simulated avalanche to the observed one. Using the same parameters, the run-out on identical terrain, but without counter-slope, is computed. Comparison between the energy dissipation in these different cases tells us which topography most efficiently retards an avalanche. With a smoothly curved valley bottom, the energy dissipation at a given point on the opposite slope is less than on a flat run-out. The explanation to this is that, when gravity retards an avalanche, a smaller portion of the energy loss is due to dissipation from viscous (velocity dependent) friction. When the avalanche travels over a retaining dam, with much smaller curvature radii, a significant portion of the energy is dissipated. The consequence of this is that defence structures are not so dependent on the counter-slope inclination as on the curvature radius in the transition to up-gradient. Further, introduction of a counter-slope in an avalanche path may conserve energy rather than causing dissipation - if the inclination is not altered abruptly enough. If the counter-slope is smooth, and not large enough to completely stop the avalanche, it will not reduce destructive energy on the downstream side. In fact, the destructive energy may be partially conserved until the run-out.