Avalanche Run-Out on Counter-Slopes

Counter-slopes in avalanche paths influence avalanche flow in similarways as defence structures. Measurements and calculations of run-outon such slopes will improve our understanding of retaining damdesign. We have studied three different, well described avalancheevents; for each case we have calculated the run-out distance usingthe Noren, Irgens and Schieldrops continuum model (NIS), adjustingthe parameters to fit the simulated avalancheto the observed one. Using the same parameters, the run-out onidentical terrain, but without counter-slope, is computed. Comparisonbetween the energy dissipation in these different cases tells us whichtopography most efficiently retards an avalanche. With a smoothly curved valley bottom, the energy dissipation at agiven point on the opposite slope is less than on a flatrun-out. The explanation to this is that, when gravity retards anavalanche, a smaller portion of the energy loss is due to dissipationfrom viscous (velocity dependent) friction. When the avalanche travelsover a retaining dam, with much smaller curvature radii, a significantportion of the energy is dissipated. The consequence of this is that defence structures are not sodependent on the counter-slope inclination as on the curvature radiusin the transition to up-gradient. Further, introduction of acounter-slope in an avalanche path may conserve energy rather thancausing dissipation – if the inclination is not altered abruptlyenough. If the counter-slope is smooth, and not large enough tocompletely stop the avalanche, it will not reduce destructive energyon the downstream side. In fact, the destructive energy may bepartially conserved until the run-out.