Rock avalanche mobility: The role of path material

Rock avalanches are high velocity flows of fragmented rock that can dramatically alter landscapes, and impact people and infrastructure far from their source. These catastrophic events have been studied for over a century, however, a consensus regarding the mechanism(s) that govern their motion has yet to emerge. This work details the results of the back-analysis of 45 rock avalanche case histories using a semi-empirical runout model that simulates motion over 3D terrain. These simulations account for topographic effects and bulk basal shear resistance variation during motion. For a subset of the cases, we find that a volume-dependent mechanism appears to influence basal resistance in the source zone. However, once the material has vacated the source zone, our results suggest that the character of the path material strongly controls mobility, and appears to dominate over other, potentially volume-dependent, mechanisms that may also be at work. The results presented in this paper support the longstanding hypothesis that the interaction of flowing debris with the underlying substrate is an important consideration in rock avalanche runout prediction, and that this mechanism warrants further research.