Rock–Snow–Ice Avalanches

Abstract Rock avalanches which occur in glacial environments are controlled in their event dynamics and mode of propagation by the interplay between the detached rock and the icy component during all phases of motion, from initiation to final deposition. Because of the presence of ice and snow, the flow mobility is enhanced with respect to rock avalanches of comparable magnitude evolving in nonglacial settings by up to 25–30%. The high mobility, together with other possible secondary effects, caused by a change in flow behavior during propagation when glacial rock avalanches impact lakes or entrain and melt ice and snow at the flow base, determine glacial rock avalanches destructiveness. In recent decades, many among the most disastrous rock avalanches have occurred in glacial environments. In the future, possible increases in failure events occurring in formerly glaciated and permafrost areas are likely because of ongoing changes in climatic conditions. Several factors converge to determine the mode of propagation and the high mobility observed for ice-rock avalanche events: (1) the debris-glacier interface provides a low-friction surface; (2) the basal topography may favor propagation due to funneling or air launching of the debris by moraines; (3) the detached material usually contains ice and snow in quantities which can be further increased by entrainment and reduces friction within the moving mass; (4) the snow and ice at the base of the flow can supply meltwater due to frictional heating, or compression of snow along the glacier surface increasing saturation and further reducing the flow resistance. The role of each factor has been analyzed based on post-event documentation, laboratory experiments, and numerical modeling. Despite a certain degree of the uncertainty of the results, which deserve more investigation, the presence of an icy basal material has been found to be particularly relevant at determining the flow mobility, either due to the smooth low-friction surface provided and lubrication or liquefaction effects of the propagating material due to melting processes.