A fragmentation-spreading model for long-runout rock avalanches

TLDR

Large rock avalanche deposits are dominated by intensely fragmented debris, and fragmentation is thought to shape their mass distribution and long runout. This study proposes that rock fragmentation is a key driver of the peculiar mass distribution and extended runout observed in large rock avalanches. Fragmentation generates high‑velocity fragments in all directions, creating an isotropic dispersive stress that, through a longitudinal dispersive force, decelerates the rear and accelerates the front of the moving mass. The model predicts greater longitudinal spreading and longer runout for fragmenting avalanches compared to non‑fragmenting granular flows.

Abstract

Based on the observation that deposits of large rock avalanches consist predominantly of intensely fragmented rock debris, it is proposed that the processes of rock fragmentation are significant causes of the peculiar distribution of mass in these deposits, and of the correspondingly long runout. Rock fragmentation produces high-velocity fragments moving in all directions, resulting in an isotropic dispersive stress within the translating rock mass. A longitudinal dispersive force consequently acts in the direction of reducing mass depth and tends to cause the rear part of the avalanche to decelerate and halt and the front part to accelerate. The result is greater longitudinal spreading of the travelling mass compared with nonfragmenting granular avalanches. The longer runout results from this additional fragmentation-induced spreading.