Simulation of Initiation, Transport, and Deposition of Granular Avalanches: Current Progress and Future Challenges

Since 1989 models to route debris flows and avalanches for hazards mitigation have been constructed using the seminal work of Savage and Hutter. With this approach a Saint Venant model for wet or dry granular flow is constructed by depth integrating equations for mass and momentum conservation, evaluating stress using bulk mixture values and a Coulomb failure criterion. Such models rely on just three forces to determine whether motion will occur: the force giving downslope acceleration, drag along the bed during flow, and the stress gradients derived from variations in thickness of the flow. With this construction most avalanche models simply begin with a force imbalance set large enough to reproduce the runout and deposits observed. However research into granular flow mechanics has advanced our knowledge considerably in recent years, allowing construction of a new and more powerful class of models that incorporate the effects of changes in internal structure in the flow, and explicitly include phenomenon such as fluid-solid coupling during rapid deformation of saturated granular mixtures. The defining feature of these more sophisticated models is that they can evolve from a stable stress state into an unstable state such that, given certain conditions, an initially stable rock or soil masscanbegin to creep or deform slowly well before it eventually accelerates rapidly andflows downhill. The contrast between simple and sophisticated models is illustrated by comparison of a simple model for an estimated rockfall hazard in California using a Savage and Hutter approach with a sophisticated, fully coupled fluid-solid model that successfully simulated initiationand transport of experimental debris flows without arbitrarily adjusting any model parameters.