The physics of debris flows — A conceptual assessment

Debris flows exhibit conspicuous dynamic interactions among their solid and fluid constituents. Key features of the interactions are neglected in traditional theories that treat debris flows as viscoplastic continua or as uniformly dispersed grain flows, but improved understanding of grain-grain and fluid-grain interactions has emerged from recent experimental and theoretical research. Grain-flow research has extended the concepts of statistical thermodynamics to consider inelastic grain collisions and to predict energy-dissipation, velocity, and grain-concentration distributions in flowing, granular materials. Research on fluid-grain interactions has focussed on fluctuating solid and fluid stresses in the vicinity of colliding grains and on energy dissipation in deforming solid-fluid mixtures. Insights born from these new approaches have practical ramifications for interpretive and predictive studies of debris flows.