Phase Separation in Debris Flows and Their Feedback on the Bulk Dynamics

Debris flows pose significant threats to people and infrastructure in mountainous regions. Their destructive potential largely arises from the deep, dry granular front, followed by a progressively thinner and increasingly watery tail. This phase separation behaviour has been rigorously described in previous work using a depth-averaged theory that accounts for the vertical structure of the flow, velocity shear and relative motion between grains and fluid. This paper numerically solves these equations using a high-resolution shock-capturing scheme to investigate the case of dry and wet granular inclined flows onto a horizontal run-out pad. In this case, comparisons between numerical results and experimental data reveal that the shear-induced grain forward transport, which is completely missed by other debris flow models, exerts stronger influence on debris flows than mobility difference-driven transport that is assimilated into many debris flow models. This shear-induced grain forward transport is responsible for the formation of phase separation and crucial to quantitatively describe the feedback of phase separation on the overall dynamics. The mobility difference-driven transport indeed leads to phase separation, but it does not adequately describe the feedback of phase separation on the debris flow dynamics.