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.
Xiannan Meng (2026) studied this question.