Erosion and entrainment in dry granular flows: Insights from large-scale experiments

Erosion and entrainment of substrate are pivotal in amplifying the scale of granular flows and altering their dynamic behaviors. However, the mechanisms behind erosion and entrainment remain poorly understood, with numerous conflicting interpretations. Here, a series of experiments were conducted using a large-scale apparatus, where a dry granular mass descended through a steep rigid channel, subsequently eroded and entrained substrates with varying conditions laid on a gentle plane, and finally deposited on a horizontal platform. Substrate deformation responses under erosive loading, post-entrainment runout, and deposit characteristics were captured by multi-point, multi-source monitoring devices. Two long-standing issues are emphasized: the mechanisms governing substrate erosion and entrainment, and the effects of substrate entrainment on granular-flow dynamics. The experiments reveal that substrate erosion and entrainment involve multiple dynamic processes that are spatially dependent across different erodible areas. In the proximal area, impact-induced ploughing dominates the interaction process and induces substantial energy dissipation. In the middle area, eroded substrate grains are readily entrained into the granular flow, forming a coupled basal deformation layer that enhances granular-flow runout. In contrast, entrainment in the distal area is strongly suppressed, thereafter inducing upward shear localization and intensified collisional dissipation. Furthermore, fine substrate enrichment at the granular-flow base after entrainment forms a low-friction shear-deformation layer capable of lubricating granular flow. These results indicate that the energy feedback associated with substrate erosion and entrainment is fundamentally controlled by spatially specific erosion–entrainment mechanisms and their associated rheological transitions. Our experiments provide important insights for explicitly modelling and further understanding erosive rock avalanches.