Revealing the effective basal condition of geophysical granular flows

Geophysical granular flows, such as landslides, rock avalanches and pyroclastic flows, remain difficult to model due to their unexpectedly high mobility. Several mechanisms have been proposed to interpret this low dissipative behavior, including the flow-substrate interaction. This study addresses this issue through dedicated, well-controlled experiments on steady granular avalanches of idealized and natural flowing materials (i.e., glass beads, sand, volcanic material) along a wide variety of inclined surfaces. Our observations reveal that the basal surface condition significantly influences the propagation and deposition dynamics of granular avalanches. In particular, we identify two main types of effective basal condition - smooth and rough - based on a single dimensionless parameter: the roughness-to-grain size ratio λ / d with a critical transition at λ / d ≈ 1 0 − 1 . The dynamic modeling of granular avalanches on smooth and rough inclines is then established based on initial flow conditions, material properties, and surface characteristics. We propose a unified flow rule governed by distinct functional relationships of the inflow Froude number, depending on the repose angle and grain size of flowing material, and the basal friction angle. These results highlight the importance of accurately constraining material and basal properties in order to improve field prediction of geophysical granular avalanches.