Erosion-deposition regime formation in granular column collapse over an erodible surface

In this paper, the collapse of a granular column over an erodible bed is investigated experimentally and numerically. Four series of experiments are performed using different materials in order to understand the occurrence of the erosion and its evolution. Three different erosion-deposition regimes are observed in experiments, which primarily depend on the difference in material properties between the column and erodible bed but vary little with the initial aspect ratio of the column. Once the erosion process occurs, the collapsing flow has both longer initial acceleration and final deceleration phases but a shorter intermediate period of constant-velocity flow, a phenomenon that is more prominent when the flow front of collapsing material eventually penetrates into bed material. However, the overall flowing duration satisfies a proportional relationship with the free-fall time of granular column tau(c) for both erosion regimes, almost identical to that observed on a rigid substrate. Furthermore, the mechanism of the erosion is discussed based on the evolution of velocity field and the migration of the interface between static and flowing materials. The proposed equation of motion for the flow front shows that the particle density ratio of the column and the erodible bed plays a crucial role in the formation of different erosion-deposition regimes. In addition to reproducing various deposit morphologies observed in our experiments, our numerical simulation results also confirm the existence of critical particle density ratios that correspond to regime transitions and are almost independent of the initial aspect ratio of the column.