Kinematics of scarp retreat in idealized tilted channel experiments

In some landslides, collapse is accompanied by the upslope retreat of a well-defined scarp whose speed controls the rate of mobilization of debris. Here we examine the evolution of such scarps in an idealized laboratory setting. We conduct tilted channel experiments involving retrogressive dry granular landslides over an erodible substrate. After first tilting up a deep sand layer to close to the angle of repose, then imposing an abrupt base-level drop, granular flow is induced at the downstream outlet. This flow generates an upstream-traveling wave with a well-defined scarp at the upstream tip. Downstream of the moving scarp, sand flows as an avalanching layer of finite depth over the erodible but stationary substrate, and outflows over the lowered outlet sill. A series of such experiments were conducted to determine the influence of channel width and base-level drop height on the speed of scarp retreat and other flow properties. Measurements included the time-evolving profile of the free surface, surface velocities acquired using particle tracking velocimetry, and the time-evolving mass outflow rate at the downstream outlet. Dimensional analysis clarifies the physical mechanisms governing the rate of scarp retreat. These results will help guide and validate numerical models of granular landsliding over erodible substrates.