Controls Over Sediment Flux Along Soil-Mantled Hillslopes: Insights From Granular Dynamics Simulations

Sediment flux from soil-mantled hillslopes controls the volume of sediments delivered to rivers, the rate of rock exhumation, and the topographic evolution of hillslopes. Recent studies have inferred a nonlinear relation between hillslope gradient and sediment flux, but the functional forms proposed to date generally lack mechanistic explanations. Furthermore, although environmental disturbances have been invoked as facilitating sediment mobilization on soil-mantled hillslopes that reside below the angle of repose (the threshold for slope-driven motion), the way in which disturbances control the flux has received little investigation. Here we develop and employ a discrete element granular dynamics numerical model to study the relations between flux, slope, and disturbance characteristics at the grain scale. The numerical grains are subjected to gravitational body forces, contact forces, and random external perturbation forces, which are used to represent natural disturbances and which are characterized by perturbation magnitude and wavelength. Simulation results reveal an abrupt transition between two regimes. Low and intermediate slopes show granular creep, whereby grain velocity rapidly decays with depth. High slopes, albeit still below the angle of repose, show deep, granular slides, where all available material participates in the sliding motion. These two regimes can be described by two different theories for flow down an inclined plane. The simulations reveal that the external perturbations effectively reduce the angle of repose and that the nonlinearity in the slope-flux relation emerges from each regime separately and from the transition between them in particular. Simulations further demonstrate a positive correlation of sediment flux with perturbation magnitude and wavelength.