  Gravity-driven flows of granular matter are involved in a wide variety of
situations, ranging from industrial processes to geophysical phenomena, such as
avalanches or landslides. These flows are characterized by the coexistence of
solid and fluid phases, whose stability is directly related to the erosion and
sedimentation occurring at the solid-fluid interface. To describe these
mechanisms, we build a microscopic model involving friction, geometry, and a
nonlocal cooperativity emerging from the propagation of collisions. This new
picture enables us to obtain a detailed description of the exchanges between
the fluid and solid phases. The model predicts a phase diagram including
erosion, sedimentation, and stationary-flow regimes, in quantitative agreement
with experiments and discrete-element-method simulations.
