Frictional dependence of shallow-granular flows from discrete particle simulations

A shallow-layer model for granular flows is completed with a closure relation for the macroscopic bed friction or basal roughness obtained from micro-scale discrete particle simulations of steady flows. We systematically vary the bed friction by changing the contact friction coefficient between basal and flowing particles, while the base remains geometrically rough. By simulating steady uniform flow over a wide parameter range, we obtain a friction law that is a function of both flow and bed variables. Surprisingly, we find that the macroscopic bed friction is only weakly dependent on the contact friction of bed particles and predominantly determined by the properties of the flowing particles.