Flow resistance in the transition from dense to dilute granular-fluid flows

Substantial research work has been focusing on the flow resistance of dense granular-fluid geophysical flows, e.g., debris flows. However, the mechanism of flow resistance as the dense debris flows transition to the dilute debris flow range (volumetric solid concentration 60% to 40%) remains an unsolved problem. Based on the accurate measurements of normal/shear stresses and pore fluid pressure at the flume base, we analyze the flow resistance of a series of controlled debris flow model tests, covering the flow regime from friction dominated to viscous/collisional dominated. We find that the flow resistance, excluding the Coulomb frictional component, can be well described by a visco-collisional scaling relationship. The solid–fluid interaction in the dilute range would facilitate a quick rebalance against the gravity driven force in the transient flow condition. Finally, a heuristic model is proposed to unify the flow resistance for dense and dilute debris flows.