Flow rule, self-channelization and levees in unconfined granular flows

TL;DR

This study experimentally investigates unconfined granular flows on inclined planes, revealing how flow confinement, levee formation, and deposit morphology evolve, highlighting a nonlocal rheology near jamming and the buildup of static layers.

Contribution

It introduces a new understanding of flow confinement, levee disappearance over time, and the static layer formation during unconfined granular flows.

Findings

Flow gets confined by quasistatic banks during long transients.

A scaling law for velocity extends to unsteady flows.

Levees disappear for long flow durations, and static layers form.

Abstract

Unconfined granular flows along an inclined plane are investigated experimentally. During a long transient, the flow gets confined by quasistatic banks but still spreads laterally towards a well-defined asymptotic state following a nontrivial process. Far enough from the banks a scaling for the depth averaged velocity is obtained, which extends the one obtained for homogeneous steady flows. Close to jamming it exhibits a crossover towards a nonlocal rheology. We show that the levees, commonly observed along the sides of the deposit upon interruption of the flow, disappear for long flow durations. We demonstrate that the morphology of the deposit builds up during the flow, in the form of an underlying static layer, which can be deduced from surface velocity profiles, by imposing the same flow rule everywhere in the flow.