River-bed armoring as a granular segregation phenomenon

TL;DR

This study investigates how granular physics explains river-bed armoring, revealing that segregation occurs through rapid shear-driven surface processes and slow creeping diffusion beneath, supported by experiments and modeling.

Contribution

It demonstrates that river-bed armoring results from granular segregation mechanisms, combining experimental observations with continuum and discrete element models.

Findings

Surface segregation driven by bed-load transport occurs rapidly.

Deeper creeping grains cause slow, diffusion-dominated segregation.

River beds armor primarily through below-surface granular segregation.

Abstract

Gravel-river beds typically have an "armored" layer of coarse grains on the surface, which acts to protect finer particles underneath from erosion. River bed-load transport is a kind of dense granular flow, and such flows are known to vertically segregate grains. The contribution of granular physics to river-bed armoring, however, has not been investigated. Here we examine these connections in a laboratory river with bimodal sediment size, by tracking the motion of particles from the surface to deep inside the bed, and find that armor develops by two distinct mechanisms. Bed-load transport in the near-surface layer drives rapid segregation, with a vertical advection rate proportional to the granular shear rate. Creeping grains beneath the bed-load layer give rise to slow but persistent segregation, which is diffusion dominated and insensitive to shear rate. We verify these findings with a continuum phenomenological model and discrete element method simulations. Our results suggest that river beds armor by granular segregation from below --- rather than fluid-driven sorting from above --- while also providing new insights on the mechanics of segregation that are relevant to a wide range of granular flows.