Incipient Motion in Granular Beds Driven by Shear Flows

Granular packings on Earth's surface are regularly subjected to shear flows, as they are typically exposed either to moving water or air. These flows agitate the packings, and may erode them by removing grains given sufficient flow strength. Incipient motion of the packings (that is, the point in parameter space when a nonzero net downstream flux of grains appears) is typically thought to occur at a critical value of the Shields number, which balances the shear stress delivered to the grains with the weight of a grain. However, such a simple framework cannot directly account for additional but common physics, including granular packing effects, turbulent flow fluctuations, or the stress delivered to the bed by other mobile grains. I will discuss both simple numerical models and laboratory experiments aimed at elucidating the consequences of these effects on describing incipient motion, with particular application to the armoring phenomenon observed in gravel-bedded rivers.