Particle shape dictates critical shear stresses for sediment motion

Existing sediment transport models concentrate on spherical grains, leaving grain shape effects on sediment transport unclear. We present two-dimensional Newtonian simulations of angular sediment grains driven by a model flow to study the influence of grain shape on the yield stress of stationary beds. We evaluate contacts between polygonal grains with a novel algorithm and parameterize a model flow with bed porosity. Simulations reveal wide statistical distributions for critical shear stress that vary systematically with grain shape and depend on bed preparation. Yield characteristics of idealized disks and irregular grains display extreme differences, which we describe from grain-scale mechanics. These findings call into question the assumption of a single yield stress for sediment transport, and they suggest sediment transport understanding requires careful accounting of particle shape.