The equilibrium and entrainment of a sediment grain

Abstract

Entrainment of sediment from loose, granular beds always requires grains to be moved up and away from initial stable equilibrium positions. At the entrainment threshold each grain passes momentarily through an unstable equilibrium point. Grain motion prior to entrainment is essentially in place, and is more or less smooth depending on the turbulence of the flow. A constrained Wiener process can be used to model this motion and provides a straightforward measure of how easy it is to entrain a grain. This allows the form of the entrainment threshold function to be predicted, and points to reasons for the systematic deviations that are found between measured data and threshold functions based on time-averaged flow.