Stochastic Incipient Motion Criterion for Spheres under Various Bed Packing Conditions

Incipient motion criteria based solely on time-averaged bed shear stress may underpredict sediment transport. The focus of this study is on the stochastic aspect of the problem of incipient motion. Specifically, the role of near-bed turbulent structures and bed packing density on the commencement of sediment motion is investigated. The cornerstone of the proposed model is based on the concept that the particle motion is governed by the intermittent nature of near-bed turbulence. Based on this mechanism, in this article we provide a quantitative model for predicting the commencement of sediment entrainment for the first time under three representative bed packing densities corresponding to the isolated, wake interference, and skimming flow regimes. The performance of the proposed model is compared to published experimental data and the "conventional" approach that is based on the consideration that flow parameters are statistically well represented by a normal distribution.