Beginning of sediment transport of incoherent grains in shallow shear flows

In this paper the main results of an experimental investigation on the begi nning of sediment transport of incoherent granular materials in shallow vis cous flows are presented. The experiments, performed in a laboratory flume in both laminar and smooth turbulent flow conditions, complement the data a lready available in literature, and confirm that the hyperbolic trend shown by the Shields curve when the grain Reynolds number X decreases to zero is not supported by experimental evidence. In addition, they confirm the Yali n and Karahan (1979) hypothesis on the existence of a distinct curve for th e inception of sediment transport in viscous dominated hows. On the basis o f the observed phenomenology, we propose to interpret the process as a func tion of the probability distribution of the repose angle of the grains of t he erodible bed. Accordingly, a simple mechanical relation is derived that explains the pattern exhibited by the experimental data.