Beginning of sediment transport of incoherent grains in shallow shear flows

ln this paper the main results of an ex peri men tal investigation on the beginning of sediment transport of incoherent granular materials in shallow viscous llows are presented. The experiments, performed in a laboratory !lume in both laminar and smooth turbulent llow conditions, complement the data already 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 Yalin and Karahan (1979) hypothesis on the existence of a distinct curve for the inception of sediment tran sport in viscous dominated llows. On the basis of the observed phenomenology, we propose to interpret the process as a function of the probability distribution of the repose angle of the grains of the erodible bed. Accordingly, a simple mechanical relati on is derived that explains the pattern exhibited by the experimental data.