Inception of Sediment Transport

Sediment transport inception curves corresponding to both laminar and turbulent flows are presented. Using these curves one can determine the critical bed shear stress (corresponding to the beginning of sediment transport) depending on the properties of the fluid and of the cohesionless bed material. The laminar flow curve has been revealed on the basis of laboratory measurements carried out by the writers. The curve corresponding to turbulent flows has been established mainly using the available data of previous researchers. It has been found that the laminar flow curve lies in the Shields' plane higher than the turbulent flow curve and thus it yields larger critical values of the dimensionless bed shear stress. Both curves tend to become indistinguishable from each other when the grain size Reynolds number decreases, i.e., when the influence of viscosity on the turbulent flow at the bed increases. For larger values of this number the turbulent flow curve can be identified with the classical Shields' curve.