Inception Sediment Transport Relationships at High Slopes

This study proposes relationships between mean streamflow depth and cohesionless particle bed characteristics at the beginning of the sediment transport at high slopes in low submergence conditions. The inception of sediment transport is theoretically approached by the incipient motion condition. A previous theoretical analysis of the equilibrium of a gravel particle based on the exposure of the particle itself to the streamflow is generalized by introducing the relative position of the exposed part of the particle with respect to the bottom and by considering the case of a partially submerged particle. Using a low-submergence flow velocity profile in the particle balance of forces at the limit of equilibrium leads to a relationship between the mean flow depth and the sediment size. Another velocity profile of low-submergence flow is added to that used in the previous study, and two analytical expressions of flow depths corresponding to the beginning of sediment transport are obtained for both fully submerged particles and partially submerged particles. The theoretical flow depths given by the analytical expressions are then compared with those from previous experimental tests at the beginning of sediment transport at high slopes with fully submerged particles. The comparison shows a satisfactory agreement between theoretical and experimental values of flow depth corresponding to the degree of exposure and the relative position of the exposed part of the surface layer bed particles.