ABSTRACT
Forces exerted on bed grains by turbulent flow have been considered in some detail in order to quantify the enhanced entrainment of sediment particles in regions of strong turbulent agitation. This has resulted in a sediment pick-up function relating the entrainment rate to the time-average and the standard deviation of the instantaneous bed shear stresses. The expression is restricted to uniform cohesionless bed material, and has been calibrated with experimental pick-up rates for a plane bed. In combination with estimates for particle step lengths and a suitable flow model, local bed-load transport rates on river-bed dunes can be calculated. The relations allow for non-negligible erosion all over an upstream dune face, even though the time-averaged bed shear stress on the lower part may be well below the Shields critical value. The derived expressions will be applied in a numerical model for the prediction of bed-form behaviour under unsteady hydraulic conditions.
