Abstract
Spatially and temporally averaged constraints on the extraction of momentum at the sediment boundary are inadequate to describe the episodic and spatially variable nature of bedload sediment transport. A model is proposed for the instantaneous transport of bedload sediment i a turbulent boundary layer having an arbitrary near-bed structure. The instantaneous force on individual grains is calculated. based upon a function for the reduction of velocity caused by upstream, protruding grains. Instantaneous forces calculated in this manner are in agreement with the highly spatially-and temporallyvarying drag forces measured of particles in sediment beds. When coupled with a simulation of the motion of all grains comprising a three-dimensional bed, the dynamic boundary condition predicts reasonable transport rates.
