| 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 in a turbulent boundary layer having an arbi-trary 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 in agreement with the highly spatially-and temporally-vary-ing drag forces measured of particles in sediment beds．[*]When coupled with a simulation of the motion of all grains comprising a threedimensional bed，the dynamic boundary condition predicts reasonable transport rates． |
