Comparison of Sediment-Pickup Rates over Plane Bed and Dunes

Abstract

Sediment-pickup rate is useful for deriving bedload function and specifying boundary conditions for numerical modeling of sediment transport. Previous pickup functions were developed based only on data measured under the plane bed condition, and they may not be applicable for dune-covered beds. The present study compares pickup rates for three bed conditions, i.e., plane bed, fixed dunes, and mobile dunes. All pickup functions are formulated by involving the densimetric Froude number instead of the Shields number. The results show that the sediment-pickup rates measured over fixed dunes can be described using the pickup function developed for the plane bed. However, the presence of mobile dunes causes a considerable reduction in the pickup rate when the densimetric Froude number is greater than 5, which is largely due to flow–particle and particle–particle interaction. This study also shows that under mobile dune conditions, the particle step length for dune bed conditions scales with the dune length rather than the particle diameter.