Modeling Fractional Bedload Transport Rates: The Importance of the Coarsest Bedload Fractions

Numerical experiments suggest that the output of morphodynamic models that implement fractional bedload transport equations can be substantially altered by relatively small differences in the transport rates assigned to the coarsest sediment size fractions in the sediment supply. The accuracy of fractional bedload transport relations in predicting the mobility of the coarsest fractions in the bedload is therefore critical for obtaining realistic predictions of equilibrium channel conditions for a given flow and sediment regime. Bedload samples from 4 locations in the Trinity River of Northern California were analyzed to evaluate the performance of the Wilcock-Crowe bedload transport equations for predicting fractional bedload transport rates. These equations performed well for predicting total bedload transport rates (68% of all predictions within a factor of 2 of sampled rates). The equations nonetheless consistently under-predicted the transport rates of the largest fractions in the bedload, frequently by more than an order of magnitude. Modifications to the Wilcock- Crowe equation for determining τ*rm and the hiding function used to scale τ*rm to other grain size fractions are proposed.