Particle densimetric Froude number for estimating sediment transport

It has been established that for ratios of flow depth to bed particle diameter less than ten (flow on very rough boundaries) neither the Reynolds number of the solid loose particles at a stream bed nor the Shields parameter are adequate variables to predict critical flow conditions for the initiation of motion. A particle densimetric Froude number F*=U/[(s-1)gD](1/2) (where U=mean velocity, s=ratio of sediment and fluid densities, g=acceleration due to gravity, and D=characteristic diameter of bed particle) is here proposed as an alternative criterion to predict hydraulic conditions for the initiation of motion. Values of critical F* were computed after calibration with available experimental data sets. After the critical conditions for the initiation of particle motion were exceeded, transport of bed particles was established. In order to evaluate the performance of a transport equation that contains F* in sediment transport, a set of the most employed formulations to estimate bed material transport in steep slope macrorough flows were tested. The comparison of the results shows that F* can be used to accurately predict sediment discharge.