Measures of Flow Intensity as Predictors of Bed Load

Abstract

The relative merits of different measures of flow intensity as predictors of bed‐load transport rates are assessed. Tractive stress, when based on depth and slope, is a poor predictor, especially in narrow channels. Correct application requires elimination of wall and bedform roughness, leaving only that stress acting on the bed grains. When this is done, excellent correspondence with bed‐load rates is found in the experiments of Gilbert, Meyer‐Peter, and G. P. Williams, analyzed here. Vertically averaged velocity requires a correction for depth and particle size; then, it is directly comparable with grain tractive stress. Stream power has limitations as a predictor because it is the product of vertically averaged velocity and bed tractive stress, and hence subject to the problems that those variables individually experience. Unit stream power is a good predictor of concentration in some circumstances. It performs poorly, however, in the data set of Williams, and reasons for this are given.