Critical discharge for entrainment of poorly sorted gravel

Abstract

AbstractEntrainment criteria for gravel‐sized particles on river beds are usually defined in terms of shear stress, but some workers have proposed that critical‐discharge relationships are more reliable and more convenient. The critical shear stress in poorly sorted gravels is known from recent work to depend much more on relative than absolute grain size. It is shown that this effect can be included in a semi‐theoretical equation for the critical unit discharge to move individual grain sizes in a poorly sorted bed. Comparisons with published data on critical discharges in a variety of rivers show that both the sensitivity of critical discharge to grain size, and the absolute levels of critical discharge, are predicted quite well. However, any prediction of critical discharge is sensitive to the assumptions made, first about flow resistance and secondly about the critical stress to move average‐sized particles.