CRITICAL SHEAR-STRESS OF NATURAL SEDIMENTS

The critical shear stress of individual fractions tau(ci) in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size of the mixture. For strongly bimodal sediments, tau(ci) increases with grain size, an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes tau(ci) to deviate from size independence in the direction of unisize (Shields) values. A quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of tau(ci) is observed and to predict the variation of tau(ci) with grain size in bimodal sediments. Because properties of sediment grain-size distributions other than bimodality appear to have little influence on tau(ci) the trends presented here for 14 unimodal and bimodal sediments may be quite general.