Critical Shear Stress of Natural Sediments

Abstract

The critical shear stress of individual fractions τci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture. For strongly bimodal sediments, τci increases with grain size, an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes τci 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 τci is observed and to predict the variation of τci with grain size in bimodal sediments. Because properties of sediment grain‐size distributions other than bimodality appear to have little influence on τci the trends presented here for 14 unimodal and bimodal sediments may be quite general.