The Critical Shear Stress of Natural Sediments
The Journal of Hydraulic Engineering
The critical shear stress of individual fractions "rci 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, ~'ci increases with grain size, an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes "rcl 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 "r a is observed and to predict the variation of %, with grain size in bimodal sediments. Because properties of sediment grain-size distributions other than bimodality appear to have little influence on "rc~, the trends presented here for 14 unimodal and bimodal sediments may be quite general.
Wilcock, P.R., 1993. The critical shear stress of natural sediments, The Journal of Hydraulic Engineering, 119(4):491-505.