Comparison study on the conditions for incipient motion of cohesionless sediment based on the concept of equivalent grain size

A uniform expression is presented to describe the incipient motion of nonuniform sediment based on the concept of equivalent grain size.Using eight formulas,a comparison study is conducted to explore the issues of hiding and exposure and the aspects of sediment sorting due to heterogeneity.Results show that the hiding and exposure effect is well described in the modeling of nonuniform sediment transport by all eight formulas.Coarse sediment is usually underrepresented by its equivalent particle size,which can artificially ease the condition for incipient motion of coarse sediment.The opposite is true for fine sediment.However,there are still some differences among the formulas,mainly in dealing with the hiding effect of fine sediment.Different formulas use different feature sizes to simulate the hiding and exposure effect of nonuniform sediment.Among those,HAN Qi-wei,ZHANG Qi-wei and LIU Xing-nian formulas can better handle the sorting function for sediment incipient motion.For fine sediment,the performance of Egiazaroff,Hayashi and QIN Rong-yu formulas is less satisfactory due to clear-water scouring.