Flow resistance and sediment transportation in streams with step-pool bed morphology

Sediment transportation is changeable during a flood both in quantity and quality due to the interactions among the bed morphology, flow structure and bed material. Three subjects, which are associated with sediment phenomena in mountain streams, are chosen for discussion in the present study. (1) Studies on step-pool bed morphology, typical of mountain rivers, is reviewed with attention focused on the mechanism of its birth and geometric feature. (2) A resistance equation of the flow over step-pool bed forms is presented. In the derivation of this formula, the energy loss due to a strong eddy formed in the separation zone downstream of the wave crest is assumed and evaluated in terms of entrainment velocity. (3) Discussions are devoted to a mathematical model for predicting the sediment discharge rate of through puts or over-passing loads on stable step-pool bed systems.