Experiments on Saltation of Sand in Water

Abstract

A high-speed video system was used to study saltation of sand in an open channel flow. High rates of image acquisition provided the information needed to resolve details of the Lagrangian properties of particle motion. Saltation of sand is described in terms of statistical properties of particle trajectories such as mean values and standard deviations of saltation length, height, and streamwise particle velocity. Results are compared with available empirical data. Saltation height seems to be independent of particle size when made dimensionless with the particle diameter; however, the dimensionless saltation length appears to increase as the particle size decreases. Particle collision with the bed is also analyzed. Observations of this process indicate that most of the interactions between saltating particles and the bed are of the collision-rebound type, which contradicts previous discussions on the subject. The estimated value of the dynamic friction coefficient is about half that proposed by Bagnold, in agreement with previous empirical evidence. Friction and restitution coefficients at collision are also estimated from the experimental observations. Nonvanishing values of the latter and values of the former lower than unity are obtained in agreement with previous work with gravel-size particles. Observations of particle resting time, particle reentrainment into saltation, particle rotation, and particle transverse motion during saltation are also presented, providing new insights on the physical processes associated with the saltation phenomenon.