GRAVEL SALTATION .1. EXPERIMENTS

Laboratory observations of the saltation of natural gravel particles in a steep, movable-bed channel are reported. Standard video-imaging techniques were used to measure and analyze particle motion. The saltation of gravel particles 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. The results obtained are compared with available empirical data, and a general good agreement is obtained. Particle collision with the bed is also analyzed, and friction and restitution coefficients are estimated from the experimental observations. Nonvanishing values of the restitution coefficient and values of the friction coefficient lower than unity are obtained, which contradicts previous discussions on the subject. The dynamic friction coefficient associated with particle motion is also estimated from the experimental data, and a,mean value of 0.3 is obtained, which is about half of that proposed by Bagnold (1973) but similar to those found in previous experiments.