Particle motion of bed-load sediment moving over a smooth bed

The continuous motion of bed load moving particle over a smooth bed was experimentally studied. Particle motion was filmed by a CCD camera and an ad hoc Image Processing software was applied to track particles. The streamwise components of instantaneous particle velocity and mean particle velocity within each trajectory were quantified. Statistical properties of particle trajectories were investigated for three different hydrodynamic conditions. A clear dependence of the statistical moments of particle velocity on shear velocity was observed. The probability density function of the instantaneous streamwise velocity is well fitted by a Normal or a Gamma distribution, depending whether or not one single motion mode (sliding, rolling, saltation) prevails on the other ones. Finally, the correspondence between the autocorrelation time scale of instantaneous velocity and the bursting time scale was identified as a footprint of the hydrodynamic properties of the turbulent flow field on particle motion.