Bed particle dynamics at entrainment

The paper reports a high-resolution experimental study focused on statistical properties of particle trajectories starting with detachment from the bed. This local range of particle motion can either be considered to start from a collision with the bed of an already mobile particle, or from the entrainment of a previously immobile particle. Both approaches are investigated and compared based on the particle diffusion concept. From the point of entrainment, local particle diffusion in all three coordinate directions displays an exponent significantly greater than that of ballistic diffusion. In contrast, particle motion after collision with the bed demonstrates ballistic diffusion in all three coordinate directions. The results highlight clear differences between the diffusion of an already mobile particle and one starting from a position of rest. These differences are attributed to variations in physical mechanisms acting at the initial phase of particle motion after detachment from the bed.