Particle motion and diffusion at weak bed load: accounting for unsteadiness effects of entrainment and disentrainment

A study of weak bed load over a plane bed is presented with a particular focus on the motion and diffusion of individual particles that were tracked along their movements. Grain position, velocity and acceleration were measured with particle-scale resolution; ensemble statistics over a significant sample of particle trajectories were then computed, enabling unsteady phases of particle motion (corresponding to entrainment and disentrainment) to be identified. Based on the experimental findings, a physically-based conceptual model for particle kinematics consisting of an initial (entrainment) unsteady phase, a quasi-steady phase, and a final (disentrainment) unsteady phase is explored. The unsteadiness effects in particle dispersion due to entrainment and disentrainment, typically overlooked in previous studies, are explicitly quantified by analysing the time evolution of the particle position variance. Starting with a previously published model of particle diffusion, a refined model is proposed which explicitly accounts for the unsteadiness effects in particle motion.