Traveling or jiggling: particle motion modes and their relative contribution to bed‐load variables

The motion state of a particle is a crucial aspect of sediment transport problems. In this paper, we conceptualized three states: stillness, “transport”, and “non-transport”, considering that not all the particle motions contribute significantly to the mean sediment transport rate. Starting from a data set of bed-load particle tracks obtained from particle tracking velocimetry, we removed the bias from experimental uncertainty and applied one-dimensional, instantaneous, and non-parametric criteria for distinguishing the states. We described the kinematics of particles in transport and non-transport states, presenting some sample trajectories and the distributions of particle velocity and acceleration. While the transport state presents a clear distinction between stream-wise and transverse particle velocity, the non-transport state is related to isotropic particle jiggling, and does not significantly contribute to the bed-load rate. Vice-versa, the particle motions in the non-transport state are relevant for other summary indicators of the transport process, such as the mean number of moving particles and mean particle velocity. We discuss how applying the proposed non-parametric criterion for state separation is beneficial compared to parameter-dependent alternatives available in the literature. Finally, we provide an outlook on possible applications of our concept for the investigation of other sediment transport processes (incipient motion, solid-fluid interface, creeping flow).