Abstract

Individual grains move through gravel bed rivers in cycles of motion and rest with variable characteristics, so tracer grains spread apart as they transport downstream in a type of diffusion. Experiments and Newtonian simulations have demonstrated nuanced diffusion characteristics, with at least three distinct ranges of behavior as the observation time of tracers increases and each range exhibiting a different spreading rate. Although these observations are nearly 20 years old, no physical model has been developed to describe them, leaving us uncertain of the generating processes. In this work, we develop the first physical model describing three bedload diffusion ranges by incorporating sediment motion, rest, and burial into a random walk concept of individual bedload trajectories. Using the model, we attribute multiple bedload diffusion ranges to the interplay between motion, rest, and burial processes, and we relate the multi-range diffusion characteristics to measurable transport parameters.