Abstract

The interaction between two complex systems, turbulent flow and multi-sized bed sediment causes considerable fluctuations in sediment transport rates and displays different characteristics depending on the scale of the observation. To explore this phenomenon, we measured grain-size specific transport rates in a flume experiment at 1 s resolution. The analysis of the time series of these transport rates reveals either individual or collective transport regimes at different sampling intervals. We compare the sampling intervals of these regimes for differently sized particles with the time scales of turbulent structure formation. This comparison informs the presentation of a heuristic model, in which small suspended particles swirl in the wake vortices of keystones until large to very-large-scale coherent structures transport those suspended particles downstream. Keystones consequently destabilize and in turn, collectively entrain another group of sheltered particles. We suggest that interactive cascades, such as this one, should be incorporated in sediment transport models.