No abstract

Bed load sediment transport is typically formulated as a nonlinear function of the shear stress exerted on the bed in excess of a critical value. However, due to the inherent spatial variability in grain packing and protrusion on water‐worked beds, the critical stress used in transport models represents a spatial averaging of the critical entrainment stresses of many individual particles on the bed. We perform a series of flume experiments in which we quantify, for the first time, the evolution of topography on the particle scale during low‐flow periods. We link this topography to subsequent bed load sediment transport rates and explore implications for particle critical stresses. We exploit the observed dependence of bed load flux on antecedent low‐flow duration to isolate the relationship between grain protrusion and bed load flux over a wide array of transport rates at the same applied stress. A synthesis of high‐resolution bed topography surveys and bed load flux data show that bed load transport rates characteristic of gravel channels are governed by the portion of grains that protrude highest above the bed and that this portion corresponds to ~1–5% of the total bed elevation distribution in our experiments. This result supports the argument that only a small portion of grain entrainment thresholds for a riverbed is exceeded during transport. Further, these results emphasize that subtle changes in bed topography can have dramatic effects on bed load sediment transport. We also find that transport of these highest protruding particles enhances the local erosion of surrounding grains.

To explore the causes of history‐dependent sediment transport in rivers, we use a 19‐year record of coarse sediment transport from a steep channel in Switzerland. We observe a strong dependence of the threshold for sediment motion (τc) on the magnitude of previous flows for prior shear stresses ranging from 104 to 340 Pa, resulting in seasonally increasing τc for 10 of 19 years. This stabilization occurs with and without measureable bedload transport, suggesting that small‐scale riverbed rearrangement increases τc. Following large transport events (>340 Pa), this history dependence is disrupted. Bedload tracers suggest that significant reorganization of the bed erases memory of previous flows. We suggest that the magnitude of past flows controls the organization of the bed, which then modifies τc, paralleling the evolution of granular media under shear. Our results support the use of a state function to better predict variability in bedload sediment transport rates.

This file contains a note on the nature of the data utilized in the Review.