Saltating or rolling stones?

A longstanding problem in the study of bed load transport in gravel-bed rivers is related to the physical mechanisms governing the bed resistance and particle motion. Although a number of experimental investigations have been conducted over the last three decades, there seems to be a substantial gap between the field measurements and the predictions of theoretical models, although these models provide a correct description of bed load transport for lab experiments. To elucidate this point, we investigated the motion of coarse spherical glass beads entrained by a shallow turbulent water flow down a steep two-dimensional channel with a mobile bed. This experimental facility is the simplest representation of bed load transport on the lab scale, with the tremendous advantages that boundary conditions are perfectly controlled and a wealth of information can be obtained using imaging techniques. Bed load equilibrium flowswere achieved (i.e. neither erosion nor deposition of particles occurred on average, over sufficiently long time intervals). Flowswere filmed from the side by a highspeed camera. Using an image processing software made it possible to determine the flow characteristics such as particle trajectories, their state of motion (rest, rolling or saltating motion), and flow depth. In accordance with earlier investigations, we observed that over short time periods, bed load transport appeared as a very intermittent process although the bed load rate was relatively intense. A striking result was that whereas for gentle slopes particles were mainly transported in saltation, the rolling regime played an increasingly important role at steep slopes. These experimental results suggest that to some extent, the mismatch between bed-load formulas and field data may be the consequence of a misinterpretation of the role played by the rolling particles, at least for steep slopes.