Experiments on gravity-driven particle flows in a turbulent stream

Bed load transport in water courses is a longstanding problem in the mechanics of two-phase flows because the involved physical processes are poorly known or overly complicated. The motion of coarse spherical glass beads entrained by a shallow turbulent water flow down a steep two-dimensional channel with a mobile bed was experimentally investigated. Focus was put on bed load equilibrium flows, that is, neither erosion nor deposition of particles over sufficiently long time intervals occurred. Flows were 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. Our first striking result was that, over short time periods, bed load transport appeared as a very intermittent process. Whereas for moderate slopes particles were mainly transported in saltation, the rolling regime was dominant for steep slopes.