Experimental study of bed load transport on steep slopes with a two-size mixture of spherical particles

Bed load transport is a longstanding problem despite its major implication in river morphodynamics. The physical processes ruling coarse-particle/liquid systems are indeed poorly known, impairing our ability to compute local and even bulk quantities such as the sediment flux in rivers. We present an experimental study of a two-size mixture of coarse spherical glass beads entrained by a shallow turbulent water flow down a steep channel with a mobile bed. The particle diameters were 4 and 6 mm, the channel width 6.5 mm and the channel inclination12.5%. The water flow rate and the solid discharge were kept constant at the upstream entrance. They were adjusted to obtain bed load equilibrium, that is, neither bed degradation nor aggradation over sufficiently long time intervals. Flows were filmed from the side by a high-speed camera. Using image processing algorithms made it possible to determine the position, velocity and trajectory of each spherical particle thanks to a PTV algorithm (particle tracking velocimetry). Transitions of the state of motion (rest, rolling or saltating) and flow depth were also determined. New data were compared to previous results obtained with spherical particles of uniform size. They confirm that the free surface acting as a physical barrier by truncating the saltation trajectories is very important on steep slopes. The use of a two-size mixture with the 4 mm beads tending to be blocked in the 6.5 mm wide channel resulted in a bed mainly formed by these 4 mm beads. This particular structure explained the single peak vertical distribution of the solid discharge contrary to the uniform case where several peaks corresponding to rolling were observed.