Experimental study of bed load on steep slopes

An important issue in the study of bed load transport in gravel-bed rivers is related to the physical mechanisms governing the bed resistance and particle motion. In order to better understand the physical processes ruling bed load solid transport at the grain scale, an experimental study of the motion of coarse spherical glass beads entrained by a turbulent and supercritical water flow down a steep channel with a mobile bed was set up. The initial width was 6.5 mm slightly larger than the particle diameter of 6 mm to create a two-dimensional particle flow allowing recording from the side all particles with a high speed camera. The overall aim of this research is to measure the trajectories of all particles and to analyze the fluctuations of the solid discharge as well as the state of movement, rolling or saltation. This paper is focussing on global hydrodynamic and equilibrium solid transport results. The channel inclination ranged from 7.5% to 15% and a doubled width of 12.7 mm was also studied. Data were corrected with the Einstein's method to account for side-wall effects. Non-dimensional solid discharge vs. Shields numbers results are in good agreement with classical incipient motion Shields numbers and compare favorably with two specific semi-empirical bed-load formulae. Those results show that upscaling of data obtained at the particle scale to the reach scale is possible despite the unusual hydraulic configuration of this narrow channel.