Particle velocity and concentration profiles in bedload experiments on a steep slope

摘要

Depth profiles of particle streamwise velocity, concentration and bedload sediment transport rate weremeasured in a turbulent and supercritical water flow. One-size 6mm diameter spherical glass beads were transported at equilibrium in a two-dimensional 10% steep channel with a mobile bed. Flows were filmed from the side by a high-speed camera. Particle tracking algorithms made it possible to determine the position, velocity and trajectory of a very large number of particles. Approximately half of the sediment transport rate was composed by rolling grains, and the other half by saltation. This revealed a complex structure, with several concentration and flux peaks due to rolling, and one peak due to saltation.With an increase of the sediment transport rate, the depth structure remained the same at the water/granular interface, with peak value increases but with no shift in elevations. The saltation region expanded towards higher elevations with an increase of the particle velocity commensurate to the water velocity. The proportion of the sediment transport rate in saltation did not vary significantly. The particle streamwise velocity profiles exhibited three segments: an exponential decay in the bed, a linear increase where rolling and saltation co-existed, and above this, a logarithmic-like shape due to saltating particles. These results are comparable to profiles measured and modelled in dry granular free surface flows and in more intense bedload such as sheet flows.