RELATION BETWEEN GRAIN VELOCITY AND SEDIMENT CONCENTRATION IN OVERLAND-FLOW

Previous flume studies of the velocity of saltating grains have almost all measured single grains traveling in otherwise clear water. Howeve r, such measurements disregard the likelihood that grain velocities ar e affected by the presence of other grains. In this laboratory study o f overland flow we show that grain velocity is negatively related to s ediment concentration. This negative relation is attributed to two mec hanisms. The first mechanism is a decrease in water velocity as sedime nt concentration increases. Water velocity decreases because an increa sing proportion of potential energy is dissipated in transporting sedi ment rather than being converted to kinetic energy. The second mechani sm is an increase in the dispersion of the normal component of grain m omentum as grain collisions increase with sediment concentration. The dispersive stress so created opposes bed load movement and causes grai n velocity to decrease. Multiple regression analyses reveal relations in the experimental data that are consistent with these two mechanisms .