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. However, such measurements disregard the likelihood that grain velocities are affected by the presence of other grains. In this laboratory study of overland flow we show that grain velocity is negatively related to sediment concentration. This negative relation is attributed to two mechanisms. The first mechanism is a decrease in water velocity as sediment concentration increases. Water velocity decreases because an increasing proportion of potential energy is dissipated in transporting sediment rather than being converted to kinetic energy. The second mechanism is an increase in the dispersion of the normal component of grain momentum as grain collisions increase with sediment concentration. The dispersive stress so created opposes bed load movement and causes grain velocity to decrease. Multiple regression analyses reveal relations in the experimental data that are consistent with these two mechanisms.