Velocities and concentrations in oscillatory flow over beds of sediment

Abstract

Measurements are reported of the velocity and concentration distributions both within and above two beds of sediment in oscillatory flow. The experiments were carried out in an oscillatory flow water tunnel, the velocities were measured with a laser-Doppler anemometer and the concentrations of sediment with resistance probes operating through the sidewall of the tunnel. The sediments studied consisted of nylon granules of median diameter 4.0 mm and Perspex of median diameter 0.7 mm. Most of the beds were plane for the tests with the 0.7 mm sediment and rippled for those with the 4 mm sediment.The measured velocity profile could be divided into three regions: a central region in which the amplitude and phase of the velocity increased almost linearly with height and two outer regions in which the variation in velocity with height was much less rapid. It is suggested that at very high sediment transport rates the central region covers almost the entire depth of the moving bed but that at lower transport rates the outer regions are more significant.The effect of sediment movement on the velocity distribution above the bed is very marked. Bed roughness length is increased and the velocity amplitude falls off more slowly with height than for fixed beds under similar conditions.Within the bed the shear stress increases almost linearly with depth. Apparent viscosity also increases steadily with depth below the surface of the bed.The measurements of concentrations are in good agreement with the results of other investigators in the region above the moving bed. Within the bed the time-mean concentration rises steadily, with distance below the initial bed surface, towards the limiting value for a stationary bed. The concentration record also shows a fluctuation during the course of the cycle at twice the frequency of the fundamental oscillation. The amplitude of this fluctuation in concentration decreases with depth below the initial bed level. The phase variation with height is close to that of zero velocity gradient, within the moving bed.