Sediment transport in oscillatory flow
The effect of non-cohesive sediment on wave-induced bottom boundary layers was examined experimentally. Fluid velocity and sediment concentration were measured simultaneously in a sheet regime generated in a u-shaped oscillating water tunnel. A major obstacle to the understanding of fluid-grain flow has been the absence of suitable measuring devices. Thus, previous experimental investigations have been concerned primarily with low sediment concentration flows. For this study, a probe was developed to measure the instantaneous variations in sediment concentration based on the electrical conductivity of the fluid-grain mixture. Unlike earlier concentration devices, this probe is non-intrusive and is capable of measuring a wide range of sediment concentrations; from close packing within the bed to low concentration suspended load. Horizontal fluid velocities were measured simultaneously using Laser Doppler Anemometry in backscatter mode. Height and time-dependent velocity and concentration profiles were obtained for differing wave and sediment conditions. Values of the shear stress calculated from the momentum integral were found to be an order of magnitude larger than in sediment-free flows. The variation in shear stress with distance from the bed is clearly dependent on the thickness of the movable bed and also on the sediment flow regime. As expected, the eddy viscosity varied significantly during the wave cycle. The time-mean eddy viscosity decreases with height above the movable bed and at large distances from the bed, fluctuates about the mean. The sediment concentration measurements provide a comprehensive data set for sediment transport in sheet flow and near sheet flow regimes. The sediment concentration was found to be time-dependent with the amplitude and form of the temporal variation dependent on distance from the bed, wave amplitude and velocity, and sediment characteristics. With increasing wave amplitude and velocity, the number of peaks in the concentration profile increased while the magnitude of the peaks decreased. Fluid velocity and sediment concentration measurements were used to calculate rates of sediment transport which were compared with predictions from existing models.