SEDIMENT TRANSPORT IN OSCILLATORY SHEET FLOW

Motivated by an interest in high rates of sediment transport, an experimental investigation was conducted to examine sediment motion in conditions of intense sediment transport. Fluid velocity and sediment concentration were measured simultaneously in a sheet flow regime generated in a u-shaped oscilllatory water tunnel for two types of noncohesive sediments. A probe was developed to measure the instantaneous variations in sediment concentration based on the electrical conductivity of the fluid-grain mixture. Unlike previous concentration devices, this probe is nonintrusive and is capable of measuring a wide range of sediment concentrations, from extremely dense concentrations within the bed to low-concentration suspended load well above the moving layer of sediment. Horizontal fluid velocities were measured simultaneously using laser Doppler anemometry in backscatter mode. The sediment concentration measurements provide a comprehensive data set for sediment transport in sheet flow and near sheet flow regimes. The temporal variation of sediment transport over the half cycle of oscillation was examined. Most of the sediment was transported within the moving bed layer, with the total transport occurring between the stationary bed height and a distance about twice the thickness of the moving bed layer. The temporal variation in sediment transport was found to be proportional to cos(sigma-t). The magnitude of the sediment transport increased with increasing bottom shear stress. Fluid velocity and sediment concentration measurements were used to calculate instantaneous and mean rates of sediment transport, which were compared with predictions from existing models.