Sand Transport in Oscillatory Sheet Flow with Mean Current

Abstract

The sheet-flow layer, under a combination of waves and mean current, is typically a few millimeters thick and characterized by very high sediment concentrations. Small conductivity probes, capable of measuring high sediment volume concentrations, were employed to estimate sediment fluxes within the sheet-flow layer. Two probes, spaced 20 mm apart in the streamwise direction, were deployed upward through the sand bed in a water tunnel. Using correlation techniques, the sensors provided concentration information and estimates of particle velocity. These measurements yielded direct measurements of sediment flux. Two different sand sizes (120 and 320 μ) were investigated with mean velocities between 0.3 m/s and 0.5 m/s and oscillatory amplitudes between 0.9 m/s and 1.5 m/s that were created using monochromatic, sinusoidal waves of 7.2-s period. Mean sediment-velocity estimates, averaged over a wave period, were smaller than predictions of water velocity from a combined wave/current boundary layer model. On the other hand the oscillatory component of the velocity was approximately in-line with predictions from the model. Flux estimates clearly show that the bulk of the sediment transport takes place within the sheet-flow layer.