Flow tunnel measurements of velocities and sand flux in oscillatory sheet flow for well-sorted and graded sands

Oscillatory flow tunnel measurements of velocities and fluxes for sands in oscillatory sheet flow conditions are presented. The experiments involved a range of well-sorted and graded sands in two asymmetric flows. Velocities were measured using an ultrasonic velocity profiler (UVP) capable of measuring deep within the sheet flow layer. Velocity profiles are found to be similar for the same flow but different sand beds and display expected features of oscillatory boundary layer flow. The near-bed velocity leads the main flow velocity by approximately 21° and a small offshore-directed current is generated near the bed. Measures of the boundary layer thickness are in good agreement with those predicted using an equation formulated for the boundary layer thickness over fixed beds. Velocity data have been combined with concentration data to produce time-dependent sand flux profiles covering the sheet flow and suspension regions. There are fundamental differences in the transport processes of sands of different size and grading, caused by unsteady effects which dominate in the case of fine sand and are largely absent in the case of coarse sand. (1) Time-averaged flux is onshore-directed (positive) in the case of coarse sand and is confined to a region immediately above the bed; in contrast, time-averaged flux in the case of fine sand extends high above the bed, is offshore-directed (negative) in the sheet flow layer and becomes onshore-directed in the suspension layer. (2) Net transport in the case of coarse sand is directed onshore. As the percentage of fine sand in the bed increases, offshore transport becomes increasingly dominant as the percentage of fine sand increases. (3) Net transport in the suspension layer is onshore while net transport in the sheet-flow layer may be onshore or offshore depending on sand size and grading.