Vertical and horizontal structure in suspended sand concentrations and wave-induced fluxes over bedforms

High resolution measurements of suspended sand concentrations have been made using a multi-transducer acoustic back scatter sensor over both steep and flat bedforms under low energy swell conditions with weak currents present on a macro-tidal beach in the southwest of England. Similar measurements were made over steep bedforms in a large-scale laboratory wave basin using a wave record simulated from field data. A detailed interpretation of the suspension process associated with steep vortex type ripples was made possible with knowledge of the position of the acoustic sensors relative to the bedform geometry. Analysis of time-averaged, wave ensemble-averaged suspended sediment concentrations, and velocity-concentration phase angles indicates that the bedforms are an important control on the suspension patterns produced by wave-induced flows. Steep asymmetric ripples under shoaling waves produce greater amounts of suspension than low steepness ripples due to the effects of vortex ejection associated with the steep ripples. Over rippled beds, we find evidence for a significant phase coupling between the resuspended sediment and the bedforms in the near bed region (<10 cm elevation); at higher elevations the coupling appears to be much less significant. This means that suspended sand transport rates in the near bed region, computed from fast response sensors, are highly sensitive to the positioning of the sensors, both horizontally and vertically, relative to the sea bed.