VERTICAL AND HORIZONTAL STRUCTURE IN SUSPENDED SAND CONCENTRATIONS AND WAVE-INDUCED FLUXES OVER BEDFORMS

High resolution measurements of suspended sand concentrations have bee n made using a multi-transducer acoustic back scatter sensor over both steep and flat bedforms under low energy swell conditions with weak c urrents present on a macro-tidal beach in the southwest of England. Si milar measurements were made over steep bedforms in a large-scale labo ratory wave basin using a wave record simulated from field data. A det ailed interpretation of the suspension process associated with steep v ortex type ripples was made possible with knowledge of the position of the acoustic sensors relative to the bedform geometry. Analysis of ti me-averaged, wave ensemble-averaged suspended sediment concentrations, and velocity-concentration phase angles indicates that the bedforms a re an important control on the suspension patterns produced by wave-in duced flows. Steep asymmetric ripples under shoaling waves produce gre ater amounts of suspension than low steepness ripples due to the effec ts of vortex ejection associated with the steep ripples. Over rippled beds, we find evidence for a significant phase coupling between the re suspended sediment and the bedforms in the near bed region (<10 cm ele vation); at higher elevations the coupling appears to be much less sig nificant. This means that suspended sand transport rates in the near b ed region, computed from fast response sensors, are highly sensitive t o the positioning of the sensors, both horizontally and vertically, re lative to the sea bed.