ACOUSTIC MEASUREMENTS OF SUSPENDED SAND ON THE SHOREFACE AND THE CONTROL OF CONCENTRATION BY BED ROUGHNESS

Acoustic backscatter measurements of the concentrations of sand in suspension on the shoreface, seawards of the breaking zone, during a mild strom event show that sand concentrations increase initially but then rapidly decrease as the wave energy increases: it is suggested that the bed roughness is a major control on suspended sand concentration and that the decrease in concentration is due to decreasing ripple steepness after the break-off bed shear stress is exceeded. No direct measurements of bedforms were available and the combined wave-current interaction model of Grant and Madsen (J. Geophys. Res., Vols. 84 (1979) and 87 (1982)) with extensions to movable bed roughness (ripple dimensions from Carstens et al's. 1969 laboratory data (U.S. Army Corps Eng. CERC Tech. Mem. 28) and to sediment-induced self-stratification (Glenn and Grant, J. Geophys. Res., Vol. 92, 1983) was used to predict the current and suspended sand concentration profile. Matching the measured and modeled concentrations at 2 cm above the bed, the values of the resuspension coefficient gamma-0 were found to decrease as excess skin friction increased, in a manner similar to that suggested by Drake and Cacchione (Cont. Shelf Res., Vol. 9, 1989) but were an order of magnitude larger. Using the much smaller ripple dimensions of Nielsen (J. Geophys. Res., Vol. 86, 1981) changed the model results very little. Significant differences were found between the time-averaged suspended sand profiles and those predicted by the model. Sand transport fluxes have also been computed using the instantaneous products of measured suspended concentrations and currents (the current being the sum of steady and wave-induced components). The wave-induced fluxes show considerable variability from run to run but the general pattern is of a transport profile having a shoreward maximum at 5-10 cm above the bed and offshore transport in the few centimetres closest to the bed: little net transport occurred above 15 cm. These fluxes show a weak dependence on the wave energy, becoming more shoreware and the height of the maximum shoreward transport decreasing as the wave energy increased, again consistent with the influence of the ripple steepness.