VARIABILITY OF SUSPENDED SAND CONCENTRATIONS, TRANSPORT AND EDDY DIFFUSIVITY UNDER NON-BREAKING WAVES ON THE SHOREFACE

Suspended sand concentration profiles and current speed measurements w ere made in 1.6 +/- 0.4 m water depth on the seaward side of a bar bey ond the break point at Stanhope Lane beach, Prince Edward Island, Cana da, during a 4 day period. Two mild storm events occurred and 7 min-lo ng bursts of 4.4 Hz data were recorded approximately every hour. Conce ntration profiles, with a resolution of 5 mm and 7.5 mm, were obtained using a 3 MHz acoustic backscatter (ABS) instrument. Some data on bed form dimensions were obtained by divers between storm events. Multipl e bed echoes from the ABS also provide further information on the ripp le heights but no direct observations were available at times of highe r waves. During the 4 day period the local position of the bed decreas ed by about 8 cm, erosion occurring in two short (congruent to 6 h) pe riods, one during each storm event. Mean currents at this location wer e weak ((0.1 m s(-1)). The resuspension coefficient gamma(o), calculat ed from the concentration at 2 cm above the bed, decreased as the wave height increased (in the break-off range) supporting the observations of VINCENT et al. [(1991) Marine Geology, 96, 1-18]. Large inter-burs t variability was observed in the concentration profiles and in the ed dy diffusivity and suspended transport fluxes computed from these prof iles. This Variability was due to the short length of record (congruen t to 7 min) relative to wave groups and to the location of the ABS rel ative to bed forms; to obtain consistent concentration and transport f luxes it is necessary to average many bursts over a time scale that is long compared to both groupiness and bed form mobility. Except at the beginning and end of events when wave conditions were changing rapidl y, averaging over groups of bursts with similar wave conditions produc ed eddy diffusivity profiles characterized by a linear epsilon(s) grad ient of (20 +/- 2.5) cm s(-1) from the sea bed to 20 +/- 5 cm, beyond which epsilon(s) slowly decreased, and a suspended sand transport whic h was all below 20 cm and dominated by a jet-like shoreward how in the lowest 3-4 cm above the sea bed. Bursts obtained when conditions were in, or close to, the equilibrium range of bed forms showed concentrat ion profiles which were partly exponential (hence epsilon(s) was const ant with height) and transport profiles with more structure. Sensitivi ty tests indicate that concentrations and eddy diffusivities may be un derestimated by about 20% by using the modal size for the sand in susp ension rather than a distribution of sizes. Towards the end of the sec ond storm a ''low suspension event'' of congruent to 6 h duration occu rred and suspended sand concentrations about 5-10 cm decreased by 1-2 orders of magnitude while the near bed concentrations remained approxi mately constant, suggesting that the bed forms responsible for vortex ejection of sand higher into the water column had been wiped out and t he sea bed had become (temporarily) flat. Wave heights did not change significantly but wave period increased from 4.8 to 6.2 s, decreasing the maximum bed shear stress. The low suspension period may have a tra nsient response of the sea bed to changed hydrodynamic forcing prior t o the subsequent growth of bed forms with a different wavelength.