Measurements of the acoustic backscatter from suspended sand over a rippled bed and of currents were made near the crest of a ridge-and-runnel beach on the macrotidal North Norfolk coast of the United Kingdom. Two 12-min records were taken close to high water in a depth of 1.7¿0.2 m. Visual inspection of the data shows that sand was intermittently suspended to heights of 20--30 cm but that these events were not always correlated with strong wave-induced currents: the structure of some of the suspension events suggest that they are due to vortex shedding from a rippled bed. The observed time-averaged concentration profiles were compared with predictions generated using Glenn and Grant's (1987) combined-flow model. The nondimensionalised concentrations showed good correspondence in the first 8 to 10 cm above the wave boundary layer, supporting the use of a value of 0.74, drawn from atmospheric data, for the constant of proportionality between sediment and momentum diffusivity. Above this height, observed values of the proportionality constant tended to approach 1.0, which may have been due to a decrease in sediment settling velocity with height (which is consistent with finer sand being suspended higher in the flow), or alternatively to nonuniform turbulence in the vicinity of the wave ripples. Absolute concentrations measured 2 cm above the bed were consistent with values of O(10-4) for the empirical constant &ggr;0 in Smith and McLean's (1977) pickup function. Concentrations within the wave boundary layer were observed to decrease much more rapidly than predicted by the model. Sand transport fluxes were computed for both the cross-shore and alongshore components of the flow using a current profile based on the combined-flow model of Glenn and Grant (1987). For the two records, vertically integrated fluxes of 1.5-mg/s/cm width and 0.89-mg/s/cm width directed towards the shore were calculated for the wave oscillatory component of the transport; the mean flow produced shoreward vertically integrated fluxes of 5.1- and 10.5-mg/s/cm width. The uncertainty associated with these estimates is considerable (¿50%). The vertical profile of the total flux (wave oscillatory and mean) for both records showed a strong shoreward transport close to the bed, an offshore transport between 5 and 15 cm above the bed, and a further weak onshore flux above this, demonstrating the necessity for measurements of the whole concentration profile (rather than a few point samples) for the calculation of sand transport over a rippled bed in a wave-dominated environment. ¿ American Geophysical Union 1990 |