SEDIMENT EDDY DIFFUSIVITIES IN THE NEARSHORE ZONE, FROM MULTIFREQUENCY ACOUSTIC BACKSCATTER

Acoustic backscatter measurements made on the seaward face of a submer ged nearshore bar are used to determine the vertical eddy diffusivity profile for sand in suspension. Suspended sand size and concentration estimates, obtained from the multifrequency backscatter inverse, permi t the sediment eddy diffusivity to be determined by assuming a balance between settling and vertical diffusion after correcting for the vert ical wave flux. The resulting diffusivity profiles exhibit a nearbed l inear region scaling approximately with the mean maximum friction velo city. Above this region, the data do not collapse to a universal form using a single vertical length scale.