COHERENCE SCALES OF WAVE-INDUCED SUSPENDED SAND CONCENTRATION FLUCTUATIONS

Multifrequency acoustic backscatter measurements made in the nearshore zone are analyzed to determine the correlation structure of the insta ntaneous susperided sediment concentration field at horizontal separat ions of 20 cm. Large-amplitude backscatter fluctuations, representing suspended sediment concentrations more than a factor of 10 greater tha n the mean and with vertical coherence scales many times greater than the logarithmic wave boundary layer thickness, are shown to be spatial ly correlated at horizontal separations less than the wave orbital amp litude over successive wave cycles. The time lag between the appearanc e of fluctuations in different acoustic beams is inversely proportiona l to the fluid velocity, showing that the fluctuations in backscatter amplitude contain real information on fluctuations in suspended sedime nt concentration, advected with the fluid. It is found that the instan taneous suspended sediment field in shore-normal cross section is punc tuated by spatially discrete clouds extending well above the (logarith mic) wave boundary layer. The fluctuations appear to be present even i n the absence of small-scale ripples. It is suggested that the sedimen t clouds may be due to vortex shedding from megaripples, other bed for ms of intermediate scale, or coherent structures in combined wave-curr ent flows.