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 instantaneous susperided sediment concentration field at horizontal separations of 20 cm. Large-amplitude backscatter fluctuations, representing suspended sediment concentrations more than a factor of 10 greater than the mean and with vertical coherence scales many times greater than the logarithmic wave boundary layer thickness, are shown to be spatially correlated at horizontal separations less than the wave orbital amplitude over successive wave cycles. The time lag between the appearance of fluctuations in different acoustic beams is inversely proportional to the fluid velocity, showing that the fluctuations in backscatter amplitude contain real information on fluctuations in suspended sediment concentration, advected with the fluid. It is found that the instantaneous suspended sediment field in shore-normal cross section is punctuated by spatially discrete clouds extending well above the (logarithmic) wave boundary layer. The fluctuations appear to be present even in the absence of small-scale ripples. It is suggested that the sediment clouds may be due to vortex shedding from megaripples, other bed forms of intermediate scale, or coherent structures in combined wave-current flows.