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.