Acoustic profiling of suspended sediments in the marine bottom boundary layer

An acoustic (3 MHz) backscattering profilometer has been developed for study of sediment transport in the bottom boundary layer. Laboratory experiments indicate reasonable correlations between backscattering intensity and mass concentrations for well‐sorted fine to coarse quartz sands typical of those found in marine sediments. Departure of the calibration curves from their expected values is thought to be due to the non‐spherical shape of the quartz grains. Field experiments on the inner shelf using the profilometer combined with velocity profiles show individual profiles (taken at 1 Hz) during erosion events are characterized by highly variable concentrations. Surface wave groups rather than individual waves appear to be correlated with groups of profiles with high concentrations extending 25‐100 cm above bottom. Averaging produces smooth profiles which fit well log‐normal distributions predicted by steady flow suspended load theories.