Relating acoustic and optical measurements to particle and floc concentrations in the bottom boundary layer

Abstract

Profiles of optical and acoustic properties were measured by moving instruments vertically in the bottom boundary layer, between the bottom and about 2 m above the sea floor, at a sandy inner shelf site 12 m deep. Profiles were performed every two hours for 36 days, spanning a range of wave and current conditions. Acoustic instruments on the profiling arm included a three-frequency acoustic backscatter profiler and a 1.5-MHz acoustic Doppler velocimeter. Optical instruments on the arm measured backscatter, attenuation, and absorption. Stationary instruments on the main tripod measured waves, currents, Reynolds stress, and vertical temperature/salinity gradients. The acoustic backscatter measurements were coupled with optical measurements and recent models for the backscattering and absorption by flocs to provide time series of particle-concentration and size profiles. Remarkable changes in particle sizes, concentrations, and inferred densities and settling velocities were observed. These observations demonstrate the value of the traditional Rouse profile assumptions often used in suspended-sediment transport calculation, but also reveal the rich temporal and spatial complexity of the near-bottom particle field.