Acoustic Backscatter Measurements of Estuarine Suspended Cohesive Sediment Concentration Profiles

Abstract

Methods of monitoring suspended sediment concentration profiles have largely relied on in situ point sampling by water bottles, coupled with optical instrumentation. These intrusive methods can affect the measurements, and optical instrumentation must usually be raised and lowered through the water column at rates too slow to obtain near instantaneous profiles. At the extremely high suspended sediment concentrations sustained by cohesive sediments in estuarial waters optical attenuation is severe, and optics cannot provide data on concentrations or gradients without very short path lengths, or use of multi-sensors. Remote sensing acoustic backscatter techniques overcome these difficulties, and provide high resolution spatial (~ 1-10 cm) and temporal (~ 0.1-1 s) concentration profiles. Despite their use for over a decade, acoustic backscatter measurements in the field have been confined almost wholly to non-cohesive sediments. Measurements of suspended cohesive sediment concentration profiles in Changjiang Estuary demonstrate the capability of acoustic instrumentation to obtain high resolution profiles at dynamic scales in a difficult environment with high gradients and concentrations.