Comparison between optical and acoustical estimation of suspended sediment concentration: Field study from a muddy coast

The aim of this study is to evaluate the ability of a 1.5 MHz Pulse-coherent Acoustic Doppler Profiler (PCADP) to measure suspended sediment concentration (SSC) in a muddy environment where SSC varies over several orders of magnitude. Two seasonal deployments were conducted south of the Atchafalaya-Vermilion Bay system along the Louisiana coast, USA. During a low discharge period of the Atchafalaya River, acoustical estimates of SSC were in good agreement with OBSs deployed. The second deployment was conducted during a high discharge period of the Atchafalaya River. The passage of a cold front across the coast resulted in a rapid advection of the river plume to the study area. High sensitivity of the OBS to fine-grained suspended sediments led to high values from the OBS readings. On the other hand, the PCADP barely sensed the transportation of fine-grained sediments and significantly responded to the combined wave and current shear stress close to sea bed. To reduce the influence of temporal variation of grain size distribution, acoustical backscatter measurements were calibrated over a time span of a few hours. The resulting SSC estimates from the time-adaptive calibration of the PCADPs were in good agreement with data from OBS sensors.