Observations of suspended sediment from ADCP and OBS measurements in a mud-dominated environment

The ability of a 1.2-MHz Acoustic Doppler Current Profiler (ADCP) to measure suspended sediment concentration (SSC) and particle size variation in a mud-dominated environment has been investigated. Experiments were conducted in the Bay of Banten, Indonesia, where clays and silts in the range of 3-55 μm are prevalent. The ADCP backscatter depends both on SSC and on the size of the scatterers. Over the time span of several separate deployments, which lasted 20 days at most, SSC was found to be proportional to the acoustically normative grain size squared. Using this relation, the ADCP could be calibrated to yield depth profiles of SSC. The obtained calibrations, however, were spatially and seasonally dependent. Differences between the calibrations could not be completely ascribed to variation in grain size distributions, due to the largely unknown influences of aggregates and organic scatterers. The ADCP backscatter measurements provided insight into diurnal events of erosion and subsequent deposition. An increase or decrease of SSC generally coincided with a raise or decline of the average grain size in the sediment suspension (respectively).