The dynamics of the inner-shelf suspended sediments based on ADCP records and Rouse theory

The present study investigates the suspended sediment dynamics on the southern Brazilian continental shelf in the fluvial influence region of the Rio de la Plata and Patos Lagoon. Estimates of suspended sediment concentration (SSC) in the water column were obtained using data from a Doppler-effect acoustic Doppler current profiler (ADCP) and were compared with theoretical profiles obtained using Rouse's theory. To construct the theoretical profile, sediment settling velocity and shear velocity are necessary. The former was iteratively estimated to minimize the error between observed and modeled profiles. The latter was obtained from ADCP measurements. The comparison between observed and modeled profiles underwent a validation process considering the shape of the observed profile and environmental energy conditions. For the period of analyzed data, there was agreement between observed and modeled profiles 56 % of the time. When energy conditions were low (39 % of the time), and the gradient profile of SSC had higher concentrations near the surface than bottom (5 %), Rouse's theory did not apply. These results indicate that the main source of sediment in the water column is due to exchanges with the bottom. Although the study area is near the mouth of Patos Lagoon, the role of horizontal advection was negligible. By indirect inference, it was obtained that the median sediment size was 25 μm, with maximum values around 60 μm, which coincides with values suggested in the literature for coastal waters, with a median settling velocity of 0.14 cm/s. During periods of more energetic hydrodynamic conditions, downwelling events were associated with northward longitudinal currents, while upwelling occurred with southward currents. When the flow was directed northward, more material was transported near the bottom compared to southward flow.