Circulação e fluxo de material particulado em suspensão no principal canal de acesso à baía de Sepetiba

Abstract

Between August 6, 2011 and February 25, 2012, continuous time series of water column current speed and direction, echo intensity, as well as nearbed pressure and water temperature were obtained in a station located at the rim of Sepetiba bay main access channel (23°0'16.5"S and 43°59'29.4"W, local depth 25 m). The data set was recorded by a 600 kHz WorkHorse BroadBand Sentinel Acoustic Doppler Current Profiler (Teledyne-RDI). Strong flood currents (up to 1.02 m/s) were documented in the nearbed area, particularly under springtide conditions. It was observed that channel orientation strongly influence current direction. The analysis of the alongchannel current velocity component substantiated previous claims that tides strongly control Sepetiba bay circulation. The main tidal harmonic components that were identified through spectral analysis are M2, M4, M6 and M8. Moreover, tides are strongly asymmetric: floods are up to 2 hours shorter than ebbs and are associated with the strongest flows. For this reason, it was concluded that at least in the sector that was monitored during this study, Sepetiba bay is a flood-dominated environment. Another interesting observation is that under the influence of strong S-SW winds, water gets piled up inside the bay and water level is directly related to the local atmospheric pressure variability. In several occasions, under E-NE winds, South Atlantic Central Water (SACW) was observed inside Sepetiba bay, extending from its entrance up to the Guaíba island area. Furthermore, several low frequency oscillations could be related to atmospheric pressure oscillations. On the other hand, higher frequencies could be associated with tides, tidal co-oscillations and local winds. Suspended Particulate Matter (SPM) concentration, which was quantitatively determined from the corrected echo intensity signal, exhibited higher values throughout the water column during springtide conditions. The cumulative SPM flux was predominantly directed towards the bay, except close to the bottom, where the lowest measurement, at 2.2 mab, was directed to the shelf. This pattern of cumulative SPM flux, that rotates from going toward the southeast close to the bottom to the NE close to the water surface, suggests the occurrence of current deflection due to shear. When looked into the time scale of a complete tidal cycle (13 hours) SPM fluxes are larger during floods, when higher shear stresses resuspend and redistribute SPM throughout the water column, whereas during ebbs higher concentrations are more limited to the nearbed area, particularly under neap tide conditions.