Dynamics of the Patos Lagoon coastal plume and its contribution to the deposition pattern of the southern Brazilian inner shelf

摘要

The southern Brazilian shelf (SBS) is a region influenced by fresh water. The initial deposition of suspended sediments carried by this fresh water presents important ecological consequences for the area. The aim of this study was to investigate the contribution of the Patos Lagoon coastal plume to the deposition pattern observed along the inner continental shelf by providing estimates of estuarine-shelf suspended sediment exchange. The study was carried out through three-dimensional numeric modeling experiments on coupled hydrodynamic and morphodynamic processes. Results were analyzed using the dynamic method proposed by Garvine (1995) to study persistent plumes. The results were also analyzed using wavelet analysis. Results showed that the Patos Lagoon coastal plume behaves as a hypopycnal plume covering the first meters of the water column. The Patos Lagoon presents a dominant ebb condition with a mean discharge of 2088 m~3 s~(-1) and an exportation rate of suspended matter of approximately 1.3 x 10~7 t yr~(-1). In the adjacent coastal region deposition prevails in sheltered regions up to 10 m deep, with enrichment in silt reaching up to 10% in the deceleration region of the Patos Lagoon coastal plume. The dynamic balance indicates a behavior typical of small to average-sized plumes, with a well-developed displacement along the coast that is mainly controlled by the alongshore wind component. The Coriolis force and bed shear stress present significant contributions during periods of moderate to high freshwater discharge when large-scale plumes are formed. Transversally to the coast the force balance is associated with the Coriolis effect and wind influence, with a less important contribution from bed shear stress. The inner continental shelf adjacent to the Patos Lagoon entrance is dominantly influenced by plume occurrence. In this region northeasterly winds induce the formation of southwestward currents near the coast. These currents deflect around jetties and generate recirculation zones to the south. The formation of a cyclonic eddy occurs in this area and creates convergence zones that intensify the vertical velocities promoting the deposition of suspended sediments.