Spatial gradients in the flow of southern Patos Lagoon

The necessity of improving our understanding of mixing, dispersion of salt, sediment, larvae and pollutants, and the increasing need of offering safe navigation conditions in the Patos Lagoon access channel highlighted the need of increasing knowledge on the flow spatial gradients in the area. Three-dimensional simulations were carried out using the TELEMAC-3D model (EDF, Paris, France) in order to simulate the estuarine flow features in the vertical. Results indicate that the basic mechanism controlling the along-estuary flow and salinity distribution in the estuarine zone during the studied period is given by the pressure gradient generated between the coastal ocean and the lagoon, resulting from the local and non-local wind action and freshwater discharge. Results also show that whatever be the cause of the transverse flow, the strength of lateral circulation is important. The transverse flow in the main lagoon is driven by the local wind forcing and is manifested as a series of gyres. Results show that in shallower areas, lateral pressure gradients resulting from the interaction between longitudinal barotropic pressure gradients and the morphology seem to control the transverse flow. In areas of complex morphology and subject to density stratification, however, the transverse flow results from the interaction between barotropic and baroclinic forces and bathymetry. Results also indicate that the presence of transverse density gradients reduce vertical momentum exchange and inhibit mixing in some areas due to damping of turbulent diffusion and internal friction