Elisa Helena Leão Fernandes

Spatial Gradients in the Flow of Southern Patos Lagoon

Abstract 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.

On the Dynamics of Mangueira Bay—Patos Lagoon (Brazil)

Abstract Mangueira Bay is a semi-enclosed system that is very important to the communities living around the city of Rio Grande. It is influenced by multiple and conflicting human impacts. This embayment is connected to Patos Lagoon Estuary through a narrow inlet and its dynamics seem to be related to the dynamics of the estuarine area. This study aims to investigate the mechanisms that control the dynamics of Mangueira Bay under the predominant forcing conditions and evaluate the influence of baroclinic estuarine circulation on the dynamics of the area. The TELEMAC-2D model was used to carry out two- and three-dimensional simulations of Patos Lagoon Estuary. The modeling results indicate that the dynamics of Mangueira Bay are directly related to the dynamics of the main access channel, which are controlled by a combination of remote and local wind effects. A time lag occurs between the excursion of salt in Mangueira Bay and the excursion of salt in the main channel, with salinity penetrating in the embayment when winds from the SW become weaker. Marked horizontal stratification is present throughout the embayment, with salinity gradients changing direction as a function of ebb and flood currents. The area where the embayment connects to the main channel presents higher salinity variability and vertical stratification, while the inner limits are less affected by the dynamics of the channel.