Hidrodinâmica e mecanismos do transporte do sal no Estuário dos Bons Sinais

Abstract

Estuaries are unique environments, located in the transition zone between the open ocean and the river, in which ocean waters are measurably diluted by freshwater from continental drainage. According to their characteristics, these ecosystems are extremely sensitive and contain very important habitats for biodiversity, biomass production and the preservation of numerous oceanic and terrestrial species. However, estuarine hydrodynamics are basically governed by three factors, river discharge, tidal currents and wind tension, subject to modifications by the Earth's rotation, bathymetry and estuary geometry. The aim of the work is to study the Hydrodynamics and Mechanisms of Salt Transport in the Bons Sinais Estuary under different tidal conditions. The campaign to acquire hydro oceanographic data was carried out under spring tide conditions and two tidal conditions (spring and neap), over a period of approximately 14 days. The estimate of the average rainfall input presented a maximum value in January with 4.7 m3⁄s and a minimum in October, with an intensity equal to −3.4 m3⁄s, presenting the maximum freshwater input in the month of January. The total freshwater discharge (Qf) due to the discharge of freshwater from the drainage basin and average rainfall input through the free surface of the estuarine channel, presented a maximum value in the month of February with an intensity equal to 46.10 m3⁄s and −3.30 m3⁄s representing the minimum in October, with a variation of 15.79 m3⁄s. The temperature, salinity and density in the water column showed homogeneous behavior in both tidal conditions, although stratified at times in the syzygy and quadrature conditions. The main salt transport mechanism was the portion of river discharge with a participation of 90.06% in the spring tide and 69.68% in the neap tide condition in terms of total salt transport. Thus, in the syzygy condition, EBS is characterized as an importer and in quadrature as an exporter of salt. In both tidal conditions, the estuary presented dimensionless Richardson number values greater than 0.25, showing the dominance of stratification over turbulence. According to the diagram proposed by Hansen & Rattray (1966), the estuary was characterized as well mixed and weakly stratified type 1a in both tidal conditions. In order to characterize the movement regime in the Bons Sinais estuary, the dimensionless Reynolds number was calculated under spring and neap tide conditions. And therefore, EBS is characterized by a fully turbulent regime with intensified mixing.