Assessment of tide and wind effects on the hydrodynamics and interactions between tijucas bay and the adjacent continental shelf, Santa Catarina, Brazil

Abstract

The importance of understanding coastal processes and their relationships to continental shelves has become increasingly important as coastal areas experience great socio-economic growth. In this context, Tijucas Bay and its interactions with the adjacent continental shelf was a subject of study through the use of a numerical model (MOHID Water Modeling System) for analysis of its hydrodynamics, exchanges of water masses between the bay and the continental shelf and the water’s residence time in the bay. Simulations were run for 30 or more days. Boundary conditions for the simulations were tides, constant river discharge (24 m3.s-1) and winds. Wind time series represented typical conditions in the region, with NE winds (3 m.s-1) forcing the domain for five days, then turning in the counterclockwise direction for 12 hours until they were oriented in the SE direction (8 m.s-1), remaining two days in this direction and again turning counterclockwise for 12 hours and ending in the NE direction. Three scenarios have been proposed for the experiment; scenario 1 was forced by tides and river discharge, and scenarios 2 and 3 were forced by tides, river discharge and winds. The difference between scenarios 2 and 3 was the wind direction at the beginning of the simulations: scenario 2 began with NE winds, and scenario 3 began with SE winds. The results showed that the hydrodynamics and water exchange of Tijucas Bay were strongly influenced by tide and wind. The tide provided the input and output water pattern in the bay, while the wind accelerated the process, increasing the speed of exchange between the bay and the adjacent shelf. Superficial layers were the most affected by the winds. The easternmost portion of the bay exhibited the greatest current speeds, with a tendency to form a gyre current with water input at one end and output at the other, depending upon wind direction. In the shallower regions, currents exhibited their greatest speeds, while in the deeper areas, the inverse was found. The residence time for scenario 1 was 75 days, and for scenarios 2 and 3, the residence times were 19 and 15 days, respectively. The hydrodynamics of Tijucas Bay is a sum of processes related to its forcings; however, it is important to highlight the role of winds as one of the major determinants of the dynamics of this region, directly affecting transport throughout the bay.