Condições hidrodinâmica e estimativa do tempo de residência no complexo estuarino lagunar Mundaú/Manguaba (AL) através de modelagem computacional

Abstract

The Mundaú and Manguaba lagoons compose the Complexo Estuarino Lagunar Mundaú-Manguaba (CELMM), located in Alagoas, in the Northeast of Brazil, are typical coastal lagoons connected to the ocean by narrow channels and suffering an accelerated process of environmental degradation due to deteriorating anthropogenic practices such as sanitary sewage, industrial effluents, the exploitation of natural resources. Modeling in estuarine environments has been widely used as aids in decision making in environmental issues. The study aimed to study the regions most susceptible to problems related to water quality within the CELMM according to the residence time calculated from the Mike 21 computational model. The residence time in this environment was comprised by a conservative tracer containing 1kg/m³ of a hypothetical substance within the CELMM and outside, in the coastal region, containing 0 kg/m³. It was simulated the residence time in the period of 2 months, July and August, with climatological data of flow during the years of 1978 to 2016 and with data of real of flows in 2016. The results of the simulation showed that the lagoons are strongly influenced by the fluvial discharges from the main affluents rivers, Mundaú river and Paraíba do Meio, where high and constant climatological river discharges resulted in renewal times of 14 and 30 days, respectively for Mundaú and Manguaba. The areas that needed more time to reach 37% of the tracer concentration were: near the banks, east of Mundaú and north of Manguaba. The absence of flow with real data for July in the Manguaba lagoon resulted in a delay in residence time where the simulations in the 2-month period were not sufficient to reach 37% of the tracer concentration in the entire lagoon area. Circulation within the CELMM showed to be very limited due to the narrow and irregular channels of connection with the ocean, with low communication efficiencies between the lagoons, reflecting in a reduction of the amplitude of the tide inside the environments, the maximum currents velocities were found in periods of flusing tide.