Effects on circulation and water renewal due to the variations in the river flow and the wind in a Brazilian estuary lagoon complex

Abstract

The Mundaú-Manguaba Estuary Lagoon Complex is located on the coast of Alagoas state in Northeastern Brazil, and consists of two shallow lagoons, Mundaú and Manguaba, that form a system of choked lagoons which are connected to the Atlantic Ocean by a series of narrow channels with a single outlet which dynamically alters its position. This study uses the Hydrodynamic Environmental System, SisBaHiA® to investigate how variations in river discharge and wind influence hydrodynamic circulation, water renewal, salinity and temperature in the lagoons. The free surface positions, obtained by model, were compared with the free surface positions measured at two points of the complex, showing good agreement. The analyses were carried out for dry and wet seasons and extreme events with very high freshwater discharge. The channel system of the lagoons is an efficient filter in reducing tidal variability inside the lagoons. The tidal ranges in the Manguaba and Mundaú Lagoons are 90% and 80% lower, respectively, as compared with the values in the open boundary. The residence time calculated varied between 11 and 365 days and between 2 and 180 days for the Manguaba and Mundaú Lagoons, respectively, making it possible to identify possible stagnation areas. The results from the salt and heat transport model show a prolonged period with low salt concentrations and slow salinity recovery after the rainy season; the water temperature in the lagoons shows little spatial and temporal variation.

Seasonal variations in residence time and water age in the Mundaú-Manguaba Estuarine Lagoon System, AL (Brazil)

“…• Analyze the characteristic hydraulic times (CHT), considering the concepts of Residence Time (RT) and Water Age (WA), in different sectors of the lagoon system. This study deepens aspects of similar studies, relevant to the MMELS, among them: Larsson and Nilsson (2014); Lima (2017); Souza (2017) ;Brito Jr. et al (2018); Cunha et al (2021) and Pinheiro et al (2021). Most of the cited works discuss the RT.…”

“…In this region, high RT are not only due to the longer distance in relation to the communication channels, but also to the hydrodynamic circulation, which provided, due to the strong influence of the winds, a greater residence of the waters in this region, moving, preferably, counterclockwise. Cunha et al (2021) also found high values in this region, with RT exceeding 200 days. Thus, the northern region is a critical location for the input of pollution, since the polluting loads, coming, for example, from the Paraíba do Meio R. and the urban drainage channel of Pilar, tend to remain in this region for a long time, degrading the water quality in this portion of the lagoon.…”

“…In the midst of the various computational tools available, such as MIKE 21/3 (DHI, 2021) and Delft3D (Deltares, 2021), the SisBaHiA ® was adopted due to its suitability to the study's purposes and its proven applicability and efficacy in the environmental modeling of estuarine systems, in addition to its accessibility. Examples of applications include the work developed by Lima (2016), Silva (2019), Aguilera et al (2020); Dalazen et al (2020) and Cunha et al (2021). SisBaHiA ® is widely used in the development of studies and projects focused on water resources and environmental management, as verified in the sections "Applications -Projects" and "Research -Theses", in http://www.sisbahia.coppe.ufrj.br/.…”

“…The HM is composed of a 2DH module, vertically averaged, and a 3D module. According to Oliveira and Kjerfve (1993), ANA (2012c), Lima (2017) and Cunha et al (2021), the Mundaú and Manguaba L. are shallow, subject to the action of tides and frequent winds coming from E to SE. Because of that, the water column is mostly well mixed.…”