Modeling the local wind effect on the hydrodynamics of the Buranhém River estuary and its coastal plume 

Delft3D. Numerical modeling. Estuarine dynamics. Porto Seguro.

Understanding the interaction between physical forcings such as tides, river flow and winds is essential in studies of estuarine and coastal hydrodynamics. It is expected to contribute to possible measures of environmental management, since beaches are the main tourist and economic attractions of Porto Seguro and, at the same time, influenced by the multiple phenomena that occur in the hydrographic basin of the Buranhém river, its estuary and the marine area. The objective of this work was to model the effect of local wind and its interaction along with river discharge and tides on the hydrodynamics of the Buranhém river estuary (Porto Seguro, Bahia, northeastern Brazil) and its coastal plume. The numerical model Delft3D was chosen, which is justified by the fact that it was already calibrated and validated for the study area regarding the effect of tides and river discharges, in Rocha (2019). In the present study, the vertical discretization with 10 Z layers was adopted, in order to try to model the vertical salt stratification, identified in the field especially under neap tides. For the calibration and validation steps, real wind data were incorporated. A quantitative evaluation of the results was adopted, using the statistical parameters Skill and RMSE. To understand the possible effects of wind and plume reproduction, a freshwater flow of up to 75 m3/s was considered, along with the following conditions: no wind, NE, S and SE winds, and intensity of 10 m/s for these three scenarios. In addition to the locations used in the calibration and validation (P1 - Quinta do Porto; P2 - Navy and P3 - Marina Buranhem), other points were incorporated in the adjacent coastal region, such as Recife de Fora (P6) and beaches where bathing analyzes are performed, such as Mundai (P4), Taperapua (P5) and Mucuge (P7). The results were exclusively representative of surface conditions, since modeling in all vertical layers was not possible. For the calibration and validation periods, the local wind effect was of secondary importance in relation to tides and river discharge on the hydrodynamics of the Buranhém river estuary. On the plume simulations, under idealized conditions, results suggest that the wind can influence the estuarine hydrodynamics. The geormorphology and topobathymetry of the region were relevant factors in the exchange of water between the hydrographic basin, the estuary and the adjacent marine region. An example of this is the presence of beachrock/sandstone at the mouth of the studied system, which reinforces the importance of a reliable representation of the characteristics of the environment for an adequate modeling. In the adjacent coastal zone, the impact of the local wind is evident when comparing the results for the considered scenarios. Under NE wind conditions, the plume is expected to reach coastal regions south of the mouth of the Buranhém River, such as Mucuge Beach (P7). On the other hand, under S and SE wind conditions, the plume tends to move to northwards, without being able to directly reach Recife de Fora (P6) in the simulated conditions. The plume moved very close to the coastline, where beaches such as Mundai (P4) and Taperapua (P5) are located. Longitudinal and transverse velocity components in the study area reached up to 0.6m/s. In the adjacent coastal environment, the influence of tides on current modulation was evident. Under extreme events of flow and wind, the Buranhem river plume can affect the estuarine hydrodynamics, in addition to the coastal one, including stretches of the coastline where important tourist attractions in the municipality are located.