Intrusão salina no estuário do Rio Buranhém (Sul da Bahia) através de modelagem numérica

Abstract

The municipality of Porto Seguro, Bahia, is a well-known tourist destination in the Brazil. However, its urban infrastructure is overwhelmed, especially during the summer. The annual floating population is approximately 10 times greater than than its fixed population. The combination of fixed population growth and floating, in the medium and long term, will put even more pressure on the sanitation of the municipality, becoming indispensable studies that can predict the possible environmental impacts to aid in urban planning. By 2025, according to the National Water Agency, it is estimated that the Buranhém River will also become used as a source of supply for the city. More hydrological data Recent studies indicate a tendency to decrease the fluvial flow, mainly due to the degradation of forest cover in the respective hydrographic basin and influence of phenomena such as the El-Niño Southern Oscillation (ENSO). The river flow suffers large annual variation, with the historical average annual flow of 22 m³.sˉ¹. the region is subjected to a regime of microtidal to astronomical mesotidal with variations above 2 m, with average speeds close to 0.4 m.sˉ¹ in the flood and 1 m.sˉ¹ in the ebb. The present work has the general objective of studying the saline intrusion in the estuary of the Buranhém river in low flow scenarios recorded in series historical. For this, the implementation of the numerical model was carried out hydrodynamic Delft3D, as a subsidy for local sanitation planning and for contribute to studies related to sediment transport and quality of Water. Scenarios were simulated under different tidal conditions and river flows. The model was calibrated and validated using field velocity data from currents and variation of water level and surface and bottom salinity, observed with CTD's at three fixed points and ADCP at a fixed point anchored in the estuary. Field data and modeled results were compared quantitatively through the Skill parameter.For the purpose of intrusion analysis salina, in addition to the typical tides of the region, the model was forced with minimum flows recorded in historical series (2, 4, 6 m³.sˉ¹) and in a minimum flow condition extreme of 0.2 m³.sˉ¹. The observed salinity data indicate an environment with a tendency to vertical stratification in the slump tide compared to the syzygy tide, which is a well-mixed estuary. Already the results modeled only reproduced characteristics of a well-mixed environment both in square tide as in syzygy. Average values ​​of Skill were obtained greater than 0.97 for the water level, under quadrature and syzygy tides with variations from 1 m to 2.15 m, respectively. The salinity results of surface and bottom had average Skill values ​​greater than 0.90, while the of horizontal speeds showed average Skill values ​​greater than 0.77. You field data showed a saline intrusion of 5.1 km under high tide. square of 08/04/2016 at 12:00 and 9.4 km, under syzygy tide of 07/31/2016 at 18:00 The model for the same period represented under squaring tide a saline intrusion of 4.1 km and syzygy of approximately 6 km. In the simulation of extreme minimum flow condition (0.2 m3.s-1) and syzygy tide the model represented an intrusion of approximately 17 km upstream of the estuary mouth. With minimum flows of (2, 4, 6 m³.sˉ¹) the intrusion gradually decreased with results of 13, 11 and 9.5 km, respectively. Confirming the scenario use of the Buranhém river as a source for water supply in Porto Seguro, studies such as this one may serve as a reference to determine a region safer freshwater abstraction