Dispersal of a small wave-protected river plume from drifter observations

Abstract

Using low-cost surface drifters, this study investigates the influence of the Buranhém River plume on the coastal region of the city of Porto Seguro (in the state of Bahia). The results highlight the importance of small river plumes on the coastal region and their possible influence on tourist beaches and high biodiversity regions. Three drifters, mounted with GPS receivers and temperature and salinity sensors, were simultaneously released into the mouth of the river in six experiments between December 2019 and December 2020, during ebbing spring tides. In each experiment, vertical salinity profiles were measured hourly at the position of the central drifter for 5 h, in order to investigate the along-track stratification of the plume. Results show that there are two distinct dispersion patterns in the Buranhém River plume, and that they are controlled mainly by wind and river discharge. Southward (northward) dispersion with curvilinear shape forced by northeasterly (south and southwesterly) winds were observed during the experiments. The maximum vertical salinity gradient confirmed the shallow character of the river plume (< 3 m in depth), more highly stratified during high river discharge and northward dispersion than in the opposite situation of northeasterly winds and low and medium river discharge. The results suggest that the Buranhém River plume has a larger influence over the northern beaches of Porto Seguro, with its reach being defined by the river discharge. Beaches to the south of the river inlet are less influenced by the river plume. Such unprecedented findings form the background for future studies and for management of this highly visited tourist area in Brazil.