On the path of plumes of the Río De La Plata Estuary main tributaries and their mixing scales

Resumo

With a length of 300 km and a width that narrows from 220 km at its mouth to 40 km at its upper end, the Río de la Plata is one of the largest estuaries of the world. Its three main tributaries - contributing to a total mean runoff of 22,000 m3 s-1- have different properties and are object of diverse environmental impact due to dissimilar development conditions on their shores. The knowledge of the paths of the plumes of those tributaries along the estuary and their mixing scales is necessary for management purposes. In this paper, advection-diffusion equations for passive tracers are coupled to the three-dimensional Hamburg Shelf Ocean Model and validated by means of a case study. Then, simulations in which each tributary is characterized by a different dye tracer are done for scenarios resulting of the combination of diverse characteristic atmospheric forcing and runoff conditions. The impact of bathymetry and Earth’s rotation on plumes paths and mixing is also evaluated. Results indicate that, for mean to low discharge conditions, the path of the waters of the tributaries is in the form of two main plumes. The different water speeds of both tributaries, the presence of a bend immediately after their confluence and the varying geometry and bathymetry of the estuary favor a rapid mixing between the Uruguay and Paraná Guazú-Bravo waters, which then flow along the northern portion of the upper estuary channel. The Paraná de las Palmas waters, instead, occupy the southern shallow region of Playa Honda in the upper estuary and then flow following the southern coast. Downstream Colonia, at the intermediate estuary, the occurrence of another large bend and a change in bathymetric features force the flow to concentrate in the central part of the estuary and favor further mixing between the plumes. For high discharge conditions the northern part of the upper estuary is divided into two regions, one with a larger concentration of Uruguay waters between Oyarvide and Martín García islands and the coast and another with a larger concentration of Paraná Guazú-Bravo waters south of the islands. In this case there is a much larger concentration of Uruguay waters along the northern coast, which might be enhanced if a peak runoff occurs for the Uruguay but not for the Paraná River. These conclusions are consistent with what can be inferred from the conductivity field observed at the intermediate estuary, the bottom sediments distribution and satellite images. Results indicate that even though strong winds can favor the mixing of the plumes, especially along Argentinean coast, they preserve their pattern and the effect of the storms only persists for a few days. For mean runoff, the elapsed time to the arrival of the leading edge of the Paraná de las Palmas, Paraná Guazú-Bravo and Uruguay tracer clouds at 88 Buenos Aires is of around 3, 5 and 7 days, respectively. The elapsed time to the peak concentration of the tracer cloud for a typical mean runoff scenario is of around 20 days at Buenos Aires. For that condition, the flushing times of the upper and upper intermediate estuary are of around 10 and 60 days, respectively. Nevertheless, mixing scales can be half (twice) those values for high (low) runoff conditions.