MODELLING THE PROCESSES THAT DRIVE THE SUSPENDED SEDIMENTS DISTRIBUTION IN THE RÍO DE LA PLATA ESTUARY

MOREIRA, D.; SIMIONATO, C. G.

Busan

Congreso; International Coastal Symposium 2018; 2018

The impact of the diverse mechanisms driving the suspended sediments distribution in the wide, shallow, microtidal and mighty Río de la Plata (RdP) estuary is studied by means of a set of process-oriented numerical simulations. Simulations are idealized but well reproduce both qualitatively and quantitatively the main features of the suspended sediments observed distribution. Naturally, the diameter of the sediments that deposit decays with the distance to the sources (with sands and silts dominating in the upper estuary and fine silts and clays over the Barra del Indio), but model results show that the large width and the geometry of the estuary play an important role in sedimentation. The widening and deepening, and the associated significant reduction of the currents speed that occurs after (i) the confluence of the tributaries and (ii) downstream the Barra del Indio Shoal, favours sediments deposition downstream those areas. Tides have a significant impact on the lateral mixing and the re-suspension of bottom sediments; this last augments the concentration of fine sediments in the layers close to the bottom but their energy is not enough to rise them up to the surface. The model reproduces the increment in the concentration of fine sediments observed in the areas where tidal dissipation energy by bottom friction maximizes, but shows that tides alone cannot account for the observed maxima. Winds enhance horizontal mixing, smoothing the pattern produced by the tides. Wind waves are an important forcing for the vertical mixing of the sediments. Their effect is most evident along the southern coast of the RdP and the Barra del Indio Shoal, where wind waves rise to the surface the sediments resuspended by tides. The bottom salinity front acts retaining the sediments upstream the Barra del Indio shoal; there, estuarine currents and flocculation play an important role in deposition.