Processes that determine the fine sediments transport in the Rio de la Plata estuary

MOREIRA, D.; SIMIONATO, C. G.

Montevideo

Congreso; INTERCOH 2017 International Cohesive Sediment Dynamics Meeting; 2017

IMFIA Universidad de la República del Uruguay

Numerical models are excellent tools for conducting process studies and testing hypotheses based on observations. They constitute a 'laboratory' in which the various forcing can be 'put' or 'removed', or modified the environmental conditions (depth of the background, geometry, contour conditions, etc.).El Río de la Plata (RdP) es uno de los estuarios más turbios del mundo y de los más importantes en cuanto a descarga continental y área de drenaje. Lo que se sabe hasta el momento en cuanto a la distribución de los sedimentos finos suspendidos, según trabajos anteriores, y en base a estudios recientes basados en nuevos conjuntos de observaciones satelitales e in situ (Moreira et al., 2013 y 2016), ha permitido establecer algunas hipótesis acerca de cuáles serían los procesos que determinan la concentración de sedimentos suspendidos en cada parte del estuario; sin embargo, el potencial rol de otros procesos y de la propia geomorfología permanece desconocido. El objetivo de este trabajo es, mediante el uso de simulaciones numéricas orientadas a procesos, analizar el rol de cada uno de los forzantes y de las condiciones ambientales del RdP en la determinación de la distribución de sedimentos finos que se observa.The Rio de la Plata (RdP) is one of the most turbid estuaries in the world and the most important in terms of continental discharge and drainage area. What is known, in terms of the distribution of suspended fine sediments, according to previous studies, and based on recent studies with new satellite and in situ observations (Moreira et al., 2013 and 2016), has allowed to establish some hypotheses about which would be the processes that determine the concentration of suspended sediments in each part of the estuary; However, the potential role of the other processes and geomorphology itself remains unknown. The objective of this work is, through the use of numerical simulations oriented to processes, to analyze the role of each forcing and the environmental conditions of the RdP in the determination of the distribution of the observed fine sediments.The main results can be summarized as follows. The distribution of suspended sediments in the RdP presents two different regions with well-marked gradients: (i) one at the upper and intermediate estuary, which is perpendicular to the axis of the RdP, with higher concentrations along the Argentine coast and (ii) the other occurs in the exterior RdP, where the concentration of suspended sediment decreases abruptly downstream of the Barra del Indio Shoal, so that it is parallel to the axis of the estuary. The tributary rivers contribute sediment in a differentiated way, the grate amount of sediments is incorporated in the system by the Paraná River (through Guazú-Bravo and Parana de las Palmas branches). The Uruguay River has a much lower sediment concentration discharge. This would be responsible, at least in part, for the north-south gradient in the observed sediment concentration. After the sediments gets into the RdP, the dynamics of the currents are reduced and, consequently, the different textures are gradually deposited according to their size and falls velocities. These would also be influenced by the morphological and hydrodynamic conditions of the estuary (tide, wind and waves). In the RdP the tidal wave comes from the southeast and propagates as a Kelvin wave, first along the Argentine coast and then by the Uruguayan, losing its energy by bottom friction along this path. This characteristic would contribute to the increase of the concentration that is observed on the southern coast. On the other hand, the turbidity response to the wave (generated by the wind) and is almost instantaneous, increases few hours later, and has a greater impact on the intermediate and external regions. Finally, the flocculation processes that occur in the salinity front contribute, together with the decrease in the velocity of the currents as the RdP widen and the gravitational circulation associated to the estuary, in the decrease of the concentration and its respective gradient, in that zone.