Morphology, fluid dynamics and mixing at the Rio Paraná-Paraguay confluence, Argentina

PARSONS D,; BEST, J; LANE, S,; KOSTASCHUCK, R,; ORFEO, O.; HARDY, R,

San Carlos de Bariloche

Congreso; IV Congreso Latinoamericano de Sedimentología - XI Reunión Argentina de Sedimentología; 2006

Knowledge of flow, sediment transport and bed morphology at channel confluences has stemmed from a mix of laboratory modeling, numerical simulation and field-study of small scale junctions. Although yielding valuable insights into the variables that dictate confluence form and flow, we cannot currently predict how well existing theoretical and conceptual confluence models may upscale, and what complications may be dictated by junction size. This paper presents results from an integrated field study examining bed morphology, sediment transport and fluid dynamics at one of the world’s great junctions: Rio Paraná and Rio Paraguay, Argentina. This site possesses a scour of ~33 m, with a marked turbidity contrast between the Paraná and the sediment-rich Paraguay rivers, thereby allowing tracing of the two flows as they combine. Study of the Paraná-Paraguay confluence applied: 1) multibeam echo sounder (MBES) surveys that reveal the main features of scour, as well as the distribution of bedforms; 2) acoustic Doppler current profiling (aDcp) surveys, with transects and at-a-point measurements to investigate the nature of the 3D-flow field and the turbulent characteristics of the mixing layer; and 3) aDcp backscatter intensity, together with targeted surveys using a Laser In-Situ Scattering and Transmissometry (LISST) instrument, which provided semi-quantitative estimates of suspended sediment concentrations. All of these surveys have used differential GPS and allied navigation software that permit precise positioning and relocation of transect lines within the junction. MBES surveys reveal a ~33 m deep scour that reaches its maximum in the region of flow constriction within the tributary (Rio Paraguay) before it reaches the junction, this being dictated by bedrock control. At the junction, the Rio Paraguay is, therefore, deeper than the Rio Paraná, and the bed discordance lies within the major channel. MBES surveys also show the orientation of dunes and, thus, the steering of sediment pathways within both the scour and the junction. aDcp results reveal the junction is marked by the descent of the velocity maximum within the Rio Paraguay to the deepest parts of the scour. Fluid upwelling is intense towards the edge of the mixing layer and against the bed topography of the confluent Rio Paraná, and convergent flow cells are absent at the centre of the junction, in direct opposition to some models of smaller scale junctions. aDcp records also visualize the deformation of the mixing layer between the two flows and show the denser, sediment-laden, fluid from the Rio Paraguay upwelling into the Rio Paraná. These results suggest the fluid dynamics of this large junction may be dominated by bed discordance between the channels. They also show the role of denser water from the Paraguay flowing beneath that of the Paraná. The difference in density between two confluent streams, which is ignored in current models of junction flow, appears critical and is likely to be important in other confluences where a turbidity or temperature difference exists between the incoming streams. The paper will examine the major flow features at this giant confluence and assess how this compares to past work on junctions several orders of magnitude smaller.