Sediment dynamic of a large river confluence with marked contrast of transport in suspension: the Bermejo and Paraguay Rivers (Argentina)

PARSONS D,; BEST, J.; LANE,; ORFEO; KOSTASCHUCK R,; AMSLER M,; HARDY R,

San Carlos de Bariloche

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

Asociación Argentina de Sedimentología

River channel confluences form key nodes within all fluvial networks and are sites where rapid and large changes occur in channel hydraulic geometry, discharge and fluid mixing. The progress in understanding flow, sediment transport and bed morphology at open channel junctions has come basically from three complementary approaches: laboratory experiments, field quantification and numerical modelling. This contribution presents new results of an integrated field study of bed morphology, sediment transport and fluid dynamics at the confluence of the Río Bermejo and Río Paraguay. In a regional context, the Río Bermejo is the main contributor of suspended sediments to both the Rio Paraguay and Rio Paraná drainage systems. Suspended sediment concentrations supplied by the Río Bermejo are often extremely high, and can reach levels over 25 g L-1. The sediments, sourced from the Chaco plain and carried by the Rio Bermejo, are the main constituents in the building of the large Río de la Plata delta, near Buenos Aires. Moreover, the high levels of suspended solids have profound implications for water quality downstream of the Bermejo, with fluid mixing rates at the confluence having particular importance for water supply in the region. The complete study applied: 1) multibeam echo sounder (MBES) surveys that revealed the main morphological features within the confluence zone, as well as the distribution of bedforms; 2) acoustic Doppler current profiling (aDcp) surveys, with moving-boat transects and at a-point measurements to investigate the nature of the 3D flow field and turbulent characteristics of the mixing layer between the combining flows; 3) aDcp backscatter intensity, together with targeted surveys using a Laser In-Situ Scattering and Transmissometry (LISST) instrument, providing semi-quantitative estimates of suspended sediment concentrations and mixing rates; 4) profiles of suspended sediment samples at key locations through the junction; and 5) historical records of discharge and sediment concentrations derived from rating curves of both rivers. This paper examines the sediment dynamics at the confluence area considering vertical and lateral distribution of suspended sediment concentration. From the sampling strategy, it is possible to observe clear differences not only at the cross sections, but also in the flow directions, indicating a complex process of flow mixing, where the suspended load dispersion is several times wider near the channel bed than near the surface. The thesis of this paper is that the consistent and significant difference in the densities of the combining flows at the junction, as controlled by suspended solid concentrations, has a dramatic role in confluence flow structure and mixing dynamics, with the higher-concentration Rio Bermejo fluid forming a density driven underflow as the flows combine. The complete study applied: 1) multibeam echo sounder (MBES) surveys that revealed the main morphological features within the confluence zone, as well as the distribution of bedforms; 2) acoustic Doppler current profiling (aDcp) surveys, with moving-boat transects and at a-point measurements to investigate the nature of the 3D flow field and turbulent characteristics of the mixing layer between the combining flows; 3) aDcp backscatter intensity, together with targeted surveys using a Laser In-Situ Scattering and Transmissometry (LISST) instrument, providing semi-quantitative estimates of suspended sediment concentrations and mixing rates; 4) profiles of suspended sediment samples at key locations through the junction; and 5) historical records of discharge and sediment concentrations derived from rating curves of both rivers. This paper examines the sediment dynamics at the confluence area considering vertical and lateral distribution of suspended sediment concentration. From the sampling strategy, it is possible to observe clear differences not only at the cross sections, but also in the flow directions, indicating a complex process of flow mixing, where the suspended load dispersion is several times wider near the channel bed than near the surface. The thesis of this paper is that the consistent and significant difference in the densities of the combining flows at the junction, as controlled by suspended solid concentrations, has a dramatic role in confluence flow structure and mixing dynamics, with the higher-concentration Rio Bermejo fluid forming a density driven underflow as the flows combine.