On the influence of density contrasts between mixing flows at channel confluences: the Río Bermejo and Río Paraguay, Argentina

BEST, JAMES; PARSONS, DANIEL; KELLMAN, PHIL; AMSLER, MARIO; SZUPIANY, RICARDO; KOSTASCHUK, RAYMOND; LANE, STUART; HARDY, RICHARD; ORFEO, OSCAR

Santa Fe, Argentina

Workshop; Workshop on Morphodynamic Processes in Large Lowland Rivers; 2007

Facultad de Ingeniería y Ciencias Hídricas (UNL) - National Center of Earth-Surface Dynamics (Universidad de Minnesota/USA) - Universidad de Illinois (USA)

ABSTRACT. This paper presents field observations of the bed morphology, flow dynamics and distribution of suspended sediment at the junction of the Río Paraguay and Río Bermejo, Argentina. We show that the flow dynamics at this site are dominated by the density contrast between the flows, created by differences in suspended sediment concentration, which results in the Río Bermejo always having a density excess over its confluent partner Río Paraguay. The denser Río Bermejo thus flows underneath the Río Paraguay, with subsequent vertical fluid upwellings of sediment-rich fluid acting against the local density gradient. We illustrate the dynamics of flow at the junction and how this influences flow in the downstream post-confluence channel. Finally, we reason this mechanism of flow is more likely at larger lowland river junctions, where the probability of differences in sediment yield from the contributing catchments may be greater. This paper presents field observations of the bed morphology, flow dynamics and distribution of suspended sediment at the junction of the Río Paraguay and Río Bermejo, Argentina. We show that the flow dynamics at this site are dominated by the density contrast between the flows, created by differences in suspended sediment concentration, which results in the Río Bermejo always having a density excess over its confluent partner Río Paraguay. The denser Río Bermejo thus flows underneath the Río Paraguay, with subsequent vertical fluid upwellings of sediment-rich fluid acting against the local density gradient. We illustrate the dynamics of flow at the junction and how this influences flow in the downstream post-confluence channel. Finally, we reason this mechanism of flow is more likely at larger lowland river junctions, where the probability of differences in sediment yield from the contributing catchments may be greater.