Flow fields, bed shear stresses and suspended bed sediment dynamics in bifurcations of a large river

SZUPIANY, RICARDO N.; AMSLER, MARIO L.; HERNANDEZ, JAVIER; PARSONS, DANIELS; BEST, JAMES; FORNARI, EVANGELINA; TRENTO, ALFREDO

WATER RESOURCES RESEARCH

AMER GEOPHYSICAL UNION

Año: 2012 vol. 48 p. 1 - 20

0043-1397

Channel bifurcations associated with bars and islands are important nodes in braided rivers and may control flow partitioning, and thus affect downstream confluences, as well as the formation/dynamics of bars. However, the morphodynamic processes associated with bar formation are poorly understood, and previous studies have largely concerned laboratory experiments, small natural streams, or numerical analyses with large Froude numbers, high slopes and low Shields stresses. In these cases, the morphologic changes at bifurcations are relatively rapid, with predominant bedload transport and the suspended load playing a minor role. In the present paper, the evolution of the flow structure and suspended bed sediment transport along four expansion-diffluence units in the Rio Paraná, Argentina, are described. The Rio Paraná is a large multichannel river with a bed composed of medium and fine sands, and possesses low Froude numbers and high suspended bed material transport. Primary and secondary flow velocity components were measured with an acoustic Doppler current profiler (ADCP) along the expansion-diffluence units, and the backscatter signal of the ADCP was calibrated to allow simultaneous measurements of suspended bed sediment concentrations. The interactions between these variables show that the cores of primary flow velocity and suspended bed sediment concentration do not necessarily follow the thalweg at the bifurcation, and that inertial effects on the suspended bed sediment may influence the morphodynamics of bar formation. It is suggested that changes in flow stage, as well as the presence of vegetation, may further increase the deposition of suspended bed sediment at the barhead. This study suggests that the ratio of suspended bed material to bed load is an important factor controlling the morphodynamics of bifurcations in large sand-bed braided rivers.