Mean flow structure in a large confluence-diffluence: Río Paraná, Argentina

L BEST, JAMES; PARSONS D,; LANE J,; KOSTASCHUCK R,; OSCAR ORFEO; HARDY R,

Amsterdam

Congreso; 8th International Conference on Fluvial Sedimentology; 2005

Confluence-diffluence units are key elements within multi-channel rivers and present a complex and linked pattern of flow and sediment transport between these two nodes. Past work has seen much progress in our modelling of river confluences and increasing attention is being paid to bifurcations, and the important role of bifurcation asymmetry. However, most of these studies have been conducted in laboratory flumes or within small rivers (depths less than 3m and widths <100m). This paper presents a study of the bed morphology and flow within a large confluence-diffluence unit in the Rio Paraná, Argentina, which aims to compare the features of mean flow with studies from smaller rivers. The field methodology involved echo-sounder surveys of the bed from a small survey vessel along 36 cross-stream traverses approximately perpendicular to the channel edges. These bed surveys were accompanied by simultaneous acoustic Doppler current profiling (aDcp) surveys that have been used to trace the pattern of velocity magnitude, vertical velocity and flow direction within the confluence-diffluence unit. Bed surveys and aDcp were located using RTK DGPS that allowed accurate differential positioning of the survey lines. This paper will present results detailing the morphology and size of dunes within the confluence-diffluence unit and the 3D flow structure within this reach. Bed surveys show that the maximum scour depth at the confluence is up to 16 m and that one channel is dominant in its discharge contribution at the junction, thereby forming a discordant confluence in which a tributary mouth bar, with steep face dipping into the central scour, is present. The deeper channel thalweg crosses the main channel downstream of the confluence and is found to route most of the discharge into the diffluent channel that is on the opposite side of the channel to the upstream confluence. aDcp surveys and dune orientations show the progressive division of flow upstream of the diffluence. These surveys also reveal that the flow within this large reach is dominated largely by the bed roughness, and that the coherent secondary flow cells that have been identified in smaller channels and assumed to be present within large rivers, are absent. The division of flow downstream of the junction occurs well before the diffluence and suggests the linkage of these two nodes through the control of the central scour. This paper will examine the nature of the flow field through the confluence-diffluence unit and its links to the bed topography in this large sand-bed channel.