Morphology, form roughness and the absence of channel scale secondary flows at bifurcations in a large river.

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

S. M. de Tucumán, Argentina

Congreso; 9th International Conference on Fluvial Sedimentology; 2009

Univ. Nac. de Tucumán - Fundación Miguel Lillo

Bifurcations around mid-channel bars are key elements within braided river sytems, having a major impact upon the routing of flow and sediment, upon channel change and bar sedimentology. Although much progress has been made in understanding a range of fluvial processes, increasing attention is being paid to bifurcations and the important role of bifurcation asymmetry and flow and sediment routing at these sites. Most studies have been conducted in laboratory flumes or within small rivers with low width:depth (aspect) ratios. This paper presents results of a field-based study that details the bed morphology and 3D flow structure around two large bifurcations in the Río Paraná, Argentina, with a width:depth ratio of approaching 200. Flow within both bifurcations is dominated largely by the bed roughness, in the form of sand dunes; coherent, channel-scale, secondary flow cells, that have been identified as important aspects of the flow field within smaller channels, and assumed to be present within large rivers, are found to be generally absent in these reaches. This paper discusses these findings and outlines how this has profound implications for flow mixing rates, sediment transport rates and pathways, and thus the interpretation of large river morphology and sedimentology