Morphology flow and sediment transport over a natural 3D dune field: Rio Paraná, Argentina

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

San Carlos de Bariloche

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

The morphology, flow and process mechanics of river dunes have attracted much interest over many years. However, many of these studies have concentrated on two-dimensional (2D) bed features, their associated flow structures, and bed stress distributions. This morphological simplification imposes inherent limitations on our interpretation and understanding of dune form and flow dynamics, as natural dunes are invariably three-dimensional (3D), with an associated fully 3D flow structure. For example, studies over 2D forms neglect the significant effect that lateral flow and secondary circulation may have on the flow structure and dune morphology. This paper details a field investigation of the interactions between the 3D morphology, 3D flow structure and sediment movement over large alluvial sand dunes in the Rio Paraná, NE Argentina. A large (0.25 km wide, 1.8 km long) area of dunes was surveyed using a Multibeam Echo Sounder (MBES), providing a high-resolution (millimetric precision), 3D view of the river bed. Simultaneous to the MBES survey, 3D flow information and suspended sediment concentrations were obtained with an acoustic Doppler current profiler (aDcp) and an in-situ laser scattering device (LISST), respectively. Repeat MBES surveys reveal the patterns and mechanics of dune movement and, thus, sediment transport rates and directions, and also highlight the important role of superimposed bedforms in the morphological dynamics of the dune field. Through mooring the research vessel at a number of points over the dune field, the study also investigated the links between large-scale turbulence and sediment suspension over the dunes, providing field evidence of the initiation and evolution of coherent structures and their eruption on the flow surface. Wavelet analysis was applied to aid identification of coherent flow structures at a range of frequencies from 1 to 0.001 Hz, although two frequencies are dominant: higher frequency structures with periods of between ~50 and 100 seconds (0.02-0.01 Hz), and larger structures with periods > ~200 seconds (0.005 Hz). These are interpreted as the manifestation of vortex shedding and wake-flapping frequencies, respectively.