On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

Abstract

The links between large-scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three-dimensional velocity, and at-a-point suspended sediment concentration and particle-size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee-side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune-related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.