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 Rio Parana, 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.