LAUSR

Large-scale coherent flow structures (CFS) above dunes are the dominant source of flow resistance and constitute the principal mechanism for sediment transport and mixing in sand bed river and estuarine systems. Based on laboratory observations, CFS formation has been previously linked to flow separation downstream of high-angle dunes with lee-slopes of ~30°. How CFS form in natural, deep rivers and estuaries where dunes exhibit lower lee-slopes and intermittent flow separation is not well understood. Here, we present Particle Image Velocimetry (PIV) measurements from an experiment where dune lee-slope was systematically varied (30°, 20°, 10°), while other geometric and hydraulic parameters were held constant. We show that CFS form downstream of all three dune geometries from shear layer vortices in the dune lee. The mode of CFS formation undergoes a low-frequency oscillation with periods of intense vortex shedding interspersed with periods of rare vortex shedding. Streamwise alignment of several vortices during periods of intense shedding results in wedge-shaped CFS that are advected above the dune stoss-side. Streamwise length-scales of wedge-shaped CFS correspond to large-scale motions (LSM). We hypothesize that the advection of LSM over the dune crest triggers the periods of intense shedding in the dune lee. LSM are weaker and smaller above low-angle dunes; however the low-frequency oscillation in CFS formation periods persists. The formation of smaller and weaker CFS results in a reduction of flow resistance over low-angle dunes.