Schematic of Flow Over a Dune

This is from Best (2005), a great schematic of fluid flow over a dune.

Best, J. (2005). The fluid dynamics of river dunes: A review and some future research directions. Journal of Geophysical Research, 110(F4), 1-21. doi: 10.1029/2004JF000218.

1.  A region of flow separation is formed in the lee of the dune, with reattachment occurring approximately 4–6 dune heights down- stream of the crest. 
2. A shear layer is generated bounding the separation zone, which divides this recirculating flow from the free stream fluid above; large-scale turbulence is generated in the form of Kelvin-Helmholtz instabilities along this shear layer, and as the free shear expands, it creates a wake zone that grows and dissipates downstream. A third region is one of expanding flow in the dune leeside. 
3. Downstream of the region of reattachment, a new internal boundary layer grows as flow reestablishes itself and develops a more logarithmic velocity profile. 
4. Maximum horizontal velocity occurs over the dune crest and bed shear stresses here may be sufficient to generate upper stage plane bed conditions: the exact nature of flow here will determine the rate and periodicity of sediment supply to the dune leeside and hence the nature of sediment sorting in the deposits of the leeside.