It is necessary to investigate 3-D structure of separated vortices and the associated kolk-boil vortex behind sand dunes for predicting the transport of suspended sediment in actual rivers. Especially, the flow structure of y-z (vertical-spanwise) cross-sectional plane is very important to estimate the diffusion of pollutants and suspended sediment. Therefore, in this study, we conducted a large-eddy simulation (LES) and PIV measurements of turbulent structure over fixed 2-D dunes to examine the instantaneous 3-D structure of kolk-boil vortex. As the results, the turbulence intensity and the generation frequency of kolk-boil vortex were compared with PIV measurement, and the LES data were in good agreement with experimental ones. Furthermore, the instantaneous structure of kolk-boil vortex obtained from LES were compared with those of PIV data, and they described well 3-D diffusion mechanism of kolk-boil vortex.
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