Abstract
Coherent vortices such as separated vortex and kolk-boils often occur especially in flood flows. The hydrodynamic behavior of the flood structures is quite different between the rising and falling stages of the flood period. In the rising stage, the concentration of suspended sediment increases due to the kolk-boil vortex because the rising stage corresponds to the growing period of dunes and because separated vortex from the dune collides intermittently at the next dune and washes out bed materials around there. Therefore, the structure of turbulence and the associated coherent vortices over dunes in rivers and estuaries is one of the most important topics in hydraulic engineering. In the present study, flow visualizations by means of dyeinjection and hydrogen-bubble techniques are adopted in order to discuss the elemental structures and unsteadiness effects on separated and kolk-boil vortices from the dune crest.
