Turbulent Structures and the Related Sediment Transport in Unsteady Open-Channel Flows

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In open-channel flows such as rivers and estuaries, various coherent vortices are often observed and greatly affect sediment transport. In the inner region which is very near the bed, bursting phenomena occur quasi-periodically and sustain the turbulence. On the other hand, in the outer region which is near the tree surface, a kolk-boil vortex occurs.There exists a strong upward-tilting streamwise vortex motion, called the "kolk" , near the bed, in a analogy to a tomado motion in the atmosphere, and the kolk vortex develops up to the free surface and then becomes a "boil". A typical boil starts life with high sediment concentration, as a raised circular or oval patch on the water surface ; in the course of time, it widens and gradually settles back down toward the river level until it subsides completely and merges with the surrounding, less-agitated parts. Such coherent vortices are generated especially in flood rivers.Therefore, it is very important to investigate unsteadiness effects on the turbulent structures and the coherent structures in unsteady open-channel flows so that the growing process of the dunes and the behavior of the suspended sediment can be explained dynamically. The authors have investigated turbulent structures in unsteady open-channel flows. It has been clarified that the unsteadiness has significant effects on the outer region near the free surface. The time variations of turbulent structure in the outer region and comparison with unsteady duct flow which has no free surface have examined.Furthermore, the detailed turbulent flow structures over the actual form of sand dunes were measured accurately by making use of two sets of fiber-optic laser Doppler anemometers (FLDA) and automatic traversing system. The three-dimensional structures of coherent vortices generated behind the dune crest and the reattachment point were measured accurately.