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Consecutive groins and embayment form dead water zones, where sedimentation and congestion of pollutants are often observed. It is thus very important to reveal mass and momentum exchanges between main-channel and side-cavity in river environment and hydraulic engineering. There exists spanwise gradient of streamwise velocity near the junction, and it produces small-scale turbulent vortices owing to shear instability. Furthermore, large-scale horizontal circulations are also generated in the cavity zone. These coherent turbulent structures play significant roles on mass and sediment transfers through a mainstream / embayment boundary. However, the relation between the turbulence and mass transfers is still poorly understood. In the present study, PIV and LIF measurements are conducted in a laboratory flume using a laser light sheets and high-speed CMOS camera, and we compared exchange properties of dye concentration and effects of sedimentation on them among different bed configurations. The both of primary gyre and secondary one are observed in the flat-bed and downward slope conditions, and in contrast, the primary gyre is prevalent in the upward slope condition. It was found that these formations of the horizontal circulations have striking impacts on the mass transfer properties between the mainstream and the side-cavity.

PIV is one of the most popular measurement techniques in hydraulic engineering as well as in fluid sciences. It has been applied to study various turbulent phenomena in laboratory experiments related to natural rivers, e.g., bursting phenomena near the bed, mixing layers observed at confluences, wake turbulence around dikes and piers, and so on. In these studies, PIV plays important roles in revealing the space-time structure of velocity fluctuations and coherent vortices. This review article focuses particularly on the applications of PIV to turbulent openchannel flows, which have been conducted for the past decade in Hydraulics Laboratory of Kyoto University. In Section 2, we introduce our experimental setup and PIV/PTV algorithm. In Section 3, we apply the PIV measurements to reveal turbulence characteristics and coherent structures in open-channel flows as well as in vegetated canopy flows. For complex flow situations, various applications of PIV to compound open-channel flows and wind-induced water waves are considered to reveal coherent vortices. In Section 4, we discuss some advanced PIV measurements in open-channel flows. The free-surface-elevation fluctuations and velocity components were measured simultaneously with two sets of cameras to examine phase-averaged parameters of turbulence. A multi-layer scanning PIV was developed to reveal 3D turbulence structure in compound open-channel flows. Our discriminator PIV/PTV was applied successfully to sediment-laden open-channel flows and revealed the fluid/particle interaction and the relationship between coherent structures and sediment concentration. Finally, we conducted simultaneous measurements of velocity and dye concentration with a combination of PIV and LIF in vegetated open-channel flow, which enables us to examine turbulent scalar flux of a passive contaminant. Ó