Hydrodynamic characteristics and related mass-transfer properties in open-channel flows with rectangular embayment zone

Consecutive groynes and embayments form dead water zones, where sedimentation and high concentrations of pollutants are often observed. It is thus very important to understand the mass and momentum exchange between the main channel and side cavities in rivers and hydraulic engineering structures. The spanwise gradient of the streamwise velocity near the junction produces small-scale turbulent vortices because of shear instability. Furthermore, large-scale horizontal circulation is also generated in the cavity zone. These coherent turbulent structures play a significant role in mass and sediment transfer at the boundary between the mainstream and embayment. However, the relation between turbulence and mass transfer is poorly understood. In this study, we performed particle image velocity and laser-induced fluorescence experiments using a laboratory flume, laser light sheets and a high-speed CMOS camera. We examined the exchange properties of a dye as a function of bed configuration and sedimentation effect. Both primary and secondary gyres were observed in the flat bed and downward-sloping bed, whereas the primary gyre was prevalent in the upward-sloping bed. Moreover, the horizontal circulation strongly affected the mass-transfer properties between the mainstream and side cavity.