An experimental open-channel laboratory study of secondary currents and surface boils is presented. Surface velocity dynamics over a large measuring field were investigated using Large Scale Particle Image Velocimetry (LSPIV). Data were analyzed for the structures of the large streamwise vortices and surface boils. The results indicate a mean multi-cellular pattern of faster and slower primary longitudinal velocities. The mean transversal velocities show a corresponding pattern. The spacing of the surface pattern of upwelling and downwelling is related to the water depth. The instantaneous pattern of the secondary currents meanders around the mean pattern. Vortex boils occur near the upwelling regions. Details of the vortex structures, in particular the growth and spreading of individual vortices when moving along the flow, are detected from instantaneous velocity maps of the water surface using a moving-camera PIV which follows the flow at the mean surface current speed.
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