Turbulent structures in partly vegetated open-channel flows with LDA and PIV measurements

The effects of vegetation growing in rivers in and at the edge of the water on turbulent structures and the associated sediment transport are not clarified yet, in spite of their importance for hydraulics and river engineering. In this study, turbulence measurements of open-channel flows with a vegetated zone at a half channel width were conducted by making use of both, a laser Doppler anemometer(LDA) and the particle-image velocimetry(PIV). It was found that the intensities of the secondary currents and the turbulent energy increase with an increase of the Froude number. The turbulence is advected spanwisely near the free surface by the secondary currents, which are generated by the anisotropy of turbulence. The horizontal vortices near the free surface are generated by the velocity inflectional instability, which increases with an increase of Froude number and the vegetation density. The bed (wall) shear stress in the vegetated zone was calculated by the Reynolds equation with the aid of the empirical drag coefficient.