COHERENT STRUCTURE AND ASSOCIATED TURBULENT CHARACTERISTICS IN OPEN-CHANNEL FLOWS WITH VEGETATION AT HALF WITH

Turbulence measurements of partly vegetated open-channel flows were conducted accurately with PIV (Particle Image Velocimetry) and LDA (Laser Doppler Anemometer). The horizontal vortices near the free surface are generated by the shear instability which increases with an increase of the vegatation density and the Froude numbers. Secondary currents near the free surface are generated by the horizontal vortices and its strength increases with an enlargement of the vegatation density and the Froude numbers. This hydrodynamic mechanics of secondary currents is quite different from non-vagatated open-channel flows, in which the secondary currents are generated by the anisotropy of turbulence. A new eddy model is proposed to explain both the secondary currents and horizontal large-scale vorticies in partly vegetated open-channel flows.