Three-Dimensional Turbulence Intensity in a Compound Channel

摘要

Experimental research was undertaken to investigate the changes in spatial turbulence intensity in a compound channel and the influence of rigid, emergent floodplain vegetation on turbulence intensity. Five tests for two various roughness values of floodplains were realized. In the first series of experiments (three tests), the surface of the main channel bed was smooth and made of concrete, whereas the floodplains and sloping banks were covered by cement mortar composed of terrazzo. In the second set of experiments (two tests), emergent vegetation (trees) on the floodplains, modeled by aluminum pipes, was added. Instantaneous velocities were measured with the use of a three-component acoustic Doppler velocity meter. The influence of floodplain trees, on the distributions of relative turbulence intensity (u'/U, v'/U, w'/U) in the main channel and on the floodplains was presented. It was found that the longitudinal (u'/U) and transverse (v'/U) turbulence values decreased from the bottom upward to the floodplain elevation (z/h = 0.56) in the main channel but remained constant above the floodplain level. Vertical relative turbulence intensity (w'/U) increased going up from the bottom until z/h = 0.15, decreased until about z/h = 0.7, and then increased again upward to the water surface. Trees on the floodplains resulted in changes in vertical distributions of the relative turbulence intensities in all three directions on the floodplains and over the bottom of the main channel. In the main channel bed of the compound channel with emergent vegetation (trees) on the floodplains, the values of longitudinal and transverse relative turbulence intensities decreased from the bottom upward to a depth of z/h = 0.35, and then these values increased upward to the water surface. Vertical relative turbulence intensity increased with the distance from the bottom until z/h = 0.15, decreased until z/h = 0.4 and then increased again upward to the water surface. The distributions of relative turbulence intensities were described with regression equations.