Abstract： Vegetated sudden-expansion open channel flow with the submerged plant of floating fern or emerging plant of lucky bamboo beds was studied in a laboratory flume. An acoustic doppler velocimeter was employed to measure the instantaneous flow velocity at different measure points of different vertical lines before and after the sudden expansion. The distribution of the mean flow velocity was calculated in order to analyze its characteristic and discuss the influence of plants in open channel on the internal structure of water flow. The experiment revealed that before vegetating, the middle current curve before and after sudden-expansion was semi-logarithmically distributed; after vegetating, because of the influence of two different plants, the distribution of flow velocity under submerged plant condition was classified into three levels: the canopy, the vegetable layer, and the intermediate bed. The intermediate bed was increasingly enlarged. Due to the different water-blocking effects on the different positions of emerging plant, the flow velocity assumes “3” style distribution. The velocity distribution of left vertical line located at the return flow region was more complex and was not semi-logarithmically distributed. After vegetating, the velocity distribution of the identical vertical line under different discharge has a great change. And the variance ratio of velocity distribution was increased along with the discharge increased.
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