Flow through layered vegetation in open channel flows: effect on velocity and discharge distribution

In natural river systems, layered vegetation like grass, shrubs, and tall bushes greatly affects the biodiversity, morphological process, and distribution of nutrients and pollutants. Previously, the effects of uniform one-layered vegetation on the flow structure and hydrodynamics have been extensively studied. However, due to the complexity of flow dynamics in the vegetated channel, multiple-layered vegetation has rarely been investigated. This paper presents a novel experiment to show the effect of three-layered vegetation on open channel flow. It contributes to our understanding of the impact of vegetation locations and heights on the velocity and discharge distributions for a mixed vegetated channel flow. Velocities at different positions along a half cross-section were measured using a mini propeller velocimetry. Observed results showed that the velocity has a distinct profile directly behind vegetation and behind the vegetation gap. The overall trend has two specific inflections about one quarter below (0.75 z/h) or near the top of short vegetation (h): the velocity remains nearly constant in the bottom layer (z/h < 0.75) and then rapidly increases until the top of short vegetation; after a gradual increase, the velocity rapidly rises to the water surface. The velocities directly behind the vegetation in the middle-after-short vegetation arrangement increase much faster than those directly behind the vegetation in the short-after-tall arrangement. The results showed that the maximum zonal discharge for a channel with mixed-height vegetation is situated at the mid-section of each half-channel, i.e., the area from 1/6 to 1/3 width of channel away from the wall. This research will attain significant importance to the engineers and practitioners defining the ecological and riverine flow pattern in the presence of riparian vegetation disseminating nutrients, pollutants, and sediments.