Author(s): Xiaonan Tang; Zixin Yang; Qinzheng Teng
Linked Author(s): Xiaonan Tang
Keywords: Vegetated flow mixed vegetation rigid vegetation velocity riparian vegetation aquatic environment open-channel
Abstract: Natural rivers and wetlands often have vegetation of different heights grown alongside the river bank, where riparian vegetation plays an important role in sediment transport, ecological environment, and habit biodiversity. Recently, the impact of vegetation on flow has been drawing increasing attention in the eco-environment and river restoration research. In the past, the majority of studies have been mainly focused on the understanding of flow through vegetation of single-layered vegetation. However, in natural riverine environments, mixed vegetation of different heights often co-grows in natural rivers and occupies only part of the channel, which results in the change of velocity both vertically and laterally, consequently re-distributing discharge. Recent studies show that mixed vegetation of different heights has significant different impact on the velocity distribution within the vegetation than single-layered vegetation does. Also, it is poorly understood how such vegetation arrangement affects the channel's velocity distribution, which is a prerequisite for flow assessment in river engineering and environmental management. In this paper, a novel experiment was designed to study the flow velocity in a water flume partially covered with rigid vegetation of three different heights that co-exists on the flume bed. The experimental study focuses on how the mixed vegetation affects the velocity profiles through the vegetation zone and the lateral change of velocity across the channel. Experiments were conducted in both partially submerged and fully submerged conditions. Dowels in three heights of 10,15 and 20 cm were used to simulate rigid vegetation in a staggered pattern for each type of dowel, and they occupy only half of the channel width. A micro propeller velocimetry was used to measure velocities at various lateral locations in a whole channel section. Experimental results showed that the vertical velocity profiles are significantly different in the non-vegetated zone and within the vegetated zone. The typical velocity of flow in the vegetated zone has a reflecting point around the top of vegetation under submerged conditions, while the velocity in the non-vegetated zone does not follow the logarithmic law of velocity, indicating the significant influence of vegetation on the flow. The results also revealed that there exists a strong momentum exchange between the vegetated and non-vegetated free-flow zones, resulting in a transition of lateral velocity from a fast-flowing non-vegetated zone to a low-flowing vegetated zone. By integrating the velocity across a section, calculated discharge through each zone shows that the flow discharge in the vegetation is almost one-third of the flow discharge in the non-vegetated zone given the same width when the mixed vegetation is under partially submerged conditions (i. e. the short and medium vegetation is fully submerged but the tall vegetation is non-submerged), indicating the retarding effect of mixed vegetation. The flow discharge in the vegetated zone against the non-vegetated zone depends on the submergence, which will be studied further. The findings of this study about how mixed vegetation affects flow would be helpful for riparian management that preserves healthy ecological and habitat zones.
Year: 2025