Open channel flow with three-layered vegetation: Effect on the velocity distribution of flow

Aquatic vegetation is widespread in rivers and influences their hydraulic characteristics. Due to individual differences from a biological point of view, the vegetation of different heights always coexists in nature. The attention on vegetation flow studies has shifted from the previous single-layer vegetation to more complex cases such as double- or triple-layer vegetation. Although only a limited number of studies on two-layer vegetation flow have shown that different vegetation heights have a significant effect on flow structure under partially submerged conditions, the effect of multi-layer vegetation on flow is unclear. In this regard, we conducted a novel experiment with triple-layered vegetation with all vegetation heights (10, 15 and 20 cm) under fully submerged conditions. A micro propeller velocimeter was used to measure the velocity at various locations in the downstream cross-section of the channel, including the positions behind short and tall plastic dowels. The measured results showed that the vertical velocity distribution was strongly influenced by vegetation height and its distribution. The observed data also showed differences in velocity directly behind vegetation and in the area behind the vegetation gap. Typically, the velocity profile has a similar profile, with almost a constant velocity from the bed to the height of about 0.75 of the short vegetation height and then a slight increase to the height of the middle vegetation. Afterwards, the velocity rises fast to the water level, where the reflection can be observed. This novel experiment reveals the effect of multiple layers of vegetation on water flow. It would also contribute to the significance of this vegetation configuration and further research on open channel flow in complex vegetation environments.