Flow Characteristics of Open-channel Flow with Partial Two-layered Vegetation

Different types of vegetation widely exists in rivers and wetlands, where the vegetation often has various heights. The influence of riparian vegetation on ecological and flow process is becoming increasingly significant in river flood risk and aquatic environmental management. Many studies have been carried out on the vegetated flow with the same height vegetation, which is far realistic in natural rivers. There is little study on flow characteristics with the co-existence of short and tall vegetation under either submerged or emergent flow condition. This paper is to present the results of a novel experimental study on a flume, which is partially covered by two-layered vegetation along one side of flume bed. Two different heights of dowels, 10 cm and 20 cm, were used to denote the short and tall rigid vegetation respectively. Experiments under two flow depths were undertaken to represent partial submerged and full submerged scenarios, and velocities at various locations were measured by Acoustic Doppler Velocimetry (ADV) and propeller velocimetry. Experimental results show that the velocity profiles differentiate significantly in the layer beyond the short vegetation height although they are almost uniform within the depth of short vegetation in different configurations. Meanwhile, a strong shear layer exists laterally between vegetation and non-vegetation zones, showing that the vegetation significantly reduces the velocity of flow. Similarly, the Reynolds stress remains almost constant in the lower zone and starts to increase from the short dowel edge. A rapid increase in the Reynolds stress at the top of the short vegetation indicates the presence of strong mixing layer.