Secondary Flow Structures in Open Channel with Patchy Gravel and Vegetated Beds

Secondary flows are present in open channel due to the complex three-dimensional flow structures as a result of the existing natural features (e.g. different boundaries) affecting the flow. This paper experimentally examined the structure of secondary flow in open channel with a patchy gravel and vegetated beds using two different experimental conditions. A high resolution three-dimensional instantaneous velocity measurement over the gravel and vegetated bed were made using the acoustic Doppler velocimeter (ADV) and the secondary flow quantities determined. The results reveal that the magnitude of secondary flow over the gravel bed in experiment one is relatively large with occurrence of down-flow, and up-flow over the flexible vegetated bed. The lateral interaction and momentum transfer between the two-bed roughness (gravel and vegetated beds) is achieved by the secondary flow, at the lower region of the flow, the transverse motion is directed from the gravel bed towards the grass bed, and at the upper region, and the flow is transported laterally in the opposite direction.