Experimental study of Flow Fields and Local Scour nearRigid Emergent Vegetation Patch

• Main Authors: Bo-Ruei Ke, 柯柏睿
• Other Authors: 詹勳全
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/36480036530734760213

碩士 === 國立中興大學 === 水土保持學系所 === 102 === In the natural rivers, woody vegetation commonly grows along the riverbank. When flows run through the woody vegetation zones, the stream processes are markedly affected. This study experimentally discusses the characteristics of flow fields and the changes of river bedform around woody vegetation zones. The experiments were produced in a flume with 20m long, 1m wide, and a fixed slope of 0.001. The woody vegetation was set in 10 square centimeters at one side of the flume. Modelled vegetation was simulated by the steel columns in the emergent flow conditions. Uniform sand with a median size of 0.88 mm was used as the bed sediment. The experimental flow was steady and flow velocity was adopted to close to the initiation of sediment motion. For the vegetation density equal to 0.05, the time-averaged three-dimensional flow fields and turbulent characteristics were measured by an Acoustic Doppler Velocimeter in the flat bed and equilibrium scour conditions. The bed morphology of equilibrium scour condition was measured by a Laser Distance Meter in the cases of vegetation density equal to 0.03, 0.05, 0.09, and 0.12. The interactions between water flows and river bed with vegetation ware investigated by observing the scour and deposition processes around the vegetation zone. When the flows passed through the vegetation zones, as a result, the approaching flow was retarded by the vegetation zone along the vegetation-bank side and accelerated in the main channel. The flow velocities also reduced downstream of the vegetation zones and the water depths dropped significantly in the streamwise direction. Near the vegetation zone, the size of the scour hole increased as the vegetation density increased. It was observed that the scouring depth in the case of vegetation density equal to 0.12 is 1.56 times of that in the case of vegetation density equal to 0.03.