Numerical Simulation on Migration Behavior of Meadering Channel
|關鍵字:||垂直水平分離;擬似三維;彎道水流;正交曲線座標系統;σ座標系統;隱式雙階分割操作趨近法;二次流;vertical-horizontal splitting;semi-3D;bend flow;orthogonal curvilinear coordinate system;sigma coordinate system;implicit two-step split-operator approach;secondary current|
驗所(2009)整合水深平均二維動床模式(Hsieh and Yang, 2003)與岸壁崩塌模式(Chiang
本研究擬參考Lin and Huang (2008)之水平垂直分割概念，進行擬似三維模式建構，
Generally, the spatial dimensions of main channel migrations can be categorized as vertical bed evolution and lateral bank variation. The vertical bed evolution refers to bed deposition or erosion which might be caused by the expansion or contraction of flowing area along the longitudinal direction. The lateral bank variation refers to bank erosion and sliding which are generally caused by the secondary current and the decrease of soil shear strength, respectively. Practically, the vertical bed evolution and lateral bank variation might occur simultaneously and interact; however, most hydraulic models only simulate the vertical bed evolution with the assumption of fixed channel width. To incorporate the effect of lateral bank variation, WRP (2009) integrated the depth-averaged 2D hydraulic model and bank stability model which enables the determination of bank stability under the variations of channel bed and water surface. In WRP (2009), the vertical velocity profile proposed by de Vriend (1977) was adopted to simulate the effect of secondary current. However, the applicability of vertical velocity profile might be limited in highly meandering channels rendering potential failure to obtain the realistic flow pattern and channel migration. In order to simulate the 3D flow pattern in the estuary and maintain the computing efficiency, Lin and Huang (2008) proposed a computation framework in which the 2D horizontal and 1D vertical flows were separately computed. In this study, a quasi 3D hydraulic model will be developed based on Lin and Huang (2008). To improve the applicability of developed model in complex channel geometries, the unstructured grid and vertical σ-coordinate will be adopted. Furthermore, the developed hydraulic model will incorporate the sediment transport mechanism and state function for density variation, and integrated with a bank stability model. Therefore, the developed hydraulic model can simulate the vertical bed evolution and lateral bank variation simultaneously in highly meandering channels. After the verification of developed hydraulic model through the comparisons between the simulation results and experimental data proposed by former researches, the influences of secondary current and bank erosion/sliding on channel migration will be intensively analyzed which may be serve as the basis for future river management planning.