蜿蜒河道側向沖刷偏移實驗與模擬分析

Abstract

本研究分別利用渠槽實驗與數值模式探討蜿蜒河道側向沖刷偏移行為。渠槽實驗方面，考慮平面型與懸臂型兩種凝聚性岸壁常見之破壞型態，分別透過河道水位洩降、岸壁沖刷兩種常見引發平面型與懸臂型破壞之成因進行渠槽實驗；在數值模式發展方面，基於極限平衡分析理論，通盤考量破壞面上所有作用力與張力裂縫因素，藉由求解Boussinesq 方程式計算時變地下水位，以考量河道水位與地下水位互制對岸壁穩定性之影響，並整合平面型破壞、懸臂型破壞等破壞型態，發展岸壁穩定分析模式；此外，為使三維動床模式更可反映彎道底床沖淤之分布，本研究進一步利用三維動床模式(RESED3D)之近底床流速取代水深平均流速，更正確地解析底床載運移角度，同時將其納入岸壁穩定分析模式，建立河道沖刷偏移模式。 接著，利用渠槽實驗分別針對河道沖刷偏移模式進行功能測試，測試方向可分為彎道底床沖淤、平面型破壞與懸臂型破壞等三部分，藉此驗證模式發展之合理性；此外，將河道沖刷偏移模式應用至現場進行預測模擬時，本研究發現張力裂縫之影響不可忽略，且經由分析提出破壞面角度修正公式，最後利用現地應用案例確認其合理性。

This study investigated the lateral scouring migration behavior of meandering channels through laboratory experiment and numerical modeling. In the experiment, two common types of failure on cohesive banks were considered, which include pla-nar failure and cantilever failure. The bank stability tests were carried out through river stage drawdown process and bank erosion respectively. In the numerical simu-lation, based on the limit equilibrium theory, a riverbank stability model was devel-oped by considering all the forces acting on the failure surface, along with the pres-ence of tension crack. The Boussinesq equation was used in the numerical model to calculate the time-varying groundwater level in the river bank in response to river stage drawdown. The interactions of the river water level and the groundwater level were examined to investigate their influences on the river bank stability. In addition, this study improved a 3D mobile bed model (RESED3D)(Chung 2012) by using near-bed velocity to replace depth-averaged velocity for better determination of the moving angle of bed-load materials. The test results collected in the laboratory experiments (including bed elevation variation in curved channels, planar failure and cantilevered failure) were used to verify the validity of numerical lateral scouring migration model. In addition, based on experiment test results this study modified the empirical formulas for determining tension crack location and bank failure angle in river bank stability analysis. The modified formulas were used in the numerical simulation model and their validity was confirmed with independent field observations.