蜿蜒河道沖淤及偏移行為之模擬分析

Abstract

河川深槽沖刷偏移行為在空間變化上可分為垂向底床沖淤與側向之河岸沖刷、崩塌 及淤積。就垂向底床沖淤而言，河道斷面窄縮或因結構物之設置，皆可能導致河床沖刷 下降，目前已有許多輸砂模式可於固定渠寬下進行模擬分析；就側向深槽偏移而言，影 響側方向深槽變化主要可分為兩種形式，其一為彎道二次流效應所引起的側向沖刷行 為，其二為河岸土體因抗剪強度降低或土體下滑力增加所引起之崩塌現象。水利規劃試 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 et.al., 2010)，考量深槽水位升降與底床沖淤對於岸壁穩定性之影響，其中對於彎道二次 流效應以de Vriend(1977)流速假設剖面反映，然而彎道二次流效應於本質上乃屬三維現 象，在蜿蜒度較大之河道中其流速假設剖面已然不適用，無法合理展示流場分布與底床 沖淤行為，因此三維模式之應用有其必要。 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， 將其概念推廣至正交曲線座標系統，並於垂直方向導入σ-座標，以適應不規則地形；並 進一步建立沉滓濃度傳輸以及交換機制，另一方面，考量河道中水位升降與沖淤行為植 入河岸崩塌機制，使模式可反映深槽側方向之變化，並嘗試模擬渠道中流場分布、底床 幾何形狀改變，且與相關試驗結果做比較，期能對河川深槽之偏移行為有所了解，提供 相關單位河川管理之參考。

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.