暴雨誘發天然壩之重建數值模擬-太麻里溪堰塞湖為例

Abstract

當邊坡土石材料受地震或降雨的作用而發生破壞，崩坍土石沿坡面運移而下可能堵塞河道而形成天然壩，造成上游產生迴水現象則形成堰塞湖。透過堰塞湖案例的統計，壩體通常在形成後短時間內潰決，然壩體相關資訊往往不易事後獲得，導致在天然壩體的發生潛勢與潰決潛勢分析上常缺乏所需參數，因此天然壩體的模擬重建有其重要性。本研究嘗試以PFC3D軟體模擬邊坡崩滑及運移至河道堆積的過程，以往學者對於由降雨誘發的地滑現象常透過降低摩擦係數的方式以啟動土體下滑，然而若過度降低相關阻抗參數易對真實的崩塌行為造成誤判，本研究為解決此問題提出以滲流力施加於邊坡地盤材料以啟動滑動體運動的方式來模擬降雨誘發的邊坡崩滑現象；模擬結果展示，以滲流力驅動邊坡崩滑的方式為簡易可行之方式。此外，為評估整體土砂變遷的過程又以階段式的沖刷模擬嘗試重建天然壩體變遷過程；本研究透過河道沖刷的階段式模擬重建，認為太麻里溪包盛社堰塞湖天然壩體短時間內的變遷過程可分為崩塌初期、刷深階段、刷深/拓寬階段而至最終的堆積型態(2009/8/28)，又透過天然壩的穩定性分析發現：羅吉斯迴歸分析相較於DBI與鑑別分析而言較為保守也較能解釋各沖刷階段的天然壩穩定性的評估。

Landslides induced by heavy rainfall or earthquake sometimes result in the blockage of river flow to form barrier lakes. Relevant data are important for understanding the formation and stability of landslide dams; the inventory of the fundamental landslide-dam data is hence essential. According to the statistics of recorded cases, landslide dams often breach in a short time. As a consequence, it is often difficult to collect comprehensive information of a landslide dam with a short life after dam breach. It is, however, possible for the reconstruction of a breached landslide dam through various methods. This study made use of the particle-flow simulation code PFC3D to model the processes of the formation and evolution of landslide dams induced by heavy rainfalls. In the past, the common approach to trigger a landslide’s initialization by the particle-flow simulation is to largely decrease the inter-particle frictional coefficient. However, this approach may lead to incorrect interpretation of the run-out process of the landslide mass and the dam formation. Different from the common approach, the present work proposed a new method by adding a seepage-force term on all particles within the sliding mass under the ground water table. Because of the action of the applied seepage force, the landslide mass can lose its stability and starts to move. The simulated results demonstrated that this approach for applying seepage forces is a simple and feasible approach to model the trigger of a landslide owing to heavy rainfall. The proposed approach was applied to a real case of landslide dam with a short life (The 2009-August landslide dam in the Tai-Ma-Li River). A couple of erosion stages due to overtopping were further modeled to simulate and to interpret the evolution of the landslide dam.