河岸退縮數值計算模式之發展與應用

Abstract

本研究之目的旨在發展並應用河岸退縮預測數值計算模式，模式中採用未飽和地下水流理論，利用數值計算獲致河岸土體內部孔隙水壓隨時間變化之分布，藉以建構河岸邊坡塊體破壞機制。此方法不但可以改進以往過去相關研究須仰賴地下水面線且須以靜水壓分布假設始能求得沿破壞面上之孔隙水壓，並可利用邊界條件的處理技巧將河川水位變動以及降雨因素之影響一併納入計算。此外，更進一步整合懸臂型破壞與水流沖蝕機制，完整地考量河岸退縮的主要關鍵因素，據以探究其互制行為與演變過程。 本研究首先以所建立的模式分別針對塊體破壞、懸臂型破壞與水流沖蝕三種破壞機制進行探討，利用案例測試結果歸納出河川水位升降決定其所提供之靜水壓力，以及土壤滲透性與降雨特性交互作用下造成土體孔隙水壓變化之差異，對塊體破壞以及懸臂型破壞二種機制均具決定性之影響，而水流沖蝕則主要與水力條件(底床坡降、河川水位、水位歷線型態)以及土壤抗沖蝕能力(臨界剪應力、沖蝕係數)相關。接續整合三種破壞機制，並由模擬結果得知河岸退縮是塊體破壞、懸臂型破壞與水流沖蝕三種破壞機制反覆循環發生的過程，而在水流沖蝕較大的條件下，河岸退縮主要受水流沖蝕的程度與範圍所影響。最後於濁水溪河段之案例應用結果顯示，此整合模式可提出不同機制下的泥砂產量，並且對於河岸退縮長度可獲一定程度之預測。

In this study, a numerical model of riverbank retreat has been developed and applied to a practical case. The governing equation of unsaturated groundwater flow is solved by implementing numerical method to obtain the transient distribution of the pore water pressure to evaluate riverbank stability with respect to mass failure. However, previous studies to compute the pore water pressure were usually based on groundwater table with hydrostatic pressure distribution hypothesis. The approach proposed in this study not only improves this shortcoming but also takes the effects of river stage variations and rainfall into account by defining boundary conditions. In addition, cantilever failure and fluvial erosion are incorporated into the model in order to further understand the interaction and the process of riverbank retreat. First, mass failure, cantilever failure and fluvial erosion are respectively investigated by a series of hypothetical scenarios. The simulated results indicate that the occurrence of mass failure and cantilever failure mainly depend on the fluctuations in pore water pressure determined by river stage variations, soil permeability and rainfall condition. Fluvial erosion is determined by hydraulic conditions (i.e. channel slope, river stage and stage hydrograph) and soil erodibility (i.e. critical shear stress and erodibility coefficient). Subsequently, mass failure, cantilever failure and fluvial erosion are combined to estimate riverbank retreat. According to the conclusions of analyses, riverbank retreat is the process of repeated failure events and is primarily influenced by the magnitude and range of fluvial erosion with remarkable fluvial erosion. Finally, the results of the study reach of Jhuoshuei River reveal that the proposed model is capable of quantifying sediment yield and well predicting riverbank retreat length.