岩質河床上固床工所受之荷載與穩定性分析

Abstract

儘管河道中之固床工之基礎往往植入岩盤，然在颱洪下不少案例中之固床工卻仍發生連鎖性之破壞，其破壞機制值得深入探索。河道中之固床工破壞與其在強烈水流下所受之作用力有密切關係，該等作用力卻不易掌握。本研究嘗試利用可處理多重物理問題之數值模擬分析軟體COMSOL Multiphysics(以下稱COMSOL)中的計算流體力學(Computational Fluid Dynamics, CFD)模組，經二相流體積分率法(包含等位函數法)模擬明渠水流之行為，以期瞭解作用於岩質河床上固床工之荷載。本研究調查現地狀況，收集案例水文資料，以紊流流場之k-ε控制方程式模擬分析建構在河道上之固床工受水流作用下造成之荷載。進而基於此荷載分析固床工結構之上舉(Uplift)、滑移(Sliding)及傾覆(Overturning)穩定性，並以流速分布概況檢核結構間縫回填之卵礫石是否遭揚起導致整體固床工不穩定現象，若以上穩定性之檢核結果皆顯示穩定狀態，代表該流況下固床工不至於因水流直接作用力導致破壞，則嘗試進一步探索固床工之其他破壞機制。 本研究選擇之固床工案例為八掌溪仁義潭攔河堰下游第三號固床工，該固床工已破壞，然而依相同設計圖之第四至第六號固床工並未發生類似跡象之破壞，其原因值得探索，故選定此案例為模擬分析對象。由模擬結果確認不論在常水期流量或洪峰流量下，完整固床工皆呈現穩定狀態，繼而探討八掌溪三號固床工之真正破壞機制，判斷其破壞機制為固床工下游端岩盤受水流沖蝕後高程逐漸下降，而在岩盤高程下降至一定程度後，水流直接作用力開始使固床工下游端進入不穩定狀態，最終導致破壞。在發現固床工破壞模式中佔大多數之下游端河床下切後，本研究接著嘗試分析固床工之消能效果，探討階梯式固床工單階消能效果與整體消能效果，配合桶頭堰之案例分析，期能提供往後固床工設計之參考。

A ground-sill structure on riverbed may fail in a large flood during torrent or typhoon in spite the structure may have a foundation onto the rock bed. Its true failure mechanism deserves attention. The failure of a ground-sill structure is closely related to the forces exerted from the strong water current (water loads). However, the water loads are not easy to know. This thesis made use of the software COMSOL Multiphysics and applied computational fluid mechanics (CFD) to model the open channel problem for river reach with ground-sill works. The simulations utilized turbulent k-ε model and two-phase flow by using level-set method with volume fraction to solve a free surface problem of open channel. The water loads on the boundary of the ground-sill structure were calculated to evaluate the loads acting on the structure. The acquired water loads were then used to evaluate the stability (the factors of safety) for various failure modes including uplift, sliding and overturning of the structure. Also, the flow velocity on the boundary was used to examine the stability of riprap. The study chose a series of similar ground-sill works on the Ba-Chang River as an example to investigate the cause of its failure. The #3 ground-sill structure has already failed, while the #4~6 structures in this series remain intact. The case study aimed to investigate the failire reason of the #3 structure. From the numerical simulation, it appears that the #3 ground-sill works would not fail because of the structure instability solely due to the water loads. Intense scouring under the foundation toe is likely the real cause to result in the failure of the ground-sill structure. Furthermore, the study evaluated the energy dissipation of the ground-sill structures using the results of numerical simulation.