軟岩河床沖刷動床數值模式之研發與應用

Abstract

台灣河川具有坡陡流急、沖淤劇烈之特性，當河床表層具有卵礫石或砂質粒徑之沖積層受到破壞時，若河川又位於軟弱岩盤帶，常造成底部之軟岩裸露，進而出現嚴重之河床沖刷與下切現象，加速河床沖刷、影響河防安全。國內外目前對於河道沖淤之數模研究，多著重在沖積層與傳統輸砂行為之模擬，而軟岩沖刷問題有別於傳統輸砂，須有額外機制之建置以描述其沖刷行為。 本研究開發具有模擬軟岩河床沖刷功能之水平二維動床數值模式，引用水力沖蝕機制之流功門檻沖蝕率公式，將該機制整合於顯式有限解析法動床模式上，考慮河床分層、分區設定，整合懸浮載源交換機制，讓模式可自動考慮岩床之沖、淤行為。以濁水溪集集堰下游與大安溪峽谷河段為例，進行軟岩河道沖刷過程之檢定、驗證與比較分析。 在現場具有明顯深槽、高灘地地形案例之模擬中，流況劇烈複雜，模擬過程可維持數值穩定，說明本模式之相關理論架構及乾溼點處理技巧等可適用於現場案例之模擬；此外，模擬底床縱剖面、橫斷面之沖刷量與實測趨勢相同，檢定驗證成果良好。水流對河床之沖蝕能力，與流速、剪應力及流功呈某種正比關係，配合遷急點沖刷機制，驗證流功門檻沖蝕率公式可適用於模擬軟岩河川之沖刷過程。

Steep slope and severe bed change are the general characteristics of Taiwan’s rivers. These characteristics often cause the bed armour layer flushed away, make the bedrock exposed, and then increase the channel incision rate. Due to the bedrock exposed and the channel characteristics, it makes the incision problem more seriously. Most mobile-bed models of past few decades aimed at the sediment transport of alluvial channel. However, the soft bedrock channel incision processes are different with the alluvial channel. Some empirical formula and mechanisms are needed to build in the model to simulate the erosion processes. In this study, a bedrock river evolution mechanism is included in a 2-D mobile-bed model, called the explicit finite analytic model. The model can consider both incision and deposition over the bedrock, by combining a stream power type of bedrock erosion rate formula with the flow and sediment transport modules. The multiple bed layers distributed in bedrock river are also considered. In the theorem of non-equilibrium suspended sediment concentration, the velocity scale is set to adjust the overloaded or under-loaded situation over the bedrock riverbed. The downstream reach of Chi-Chi Weir of Choshui River and the Taan River are chosen as the study sites for the model calibration, validation and comparison. In the field cases, the proposed model is stable in the simulation of complex river morphology with the main channel and floodplains. For the stream power threshold bedrock erosion rate formula and knickpoint evolution mechanism, the comparison of the calculated bed changes with field data show the model is capable of predicting bedrock incision, with reasonable accuracy and reliability. The simulated bed changes, thalwegs and cross-sectional profiles agree well with the measured data. It also indicates that the velocity, shear stress and critical stream power of flow could form a proper relation of erosion threshold in bedrock rivers. This model could be served as the prediction tool for the bedrock rivers.