軟岩河床沖刷行為模式之研究

Abstract

台灣河川豐、枯水期河川流量差異甚大，枯水期時河川裸露之高灘地，常有民眾植栽，於颱洪時期，坡陡流急、洪水歷程急促，致河道侵蝕嚴重，使下游護甲層流失，河床大都為軟弱岩層出露。然目前數值模式發展多侷限在砂質底床沖淤部分，對於軟岩河床沖蝕模式之研究較為缺乏。另一方面，由於現場量測資料時間間距長，自然因素與人為因素影響難以釐清，且綜觀國內、外相關水槽試驗甚少，因此可供應用模式有限。在軟岩河床沖蝕中，目前實務上在評估沖刷潛勢中包括考慮水力沖刷及因為床質顆粒跳動所造成的泥砂磨蝕效應，因此本研究將以水槽模型試驗探討水力沖刷及泥砂磨蝕效應對於軟岩河床沖蝕深度的影響，將根據水槽試驗結果探討軟岩性質、懸浮載泥砂粒徑及泥砂濃度對於軟岩沖刷率的影響，也藉由清水及濁水入流試驗，可評估水力沖刷與泥砂磨蝕對於軟岩沖刷的影響程度。而後將由水槽實驗所得出的迴歸公式植入擬似三維動床水理模式中，應用於現地實際案例的計算上，並比較不同岩床沖刷經驗式與過去歷史量測資料之差異，期能對颱洪期間河道深槽沖刷對於河道穩定與河防安全的影響有更進一步之探討及瞭解，而在未來能應用於河川沖蝕規模之評估，提供治理規劃之參考運用。

In Taiwan, due to high flow rate, unexpected elevation changes, and inappropriate vegetation and development in floodplains, channel stability is always a devastating factor that leads to damages of main river channels and riverbanks. The sandy layer overlying riverbeds is usually disappeared after typhoon events, and soft rocks are exposed over riverbeds. Nowadays, most of sediment transport studies focus on channels with sand, silt or clay materials, and only few studies can be applied to the soft rock riverbeds. In this study, laboratory experiments and numerical models are developed to study erosion behavior of soft rock riverbed. The laboratory experiments mainly concentrate on the effects of hydraulic erosion and particle abrasion on soft rock scouring. For hydraulic erosion, experiments will be conducted in clear water and regression relationships between properties of soft-rock and hydraulic bottom shear stress will be established. In terms of particle abrasion, tests under different particle sizes and concentration will be carried out to find their correlations with riverbed erosion rates. The experimental results will be incorporated into the quasi-3D movable-bed numerical model to study sediment transport in practical cases. Comparisons of simulation results and the field data from several typhoon events will be presented. The outcome from this study is expected to use as guidelines for hydraulic engineers in the designing and planning and is valuable as a reference to future improvement projects