拆壩對河道影響評估──以大甲溪石岡壩為例

壩體移除之相關研究及案例為近幾十年來熱門之議題，跨河構造物下游易有河床沖刷之問題，構造物亦可能已逐漸喪失其功能、或趨近於使用壽命。因此，針對河道變化、棲地評估，有助於突破臺灣之現況流域管理，進而探討拆壩之可行性。本研究以大甲溪石岡壩拆壩為案例，因應不同之目的，分別應用一、二、三維之河道動床數模工具，其中一維用於模擬馬鞍壩至河口之動床變化，分析拆壩可能造成之河床影響範圍；三維數模則用於橋梁河段模擬，水流通過橋墩產生之三維流場特徵以及橋墩周圍之沖淤；而二維數模模式，模擬石岡壩上下游水平二維之流場分布，計算其棲地適合度指數。結果在河床變化、生態棲地適合度上均顯示拆除石岡壩對河川環境問題有所助益。

關鍵字

動床數模；拆壩；橋墩沖淤；大甲溪

並列摘要

Dam removal is a contemporary concern that has been discussed for decades. Dams, weirs, and river-crossing structures cause substantial local scouring in downstream areas, and some of these structures progressively lose function and approach their service life limitations. Therefore, analyzing the effect of dam removal through assessing river channel migration and ecological habitat can break through current river basin management, and the feasibility of dam removal could be explored further then. This research took Dajia River as a study site and analyzed the effect of removing Shihgang Dam. According to the demands of different proposes, 1D, 2D, or 3D hydraulic numerical models should be applied. Fundamentally, the 1D model was applied to estimate the sediment transportation from Ma'an Dam to the estuary. The result indicated the affected domain of dam removal. Consequently, the flow field and local scouring near the piers of Dongshih Bridge was simulated by 3D simulation. Finally, for analyzing the habitat suitability, the consequence of 2D simulation was used to calculating the habitat suitability index. The result of the study showed that Shihgang Dam removal has a positive influence on the environment of Dajia River Basin, both on the migration of river bed and the habitat suitability.