台灣山區河道受颱風豪雨與極端降雨影響，常有大量土砂進入河道並向下游輸送。防砂壩雖具有攔阻土砂、穩定河床及調節輸砂之功能，但在長期受到洪水、高含砂水流與土砂撞擊作用下，壩體可能因局部沖刷或結構受損而形成破壞開口。當開口形成後，上游蓄積之水量與土砂將經由開口向下游釋放，進而改變壩址附近之水流集中、泥砂搬運與河床沖淤分布。本研究採用二維動床數值模式，探討防砂壩沖刷破壞後，不同束縮比、寬深比、底床粒徑及入流流量對水理特性、輸砂行為與河床變動之影響。
模擬結果顯示，破壞開口附近為水流集中、泥砂啟動與河床沖淤變化最明顯之區域。束縮比降低時，開口通流能力提高，水流與土砂較容易向下游輸送，並使上游沖刷範圍增加；寬深比改變則主要影響水流集中與下游堆積型態；底床粒徑增加後，泥砂輸送較不連續，且局部沖刷與分段式淤積較明顯；流量增加則會提高水流動能與底床擾動能力，使沖刷深度、下游堆積範圍及河床調整時間增加。整體而言，破壞開口幾何條件會控制水流通過型態與沖淤分布，而底床粒徑與流量則進一步影響泥砂搬運距離、沉積型態及河床趨於穩定所需時間。本研究成果可作為後續防砂構造物受損後河床變動評估與河道治理規劃之參考。

Taiwan’s mountainous rivers are strongly affected by typhoons and extreme rainfall events, which often deliver large amounts of sediment into river channels and transport it downstream. Check dams are commonly constructed to trap sediment, stabilize riverbeds, and regulate sediment transport. However, under long-term impacts of floods, sediment-laden flows, and sediment collision, the dam body may be damaged by local scour or structural deterioration, forming a scour failure opening. Once an opening is formed, the water and sediment accumulated upstream are released downstream through the opening, thereby changing the flow concentration, sediment transport, and riverbed scour and deposition patterns near the dam site.
In this study, a two-dimensional movable-bed numerical model was used to investigate the effects of different contraction ratios, width-depth ratios, bed-material grain sizes, and inflow discharges on hydraulic characteristics, sediment transport behavior, and riverbed morphological changes after check dam scour failure. The results show that the area near the failure opening is the most significant zone of flow concentration, sediment mobilization, and riverbed change. When the contraction ratio decreases, the flow capacity of the opening increases, allowing water and sediment to be transported downstream more easily and increasing the upstream scour range. Changes in the width-depth ratio mainly affect flow concentration and downstream deposition patterns. As the bed-material grain size increases, sediment transport becomes less continuous, and local scour and segmented deposition become more evident. Increasing the inflow discharge enhances the flow energy and bed disturbance capacity, resulting in increases in scour depth, downstream deposition range, and riverbed adjustment time.
Overall, the geometric conditions of the failure opening control the flow passage pattern and scour-deposition distribution, while bed-material grain size and inflow discharge further influence sediment transport distance, deposition pattern, and the time required for the riverbed to approach stability. The results of this study can serve as a reference for future assessments of riverbed changes after damage to check dam structures and for river channel management planning.

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