氣泡與水泡工法對於減緩壩體下游沖刷之探討

Abstract

防砂壩其下游以鋼筋混凝土施作之消能設施，經常遭遇沖刷問題，本研究將探討氣泡工法與水泡工法的消能效果，上述工法分別在水流中設置管線打入空氣或水泡，來削減水流動能，達到減緩水流沖擊力，解決下游沖刷問題。本研究將寒溪(番社坑溪)之寒溪壩作為模擬對象，利用臺灣大學水工試驗所之多功能渠槽進行縮尺試驗，同時以FLOW-3D進行數值模擬，評估氣泡工法與水泡工法對於減低沖刷坑深度與範圍的成效，冀望氣泡工法可減輕對生態景觀之衝擊，在確保安全無虞的條件下，同時符合生態平衡的治理之道。 透過氣泡工法之設計，壩基沖刷深度與最大沖刷深度均有下降，顯示氣泡工法確實具有消能之作用，達到減緩沖刷的目的。依據實驗結果得知空氣與水流量比為0.35時，具有最佳得減緩效果。在壩基沖刷部分，減緩程度可達56.31%，在最大沖刷深度部分，整體減緩程度可達35.48%。考量上游為含砂水流時，可透過旋轉角度達到防止土砂影響氣泡工法，故30度時視為最佳之角度，在達到減緩之作用之時，亦能達到維護設備的目的。 此外考量防砂壩設置處未必有足夠之架設打氣設施之條件，故透過調整將原先打氣轉換成打水之形式，探討在改變流體性質後，其對減緩沖刷之效益。實驗依據氣泡工法之實驗結果如最佳空氣與水流量比、最佳氣泡孔入射角度等，以與氣泡工法具有相同動能之情況下設計。水泡工法結果為壩基平均減緩效益為62.01%，而最大沖刷深度平均減緩效益為44.17%，在相同條件下，氣泡工法為壩基減緩達56.54%，而最大沖刷深度為28.27%，意旨水泡工法具有比氣泡工法更能減緩沖刷現象的能力。 依據FLOW-3D多次模式驗證後，結果彙整得知，在流場變化中Z方向上流速變化可看出，氣泡工法或是水泡工法均可減緩Z方向上流速。此外考量未必會有足夠的動力設施，故模擬重力排放的方式來取代電力的抽水，當固定水位時水泡工法之流速與理論推出之流速值相近。

In this study, the Air-Bubble and the Water-Bubble Screens are investigated to reduce the kinetic energy of the water flow and solve the downstream scouring problem of the dam. The research is divided into scaled experiments and numerical simulations. First, the Hanxi Dam is selected as the object of analysis. The scaled experiments used flume channel at Hydrotech Research Institute in NTU, and the numerical simulation used FLOW-3D software. It is expected that the proposed methods can reduce the threat of scouring, and achieve a safe and aesthetic treatment without affecting the ecological landscape. The results of the Air-Bubble Screens showed that the scouring depth near the dam and the maximum scouring depth are both decreased. According to the results, the best mitigation effect is achieved when the air to water flow ratio is equal to 0.35. The scouring depth near the dam can be reduced by 56.31%. As for the maximum scour depth, the mitigation is 35.48%. The angle of 30 degrees is considered the best angle to prevent the sand from affecting the Air-Bubble Screens when the upstream is sandy water flow. Considering the lack of conditions for setting up air pumping facilities in the field. The air-bubble is replaces by the water-bubble. To explore the benefits of changing the fluid form to reduce the scouring. The outcomes from the experiments of air bubble is used as the reference for the water-bubble. For example, the air to water flow ratio and the angle of bubble incidence angles. The result of the Water-Bubble Screens was 62.01% reduction in the scouring depth near the dam and 44.17% reduction in maximum scour depth. Under the same conditions, the Air-Bubble Screens was 56.54% in the scouring depth near the dam and 28.27% reduction in maximum scour depth. This means that the Water-Bubble Screens has the ability to mitigate the scouring phenomenon more than the Air-Bubble Screens. After the FLOW-3D model is validated, the results are aggregated to understand that, the flow velocity tends to slow down in the Z-direction in the design of Air-Bubble Screens or Water-Bubble Screens. In addition, considering that there may not be sufficient electrical facilities, gravity discharge is simulated instead of electrical pumping, when the water level is fixed, the simulated flow velocities is similar to the theoretical value.