氣泡工法應用於減緩沖刷之研究

Abstract

臺灣位處之地理位置位於板塊交錯之位置，臺灣河川多坡度陡、流量變化大，因此為了穩定溪床，降低災害發生，於上游集水區多有構築水工構造物應用於野溪治理中。然因水工構造物上下游高度常相差過大產生之沖擊力，易使下游河床形成沖刷坑，而沖刷坑之持續增大可能導致基礎沖刷以及護坦破裂受損，進一步導致水工構造物破壞並喪失其原有功能及經濟價值，故如何有效的防止或減緩水工構造物下游沖刷為一臺灣河川治理之重要問題，而目前沖刷坑臺灣常見之減緩沖刷方式多為硬性工法，然臺灣近年由於生態環境之考量，多提倡以柔性工法取代硬性工法，而近年國外新興的氣泡工法為一柔性工法，已在國外有初步使用並於多方面有相關應用的例子，相較於傳統常見之硬性工程，其有較易去除以及能根據現地情形進行調整等特性，能提供一未來能應用於相關水工構造物之參考方法。 本研究將氣泡工法應用在水工構造物的減緩沖刷研究中，以宜蘭縣大同鄉寒溪村番社坑集水區防砂壩為研究範例，應用Flow-3D三維模式模擬分析多管排出氣泡改變水流流場，並減緩防砂壩下游處沖刷之情形，透過改變不同之影響沖刷之參數，包括流量、壩高、管仰角、管高度、氣泡流量等，並透過對於結果進行迴歸分析以及最佳化推演出沖刷深度最小之設計，研究結果顯示氣泡工法能確實改變水流型態，進一步達到減緩沖刷的目的，且透過PSO最佳化後結果顯示對於現地防砂壩，較佳之氣泡管設置為角度45度左右，高度為0.7~1公尺高，管徑為30公分，氣泡流速為6~7(m/s)左右，與原始沖刷深度相比其可減緩沖刷深度平均達1.164m，減緩程度達32.12%，能有效減緩沖刷深度。期許未來能提供防砂壩構築者一經氣泡工法減緩後之沖刷深度參考，以利減少下游之沖刷情形，能實際應用於現地，對於未來水工構造物之減緩沖刷工法有所貢獻。

Taiwan is located at the geographic boundaries of two plates, resulting in short rivers with steep slopes and unstable flow changes. In order to stabilize the riverbed and reduce the occurrence of disasters, many hydraulic structures are constructed in the upstream watershed for torrent management. However, the impact force caused by the altitude difference between the upper and lower reaches of the hydraulic structure is likely to cause the scouring pits at downstream riverbed and the continuous increase of the scouring pit may result in the foundation scour and the rupture and damage of the apron, and further lead to the damage of the hydraulic structure, which may also loses its original function and economic value. Therefore, effectively preventing or slowing down the erosion of hydraulic structures downstream is an important issue in the management of Taiwan’s rivers. At present, most of the common methods of reducing scouring in Taiwan are hard construction methods. Due to ecological considerations, flexible construction methods are often advocated. In recent years, the emerging air injection method is one of the flexible construction methods. It has been initially used abroad and has applied in many aspects. Compared with the traditional hard engineering method, air injection method is relatively easy to remove and can be adjusted according to the current situation, which can provide a valuable method that can be applied to related hydraulic structures in the future. In this study, the air injection method is applied to reduce the buffer brush of hydraulic structures. Taking the sand control dam in the catchment area of Hanxi Village, Datong Township, Yilan County as a research example, the Flow-3D three-dimensional model is used to simulate and analyze the multi-tube discharge air to change water flow field, and slow down the erosion of the downstream of the sand control dam, by changing different parameters that affect the erosion, including flow, dam height, tube elevation, tube height, air flow, etc., and through regression analysis and optimization to find the best design with the smallest scouring depth. The research results show that the air injection method can indeed change the water flow pattern and f reduce the scour. Based on the PSO solution, the results show that for the on-site sand control dam, the best air tube degree setting is 45 degree, the height is 0.7 to 1 meter high, the pipe diameter is 30 cm, and the air velocity is about 6 to 7 (m/s). Compared with the original scouring depth, the scour depth can be reduced by an average of 1.164m, and the degree of declining is 32.12%. The proposed method is able to effectively reduce the depth of the scour. It is hoped that in the future, it will be able to provide a baseline for the scour depth of the check dam builder, so as to reduce the downstream scour situation, which can be practically applied on-site and contribute to the future reduction of the scour construction method of hydraulic structures.