不同曲率半徑拱型固床工之沖擊力探討

在國內，根據行政院水土保持局發行之水土保持手冊(2017)，固床工定義為以保護溪床免於被洪水沖刷下切為目的所構築有效高度在 1.0公尺以下之橫向阻水構造物；而近年來，由於生態考量、美觀、創新等因素，多數固床工設計開始嘗試不同形式之創新，拱型固床工便是其中一個例子。在現今工程設計，直線型固床工因為不同規劃設計原因而常有基礎不穩固之問題，在未解決這些肇因之前，拱型固床工被破壞之可能性會更大。因此，本研究對固床工上游面之受力情況進行了多項分析與討論，包含固床工高度對安全係數之關係、水深高度與安全係數之關係以及水流與安全係數之關係等進而討論出固床工在河床中能安定、穩定之條件需求，使其不會造成滑動與傾倒。再配合水工模型試驗針對拱型固床工進行多項曲率半徑之影響探討，運用小型拉壓兩用荷重計進行受力值之量測，並由受力理論值與實測值之間求得損失修正係數k，期許能達到合理設計之目的，提供未來工程在設計非直線型固床工時作一參考依據。成果說明如下: 1. 直線型固床工造成之下游沖刷分布較均勻；上拱型固床工水流集中於渠道中心，因而導致下游渠道中心沖刷深度會較直線型固床工來得深；下拱型固床工之下游沖刷可以發現兩側沖刷深度較深，但靠近渠道中心之位置並無明顯之趨勢。 2. 上拱型固床工拱度越大時，固床工下游渠道中心處之沖刷深度較深。其表示當上拱型固床工拱度愈大，其水流的能量越趨集中於渠道中心。但是這個現象惟有於小流量時較顯著。關於下拱型固床工部分，曲率半徑之不同，和其沖刷深度並無相關及顯著之關係。流量因子對於沖刷坑之影響會較曲率半徑因子的影響較顯著。 3. 上拱型固床工其三點壓力分佈可看出大多中心點壓力較兩側大，顯示其上拱型固床工上游面受水流作用力分布中心點較大，能使水流集中於中心，與下游的沖刷坑分布一致。下拱型固床工之渠道兩側壓力較渠道中心大，顯示其下拱型固床工上游面受水流作用力分佈渠道兩側較大，故水流多分散，可減少河床中心之沖刷。 4.由實驗結果可知損失修正係數k值會隨著流量不同而改變，當流量增加時，損失修正係數k會隨之下降，接著呈現一穩定值；當雷諾數增加至其呈紊流狀態時，損失修正係數k趨近於1.0，可得知其損失值為速度頭的一倍。 5. 由損失修正係數圖進行迴歸，得出上下拱不同曲率半徑時之迴歸公式，以便求得設計時應考慮之損失修正係數k，供後續設計時參考，x為曲率半徑(公尺)，y為單寬流量(cms/s)，其公式如下: 上拱型固床工之損失修正係數公式: 𝑘=1.461−11.55xy+6.946𝑒^(−(190.8𝑦)^2) 下拱型固床工之損失修正係數公式: 𝑘=1.679−17.9xy+8.604𝑒^(−(222.7𝑦)^2) 6. 大多數固床工之設計都應斟酌實施安定分析並於固床工完工時於下游端進行被填土之作業，以確保固床工之穩定性。

關鍵字

拱型固床工；水工渠槽試驗；曲率半徑；損失修正係數 k

並列摘要

Groundsill refers to a horizontal structure in a river bed. A series of groundsills are often formed by continuous facilities to expand the protection section of the river. In addition, in recent years, many projects using innovative groundsills— including upper and lower arched groundsills—have been undertaken to maximize the benefits. Therefore, in this study, several analyses and discussions are conducted on the force on the upstream surface of the groundsills and the scouring mechanism through a flume experiment. Furthermore, the influence of multiple curvature radii on arched groundsills is discussed. The correction coefficient of loss is obtained from the theoretical value of the impact force and the measured value. This coefficient is expected to yield a reasonable design as well as provide a reference for the design of non-linear groundsills in the future. The results of this study show that the water flow is concentrated in the center of the channel by the upper arched groundsills, which results in the scouring depth of the center of the channel being deeper than that at the other sides. Regarding the downstream scouring of the lower arched groundsills, the scouring depth on both sides of the channel increases; however no obvious trend near the center can be observed. For the factor of the radius of curvature, it can be concluded from the experimental results that the influence of the discharge factor on the scouring hole is greater than the factor of the radius of curvature, and its influence is more significant. For the impact force, when the discharge increases, the pressure becomes larger. The force on the upstream surface of the groundsills is proportional to its water depth; thus, it shows an upward trend. However, the pressure distribution at the right and left sides of the channel and the center point shows that most of the center points have higher values than the other two sides, indicating that the upper arched groundsills can concentrate the flow of water in the center. This is consistent with the distribution of scouring holes downstream. The impact force of the lower arched groundsills on both sides of the channel is lower than that of the center, indicating that the impact force acting on both sides of the channel is larger. Therefore, the flow of water is scattered on both sides, thus reducing scouring in the center of the river bed. In addition, the design of most groundsills should consider the implementation of stability analysis and carry out the filling operation at the downstream when the fixed bed work is completed to ensure the stability of the groundsills. To achieve a reasonable design, the research obtains a correction coefficient of loss from the theoretical value of the impact force and the measured value. Based on the figure of the correction coefficient, the regression formula can be obtained for reference in subsequent design. The regression formula is as follows, x is the radius of curvature(m), y is the discharge(cms/m), k represents the correction coefficient of loss. The formula of the upper arched one: 𝑘=1.461−11.55xy+6.946𝑒^(−(190.8𝑦)^2) The formula of the lower arched one: 𝑘=1.679−17.9xy+8.604𝑒^(−(222.7𝑦)^2)