垂直跌水之沖刷探討

Abstract

水流通過防砂壩或固床工後形成跌水，流動過程中高位能轉換成動能衝擊下游溪床形成沖刷坑，經常導致壩趾基礎淘空或是護坦受損，進而危害構造物安全。本研究藉水槽試驗探討自由射流下沖刷坑之沖刷機制與型態，並提出沖刷坑特徵之預測模式。 本研究水槽試驗之範圍為單寬流量0.00325~0.0177cms/m，跌水高0.02~0.15 m，均質粒徑1.5~7.5mm，沖刷時間2400~14400s，在清水流下經試驗數據分析與FLOW-3D模擬，得結論如下： 1.沖刷坑深度與跌水高、單寬流量成正相關，而與粒徑大小成負相關，且隨時間成冪次方遞增直到達平衡狀態為止，而沖刷坑長度約為最大沖刷坑深度之3倍(L2/hs=3)。 2.沖刷坑深度與有效跌水高比隨時間增加而趨於穩定，最終達一臨界值，其最大沖刷坑深度將不超過有效跌水高之0.7倍。 3.沖刷坑深度與壩基掏刷深度呈線性遞增，壩基掏刷深度約為最大沖刷坑深度之0.45至0.85倍間。 4.消能效率隨著相對臨界水深之降低而增加，當臨界水深與壩高之比在0.4以下，其消能效率皆可達50%以上。 5.經過水槽試驗，可得到沖刷坑深度隨時間變化模式： h_st/H=0.9560(q^2/(gH^3 ))^0.4130 (d/H)^(-0.4448) (t_t/240)^0.0345 hst為沖刷延時t時之最大沖刷坑深度(m)，H為跌水高度(m)，q為單寬流量(cms/m)，g為重力加速度(m/s2)，d為粒徑大小(m)，tt為沖刷延時(min)。 6.透過FLOW-3D數值模擬垂直跌水之沖刷，以沖刷坑深度變化率為0.003cm/s來判斷是否達平衡狀態，調整流量係數(C=0.885)後其最大沖刷深度與水槽試驗有相當之吻合度。 7.藉實測資料與FLOW-3D模擬比對，模擬之沖刷坑深度將些微高估於實際情形，未來應用於預估沖刷坑規模時須注意。

The potential energy converted into kinetic energy by fall at check dam or ground sill always attacks the creek bed and forms a scour hole. This phenomenon results in taking out the base of hydraulic structure or damaging the apron, and even affecting the safety of the structure. This study uses flume experiments to investigate the free jet scours and does hydraulic mechanical mechanism and behavior analysis of vertical drop. The prediction models are developed and being used to predict the scour below high vertical drops. The per unit width of discharge is between 0.00325~0.0177cms/m, the height of drop is between 0.02~0.15 m, the particle size is between 1.5~7.5mm, and the test duration time is between 2400~14400s in this flume experiment. After analyzing available data and the simulation by FLOW-3D, the principle results were as follows: 1.The depth of scour is positively correlated with the height of drop, the per unit width of discharge, and is negatively correlated with the particle size. The scour depth increases with time in an exponential rate until reaching to the equilibrium, and the length of scour is 3 times than the depth of scour(L2/hs=3). 2.The ratio of the scour depth to effective drop height increases with time and eventually reaches a critical value, and the maximum depths of scour will not exceed 0.7 times the effective drop height. 3.The depth of scour at the base of hydraulic structure is about 0.45 to 0.85 times to the maximum depth of scour and linearly increasing with it. 4.The energy dissipation efficiency increases with the relative critical water depth. The energy dissipation efficiency can reach more than 50% . while the critical water depth is equal to or less than 0.4. 5.According to the result of experiment and previous studies, we propose the module of the depth scour hole with variations of time: h_st/H=0.9560(q^2/(gH^3 ))^0.4130 (d/H)^(-0.4448) (t_t/240)^0.0345 where hst = the maximum depth of scour at time t(m), H = drop height(m), q = per unit width of discharge(cms/m), g = gravitational constant(m/s2), d = particle size(m), tt = specified time(min). 6.By using FLOW-3D software to simulate the dimension of scour holes below a vertical drop, the equilibrium of scour depths is determined by the scour depth change rate of 0.003cm/s. The maximum depth of scour is quite consistent with the result of flume experiments after adjusting the discharge coefficient(C=0.885). 7.According to the measured data and FLOW-3D simulation, the simulation of the scour depth is deeper than the field result. It should be noted in the applications of predicting scour scales in future.