潛板對橋墩沖刷保護之試驗研究

Abstract

由於台灣河川坡度陡峭、豪大雨集中，橋墩附近因洪流產生嚴重的局部沖刷，造成橋墩基礎承載強度下降，進而導致橋樑倒塌。目前在橋墩保護工法上設置蛇籠、固床工、拋石、消能塊等，費時又未必能達到最佳效果，因此研究新的保護橋墩工法有其必要性。 本研究在清水沖刷條件下，於橋墩上游處設置雙片之對稱潛板(潛沒於水面下)，進行局部沖刷試驗。由於橋墩局部沖刷之以向下射流（down flow）與馬蹄形渦流（horseshoe vortex）為影響橋樑基礎沖刷中兩重要機制。本試驗探討不同潛板佈置方案、橋墩間之渦度場對於底床泥沙沖刷深度之影響，評估潛板對橋墩沖刷保護之效果。並以非侵入式彩色質點影像測速法(color particle image velocimetry，CPIV)之量測方法進行橋墩局部沖刷之流場觀測研究，了解橋墩沖刷二維流場變化。並藉由攝影機(CCD)擷取流場影像，進而影像分析，以獲得流場之速度向量與渦度場分佈。當固定流量0.007cms(福祿數Fr=0.35)、均勻粒徑( mm)、橋墩直徑2cm與潛板長5cm條件下，進行橋墩局部沖刷與渦度場之試驗結果顯示出，當潛板沖擊角度約為15度、潛板前緣與墩鼻端的距離為3倍之橋墩直徑、潛板前端開口為0.5倍之橋墩直徑、潛板入水深度約為0.48倍之平均水深，減少沖刷深度達到54.6％，且在較大流量下潛板最佳化佈置也能顯示出作用。

Due to rivers with steep gradients and excessively concentrated rainfalls in Taiwan, floods generate severe local scours around bridge piers that reduce the bearing strength of pier foundations; more seriously they will result in the collapse of bridges. Present engineering methods of constructing gabions, groundsill works, ripraps, and armor units for pier protection are taking lots of time and uncertainly reaching its best effectiveness; therefore, it is necessary to develop the new modes of engineering methods for bridge pier protection. This research is to study on the local scour under the condition of clear water scouring with a pair of symmetrical submerged vanes upstream of bridge pier. Local scour at bridge piers occurs mainly because of the down flow and horseshoe vortex, the two major mechanisms. This experiment is conducted to study different layouts of submerged vanes and vortex fields between the vanes and the bridge piers that both affect the scour depth on the river bed, and estimate the protective effectiveness of submerged vanes against the local scour around bridge pier; moreover, it studies on the field observation of local scours at bridge piers with the method of non-intrusive color particle image velocimetry to understand the variation of 2-D flow field of local scours at bridge piers. In addition, the field images captured by charge-coupled devices will be analyzed to obtain the velocity vector of the field and the layouts of vortex field. When the constant flow is 0.007cms (Froude number, Fr = 0.35), same bed material ( mm), the diameter of the pier is 2cm, and the length of submerged vane is 5cm, the result of this experiment testing on local scours at bridge piers and vortex fields by estimating the river bed and conducting field experiments shows when the angle of attack of the submerged vane is round 15°, the distance between the submerged vane and pier is three times length of the pier diameter, the front opening of submerged vane is 0.5 times length of the pier diameter, and the submerged water is around 0.48 times than the average depth of water; reduce the scouring depth to 54.6%, further, under the condition of greater scouring submerged vanes with the most effective layouts still function well.