導流板對橋墩沖刷保護之試驗研究

Abstract

天然河道因設置橋墩使水流通水面積減少，導致橋墩周圍流場產生劇烈變化，並改變原有輸砂行為，對底床造成局部沖刷，進而使橋墩基礎受到侵蝕而裸露，造成橋樑之破壞。一般橋墩保護方式如固床工、拋石與蛇籠等，皆為剛性保護工法，因此，本研究建議利用導流板以引導水流之方式，降低水流於橋墩沖刷之能力。 本研究在清水沖刷條件下，於直線形水槽進行模型佈置，採用對稱之雙片導流板置於圓柱橋墩前方，進行局部沖刷試驗。由於造成橋墩局部沖刷原因為水流沖擊橋墩後所產生之向下射流(down flow)與馬蹄形渦流(horseshoe vortex)，故減少此沖刷之效應，為試驗之主要目的。試驗中將探討不同導流板佈置方案對橋墩局部沖刷之影響，評估導流板對橋墩沖刷保護之效果。 此外，由於水流在橋墩的沖刷擾動，其流場變化瞬息萬變，若使用單點之量測法量測，對於局部沖刷之流場缺乏整體、有效的了解。因此，本研究利用非侵入式彩色質點影像測速法(color particle image velocimetry, CPIV)，進行橋墩沖刷之垂直二維流場量測，藉由質問窗(interrogation window) 設定與影像分析，以獲得流速向量與渦度場分佈。 經由試驗結果顯示，在直線形水槽中，於清水沖刷條件下進行局部沖刷試驗，當導流板(長2cm)與橋墩(墩徑2cm)間距為3倍橋墩直徑、開口大小為0.5倍橋墩直徑及攻角為15度時，為導流板於橋墩沖刷保護之最佳化佈置，對橋墩最大沖刷深度減少率可達到54.1 %。

The construction of bridge piers in natural river channels reduces the cross sectional area of flow which causes drastic variation of flow field around these piers, alters the sediment transport characteristics, and develops local scour at the riverbed. These physical phenomena in term result in erosion, which would expose the pier footings and cause damages that may threaten its structural integrity. Apart from the present methods for bridge pier protection such as the grade control structures, the ripraps, and the gabions…etc., this study proposes the use of vanes as flow deflector to reduce the action of flow on bridge pier scour. This research involves a scaled model consisted of two vanes in front of a single cylindrical bridge pier for the experimental study of local scour in a straight channel under clear water condition. Local scour at bridge piers occurs through the action of the down flow and the horseshoe vortex when water impacts the bridge piers. The reduction of this scouring phenomena is the main purpose of this study, with evaluation on the effects of various arrangements of vanes on local scour and hence the efficiency of using these vanes as bridge pier protection. The flow field changes rapidly as channel bed varies through process of scouring around the pier. Under such circumstances, traditional single-point measurement at a specific time would not be able to obtain the variation of the velocity field completely and effectively. Therefore this study adopts the non-intrusive color particle image velocimety (CPIV) to measure the vertical 2-D flow field in the channel in order to obtain the velocity and vortex field through the set-up of interrogation window and image analysis. The experimental result reveals an optimal arrangement of vanes for bridge pier protection when maximum percentage reduction of scour depth (54.1%) is reached under the following three conditions: 1. when the distance between the vanes (with length of 2 cm) and the pier (with diameter of 2cm) is 3 times of the pier diameter; 2. when the spacing between the vanes is equal to 0.5times of the pier diameter; 3. when the attack angle is 15°.