近年來，台灣受到多次颱風與洪災的侵襲，造成河床嚴重局部沖刷，導致橋基裸露、基礎埋入深度不足。若無法有效維護橋梁安全，將會嚴重影響到民生經濟及生命安全。局部沖刷深度對於橋梁基礎穩定性佔有很大因素，其中水流流速更是評估局部沖刷最重要參數之一，水流流速影響作用於橋墩基礎之水流作用力大小。本研究旨在研究可在洪水期間使用，同時不會受到惡劣環境影響、簡易且經濟而有效的量測水流流速之方法。其原理為利用連接於鋼纜之重錘置入水中，水流拖曳力與浮力、重錘自重及繩索拉力會有一平衡關係，藉由水流拖曳力對於重錘產生擺角估算得到流速。由水工實驗結果得知，本研究所使用的阻力係數在臨界雷諾數下不為定值0.47須重新修正，同時平衡關係也必須加入繩索影響，因此本研究引入分段外推的數值方法，對拖曳繩錘量測流速的方法進行更詳細的分析。

關鍵詞：流速量測、水流拖曳力、繩錘、分段外推法

In recent years, Taiwan is attacked by typhoons and flood, causing severe local scour on riverbed, resulting in the bridge foundation is exposed and the foundations embedment depth is insufficient. If we can not effectively maintain bridge safety, it will seriously affect the livelihood and economic and life safety. Local scour depth represent a significant factor for the bridge foundation stability, in which Flow velocity is one of the most important parameters for assessing local scour, Flow velocity effect the strength of flow force which action on the bridge pier foundation. The purpose of this study is to research a method of flow velocity measurement which can use during flood and it will not be affected by adverse environmental impact and cost-effective. The principle for use cable connected to the plumb into the water and it will be a balance relations between flow drag force and buoyancy, plumb dead weight, and the cable tension. Through flow drag force for the plumb to produce a swing angle to estimate velocity. In the critical Reynolds, drag coefficient used in this study is not a constant value number ,which must be correct. While the balance relationship must join the cables effect. Therefore, this study introduces a numerical method of piecewise extrapolating to a more detailed analysis for the method of measuring the velocity using the cable drag plumb.
Keyword：flow velocity measurement, flow drag force, cable and plumb, piecewise extrapolating method

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