Measurements and simulation of local scour at nonuniform cylindrical piers

Abstract

台灣河川坡陡流急，每逢颱風豪雨河川水位迅速漲落，並造成上游集水區土壤沖蝕與崩塌，河川挾帶大量泥砂。繼九二一集集大地震後，近年來危橋事件屢見不鮮，辛樂克颱風(SINLAKU，2008)甚至發生后豐大橋斷橋事件，導致交通停擺影響經濟甚鉅。因此，橋墩沖刷之研究課題刻不容緩。 本研究針對清水沖刷之定量流及變量流流況，配合非均勻橋墩(即圓柱型橋墩位於圓柱型基礎之上)進行室內試驗與模擬。首先，在試驗研究方面，分別探討不同覆土深度與墩徑比，其所造成局部沖刷之差異，並提出平衡沖刷深度預估式。 在定量流沖刷模擬方面，本研究修改Mia and Nago (2003) 均勻橋墩沖刷模式，考慮墩前渦流與泥沙傳輸理論，針對定量流作用下未裸露之非均勻圓柱橋墩沖刷行為，提出一計算模式，以模擬其完整之沖刷歷程，並加入Melville (1996)之試驗資料進行模式之檢定與驗證，以提升模式之實用性。 此外，在變量流沖刷模擬方面，由試驗觀察得知變量流之非均勻橋墩沖刷行為，可從定量流之沖刷資料進行套疊。據此，本研究先行計算變量流歷線中各個時段之定量流沖刷結果，再利用不同流量沖刷體積套疊之概念，進行疊加模擬，以快速地模擬變量流墩前沖刷深度之歷程。

Many rivers in Taiwan have steep slope gradients and rapid flows. The flow stages usually vary rapidly during the typhoon or storm periods for these rivers, causing severe upstream watershed soil erosion, landslide and high river sediment concentration. Dangerous bridge events are common in Taiwan in recent years after the Chi-Chi Earthquakes. For example, the Houfeng Bridge failure due to Typhoon SINLAKU led to severe traffic and economical damages. Therefore, there is an urgent need to conduct bridge scour research. In this study, different types of steady and unsteady flow hydrographs, with non-uniform piers were used to conduct the laboratory bridge scouring experiments. The variations of local scour depth with different exposure elevations and pier ratios were investigated in the experiments, and an equilibrium scour depth prediction equation was proposed. In regard to the steady flow scour simulation, the Mia and Nago's (2003) model for the uniform pier was modified with consideration of the primary vortex concept and the volumetric sediment transport theory to simulate the entire scouring process at non-uniform piers with a discontinuous surface below the initial bed level. To increase the practicality of the model, Melville's (1996) laboratory data were included in the model verification and validation. In addition, by analyzing experimental data with steady flows, a scheme is proposed to calculate the temporal variation of scour depth with unsteady flows. In general, the simulated results based on the proposed methods correspond well with the experimental data.