橋墩水深平均二維沖刷模式之發展

Abstract

本研究旨在發展一橋墩水深平均二維沖刷模式，使水平二維模式除了可反映橋墩兩側的束縮沖刷外，亦可以合理模擬垂向水流在橋墩周圍底床造成的局部沖刷現象。本研究以謝氏(2003)的水深平均二維動床模式為主體，採用浸沒式邊界法處理計算區域中因水流無法穿越橋墩導致的內部邊界問題，並在模式中加入垂向流速之估算式與相對應的局部沖刷輸砂公式。在垂向流速推估部份，首先針對重要影響參數進行無因次分析，再根據參數的合理範圍設計多組模擬案例，以三維CFX 模式進行三維水理分析，將模擬結果進行迴歸分析後獲得橋墩周圍的最大垂向流速分布之經驗公式。局部沖刷輸砂公式部分，使用多組橋墩沖刷實驗案例進行沖刷參數的檢定分析，並利用檢定結果與對應案例之水理條件進行迴歸得到沖刷參數的經驗式。最後以其他的橋墩沖刷實驗案例進行驗證分析，結果顯示模式能合理模擬橋墩周圍局部沖刷深度變化的歷程。

This study develops a depth-averaged two-dimensional pier scouring model to simulate both the contraction scour and the local scour properties due to vertical flow. The framework of the model developed is based on the depth-averaged two-dimensional mobile bed model by Hsieh (2003), in which employed the immersed boundary method is to deal with the internal pier boundary. The maximum vertical flow velocity and the corresponding sediment transport formula are taken into account to simulate the local scour phenomenon. The maximum vertical velocity regression formula is established from the velocity field simulated by a commercial 3-D model, CFX, through several hypothetical cases designed. Several bridge pier scouring experiment results are used to conduct the model calibration and parameter regression analysis to develop the empirical formula of local scour. Furthermore, the model is verified by using experiment results other than those for calibration. The verification studies show that the variation of the scour depth with respect to time around the bridge piers could be well simulated.