Laboratory Experiment and Simulation of Scour Process around Nonuniform Pier with Steady Flows

台灣河川坡陡流急，颱風暴雨期間，其侵蝕作用相對劇烈，使橋樑基礎受到沖刷，以致裸露而破壞。非均勻橋墩中，若覆土面鄰近不均勻面，一旦洪水刷開該不均勻面，橋墩迎水面積增加，沖刷速率將急遽上昇，橋梁在短時間內即可能陷入危險狀態。因此，吾人若能掌握其沖刷歷程，不僅可作為疏散之依據，亦可提供工程上之參考。 本研究針對清水沖刷，配合不同墩徑比與數種覆土高度，進行室內試驗與模擬。除於實驗中觀測沖刷現象外，並探討各種條件下之差異性。試驗結果顯示，水流強度、墩徑比與覆土高度，皆對平衡沖刷深度有一定之影響。於天然河川中，若不均勻橋墩配合適當之覆土厚度，可有效降低局部沖刷之深度。 本研究之時變沖刷模式，乃以單墩沖刷機制為基礎發展而得，其可初步模擬不均勻橋墩之橫向擴展，及進一步之向下沖刷，直至平衡沖刷止。於應用上，其可掌握沖刷歷程之概略情形，此將有助於往後橋樑預警模式之研發。Many rivers in Taiwan have steep slope gradients and rapid flows. The bridge scour may induce foundation exposures and bridge failures during the typhoon or storm season. For a non-uniform pier, if the river bed surface is close to the discontinuous surface, the scour rate will rapidly increase once the foundation below the discontinuous surface is exposed to the flow due to the increase of the frontal projection area of the pier. Therefore, investigation of the non-uniform pier scour process is very important for the proper bridge design. In this study, laboratory bridge scour experiments were conducted using different non-uniform piers and exposure elevations under clear-water scour conditions. Besides observing the scour process, the differences among different experimental conditions were also discussed. The experimental results revealed that all three factors including the flow intensity, ratio of the upper to lower pier diameter and the distance between top of foundation and initial bed level have significant influences on the equilibrium scour depth. For a natural river, the local scour depth can be reduced if the soil covering depth above the top of foundation is thick enough. A transient scour simulation model is developed to simulate the horizontal enlargement of the scour hole near the discontinuous pier surface and the further downward scour until the equilibrium scour depth is reached. The results will increase our understanding of the non-uniform pier scour process and will be helpful for the future development of an effective bridge scour warning system.中文摘要 I Abstract II 目錄 III 圖目錄 V 表目錄 VII 照片目錄 VIII 符號表 IX 第一章 緒論 1 1.1 前言 1 1.2 研究目的 1 1.3 研究內容 2 1.4 本文架構 2 第二章 文獻回顧 4 2.1 沖刷原因與分類 4 2.2 沖刷因子之探討 6 2.3 相關保護措施 13 第三章 理論分析 15 3.1 因次分析 15 3.2 沖刷歷程理論分析 16 第四章 試驗方法 24 4.1 試驗設備 24 4.2 試驗條件之規劃 28 4.3 試驗步驟及方法 31 第五章 結果與討論 35 5.1 不同覆土深度之試驗結果 35 5.2 不同水流強度下之試驗結果 44 5.3 不同墩徑比下之試驗結果 46 5.4 沖刷過程中之沖刷坑變化情形 47 5.5 模式率定與驗證 50 第六章 結論與建議 59 6.1 結論 59 6.2 相關建議 60 參考文獻..........62 附錄A ............66 附錄B ............70 附錄C ............7