| 摘要: | 局部沖刷為河床表面物質受洪水力而從橋墩周圍搬移，此機制為台灣橋梁於颱風期間倒塌之最普遍之原因。其中案例之一即為2009 年莫拉克颱風過境所造成雙園橋倒塌事件。因此欲降低橋梁災害，準確預估沖刷深度為關鍵。然而，由於泥沙沖刷機制極為複雜，且難以完整地以傳統輸沙理論描述，因此至今工程師仍極度仰賴經驗或半經驗公式以推估最大沖刷深度。然而橋梁倒塌經常肇因於極端洪水事件，而該事件往往嚴重偏離上述公式之先決條件。另一方面，泥沙之沖刷機制非常近似於賓漢物質之運動，亦即該物質存在一臨界剪力，當水流剪力低於該臨界剪力，則該物質呈現固體行為。反之則液化，並以牛頓流體呈現。因此本計畫將發展一三維數值沖刷模式，並根基於賓漢流變理論。其中傳統輸沙理論中之床質、底床載(bed load)、及懸浮載(suspended load)將以賓漢理論中之固體區、混和區、以及清水區方式呈現。所發展之模式將與理論解或實驗資料加以驗證。為模擬極端氣候下所造成之局部沖刷，本模式將考慮多種流體間之界面，包括空氣與水，以及水與泥沙等。而界面之運動則以流體體積法(VOF)加以描述，該法已於國科會所補助之前一年計畫中完成。本研究預期所模擬之結果將可描述三維沖刷坑剖面，其中包括最大沖刷深度以及施於橋墩上之水流力。以上兩種機制為橋梁災害防治重要之參數。除預測局部沖刷之沖刷坑外，本模式並將與DEM 模式耦合以模擬橋墩所承受之多重外力，其中包括碎波力、漂流塊體之撞擊力、以及水流力。此問題關係到困難之雙向流固耦合，且將與賓漢沖刷模式結合以徹底模擬橋梁倒塌之過程。一旦本模式發展完成，將成為全球第一可完整模擬洪水與橋梁互制作用之水動力模式，此對於台灣洪水防治及保持學術領先地位將有重大意義。 Local scour is the removal of the streambed material from bridge foundations due to the flooding water, and is the most common cause of bridge failures during the typhoon events in Taiwan. One of the notorious cases is the failure of Shuan-Yuan Bridge in the event of 2009 Typhoon Morakot. To minimize the future bridge damage, an accurate estimation or simulation on the scouring depth is the key. However, due to the difficulty on describing the sediment transport mechanism, the engineers deeply rely on empirical or semi-empirical formulae which are mostly derived from the steady and equilibrium conditions. Unfortunately, the bridge failure usually occurred in the event of extreme flood which deviates tremendously from the assumption mentioned above. Because the mechanism of the sediment transport is very close to the one of Bingham material, we shall propose a 3D numerical scouring model based on the Bingham rheological theory. The bed load, suspended load, and laminated load in the conventional sediment transport theory shall be converted into the plug area, mixed area, and clear water area in the Bingham model. The model shall be validated with the analytical solution or with the laboratory experiment. In order to simulate the local scour under the extreme weather condition, this model shall consider the multiple fluid-fluid interfaces, such as the air-water and water-sediment interfaces. The interface kinematics will be described by the Volume-of-Fluid (VOF) method which has been developed successfully in the previous project sponsored by NSC. We expect the simulation results will depict the 3D scour profile, including the maximum scour depth and the flooding force acting on the bridge piles. Both of them are important for bridge hazard mitigation. Except for the prediction on the local scour, this model will be coupled with the DEM model to simulate multiple forces on a bridge, such as the breaking waves, floating rocks, and flooding current. This problem involves the difficult issue of two-way coupled fluid-structure interaction, and shall be engaged with the Bingham scouring model to have thorough simulation on the bridge failure. Once the model is developed, it will be the world-first hydrodynamic model for completely simulating the flood interacting with bridges. This is significant to the flooding hazard mitigation in Taiwan, and also maintains the world leadership in the field of flood control. 研究期間：10003 ~ 10007 |
