固床工下游軟弱岩石沖刷坑形狀與深度分析 -以中正大橋河段為例

Abstract

台灣西部地區軟岩質河道沖蝕情況嚴重，大多興建固床工保護重要河段及跨河構造物，然而固床工因與下游河床面的高差，反而引致沖刷坑的形成。以新竹頭前溪中正大橋三階固床工為例，三階固床工完工後與下游河床形成高差及坡度不連續，於民國94~96年汛期過後，因沖刷坑持續發展，固床工嚴重毀壞，下游河道深槽化，並發生溯源沖刷，影響橋墩安全。由於岩質河床沖刷坑形成為不可逆性，因此探討其形成機制及發展正確深度與形狀的估計方法有其必要性。 本研究使用COMSOL Multiphysics 多重物理量耦合軟體模擬水流條件，修改羅楚鈞(2011)軟岩質河床沖刷坑分析方法，即同樣以Annandale (2006)計算水面流功，而尾水消能改以Castillo (2014)所提出公式估計，以及Bollaert(2002)的縱向水壓力分布曲線，並以李國維(2016)斜向射流尾水深計算方式修正以往嚴重高估的沖刷坑深度。本研究以中正大橋下游固床工沖刷坑深度為例進行計算，計算結果顯示沖刷深度接近現地測量結果，而沖刷坑形狀部分差異較大。最後以室內沖刷坑試驗數據並參考Meftah and Mossa(2006)的建議，提供現有沖刷坑深度評估方式不考慮時間因素的解決方法。

Because of the serious scour of soft bedrock in western Taiwan, there are many groundsill works were constructed to protect important reaches and cross-river structures, which may cause the formation scour hole downstream the structures. For example, the Jhong-Jheng Bridge’s three-stepped groundsill works form the discontinuous slope of river and height difference between groundsill and downstream. Then, the scour hole downstream the works is developed; the groundsill work was damaged severely; and the formed knickpoint on the river channel retreated upstream. The retreat of knickpoint may endanger the bridge pier. Since the formation of a scour hole in rockbed channel is irreversible, it is necessary to study its formation mechanism and to propose a reasonable approach for calculating the depth and shape of the scour hole. This study adopts the COMSOL(Multiphysics Software) for calculating the flow conditions. The approach for estimating the shape and depth of the scour hole is modified from Lo’s method (2011). Integrating the formula for calculating stream power (Annandale, 2006), estimating the tailwater dissipation(Castillo, 2014), estimating the longitudinal water pressure dissipation curve(Bollaert, 2002), and determining the depth of tail water for the inclined water jet flow revising(Lee, 2016), the method for estimating the scour hole’s depth and shape is presented. This study takes the river channel in the Jhong-Jheng Bridge area, Hsinchu as an example to calculate the depth and shape of the scour hole downstream the groundsill work. The analytical results show that the calculated depth is closed to that surveyed in field, but there is a difference in shape of scour hole between calculated result and measured result. Finally, this study adopts the existing experimental data of scour hole model test and Meftah and Mossa’s method(2006) to propose an approach to consider time effect on the scouring depth.