河口與近海三維水理與懸浮沈積物傳輸模式之建置與發展

Abstract

河口為河流與海洋交會之處，亦為淡水與海水混合之處，故潮流、水密度以及懸浮沈積物呈現複雜之非線性交互作用。主要影響河口物質傳輸因子為潮流與上游流量。河口之水理狀況為三度空間型態，因此三維水理模式可精確模擬河口各種水理及傳輸現象。本研究目的及貢獻在建置與發展淡水河系三維水理懸浮沈積物傳輸模式，並將模擬區域延伸至淡水河近海，使模擬結果更吻合實際狀況。模式經過與觀測資料（包括水位、流速、鹽分以及懸浮沈積物濃度）檢定後應用於淡水河口物理特性之研究。 研究結果顯示，鹽分、流速以及擴散係數呈現明顯之漲、退潮不對稱；流速、紊流混合與垂直分層於大、小潮過程中出現雙週擾動週期，而小潮的河口環流較大潮為強。殘餘環流中，尤拉傳輸為河口物質往外海傳送之機制，而史托克傳輸則為往上游傳送之機制。數值模式實驗指出，淡水河口之河口停留時間不超過三天，而河口環流為加速物質傳輸之主因。經顆粒追蹤模式模擬，於Q60流量下，水體之年(age)在淡水河口約320小時、新店溪口約100小時，基隆河口約485小時。顆粒釋放實驗顯示，相較於潮流的影響，風對於河道中顆粒分布影響甚微，而有、無密度環流則會呈現不同型態之顆粒分布。漏油模擬則指出，無論漏油事件發生於河道或港口，洩漏時間點與風向為影響油污軌跡分布之重要因素。 懸浮沈積物傳輸模擬結果顯示，高剪應力與較深的地形，可能是淡水河口最大混濁帶出現於關渡橋附近之原因。懸浮沈積物濃度隨著紊流擴散係數提高而些微上升，但高游流量可使得懸浮沈積物濃度有顯著的增高，懸浮沈積物濃度於大潮時約為小潮的1.5倍。而於地形改變後，最大混濁帶位從原來的距河口7-12公里擴張到7-16公里。

Estuaries are coastal waters where the mouth of the river meets the ocean and where the freshwater of the river mixes with the saline water from the ocean, and represent the complex nonlinear interaction of tides, current, salt, water and suspended sediment. The primary factors controlling transport processes in estuaries are tides and freshwater from upstream. Hydrodynamic processes in estuaries are generally three-dimensional; hence, implementation of the three-dimensional model for estuarine system is imperative. In the present study, a three-dimensional hydrodynamic model incorporating suspended sediment module was developed and applied to the Danshuei River estuarine system and its adjacent coastal sea. The model was calibrated with measured water surface elevation, tidal current, salinity and suspended sediment concentration (SSC). The overall model simulation results are in qualitative accordance with the available field data. The validated model was then adopted to investigate the physical properties of the Danshuei River estuary. The vertical profiles of salinity, velocity, and eddy diffusivity show a marked asymmetry between the flood and ebb tides; moreover, the tidal currents, turbulent mixing and vertical stratification show large fortnightly fluctuations over the spring-neap cycle. The circulation strengths were stronger during the neap tides than during the spring tides. The Eulerian transport is always seaward, whereas the Stokes transport is the main mechanism driving the up-estuary. According to the numerical experiments, the residence time is less than three days under mean flow conditions and the density-induced circulation play an accelerating role for mass transport. The particle-tracking model indicates that the water ages were approximately 320, 100 and 485 hours at the mouths of the Danshuei River-Tahan Stream, the Hsintien Stream and the Keelung River under the Q60 flow condition. Particles released experiments show that compare to the tidal current, wind influences were insignificant in the river channel. However, particle distribution was affected by density-induced circulation. The trajectories of oil spill were sensitivity to the leaking timing and wind direction, no matter oil spill even occurred in the river channel or harbor, because the oil particles always drift on the water surface. The estuarine turbidity maximum appears around Kuan-Du Bridge, it may be due to the high bottom stress and deeper topography there. The SSC has slight raise with increasing turbulent eddy diffusivities but significant increase with high freshwater discharge. The SSC is over 1.5 times higher at spring tide than that at neap tide. The estuarine turbidity maximum extends its range from 7.0~12.0km to 7.0~16.0km (distance from the Danshuei River mouth) under the condition of bathymetric change.