河口及感潮河段為河川流域與近岸海洋水和物質交換所必經之道，所以對於此一河川水理狀況之瞭解可幫助預測污染物質自河川排放至近海的傳輸過程。淡水河系為台灣最大之感潮河川，其感潮河段包括三大支流，分別是大漢溪、新店溪及基隆河。淡水河系除了受潮汐與淡水流量所引起的正壓流（Barotropic flow）影響外，鹽分入侵所引致斜壓流（Baroclinic flow）是造成河口傳輸的另一重要機制。 本研究應用UnTRIM （Unstructured grid Tidal, Residual, Intertidal Mudflat）三維水動力數值模式對淡水河系及其近海進行潮流與鹽度之模擬，此模式是以不規則正交網格（Unstructured orthogonal grid）來模擬複雜的邊界外形，它的優點是可以只在重要的地方增加網格數目而不需要將整個模擬區域的網格數增加，能有效地降低電腦在模式運算中所耗費的時間。 模式演算時上游邊界分別是：城林橋（大漢溪）、秀朗橋（新店溪）以及江北橋（基隆河），淡水河外海為下游邊界。本研究先以 潮為外海邊界潮汐的驅動力，初步檢定Chezy係數（ ），再以五個分潮之綜合潮做細部調整，使模式模擬結果各分潮之振幅與相位角與實測數據之調和分析結果相近。模式檢定後以2000年實測數據來驗證，以實測之振幅及相位角為外海邊界條件，各流量站實測之日流量為上游邊界條件；比較各測站之實測與模擬結果之水位、流速和鹽度以及殘餘流速之時間序列變化後，顯示本三維水動力模式之正確性。 模式經檢定與驗證後，應用於探討淡水河系於上游流量為平均流量與Q75低流量作用下之河口環流與鹽分分佈之比較；另模式亦應用於探討基隆河截彎取直前後對於基隆河殘餘流速與鹽分分佈之影響。 The estuary is the primary conduit for the transport of water and material from its drainage basin to the coastal ocean. The ultimate fates of land-derived materials, such as pollutants, depend on the water movement in the estuaries. To predict the fate and transport of materials from land to the ocean requires the knowledge of water movement through this stretch of water body. The Danshuei River system is the largest estuarine system in Taiwan. It consists of three major tributaries: Tahan Stream, Hsintien Stream and Keelung River. In addition to the barotropic flows forced by the tide and river discharges, the baroclinic flow forced by seawater intrusion is another important transport mechanism in the Danshuei River estuarine system. In this study, a three-dimensional hydrodynamic model (UnTRIM-3D) was used to simulate tidal current and salinity distributions in the Danshuei River estuarine system and adjacent coastal sea. The unstructured orthogonal grid in this model is adopted to fit complicated boundaries. It is flexible to permit local mesh refinements in areas of interest only and to reduce computational time. The upstream boundaries of the computation domain are: Cheng-Ling Bridge (Tahan Stream), Hsiu-Lang Bridge (Hsintien Stream), Jiang-Bei Bridge (Keelung River) and downstream boundary is at the coastal sea. First, a single constituent tidal was used as forcing function at the coastal sea boundary for preliminary model calibration of the value of Chezy friction coefficients. The use of a single constituent tide expedites the extraction of tidal ranges from model outputs for comparison with prototype data. Five-constituent tidal was then used for fine-tune calibration by matching the observed amplitudes and phases of the five tidal constituents. Using the field data in 2000 for model verification, the model was run to simulate the prototype condition. The comparisons between model results and field data of tidal elevation, current velocity, salinity, and residual current indicate that the model is accurate and suitable for the Danshuei River estuarine system. After the model calibration and verification, the 3D model was applied to investigate the residual circulation and salinity distributions under mean and low ( ) flow conditions. The model was also used to evaluate the effects on residual current and salinity distributions in the Keelung River due to the channel regulation of Keelung River.
