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石門水庫位於淡水河上游之大漢溪上，自93年艾利颱風後水庫淤積嚴重，為求水庫永續經營，近年來水利署北區水資源局、水利規劃試驗所等單位積極研擬因應策略，包括以水庫表水與低標高濁水混合稀釋排放、排洪隧道排砂、石門大圳退水路排放底層濁水、石門電廠與永久放水口排砂、石門水庫上游主河道防洪防淤工程排砂等措施。而水庫防淤、排砂策略對下游大漢溪及主流淡水河河道之影響評估方面，包含水庫排砂對下游河道沖淤影響、河防安全影響，鳶山堰、規劃中之中庄調整池取水口濃度之影響、後村堰下游軟岩河床沖刷情形、三鶯堰規劃堰址之沖淤及藍色公路航道沖淤變化等，尚需相關研究以評估防淤策略及取水設施操作之合適性，爰成立本計畫。 二、 相關資料收集彙整與分析 收集大漢溪、淡水河歷年之水文、斷面及河床質等基本資料，並對河道歷年之沖淤情形及河道變遷進行分析。另於大漢溪溪及淡水河進行河床質調查分析(共二十五個採樣坑)及泥砂濃度監測。 三、 石門水庫防淤對下游河道之影響評估 本研究採用美國國家計算水科學及工程中心之CCHE1D、CCHE2D模式，作為相關分析評估之動床數模工具，並參考民國100年「石門水庫防洪防淤工程規劃-水工模型試驗」之物模試驗資料，及北水局提供鳳凰、薔蜜、辛樂克及莫拉克等颱風實測流量及泥砂資料，進行模擬。本計畫針對現況案、防洪防淤隧道C1案、C1延伸管案、D2案及C1延伸管案+D2修改案等五案，評估各防洪防淤方案對下游河道之影響。 新增防洪防淤隧道後，單一颱洪事件對下游河道之影響，與上游排洪排砂量有關，如艾利、辛樂克及薔蜜颱風因排洪、排砂量大，水流可將泥砂挾帶至淡水河下游沉積，明顯沉積範圍位於淡水河柑園大橋至忠孝大橋間；而鳳凰、莫拉克颱風排洪、排砂量小，水流挾砂能力小，故砂泥零星落淤在大漢溪河道，並未影響下游淡水河之沖淤。 新增防洪防淤隧道後，對下游河道之長期影響，因現況上游來砂之補充，未來48年間現況下游河道多為淤積狀態，而新增防洪防淤隧道後，淤積深度有所增加、河床漸有回淤，尤以C1延伸管案及C1延伸管案+D2修改案最為明顯(排砂量較其它方案為大)，河道淤積最為顯著，範圍為城林大橋至關渡大橋河段。 四、 河防安全評估 大漢溪採100年重現期進行模擬，淡水河採200年重現期進行模擬，且河口水位設定為EL 4.03公尺(台北地區防洪計畫建議方案)作為起算水位，用以評估單場颱洪在防洪防淤操作條件下，是否有出水高不足或溢堤之可能性。結果說明除關渡大橋至淡水河口附近，受暴潮影響下，有出水高不足及溢堤之問題外，其餘河段尚無出水高不足之問題；而新增防洪防淤隧道後之洪水位，與現況案相近，並無水位大幅上升之情形發生。另針對各模擬案例中，底床沖淤、流速變化較劇烈之河段，進行堤岸基腳附近之流場分析。 五、 含砂濃度影響評估 鳶山堰水源係由石門水庫放流或擷取河道側流量，其濁度抬升主要是上游石門水庫排洪排砂所致，一旦發生原水濁度提高，影響供水甚劇。本研究針對鳶山堰河段進行二維局部流場模擬，探討各防洪防淤方案下鳶山堰取水口附近之懸浮載濃度變化，有無影響供水之虞。另外，懸浮載濃度對水生動、植物影響，本研究則以現況案之洪峰懸浮載濃度作為基準，評估新增防洪防淤隧道後，懸浮載濃度是否與現況案相差甚多，作為評估是否有影響水生動、植物之虞。 六、 防洪防淤操作之影響說明及因應對策 未來不論石門水庫實施任何防洪防淤策略，都有助於下游河床回淤，使河道趨於沖淤平衡狀態，但近年下游河道已設置多處河濱公園、生態濕地及自行車道等親水設施，加上政府已著手規劃三鶯堰、藍色公路等設施，故有需要提出相關因應策略來面對未來可能遭遇之挑戰。本報告就三面向進行說明：1.上游集水區保育、2.下游河道管理、3.穩定供水設施。 七、 教育訓練 於民國99年12月01日辦理CCHE1D模式之教育訓練工作，主要介紹CCHE1D模式理論架構與特點、代表案例說明、本計畫建置之淡水河動床應用案例模組等，進行技術轉移工作。另於民國100年11月08日辦理CCHE2D模式之教育訓練工作，主要介紹CCHE2D模式理論架構與特點、代表案例說明、本計畫建置之淡水河動床應用案例模組等，進行技術轉移工作，內容詳見附錄二。

Shimen Reservoir is located upstream of the Tamsui river. The reservoir has been deposited seriously after the occurrence of typhoon Aere in 2004. For the sustainable operation of the reservoir, in recent years, Northern Region Water Resources Office, Water Resources Planning Institute, and other departments develop the strategies actively, including mixture diluted emissions of the surface water and low-elevation water with high turbidity sediment discharged by the tunnel spillway, low-elevation turbid water discharged in Shimen canal, sediment discharged by Shimen power plant and PRO, and upstream flood diversion and hydraulic desilting and other measures. The purpose of this project is to assess the reservoir desilting operation strategy on the bed evolution of downstream reach. II. Data Collection and Analysis Historical hydrologic, cross-sectional, and bed material data were collected in this project. Moreover, size gradations of the bed material in Dahan and Tamsui rivers (25 sample pits) suspended sediment concentrations were measured. III. Impact of Shih-men Desilting on Downstream Reach This study adopted CCHE1D and CCHE2D models as the numerical model tools, and referred to the physical model data obtained from the project entitled is "Shimen reservoir flood control and desilting project planning-hydraulic model tests", and the discharge and sediment concentration data during Fung-wong, JangmiI, Sinlaku, and Morakot typhoons provided by Northern Region Water Resources Office, WRA. In this study, five cases were considered: the existing condition, C1, C1 with extension pipes, D2 and C1 with extension pipes plus modified D2. After the construction of desilting tunnel, downstream aggradation for a given typhoon event depends on the amount of sediment released from the reservoir . Significant river deposition can be found in the reach from Kan-yuang bridge to Chung-hsiao bridge of Tamsui river. Small the amount of deposition is located in Dahan river, and the deposition at the Tamsui estuary can be neglected. For the long-term simulation (48 years), most downstream reaches the amount of aggradation increases. The more significant aggradation can be found in the cases of C1 with extension pipes and C1 with extension pipes plus modified D2. IV. Evaluation on Dike Safety To evaluate the freeboard being enough or not and the possibility of overtopping of dike, Q100 in Dahan river and Q200 in Tamsui river , and elevation of 4.03m at the downstream estuary were adopted, It can be found that not enough freeboard and dike overtopping occurred in the reach from the estuary to Guan Tu bridge. In addition, the flood stages do not change much after the construction of desilting tunnel. The safety of dike base at some critical location is also evaluated. V. Impact Evaluation of Sediment Concentration Through 2D mobile-bed simulation, the suspended sediment concentration at the intake of Yuanshan weir can be estimated. It can be seen that the duration of high sediment concentration (larger than 6,000 NTU) increases offer the construction of desilting tunnel. VI. Education and Training The theories, numerical schemes, and application case studies of the CCHE1D and CCHE2D models were introduced on December 1, 2010 and November 8, 2011, repectively.