圖書典藏及影音數位平台

由於台灣地質環境脆弱、地形變動快速，因此河川於颱洪時期的輸砂量高，水庫攔阻河川往下游輸送之土砂，除減少水庫本身壽命外，亦增加下游河床沖刷量、提升海岸侵蝕率。石門水庫自民國52年開始蓄水迄今，截至109年12月止，總庫容之淤積率已達約34％，為維護水庫安全及達成水資源永續發展目標，水庫近年來實施更新改善計畫，包含已改建完成之排砂隧道及正在進行之阿姆坪防淤隧道工程，期望水庫於颱洪時期加強水力排砂，除有助於水庫水資源的永續利用外，也能使土砂回歸下游河道，維持河川天然輸砂作用，減少對下游生態、河床與海岸之影響。爰此，本計畫期望在水庫進行操作時，能持續監測並掌握與下游水質、生態之關聯性，以了解石門水庫下游大漢溪及淡水河河道之變遷狀況。 根據今年度汛期前後三次水質調查結果，氨氮與有機物於汛期中第一次調查的濃度較汛期前高，推測因上半年度的降雨將上游汙染物帶至下游所致；然而汛期中第二次調查的濃度較前兩次低，可能因流量增加，稀釋汙染物濃度而改善水質；溶氧則於歷次調查的濃度相似。另外，烟花颱風期間，溶氧濃度較平時高，但氨氮則無明顯變化。整合環保署與本計畫水質調查成果顯示淡水河中下游屬於較嚴重污染之河段，然而受到潮汐影響，於漲潮時，海水的交換稀釋作用會使水質略受改善。 本計畫生態調查的主要項目包含底棲動物、浮游藻類、底棲藻類及魚類，用以對照汛期前後及水庫排砂對下游河道與生物相之影響。其中，底棲動物汛期前與汛期中相比，物種組成及數量差異不大，僅台北橋與挖子尾略有差異，後者主要受潮汐影響。浮游藻類及底棲藻類的物種組成與數量易受環境因子影響，尤其潮汐的影響明顯。魚類於汛期前後調查的數量及物種組成皆不豐富，歷次調查的差異物種屬於廣鹽性魚類，以鱸形目較多。 另外，本計畫選定文蛤與蟹類進行指標物種分析。綜合歷年調查結果發現，水庫排洪排砂，未造成文蛤豐度明顯變化，且由今年度調查發現，9月文蛤豐度明顯高於歷次調查結果，顯示水庫操作未對文蛤生長有不良影響。至於蟹類歷年調查結果顯示，各測站蟹類組成與數量沒有明顯季節與年份之變化，顯示蟹類並未受到水庫排洪排淤操作之影響。 綜合105年梅姬颱風與今年度烟花颱風、璨樹颱風的懸浮質分析及數值模擬成果顯示，淡水河主流的土砂來源(各支流的土砂供應量)，視各颱洪事件對集水區上游的衝擊而定。由於石門水庫排放之土砂粒徑小，多屬於懸浮質，因此過庫順流之泥砂絕大多數皆會直接出海，對河道底床與通洪安全影響甚微。颱洪時期流量增加配合石門水庫操作，會短暫增加水中懸浮固體濃度並使其粒徑下降，然而懸浮固體濃度及高濃度的維持時間很少超過魚類耐受極限，而細粒土砂於下游河床的淤積量同樣有限，因此對底棲生物無明顯影響。相反的，水庫排洪可能增加水中溶氧量並減少氨氮濃度，尤其在漲潮時可使水質略為改善。

Since the geological environment is fragile and the landform is changeable in Taiwan, the sediment transportation rates of rivers are high, leading to high deposition rates of Taiwanese reservoirs. From 1963 to the end of 2020, the percentage of deposits to the total installed capacity of Shihmen Reservoir has been accounted for 34%. In order to increase the lifespan of Shihmen Reservoir, the government has completed the desilting tunnel project of the hydroelectricitypower plant and is now constructing the Amuping Tunnel. This project monitored and assessed the potential influences of sediment flushing on water quality, ecology, and hydrological characteristics of the lower reach. According to the water quality investigation, ammonia nitrogen and organic substance concentrations increased from the dry season to the wet season. However, the concentrations decreased during the wet season, probably because of the dilution of higher water discharge. On the other hand, the oxygen concentration did not change obviously during the investigations. The water quality at the lower reach of Tamsui River was poor, mainly owing to the high nutrients of domestic wastewater. Still, the water quality tends to improve when the tide rises. The targets of ecological investigation of this project included benthic animals, algae, and fish. Among the targets, the clams and crabs were chosen to assess the influences of sediment flushing on the ecology. Based on the factors that affect the abundance of clams, we found that the particle size and redox potential of deposited sediments were the most important environmental controls. Still, the influences of sediment flushing on these controls were not evident. In terms of crabs, we also did not find a relationship between sediment flushing and the number of crabs. The investigation of hydrological characteristics included sampling sus-pended sediments during typhoon influences, analyzing particle size of riverbed material, and measuring the cross-sections of river channels and the height of deposits of the lower reach. We showed that during typhoon seasons, the particle size decreased after the desilting tunnels began to operate. However, the magnitudes of typhoons in this year were not high; therefore, the variations of particle size of sediments, and cross-section and height of river channels were not obvious. Finally, the CCHE model was used for simulating the influence of flood discharge and sediment flushing on river channels. Specifically, we investigate the effects of various lengths of desilting tunnel operation and the impacts of tides on the erosion and deposition of riverbed. The results show that by increasing the temporal length of tunnel operation, the amount of flushing sediments can increase effectively during historical typhoon conditions. Most importantly, most of the increased sediments transported from the reservoir to the lower reach can be moved to the estuary; therefore, the influence of increasing operation lengths of tunnels on river channel change is limited.