圖書典藏及影音數位平台

由於臺灣地質構造脆弱、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，亦使下游河道缺乏砂源補充。石門水庫自民國52年5月開始蓄水迄今，截至110年12月止，總庫容之淤積率已達約33.58%。石門水庫肩負北部地區重要的水資源供給重任，為維護防洪安全及達成水庫永續發展等目標，持續改善既有設施並新建阿姆坪防淤隧道等工程，加上以抽泥泵浦將淤積於庫區之淤泥抽取後暫置於水庫後池堰下游，俟颱風期水庫洩洪時，淤泥藉由洪水自然回歸下游河道後入海，可維持河川天然輸砂作用並減緩水庫淤積量。針對石門水庫下游大漢溪及淡水河河道之影響及變遷狀況，經濟部水利署北區水資源局已於100~110年間陸續執行相關計畫之研究。本計畫期望在颱洪期間水庫進行排洪排砂操作時，能持續調查下游河道水質、生態及沖淤之變遷情形，並分析其與水庫運轉操作之關聯性，以釐清各界疑慮，預期將有助於累積相關生態背景資料及河道底床變化等資料，提供未來排洪排砂操作之參據，俾利推動石門水庫永續經營之管理目標。本計畫主要以歷年及汛期前資料為基礎分析依據，今(111)年度於汛期前及汛期中(9月軒嵐諾颱風後)各進行1次河道沖淤、水質及生態調查分析，並於2場颱洪事件(9月軒嵐諾颱風與10月尼莎颱風)進行懸浮質採樣與水質調查分析。本計畫另以數值模擬分析下游河道與河口沖淤變化，探討排洪排砂操作對下游河道水質、生態及河道沖淤之影響。各工作項目說明如下：河道沖淤調查分析：在大漢溪與淡水河主流選取4處(包含鳶山堰)進行影像比對分析，選取6處(包含阿姆坪防淤隧道出口)進行河道斷面測量，選取8處(包含鳶山堰、阿姆坪防淤隧道出口與後池)進行河道底床質粒徑分析，在淡水河主流選取3處溼地進行溼地底質與高程變遷分析，並在颱洪事件時於8處位置(包含三峽河、新店溪與基隆河下游)進行懸浮質濃度、濁度與粒徑分析，以瞭解水文事件與水庫操作前後下游河道之變化情形。數值模擬分析：(1)大漢溪與其他支流對下游河道之影響探討：以民國93年艾利颱風(約為石門水庫Q100流量)及民國104年蘇迪勒颱風(約為石門水庫Q10流量)為案例，模擬現況、排砂最佳化、無石門水庫及無主要支流等共7種情境之河道與淡水河口沖淤情形。(2)現況排洪排砂放淤對下游河道之影響探討：針對111年度軒嵐諾颱風時，石門水庫實際排洪排砂操作及放淤作業，進行模擬分析並檢討對下游河道變遷之影響。水質及生態調查分析：於大漢溪與淡水河主流之城林橋、新海大橋、臺北橋、關渡大橋、挖子尾等5處進行水質調查暨河床底棲動物(含魚類)、底棲及浮游藻類等生態調查。為釐清颱洪事件時水庫操作及下游支流排水匯流對水質之影響，本計畫另於關渡大橋、臺北橋及社子大橋進行全洪程水質監測。此外，因文蛤為淡水河下游的重要經濟物種，其數量變動受到漁民關注，而蟹類則推測其數量與物種組成應能反應底床質粒徑變動情形，且較不受人為捕撈與放養影響，亦選定進行指標物種分析。綜合111年度與歷年調查分析結果，111年颱洪事件規模小，對上游集水區影響有限，因此淡水河系各主要支流的土砂運移量體小。汛期中的調查顯示，大漢溪與淡水河主流中下游河道主深槽及紅樹林溼地相較於汛期前略有淤積(深度分別為0.15~0.25公尺及0.02~0.33公尺)。根據數值模擬結果，若水庫排砂量增加，可能於柑園橋至浮洲橋，以及關渡大橋至紅樹林增加淤積量體。另外，颱洪時期的水庫操作可能增加懸浮固體與營養鹽濃度，然而淡水河歷年颱洪事件的懸浮固體濃度多未超過魚類耐受範圍，且石門水庫泥砂的有機物含量低，汛期中也會因流量增加使營養鹽濃度降低，因此歷年調查顯示石門水庫操作並未對水域生物(包含魚類、藻類、蟹類及文蛤)的豐度與物種數造成影響。

Since the geological environment is fragile and the landform is changeable in Taiwan, the sediment transportation rates of rivers are high during flooding events, leading to high deposition rates of Taiwanese reservoirs which may decrease the lifespans of reservoirs and reduce the supplement of sediments on the estuary. From 1963 to the end of 2021, the percentage of deposits to the total installed capacity of Shihmen Reservoir has accounted for 33.58%. In order to maintain security from flooding risk and achieve the goal of sustainable development of the reservoir, the government continues to improve the existing constructions, constructing the Amuping Tunnel, and conducting the sediment venting and replenishment from the Shihmen Reservoir.The Northern Region Water Resources Office of the Water Resources Agency conducted relevant projects from 2011 to 2021 to monitor the influences of reservoir operations on morphological changes of the lower reach of the Dahan River and Danshuei River. This project aimed at monitoring and assessing the potential influences of sediment flushing on the water quality, ecology, and hydrological characteristics of the lower reach.This project referred to historical data and data in the dry season as the baseline, and conducted hydrological, water quality and ecological investigations two times (during dry and wet seasons), respectively, and analyzed the variation of riverbed landform by numerical modeling to explore the influences of sediment flushing on the lower reach. The details are explained as follows:Riverbed topographic investigations: We chose 4 river sections to compare remote sensing images between dry and wet seasons, 6 river sections to measure the cross-section morphology changes, 8 places to analyze riverbed material particle sizes, and 3 wetlands to measure the elevation changes of the land surface. During typhoon events, the concentration and particle size of suspended sediments, and the turbidity were also analyzed at 8 river sections to explore the relationships between flooding events, sediment flushing operations, and the variations of riverbed topography on the lower reach.Numerical modelings: (1) The influences of Dahan River and tributaries on the topographic changes of the lower reach: Typhoon Aere in 2004 (the peak discharge was around Q100 of Shihmen Reservoir) and Typhoon Soudelor in 2015 (the peak discharge was around Q10) were chosen as the modeling cases to simulate the riverbed variations of different reservoir operations, the influences of Shihmen Reservoir and the influences of the main tributaries. (2) The typhoon event this year: The hydrological characteristics of Typhoon Hinnamnor this year and the reservoir operation during the event were chosen to model the variation of riverbed topography, and to analyze the potential impacts of reservoir operations.Water quality and ecological investigations: Along the Dahan River and Danshuei River, 5 river sections were chosen to monitor the water quality and investigate the ecology. The ecological investigations included fishes, benthic animals, and algae. Among the targets, the clams and crabs were explicitly chosen to assess the influences of sediment flushing on the ecology. Moreover, this project also monitored the water quality of 3 river sections in Danshuei River and Keelung River during flooding events to explore the potential influences of sediment flushing and main tributary on the water quality of the mainstem river.Based on the investigation and analysis results of 2022 and previous projects, the magnitudes of typhoon events in 2022 were small and had limited impacts on the upper catchment regions, so the sediment transportation rates of the main tributaries of the Danshuei River was small. The investigations during the wet season of 2022 showed that the deep channels of the middle and lower reaches of Dahan River and Danshuei River, and the mangrove wetlands were slightly deposited (the depths were 0.15~0.25 meters and 0.02~0.33 meters, respectively), which were consistent with the results of numerical modeling. In addition, the suspended sediment concentration of typhoon events in the Danshuei River over the years mostly have not exceeded the tolerance range of fish, and the increase in water discharge tended to reduce the concentration of nutrients. Therefore, biological investigations over the years showed that the operation of Shihmen Reservoir did not affect the abundance and species of aquatic organisms, including fish, algae, crabs, and clams.