| 摘要: | 摘要 翡翠水庫為供應大台北地區公共用水，其水質之好壞對民生用水影響甚大。根據翡翠水庫管理局所公佈之水庫水質卡爾森優養指標綜合評估，水質逐由「普養化」趨向「優養化」的現象，顯示水庫水質的變化值得關注。因此本研究最主要之目的是希望透過對水庫水質的密集監控，了解現地水質在時間及空間上的變化，並藉由實測資料取得現地的參數值，以提供本研究所發展的三維水理水質模式之用，並利用模式探討水庫特別之水理、水質現象，期能作為改善水庫水質之參考。 由歷史資料及本研究現地採樣結果得知，水庫水溫垂直剖面在夏季有非常明顯分層現象，上下水溫最大溫差可達15 ℃左右；而在冬季時，水溫之垂直分佈趨向均勻；而沿縱軸方向之等溫線有震盪情形，經由數值實驗結果顯示其可能是因表面風應力造成之內波現象。在強風(10 m/s)吹拂24小時後在水下30 m之內有明顯之內波現象，其振幅會受風應力之減弱及水深增加而變小，在微風(1 m/s)吹拂下則對水體幾乎沒有影響。而在水溫之時空分佈上，颱風時期和冬季水庫中上游的底層皆有冷水團之存在，其是由於颱風及冬季時期上游及支流注入庫區的水溫較平時低，在進入庫區後因其密度較重而沉入底部；而由數值實驗結果也顯示主流及支流進來的冷水會潛行至庫區底部並向下游推進，但無法在一個月的時間內潛行至大壩最底層並替換原來之底層水；而大壩出水口的放水，會加速水庫水體流動而有助於大壩底層水之替換。 在溶氧的時空分佈上，大致上是水下20m以上有較高溶氧之存在，中層溶氧降低，底層則呈現缺氧現象。而在3月時整個水庫水體之溶氧呈現非常均勻的分布，推測水庫水體當時乃處於翻轉後之垂直混合狀態。本研究利用簡單之收支方程式探討溶氧之輸入、輸出、產生及消耗。在研究期間平均入流所帶給水庫的溶氧增加為95.3 mg/m3/d，被排水所帶走的則為93.1 mg/m3/d，水體內產生之溶氧平均速率為16.2 mg/m3/d，水體內之消耗平均速率為14.5 mg/m3/d，底部耗氧相當於水體內之消耗平均速率為7.1 mg/m3/d。在水面之交換造成之淨輸出相當於消耗率為1.1 mg/m3/d。 藉由收支平衡估算所得之參數值可供水質模式之用。本研究所發展之水質模式在帶入這些由收支平衡所估算之參數值後其模擬結果與實測結果還算吻合，表示水質模式於翡翠水庫應當有其適用性。 Abstract The Feitsui Reservoir was built as a primary source for water supply to the Taipei metropolitan area. Therefore, its water quality is a very important hydrological issue. According to the Carlson trophic state index (CTSI) reported by the Taipei Feitsui Reservoir Administration, the water quality has changed gradually from oligo-/meso-trophic condition to meso-/eu-trophic condition in the last ten years. Such a change deserves attention. Because of the increasing threat of eutrophication, we decided to develop a coupled hydrodynamic-water quality model to study the processes governing the motion and quality of water in the reservoir. We hope that our study may lead to the generation of useful information for the management of the Feitsui Reservoir in the future. Prior to the development of the model, we need to compile hydrographic and chemical data from historical records and to conduct field work in order to obtain basic information on the hydrological characteristics of the reservoir. The historical records and our field data have demonstrated that the water column is strongly stratified is summer, with the maximum difference of water temperature reaching 15 ℃, and well mixed in winter. Vertical fluctuations of the isotherms are discernible in the upper layer along the main axis in the reservoir. Our numerical experiments show that the fluctuations could have been internal waves resulting from wind forcing. A strong wind with velocity of 10 m/s along the long axis of the reservoir may induce wave motion in the top 30 m within 24 hours. The amplitude of the wave decreases with increasing water depth., Weaker winds with 1/10 the velocity can hardly disturb the water column. It was observed that colder water existed along the bottom of the upper and the middle reaches of the reservoir after the typhoons and also in the early winter. Numerical experiments have demonstrated that the cold water intrusion can proceed along the bottom of the reservoir starting from the upper reach, but the cold water cannot reach the deepest part of the reservoir within a month, if no water discharge is allowed. By contrast, the bottom water renewal by cold water intrusion is much enhanced, if the water discharge is turned on at the dam. The space-time distribution of dissolved oxygen (DO) reveals the following pattern. The DO concentration is high in the top 20 m and drops in the middle water column. Significant oxygen deficiency occurs in the bottom layer. The distribution of DO in the reservoir in March is very uniform, indicating strong vertical mixing due to water column overturning in late winter. Using mass balance equations, we are able to evaluate the oxygen inflow, outflow, consumption and production rates. During the study period, the average inflow rate of DO is 95.3 mg/m3/d; the average outflow rate of DO is 93.1 mg/m3/d; the average oxygen production rate in the water column is 16.2 mg/m3/d; the average oxygen consumption rate in the water column is 14.5 mg/m3/d; the average oxygen consumption at the bottom is equivalent to a consumption rate in the water column of 7.1 mg/m3/d; the average oxygen exchange between the surface water and the atmosphere is nearly balanced with a small deficit equivalent to a consumption rate in the water column of 1.1 mg/m3/d. The parameters evaluated by the mass balance equations prove useful for application to the water quality model. It has been demonstrated that the model developed in this study can simulate important features of oxygen distribution, which is one of the most important water quality indices, in the Feitsui reservoir. |
