dc.description.abstract | 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. | en_US |