乐清湾水交换特征研究

Study on the features of water exchange in Yueqingwan Bay

采用EFDC模式模拟研究了乐清湾水交换的三维过程和时空变化特征,并通过计算水示踪剂质量浓度分析水体置换过程。结果表明,乐清湾水交换主要是由鹿西岛两侧流入的外海水体与湾内水体的交换,以及乐清湾口门西侧附近的湾内水体与瓯江北口径流冲淡水之间的交换。从口门到湾顶,水交换能力差别较大。以最窄的连屿至打水山断面为界,以南水体1个月基本可以完全交换,而以北水体2个月后仍然无法交换至湾口水平。连屿至打水山断面以北地形复杂,岛屿较多,污染物主要通过岛屿间的潮汐汊道输运,断面的瓶颈效应也使得断面以北的水体交换能力稍弱。在口门附近90%以上的水体被外海置换所需时间不到5 d,而此时湾顶水质未有太大改变;15 d左右,80%湾内水体被外海水置换;90%湾内水体被置换仅需40 d;70 d时的水体置换率达97%。

Three dimensional processes and temporal and spatial variations of the water exchange in Yueqingwan Bay are studied by statistical analysis of observed data, and also by EFDC model. A convection-diffusion model of dissolved conservative tracer was run after the hydrodynamic simulation became stable. And the water replacement was calculated with the tracer mass concentration in the Bay. The calculated results show that the rising tide flows north-westwards, presenting a floodplain form with low flow velocity, and that the falling tide flows south-eastwards and finally into the sea with higher velocity than the rising tide, and that the duration of the rising tide is greater than that of the falling tide. Because of terrestrial obstruction and seabed friction, the direction and speed of the open sea tidal wave begins to change after entering into the bay, the speed decreases from east to west of the bay-mouth and from the bay-mouth to the bay head, and the shallow water component gradually increases, so does the tidal range. The results of water replacement show that the water exchange mainly occurs between the open sea water from both sides of Luxi Island and the bay water, and between the bay water to the west of the bay-mouth and the diluted water of runoff from the north estuary of the Oujiang river. There is a great difference of water exchange from the bay-mouth to bay head. Take the narrowest section （from Lianyu to Dashuishan） as a boundary, the water to its south can be fully exchanged within a month while it can not reach the bay-mouth in two months to its north. The topography to the north of the section is complicated with a lot of islands in this area. The pollutants are mainly transported by the tidal inlets among the islands. Due to the bottleneck effect of the section, the water exchange to the north of the section is slightly weaker. The calculated average exchange rate shows that the water exchange rate is very different. It only needs less than 5 days for 90% of water near the bay-month to be replaced by the open sea water, while the pollutant concentration in the bay-head changes little. 80% of the bay water has been replaced by the open sea water after 15 days, 90% after 40 days and 97% at the 70th day.