三峡工程蓄水以来长江与洞庭湖汇流关系变化

Confluence relationship changes of Yangtze River and Dongting Lake since impoundment of Three Gorges project

三峡工程蓄水后,长江干流监利站同流量下的水位有一定降低,而城陵矶站和螺山站同流量下水位抬升。监利站流量为5000m3/s时水位下降1.0m左右,流量大于15000m3/s时无显著变化;城陵矶站流量为2500m3/s时水位抬高0.4m,流量大于10000m3/s时暂无显著变化;螺山站流量小于15000m3/s时水位无明显变化,但高水时期有所抬高,当流量为35000～45000m3/s时,蓄水后同流量下水位抬高约0.6m。长江与洞庭湖汇流区相互顶托比较严重,顶托关系与两者来流大小的相对关系密不可分。长江与洞庭湖顶托最小的最优流量比(城陵矶站与监利站流量比)在0.55～0.85之间,一般总流量越小,最优流量比越小。近段时期以来,由于四水加之三口分流减小引起城陵矶与监利站流量比减小,导致长江对洞庭湖的顶托加剧,抬高了城陵矶同流量相应水位,但另一方面由于城陵矶流量减小,降低了水位,两者可以部分甚至全部抵消。现阶段,螺山站同流量下相应城陵矶站水位没有发生明显变化,仅在高水(螺山流量大于等于25000m3/s)同流量下城陵矶水位抬高0.1～0.4m。三峡工程蓄水后,洞庭湖出流段水面比降呈逐渐减小趋势,鹿角-莲花塘平均水面比降较1991-2002年平均值减小22.8%,高水期间和枯水期间减小幅度达到30%左右。

Since impoundment of the Three Gorges project, the Yangtze mainstream water level at the Jianli station has been somewhat lowered, but the levels 1. Om at flow Q = 5000m3/s, little change at Q at Chenglingji and Luoshan has risen. Jianli level falls about 〉 15000m3/s; Chenglingji level rises about 0.4m at Q = 2500m3/s, little change at Q 〉 10000m3/s; Luoshan level changes little at Q 〈 15000m3/s, but is higher otherwise, for example, by 0.6m at Q = 350000 - 45000m3/s. The mutual backwater push is serious at the confluence area of the Yangtze and the Dongting, and this backwater effect is closely related to the flow ratio （ Chenglingji＇ s flow over Jianli＇ s flow）. The optimal flow ratio that minimizes the effect is in a range of 0.55 -0. 85. Generally, this optimal ratio decreases with the decrease in the total flow. Recent years, all the flows in the Four-Rivers and three branch channels have been reduced and the flow ratio has also been reduced. This intensifies the trend of backwater push to the lake outlet, i.e. the lake outlet level tends to be raised up by the relative higher level in the river. On the other hand, however, such a trend is counteracted partly or even fully by a falling trend at the lake outlet as a result of smaller flow there. At the present stage, the water level at Chenglingji does not show an obvious change relative to that before the impoundment if the flow is low,but it is raised by 0. 1 -0.4m if Q ≥ 25000m3/s （gauged surface gradients in the outlet reaches of the lake decrease the mean decrease in the reach from Lujiao to Lianhuatang at Luoshan）. After the impoundment, the water gradually. Relative to the period of 1991 -2002, is 22.8%, and even 30% or so in the season of high or low flow.