三峡建库前后洞庭湖对下荆江的顶托与消落作用研究

自然通江的洞庭湖与长江间复杂的关系影响着下荆江尾闾河段的水文情势变化。三峡建库后洞庭湖出口基准面的变化，使得湖对江的顶托与消落关系发生调整，进而影响防汛抗旱与生态保护。从水面比降变化出发，以顶托消落比表征湖对江的顶托消落作用，利用1991—2017年监利站、螺山站、七里山站水文资料，分析洞庭湖对下荆江顶托、消落作用及其变化规律，并根据平衡比降公式，从理论上解释了蓄水前后比降变化的合理性。结果表明：蓄水后监利-城陵矶、城陵矶-螺山水面比降均有所增大。1991—2002年、2003—2007年、2008—2017年以来，监利-城陵矶河段年内月最小水面比降的出现时间有所推迟，对应的城陵矶水位有所升高，汇流比有所减小，湖对江的顶托作用有所减弱。1991—2017年湖对江综合表现为顶托作用，三个时期顶托消落比减小的原因为顶托作用减弱而非消落作用增强。从年内变化看，湖对江表现为消落作用出现在汛后（9~10月）及2008—2017时期枯水期（1~3月）。三个时期顶托消落比均受汇流比的影响，当汇流比小于0.5，洞庭湖易产生消落作用，当汇流比大于1，洞庭湖顶托作用增强。当长江流量小于6000m3/s，湖对江均表现为顶托作用。1991—2002年时期顶托消落作用主要受洞庭湖出流的影响而2008—2017年时期主要受长江干流流量的影响。研究成果可为江河湖泊治理与保护提供科学依据。

Study of the Supporting and Falling Impact of Dongting Lake on the Lower Jingjiang River Before and After Construction of Three Gorges Dam

The hydrological changes in the Lower Jingjiang reach is related to the complex exchange relationship between Dongting Lake and Yangtze River. After the construction of the Three Gorges Dam(TGD), the change of the base level of Lake Exit makes the lake-to-river supporting and falling effect adjustment, which could have an adverse impact on flood control& draught resistance and ecological environmental protection. Based on the variation of water surface slope, supporting and falling ratio is proposed to characterize lake-to-river effect. The data at Jianli, Luoshan and Qilishan Station from 1991 to 2017 are used to analyze the supporting and falling effect and its variation law. The rationality of gradient change before and after impoundment is explained theoretically by using balanced gradient formula. The results showed that: After impoundment, the water surface slope of Jianli-Chenglingji and Chenglingji-Luoshan increased. The minimum monthly slope of Jianli-Chenglingji reach delayed from 1991-2002 to 2003-2007 to 2008-2017. The corresponding water level in Chenglingji increased, while the confluence ratio decreased, and the supporting effect of lake-to-river weakened. The lake supported the river comprehensively during 1991-2017 periods. The reason for supporting and falling ratio reduction is the weakening of supporting effect rather than the enhancement of falling effect. After the flood season(September-October) during three periods, the Lake showed a falling effect on the River, while from 2008 to 2017, the falling effect of the Lake also occurred in the dry season (January-March). Supporting and falling ratio is affected by the confluence ratio during three periods. When the confluence ratio is less than 0.5, the Lake is liable to cause falling effect on the River. When the confluence ratio is greater than 1, the supporting effect increases. When the discharge in the Yangtze River is less than 6000 m3/s, the Dongting Lake acts as a supporting force to the Yangtze River. During 1991-2002, supporting and falling effect was mainly affected by the Dongting Lake outflow, while during 2008-2017, it was mainly affected by the discharge in the Yangtze River. The research results can provide scientific basis for river regulation, water ecology and water resources utilization in the lower Jingjiang River.