三峡建坝后长江宜昌—汉口河段水沙与河床的应变

Responses of runoff-sediment flux and bedform in the Middle Yangtze River to the completion of the Three Gorges Dam

通过三峡建坝前后1950—2013年的水文、泥沙与2003年和2011年的实测河床底形资料对比,试图讨论长江中游宜昌—汉口河段水沙及河床地形等应变量对三峡建坝的响应,为人们理解河流水沙及河床冲淤对超大型水库应变的幅度提供具体案例.三峡建坝后,宜昌站和汉口站年均流量由建坝前的13 850 m3/s和22 650 m3/s(1950—2002)减少到建坝后的12 450m3/s和21 000 m3/s(2003—2013),分别减少10.1%和7.2%;年均输沙量由5.56亿t和4.02亿t(1950—2002)锐减至0.6亿t和1.5亿t(2003—2013),分别减少89.5%和62.2%;年均悬沙中值粒径则由35μm(1950—2002)和25μm(1955—1975)减少至4μm和15μm(2003—2010).同时,水流挟沙不饱和能力的增加,引起了河床的沿程冲刷及泥沙粗化,宜枝河段、荆江河段和城汉河段深泓处分别平均冲刷了3.7 m、1.5 m和0.4 m,在主槽刷深的同时,河道浅滩有不同程度的萎缩.三峡建坝已导致中游水沙及冲淤条件发生了巨大改变.

Comparation methods are used to examine the responses of hydro-sediment and river bedform after the completion of the Three Gorges Dam. Data are based on the historical data of runoff and sediment discharge （1950--2013） and in-site measured data of depth and bedload （2003, 2011）. The results demonstrate that the average annual discharge at Yichang and Hankou stations has decreased 10. 1% and 7.2% after the Three Gorges Dam completion. The average annual sediment flux of the Yichang and Hankou stations decreased from 556 and 402 Mt/a to 60 and 150 Mt/a, accounted for 89.5% and 62.2% of that have been trapped by the Three Gorges Dam. In addition, the average annual medium diameter of suspended sediment at Yiehang and Hankou stations has decreased from 35μm and 25μm to 4 μm and 15 μm, but the bed load has become coarser and coarser. The depth at the thalweg has also increased to 0.4-3.7 m due to the bed erosion after the Three Gorges Dam impoundment.