三峡水库蓄水前后长江中下游江心洲的演变及其机理分析

长江中下游河道存在数量众多的江心洲,三峡水库蓄水运行后这些江心洲的演变深为人们关注。选取2000~2011年宜昌、汉口、大通站的年均流量和输沙量,1999、2001、2003、2005、2007、2009和2011年长江中下游20个代表性江心洲的遥感影像,分析三峡水库蓄水前后水沙变化及江心洲面积变化过程,同时利用概念模型分析江心洲面积变化与流量和含沙量之间的关系。研究表明:(1)三峡水库蓄水后(2003~2011年)宜昌、汉口、大通站的年均径流量较蓄水前分别减少6.1%、7.3%、9.5%,输沙量分别减小90.3%、72.1%、66.9%,平均含沙量分别减少了89.6%、70.8%、64.8%;(2)2003年蓄水前,12个(60%)江心洲面积逐年增加,相对于1999年其平均变化率为26.8%;2003年蓄水后,只有9个(45%)江心洲面积逐年也增加但是平均变化率减少至22.1%,11个(55%)江心洲面积逐年减少,相对于1999年其平均变化率为19.4%;(3)概念模型分析表明:江心洲面积变化量跟水位、冲蚀量成正相关,而水位、冲蚀量跟流量、悬沙减少量成正相关。江心洲面积变化量主要受水位变化控制,这种关系导致长江中下游江心洲演变的一种假象:既蓄水后面积不断增加的少数江心洲实际上也处于不断冲蚀的过程。

EVOLUTION ANALYSIS OF CHANNEL BARS IN THE MIDDLE AND LOWER YANGTZE RIVER BEFORE AND AFTER IMPOUNDMENT OF THREE GORGES RESERVOIR

The middle and lower Yangtze River develops a large number of channel islands, which attract research interests after impoundment of Three Gorges Project (TGR). The data of average annual runoff and sediment discharge of Yichang, Hankou and Datong During 2000-2011 are analyzed. The 20 representative channel bars are selected as research objects based on remote sensing images in 1999, 2001, 2003, 2005, 2007, 2009 and 2011. The area change of channel bars are closely linked with the annual runoff and sediment discharge before and after the impoundment of TGR. A conceptual model is developed to analyze the relationship between changes of bars area and the runoff and sediment discharge. It shows that: 1) after the impoundment of Three Gorges Reservoir (2003-2011), average annual runoff of Yichang, Hankou and Datong decreased by 61%, 73% and 95%, respectively; sediment discharge decreased by 903%, 721% and 669%, respectively; average sediment concentration decreased by 896%, 896% and 896%, respectively; 2) before the impoundment, 12 bars (60%) area is increasing year by year, with an average rate of 268% on the basis of 1999; after the impoundment, only 9 bars (45%) area also increased, the average rate reduced to 221%, 11 bars (55%) area was reducing with an average rate of 194%; 3) the conceptual model shows that there is the positive correlation between the change of channel bars area and water level and erosion. Moreover, water level and erosion have the positive correlation with flow discharge and suspended load reduction. The area change of channel bar is mainly controlled by water level change. This relationship produces an illusion of channel bars evolution in the middle and lower reaches of the Yangtze River: the area of the minority of channel bars increases after the impoundment of TGR because the serious riverbed erosion occurred, but actually they are undergoing the erosion process