三峡水库蓄水后关洲河段河势调整对松滋口分流影响

Influence of Channel Adjustment of Guanzhou Reach on Flow Diversion at Songzikou After Impoundment of Three Gorges Reservoir

关洲河段属典型的主流年内交替型分汊河道,三峡水库蓄水后该河段河势出现了较大变化。关洲河段河势调整对下游松滋口分流变化的影响作用逐渐凸显,也越来越受关注。作者利用实测的2002—2013年的河道地形资料和1984—2017年的水文观测数据,首先分析了三峡水库蓄水运用后关洲河段的河道演变特点,然后从河道冲淤、左汊发展与左汊主汊的维持时间等3个方面研究了上游河势调整对松滋口分流的影响。研究成果表明:三峡水库蓄水后,关洲河段以枯水河槽冲刷为主,枯水河槽冲刷量约占平滩河槽总冲刷量的90%;冲刷形式由起初的冲刷下切逐渐转化为冲刷左侧低滩及河床组成较细的左汊;分汊段深泓高程由左高右低转变为左低右高;枯水期右汊的主汊地位有所减弱;左右汊主流年内易位的临界流量呈现减小的趋势;枯水河槽的冲刷将引起中小流量(小于20 000 m3/s)时松滋口分流能力的减小,但随着关洲河段河道冲刷的逐步完成这一变化会有所削弱;关洲左汊的发展有利于增加较大流量(大于20 000 m3/s)时松滋口的分流比,且这一流量持续的时间越长,即左汊维持主汊地位的时间越长,松滋口年分流比越大。但左汊的过度发展可能会引起关洲河段主流不再发生年内交替,而是长期稳定在左汊河道,建议有关部门加强对关洲左汊河道的监测。

The Guanzhou reach,located in the middle Yangtze River,is a typical anabranching channel with its mainstream alternating between the two branches within a year.After the impoundment of Three Gorges Reservoir（TGR）,the channel of Guanzhou changed significantly.The influence of channel adjustment of Guanzhou on the flow diversion at Songzikou became obvious and has been paid more and more attention.Based on the topographical data in the period 2002—2013 and the hydrological data in 1984—2017,the channel evolution of the Guanzhou reach after the impoundment of TGR was analyzed.Then the impact of upstream channel adjustment on flow diversion at Songzikou was studied from the following three aspects： the channel erosion and deposition,the development of left branch,and the duration of the left branch staying as main channel.The results show that,after the impoundment of TGR,the Guanzhou Reach was eroded continuously,and about 90% of the total channel scouring occurred in the low-water channel.The scour pattern changed from purely downward erosion to left convex bank erosion along with left branch erosion due to finer river bed materials;now the thalweg elevation in left branch is lower than that in right branch which was opposite before.The status of the right branch being the main channel was weakened in dry season.The critical flow rate for the mainstream alternation was decreased.When the incoming flow was less than 20 000 m3/s,the flow diversion ratio at Songzikou was reduced,while the reducing trend was weakened gradually with the channel developing into an equilibrium state.The development of the left branch will increase the diversion ratio at Songzikou in larger discharge,and the longer the duration of the larger discharge,that is,the duration being the main channel for the left branch,the greater the annual averaged flow diversion ratio at Songzikou.It＇s worth noting that,the over-development of the left branch may cause the mainstream of Guanzhou Reach to cease alternating between two branches within a year,and to stay in the left branch.The field monitoring should be strengthened in the future and proper engineering measures taken if needed.