三峡工程运用后坝下游分汊型河道演变与调整机理研究

Study on processes of braided river downstream of the Three Gorges Reservoir

2003年三峡水库蓄水运行以来,进入长江中下游的年平均径流量变化不大,但输沙量锐减。分汊河段的演变对水沙条件的变化十分敏感,本文对长江中下游鹅头型、弯曲型、顺直型分汊河段的演变与调整进行了系统分析,分流比及汊道冲淤情况实测资料表明,长江中下游各种分汊河段的演变调整具有较好的一致性,比降较大的汊道趋于发展,与汊道分流比的大小关系不大。对此,分析认为主要是因为来沙大幅度减少后,河道由淤积型转换为冲刷型,水流条件的作用凸显,比降较大的汊道易于多冲或是少淤,随之呈现出发展态势。

Since the impoundment of the Three Gorges Reservoir in 2003,the variation of annual runoff downstream of the dam is not obvious,but the annual average sediment transport volume reduced significantly. It is generally accepted that change of the braided channel is very sensitive to the water sediment condition. This paper makes a systematic analysis on the evolution and adjustment of goose-head pattern diverged channels,bending braided channels,and straight braided channels,which are located in the middle and lower reaches of the Yangtze River. Observed data of diversion ratio and channel siltation show that adjustments of braided channels have good consistency,which means that channels with lower gradient tend to development,and this regular pattern has little relationship to the current diversion ratio. A reasonable explanation is that the status of river has been converted from accumulation type to erosion type because of sediment reduction,so that the flow conditions have strengthened. Finally channels with lower gradient will be less deposition or more scour.