长江下游潮流界变动段三益桥边滩与浅滩演变驱动机制分析

Driving mechanism of Sanyiqiao point bar and shoal evolution in fluctuation segment of tidal current limit in lower reaches of Yangtze River

潮流界变动段的边滩与浅滩演变关联性强,同时受径潮流水动力、供沙来源及人类活动等多重影响,是航道治理与疏浚维护的重点河段。通过对三益桥河段1976-2017年期间河床冲淤、汊道分流比、三益桥边滩及浅滩演变过程的分析,明确三益桥边滩及浅滩演变的驱动机制。2012年以来三益桥边滩12.5 m水深以浅滩体体积为增大态势,大水年份边滩以淤积为主,设计航槽及深槽以冲刷为主,中水年份边滩淤积厚度小于深槽。上游五峰山弯道河势稳定,具有阻隔上游和畅洲河段河势、汊道分流比调整等传递作用,三益桥边滩淤涨(长)与上游和畅洲河段河势及汊道分流比调整的关系不显著,主要与流域来流流量大小及过程、上游河道冲刷供沙等相关。洪季三益桥上浅区碍航程度大于枯季,汛前中水流量(大通水文站流量介于26000~34000 m^3/s之间)持续天数长的年份碍航程度大于大洪水年份同时期;因此,流量过程决定边滩与浅滩的冲淤分布,中水流量持续时间长短及供沙量大小决定三益桥边滩淤积量及浅滩碍航淤积量。

A strong correlation exists between the evolution of the point bar and shoal in the fluctuation segment of the tidal current limit in the lower reaches of the Yangtze River,with simultaneous influences from the hydrodynamic force of runoff and tidal currents,sediment sources,and human activities.This segment is crucial to channel regulation and dredging maintenance.Based on analyses of riverbed scouring and silting,branch channel diversion ratio,and evolution of Sanyiqiao point bar and shoal from 1976 to 2017,the driving mechanism of the evolution of this bar and shoal was clarified.Since 2012,at a depth of 12.5 m at the Sanyiqiao point bar,the shoal-body volume has been increasing.During high rainfall years,the point bar was dominated by silting,and the designed and deep channels were dominated by scouring.In medium rainfall years,the thickness of point bar deposition was less than that in the deep channel.Wufengshan bend in the upper reaches showed a stable river regime and had the functions of blocking the river regime of the upstream Hechangzhou River segment and adjusting the branch channel diversion ratio.The relationship between siltation at the Sanyiqiao point bar and the river-regime adjustment and branch channel diversion ratio of the upstream Hechangzhou River segment was not significant.It was mainly related to the scale and process of incoming flow in the river basin as well as scouring in the upstream river segment as sediment source.During the flood season,navigation obstruction in the upper shallow area of Sanyi Bridge was greater than that during the dry season.Navigation obstruction in years with a long duration of intermediate water flow before the flood season was greater than that during the same period in flood years(flow at Datong hydrological station was 26000-34000 m^3/s).Therefore,the flow determined the scouring and silting distribution at the point bar and shoal,whereas the duration of intermediate water flow and amount of sediment supply determined the siltation volume at the point bar and shoal,respectively,obstructing navigation.