The Recent Morphodynamic Processes Of The Xinqiao Channel,Changjiang(Yangtze) Estuary

Estuary is an important interface for land-ocean interactions.Being characterized for convenient navigation and abundant resources,estuary provides a good material foundation for the development of human society.Changjiang Estuary is one of the economically developed and densely populated regions in China,its channel evolution therefore is severely influenced by anthropogenic activities.Flood channel is a vital geomorphic unit of tidal estuary that exits worldwide and suffers serious human interferences.As a typical flood channel,Xinqiao Channel(XQC)is the largest flood channel in the South Branch of Changjiang Estuary.The coupling effects of runoff,tide,and morphodynamic evolution of XQC are closely related to the stability of tidal flat and dike of Chongming Island,the safety of residents' water access,and the stability of river regime of the South Branch.Under the current scenario of rapidly fluvial sediment reduction,green seawalls construction around Chongming Island,and tidal forcing increasing,XQC is likely to undergo a great extent of changes.The study of hydrodynamic variation,sediment distribution pattern and geomorphological evolution process of XQC under the influence of human activities and runoff-tide interaction,therefore,will contribute to reveal the geomorphological evolution rules of human dominated flood channel,and also provide partial theoretical and technical support for a scientific management and utilization of flood channels.Water and sediment characteristics,suspended sediment transport mechanism,sedimentary dynamic and morphological evolution process of XQC are analyzed based on field observations and numerical model.Field data include fluvial water and suspended sediment discharges during 1958-2020 at Datong Station,water and sediment observations along XQC in 2002,2011,and 2021,surface sediment samples along XQC in June 2020,and 13 bathymetric maps of XQC from 1958 to 2020.Main conclusions are as follows:(1)Tidal current in XQC is dominated by reciprocating current and is basically consistent with the trend of river channel.Due to tidal wave propagation differences between the main channel of the South Branch and XQC,fluvial runoff entering the middle and lower sections of XQC mainly through Nanmen Channel,and the northward shift of flood current,the upper section of XQC exhibits flood dominance,showing that tidal volume and sediment transport in the flood period are greater than the ebb period while the middle and lower sections of XQC indicate the predominance of ebb.Meanwhile,Euler residual flow presents landward in the upper section of XQC,but shows seaward in the middle and lower sections,while Stokes residual flow indicates landward through the entire channel.Euler residual flow is greatly affected by runoff while Stokes residual flow has a relatively weak response to runoff.In addition,the spatial distribution of tidal asymmetry in the channel is mainly controlled by the interaction between residual current and tidal constituents.The upper section of XQC is dominated by flood,the middle section is the transition area between flood dominance and ebb dominance,while the lower section is dominated by ebb.(2)Xinqiao Channel can be divided into three sedimentary areas with obvious spatial sedimentary differentiation.Sediments in the upper section of XQC are mainly composed of silty sand,clay silt with poor sorting.The middle section of XQC is characterized by silty sand,mixed with sandy silt with better sorting.In the lower section of XQC,the sediments are mainly composed of silty sand,sand and sandy silt,with poor sorting.(3)Spatial differences of sedimentary mode along XQC are dominated by various factors.Sedimentary mode of the flood dominated upper section are mainly composed of sandy silt and clayey silt as fine sediment was transported landward to the channel top by the flood current.The top of the middle section is featured by sand as the fluvial water and sediment entering XQC through the Nanmen channel.Sedimentary mode of the middle section of XQC is dominated by silty sand due to the influence of shoalchannel lateral circulation and Biandan Shoal evolution.Sedimentary mode of the ebb dominated lower section are characterized by silty sand and sand because the high flow enters the channel through Xinxinqiao Channel as well as the tidal channel in low Biandan Shoal in flood season,and meets the landward prorogating tidal current.(4)The morphodynamic processes of XQC from 1958 to 2020 can be divided into three stages.In the first stage(1958-1997),the channel experienced cycles of erosiondeposition,together with upward-downward moving and widen-shrinkage changing,which is controlled by the coupling effect of runoff and tide.In the second stage(1997-2003),XQC suffered erosion due to the impacts of flood,with the channel extending upward and getting wide while the thalweg moving to the bank.In the third stage(2003-2020),XQC presented deposition with the sedimentation center locating at the upper section and the thalweg keeping stable due to human intervention,the northward movement of Biandan Shoal,and runoff-tide interaction.In addition,the construction of Dongfengxisha Reservoir will intercept part of water that previously enters into XQC,which may aggravate the deposition in the upper section of the channel and result in further shallowing of XQC.