Space Structure Of Sediment Transport In The North Channel Of Yangtze Estuary,China

Sediment is the link between hydrodynamic action and landform evolution.It is significance to study the spatial structure characteristics of suspended sediment and analyze the hydrodynamic and sediment transport laws.Based on the measured data,the synchronous water and sediment observation data of five stations uniformly distributed on two typical cross-sections of the North Channel main trough and the reach of mouth bars are selected.The observation data are decomposed into tidal averages and multiple data using quasi-harmonic analysis.With the fluctuation of the tide,the cross-sectional spatial structure characteristics of the suspended sand transport in North Channel and the contribution mechanisms of various dynamic factors are analyzed.A three-dimensional hydrodynamic sediment model of MIKE 3 was established to simulate the plane scouring and silting characteristics of North Channel River and the evolution trend of the river channel in the future to provide a basis for the development and utilization of North Channel River.Main research contents and conclusions are as following:(1)The half-day current in the North Channel of the Yangtze River Estuary is significantly greater than the residual current,which is the main hydrodynamic force of the North Port.The residual current is mainly driven by runoff.The quasi-harmonic analysis method was used to decompose the residual currents and tidal currents of North Channel hydrodynamics.Under the factors such as curve effect,Coriolis force,and density gradient,the cross-sectional characteristics of the flow velocity show the existence of a lateral circulation transport mode.The spatial structure characteristics of the cross section of the average sediment content in North Channel are related to the characteristics of the average tidal shear stress and cross-section velocity at the bottom of the riverbed.Moreover,the section of the reach of mouth bar is significantly higher than the main channel section.In terms of the sand content of each part,the average sand content c₀of the tide is much larger than the sand content of c₂and c₄which fluctuates with the tide.(2)The main channel sediment transport in North Channel is mainly composed of residual current transport(T₀),M₂current transport(T₂),and M₄current transport(T₄),and the spatial structure of the sediment transport volume driven by various hydrodynamic factors is significantly different.In the spatial structure of the total sediment transport volume(T)in the cross section of the North Channel main trough,the seaward sand transport on the north side of the river trough is significantly larger than the landward sand transport on the south side.The composition of sand(T₂),of which T₀is the main contribution factor.Landward sand transport is mainly composed of half-day tidal sand transport(T₂)and M₄tidal sand transport(T₄),of which half-day tidal sand transport(T₂)contributes the most.The cross-section Sediment transport in the reach of mouth bars is dominated by sea direction,and the main contributors are the residual stream transport,followed by half-day tidal transport and M₄tidal transport.(3)The spatial distribution of suspended sand in North Channel The suspended sand concentration of the main channel river section is higher than that of the inlet section and the section in the reach of mouth bars.The river channel has the characteristics of scouring and silting of"the main channel scouring and depositing on both banks".From 2006 to2011,the curvature of the upper reach of the North Channel continued to increase large,the trunk stream of the upper reach of the North Channel moves continually towards north.Since 2011,the river regime has not changed significantly due to the construction of the North-South Channel bifurcation control project and the Qingcaosha Reservoir.The model research results show that the river regime of North Channel will not change much in the next 60 years,and the river channel will continue to scour and deepen,basically meeting the shipping requirements.