Analysis Of Sediment Grain Size Changes In The Yangtze River Estuary During The Past 40 Years

Large river deltas are areas of intense land-sea interaction and a transition zone for the exchange of marine and terrestrial material.Over the last 40 years,many of the world’s large river deltas have been undergoing continuous adjustments in sediment sources,transport and sub-bed siltation,driven by both global change and human activity.As a typical tide-controlled estuarine delta subject to strong human intervention,the study of the systematic evolution of the Yangtze River delta has been a frontier and hot topic of academic interest.The current Yangtze River delta faces the threat of erosion disasters and the topography,geomorphology and hydrodynamic conditions of the estuary have changed dramatically.In order to explore the spatial and temporal trends of sediment transport and bottom-bed changes in the Yangtze estuary in the past 40 years,this study combines sediment grain size trend models,various spatial statistical analysis methods and numerical model experiments under ideal conditions to reveal the grain size changes and the evolution of in the Yangtze estuary based on the surface sediment grain size and underwater topography data from 1980 to 2020,and analyzes the intrinsic influencing factors,and the conclusions are as follows:(1)The surface sediments of the Yangtze estuary show a significant coarsening trend,influenced by the reduction in incoming sand from the basin over the last 40 years,which is consistent with the flushing pattern that the estuary has shown in recent years.The sediment types in the Yangtze estuary have not changed significantly over the years,but the overall sand content of the surface sediments has increased,with areas of high sand content showing an opposite spatial distribution to areas of high chalk content,i.e.high sand content inside the estuary and high chalk content outside the estuary,and a lower and more evenly distributed clay content overall.Intense human activities,such as deep-water channel construction and reclamation projects,have caused the sediment particle size parameters in the Yangtze estuary to show an uneven patchy distribution in space.(2)The sediment transport trend of the northern branch is a net transport to the inner part of the channel,which is consistent with the continuous deposition of the northern branch in recent years,but due to channel dredging,the sediment transport trend has changed significantly compared with 2007.The sediment trend analysis indicates that the sediment center has a larger deposition magnitude;the sediment outside the mouth is transported in a general north to south direction,which may be related to the southward transport of the longshore current.(3)The results of the multi-year digital elevation model of the Yangtze River estuary show that: the deposition of the Yangtze River estuary was more of a naturalled change before the 1980 s,and more of an anthropogenic-led scouring process after the construction of dams and reservoirs in the upper reaches,especially after the Three Gorges Dam started to store sand;the reduction of sand in the Yangtze River basin caused the estuary to shift from overall deposition to erosion,with the period 2009-2012 being a transitional period in response to the reduction of sand in the upper reaches.After 2012,the Yangtze estuary changed to an overall erosion process;human activities in the estuary,such as deep-water channel construction,caused localised changes,mainly due to changes in local hydrodynamics and sediment sources.(4)The numerical simulation results based on the FVCOM model show that the Yangtze River estuary can maintain a stable state if the high amount of sand transport before the impact of human activities is maintained,and the overall content of each sediment component is stable,and the changes of all three components fluctuate in a small range,which indicates that the sediment changes in the estuarine bottom bed that have occurred are mainly closely related to the reduction of sediment supply in the basin.By further reducing the amount of incoming sand in the basin,significant changes were found in the sand,chalk and clay sediment fractions,as evidenced by the significant increase in sand and chalk content,decrease in clay content and significant coarsening of the sediment.This shows that the sediments of the Yangtze estuary will be further coarsened if the amount of incoming sand in the basin is further reduced in the future.The Yangtze estuary will face more complex external drivers in the future,including human activities such as watershed ecological protection,estuary waterway resource development,as well as global changes and extreme climate events.It is necessary to strengthen the fine monitoring of surface sediment and sub-bed siltation changes in the Yangtze estuary,and develop numerical models with more functional modules(e.g.human activities,bio-ecological processes)to support the science for future Yangtze River Delta integration and Yangtze River grand strategy protection.