Recent Sedimentary Environment And Sediment Transport Of Typical Shoal Sections In The Yangtze River Estuary

The delta of the estuary into the sea is a delta sedimentary system.When rivers flow into the ocean,due to the widening of the estuary,the reduction of flow velocity,the interaction of tidal runoff,and the mixing of salt and fresh water,a large amount of sediment carried by the river is deposited and gradually developed into a delta.In recent decades,due to the rise of sea level caused by climate change and the impact of human activities in the basin,the sedimentary process and geomorphological characteristics of the estuary delta have changed to varying degrees.Estuary shoal is a typical sedimentary landform phenomenon in estuarine deltas,and it is very sensitive to the dynamic process of estuary and the source of sediment in the river basin.Therefore,studying the dynamic depositional environment and sediment transport mechanism of the estuary shoal is of great significance to the evolution of the estuary delta.The Yangtze River estuary is a super large estuary with medium tidal intensity.A huge amount of sediment from the Yangtze River enters the sea,forming a vast underwater delta sedimentary landform system in the estuary area,Chongming Dongtan,Hengsha Shoal,Jiuduansha and Nanhui Dongtan are the four typical shoal geomorphological units of the Yangtze River Estuary.In recent decades,due to the sharp decrease in the source of sediment in the Yangtze River Basin,the sedimentary environment and sediment transport process of the estuary shallows have undergone significant changes.Therefore,it is of great significance to discuss the dynamic deposition process,sediment transport characteristics and mechanism of the typical shoal section in the front of the Yangtze River Delta under the background of the sharp decrease of the Yangtze River's sediment into the sea.Based on the comprehensive survey data of the dynamics,sedimentation,and geomorphology of the four typical shoal sections of the Yangtze River Estuary in July 2017,combined with historical topography data and the data of the Yangtze River inflow and sediment(Datong Station),this paper analyzes the hydrodynamic,sediment and sedimentary geomorphological characteristics of typical shoal sections in the front edge of the estuary delta,and calculates the characteristics of the water and sand transport flux and net flux of the typical shoal section under the background of the reduction of sediment from the Yangtze River into the sea,studies the dynamic mechanism and influencing factors of sediment transport process in typical shoal section.The specific conclusions are as follows:1.Sedimentary dynamic environment of typical shoal section in the front of the Yangtze River DeltaThe typical shoal section of the Yangtze River Delta front is steep in the north and gentle in the south,with a water depth of 2.0 m?20.0 m and a topographic gradient of 0.42‰and 0.56‰at the Chongming Dongtan section in the north and 0.56‰in the Hengsha Dongtan section,respectively;The section and Nanhui Dongtan section are 0.31‰and 0.30‰respectively.There are six types of sediments in the typical shoal section of the front of the Yangtze River Delta:argillaceous sand,silty sand,sandy mud,sandy silt,mud and silt,among them,the content of sandy silt is the highest,which is distributed in the depth of each shoal section;the second is mud and silt,mud is mainly distributed in the deep water area(10 m?20 m)of the southern shoal section,while silt sand is mainly distributed near the water depth of 10m.The median size of the shoal sediments in the delta front of the Yangtze River Estuary is between 6.0 and 211.3?m,with an average median size of32.0?m,the median particle size of the section sediments shows a decreasing trend from north to south.The median size of sediments showed a decreasing trend with the increase of water depth.The degree of sediment sorting is poor overall,with skewness ranging from-0.17 to 0.78,with an average value of0.21,kurtosis ranging from 0.68 to 3.48,with an average value of 1.09.As the water depth increases,the velocity rotation of the typical shoal section of the Yangtze River Estuary continues to increase.In the shallow water area,the tidal current is affected by runoff,topography and other factors,presents an obvious reciprocating flow.The ellipticity of the tidal current begins to increase near the 10 m isobath outside the mouth and gradually transforms into a rotating flow;the rotating flow phenomenon in the southern section is more pronounced than in the northern section.The average velocity of the section is 0.71 m/s and the distribution of velocity at different depths is obvious.2.Characteristics of Suspended Sediment Distribution and Influencing Factors of Typical Shoals in the Front Edge of the Yangtze River DeltaThe sand content of the front shoal of the Yangtze River Delta presents a flat distribution characteristic of high in the south and low in the north,and high in the west and low in the east.Within the 5m isobath,the sand content is significantly higher than other deep water areas.As the water depth increases,the sand content decreases,and beyond the 10m water depth line,the sand content drops below 0.1 kg/m~3.The instantaneous vertical distribution of sand content is more complicated:the shallow water area in the west and the high sand area outside the southern slot are mostly oblique,and the vertical distribution of sand content in the saltwater wedge active area is mostly parabolic,and the 20 m range near the deep line is a mixed type.Under the same water depth,the sand content of each hydrological station during the observation period showed a feature that the rising tide was greater than the low tide.The distribution of suspended sand at the 2 m isobath of the estuary shoal is relatively uniform in vertical distribution,with high sand content,and the characteristics of sediment resuspension are significant;the vertical tidal periodic changes of the suspended sand around the 5 m isobath are obvious,and the sediment is separated.The layer phenomenon is significant;the 10 m isobath outside the mouth and the deep waters are little affected by runoff.Due to the large water depth,the resuspension of sediment is weakened,and the sand content is generally low.In addition,the impact of the Yangtze River runoff on the distribution of suspended sediment,the difference in suspended sediment concentration at the bottom of the surface generally shows a decreasing trend from west to east.3.Characteristics and mechanism of single-width sediment transport in typical shoal sections on the front edge of the Yangtze River DeltaIn general,the single-width section of Hengsha Shoal has the strongest sediment transport flux,followed by the Nanhui Dongtan section,and the Chongming Dongtan and Jiuduansha sections are relatively weak.In the offshore direction,the 2 m isobath is mainly affected by the diffusion of runoff from the estuary into the sea,and the single-width sand transport direction is northeast and southeast;The single-width sand transport direction of the 5 m contour line rotates clockwise as a whole;the single-width sand transport situation of 10 m and 20 m isobaths is mainly affected by the resuspension of sediment outside the mouth:Outside the North Port,part of the re-suspended sediment points to the front end of Hengsha Shoal,participating in the cross-section shaping process of Hengsha Shoal,and part of it enters the North Passage to silt with the water;the single-width sediment transport outside the Nancaokou mainly follows the depth contour to the west-south and east-south directions,participating in the coastal transport of sediment outside the mouth.In the coastal direction,during the tide cycle of the shoal section,the coastal sediment transport tends to transport southward.The single-wide sediment flux southward is strongest in Nanhui Dongtan and Hengsha Shoal.The Chongming Dongtan section has a small overall sediment transport volume.Except for a small part of the sediment discharged from Beigang Runoff,it crosses the Chongming Dongtan and enters the North Branch.The overall performance is transported to the south.The sediment transport of a single-width section of the Yangtze River Estuary shoal is significantly affected by the water transport caused by the residual current.The residual flow velocity in the study area varies within the range of 0.05?0.36 m/s,and the general trend is to the south.The residual flow velocity in the northern section is significantly greater than that in the southern section.As the water depth increases,the residual current intensity gradually decreases;From the perspective of the vertical difference of residual current,the velocity of surface residual current is about 3 to 4 times that of the bottom layer,and the effect of runoff affects the surface and bottom layer distribution of estuary shoal residual current.Affected by the fresh water of the Yangtze River,during the observation period,the main contribution of the single-width water transport in the shallow water area of each shoal section is the Euler residual flow.As the water depth increases,the Stokes residual flow will gradually replace the Euler residual flow Become a major contribution item.4.The net transport process and mechanism of suspended sediment in a typical shoal section of the estuaryIn the past 30 years,the four shoal sections of the Yangtze River Estuary have generally shown a siltation-equilibrium-local erosion trend.The evolution trend of the typical shoal sections in the delta is more consistent with the changes in the sediment flux of the Yangtze River into the sea.The direction of net sediment transport is basically the same as the direction of residual flow in the estuary shoal,and the spatial distribution of residual flow controls the overall trend of the direction of net sediment transport in the estuary delta.The net transport intensity of sediment in the estuary shoal mainly depends on the change of the water body's sand content.The water body's sand content is inversely proportional to the water depth.The distribution of high sand content in the coastal water body is closely related to the resuspension of sediments.The action of wind and waves is the coastal shoal The main driving force of sediment resuspension.In the east-west direction,the net sediment transport is generally shown as eastward net transport.The surface and bottom layer differentiation phenomenon appears in the net sediment transport,which is mainly manifested by the characteristics of sand transport from the surface layer to the east and the bottom layer to the west.With the presence of sediment,the vertical average net sediment transport direction is mostly consistent with the bottom net transport direction.The net transport of sediment in the north-south direction is generally shown as the net transport to the south.The south transportation characteristics of the Hengsha Dongtan section and the Nanhui Dongtan section are more pronounced.The net transport of coastal sediment to the south is relatively consistent in the table and bottom net transport.The southward net transport of water from the coast is the main driving force for the net transport of sediment.The resuspension of the sediment produced by wave breaking is the key factor for the high-intensity net transport near the coast and the imbalance of the coastal sediment transport.The source of sediment is the controlling factor of the scouring and silting evolution of the shoal sections on the south and north sides of the estuary delta.The shoal section on the north side of the delta mainly depends on the supply of sediment discharged from Beigang.In recent years,due to the sharp decrease of sediment sources,the underwater slope of the north shoal section has been slightly eroded,and the northern section of the delta is more sensitive to changes in the sediment flux into the sea;The south side of the shoal section is not only recharged by the sediment discharged from the runoff,but also recharged by the sand transport on the north side.The siltation rate of the shoal section slows down and is relatively insensitive to changes in the sediment flux into the sea.