Studies On Sediment Characteristics And The Influencing Factors In The Yangtze Estuary Riverbed During The Recent 30 Years

As the estuary of the largest river in China,the Yangtze Estuary has suffered a long-term and high-intensity human activities.As a result,the dynamic sedimentation process of the sediment in the estuary is very complicated,and the erosion and deposition of the river bed is drastic.The sediment of the Yangtze River has decreased sharply with the water conservancy project built in the drainage basin and estuary in recent decades,which have a significant impact on the terrain,geomorphology,and sedimentary environment of the Yangtze Estuary.More than 30 years have passed since the comprehensive survey of Shanghai coastal and tidal resources in the 1980 s.In order to understand the recent changes of the Yangtze Estuary,large-scale sediment samples and associated flow-sediment data were collected and analyzed from the Yangtze Estuary in September 2015,and sediment historical data from 1982 to 1984 as well as the topographic data are collected.The sediment historical data is corrected by the past particle size test method simulated in the lab.The sedimentary characteristics and changes of channels in the Yangtze Estuary in recent 30 years are analyzed through particle size analysis,spatial interpolation,index extraction,type classification,and erosion and deposition analysis methods.Based on the above data,the influencing factors and mechanism of action are analyzed,and the distribution characteristics of cohesive sediment and its partition method in the Yangtze Estuary are discussed,which can provide reference and basis for the evolution of the Yangtze estuary riverbed and prediction of erosion and deposition in the delta.The results are shown as follows:1.The surface sediment types(in Folk's classification)of the Yangtze estuary include 7 types,which are mud,silt,sandy mud,sandy silt,muddy sand,silty sand,sand.Sandy silt accounts for the most.The sand components are mainly distributed in the upper channel and the shoals of the entrance in the north.The silt components are widely distributed in the middle and lower river channel and the outer estuary area.The clay component is mainly distributed in the southern area of the outer estuary.The riverbed is dominated by sandy silt and silty sand.The main channel is mainly silty sand.The shoal is dominated by sandy silt.The outer estuary is mainly sandy silt,silt,mud.The median size distribution showed a downward trend from the west to the east and from the north to the south in the entrance section.The river sediments are coarser in the main channels and finer on the shoals generally,but differ from different channels.The shoal area within the 5m contour line of the outer estuary is characterized by coarser in the north and finer in the south.The content of fine sediments in the Yangtze Estuary gradually increased from the West to the East,with the highest content in the outer estuary.The areas with a content of less than 32 ?m are mainly distributed in the 10 m-20 m area outside the mouth.The higher content of fine sediments in the mouth is mainly distributed in the North Branch,South Branch,and north of Chongming East Beach.On the other side,the areas with low content of fine sediments are distributed in the upper North and South branches,the shoal area within 5 m of the North Channel and the east side of Guyuansha.2.Sedimentary characteristics of the river channels in the Yangtze Estuary have significant differences.In the upper South Branch,the median particle size shows a gradual decrease from upstream to downstream and the cohesive sediment content shows a gradual increase.The sediments are coarse both in the main channel and shoal,which are mainly sand and silty sand,and belong to the deposition of river facies under the effect of runoff.The upper North Channel is affected by the Qingcaosha reservoir project,and the sediments are coarser on the shoals and finer in the main channels generally.Sedimentary characteristics of the upper South Channel generally extend the characteristics in the upper North Branch.The promote siltation project of Hengsha East Beach in the lower North Channel leads to the reduction fine particle sediment source.In addition to strong wind waves,the cohesive sediment content on the shoals is significantly less than that in the main channels.Sediments are coarser than the upper channel.The North Passage is disturbed by man-made factors and the sediments are obviously finer than other channels,which shows the characteristics of suspended sediments.The sediments in the south Passage are coarser in the main channels and finer on the shoals generally.The type is dominated by sandy silt.The cohesive sediment content on the north shoal is less than that in the south,reflecting the difference in erosion and deposition environment between north and south.Sediments in the North Branch are mainly silty sand,sandy silt,and sand.The median grain size of sediments generally varies from upstream to downstream with coarse and fine features,but the characteristics of the main channel and shore are different.Sediments in the upper North Branch are coarse both on the shoals and in the main channels.In the middle branch,sediments are coarser on the shoals and finer in the main channels.Sediments in the lower branch are coarser in the main channels and finer on the shoals.That reflects the differences in sedimentary dynamic environment of the North Branch.3.Compared with the 1980 s,the median size of sediments in the Yangtze River Estuary is mainly reduced,which is significantly in the North Passage and Sanhegang in the North Branch.On the other hand,the median grain size has increased in the South Passage,lower North Channel and outer estuary of the North Branch.The influence of river material changes is smaller in the outer estuary below 10 m,where the change rate of median size and fine sediment content is less than the areas above 10 m.That means the sedimentary environment is basically stable.The cohesive sediment content of the Yangtze Estuary shows an overall upward trend,reflecting the enhancement of suspended sediment sedimentation in the channel.The increase in Sanhegang in the North Branch,the northern side of the in Chongming Eastern Beach,and the middle North Passage is significant.On the other hand,the composition of cohesive sediments decreased on the South Passage,the entrance of the North Channel,and east side of Guyuansha,reflecting the enhancement of wind waves in the outer estuary.4.The main factors affecting the sedimentary environment of the Yangtze Estuary include changes in the source of material and dynamic conditions.The difference of sediments origins is the leading factor which cause the sedimentary environment deviation.Sediments in the South Branch and upper South Channel show a characteristic of riverine sediments,while the sediments in the North Channel,North Passage and South Passage show a mixture of riverine and marine sediments.The erosion and deposition of the terrain changed the local sedimentary environment of the channel.The ebb and flow of sediment transport and sedimentation is an important factor affecting the vertical distribution of the channel.The sedimentary environment of the entrance section differences from north to south are strengthened by the wind wave.The salinity changes have affected the sedimentation process of the fine sediments in the river channel.The resuspension of sediments is an important source of fine particulate matter for the riverbed.5.The content of clay particles plays an important role in sediment properties for estuarine sediments.Significant differences are shown in sediments with different cohesive sediment contents.The critical start-up and erosion stress are quite different from those of general sediments especially for cohesive sediments after the consolidation,which are important for the evolution of estuary channels and the erosion and deposition of outer estuary.The area with clay content(< 4 ?m)above 15% in the Yangtze Estuary is mainly located outside the 10 m contour line,especially for the outer estuary of the South Passage,which contains over 30% of clay.It has strong antierosion ability.The terrain is basically stable.The area with clay content(< 4 ?m)below 15% in the Yangtze Estuary is mainly located in the upper North and South Branch,the entrance of the North Channel,and east side of Guyuansha within the 5 m contour line,mainly silty sand,silt and sand.The comparison shows that the region which <4 ?m content above 15% has good similarity with the region <32 ?m content above 50%.Thus,the <32 ?m content above 50% can be used as the standard for defining the cohesive sediment in the Yangtze Estuary.