On The Characteristics And Influencing Factors Of Sediments Over The North Branch And Offshore Area Of The Yangtze Estuary

Sedimentary characteristics are the result of long-term action of sedimentary environment,which contains abundant information of sedimentary environment.Because of the interaction of runoff and tidal current,the mixing process of salt and fresh water,and the frequent action of wind wave,the dynamic process is complicated in the estuary,the flocculation,subsidence and resuspension of sediments also have an important impact on the sedimentary environment,which lead to the complicated sedimentary characteristics.The North Branch of the Yangtze Estuary is a typical declining tidal estuary.Since the South Branch of the Yangtze River transformed to the main channel to the sea in the middle 18 th century,the North Branch channel has been shrinking.Especially in the past 30 years,the amount of sediment coming from the basin has dropped sharply,which has a profound impact on the sedimentary environment of the channel and the underwater delta of the North Branch.Therefore,under the dual effects of natural evolution and human activities,it is of great theoretical and practical significance to study the sedimentary characteristics,mechanism and trend of the declining tidal estuary of the North Branch for the research of estuarine geomorphological evolution,engineering regulation measures and ecological environment protection.Based on the field surveys of surface sediments,fixed-point hydrological observation,as well as the historical data of sediment survey in the coastal zone in the 1980 s and the topography in the past 30 years,the recent grain size characteristics and spatial differences of sediments in the North Branch and offshore area of the Yangtze Estuary is analyzed by laboratory experiment analysis,experimental analysis and correction,combined with interpolation and digital methods of elevation.Based on the corrected historical grain-size data,the grain-size variation characteristics and mechanism in the North Branch and offshore area of the Yangtze Estuary in the past 30 years are discussed.On this basis,the role and mechanism of different factors in the evolution of sedimentary environment in the North Branch are studied.At the same time,the evolution trend of sedimentary environment in the North Branch under the influence of both natural and human activities are discussed in combination with the process of human activities such as the sharp reduction of water and sediment from the Yangtze River and reclamation project construction.The main conclusions are as follows:1.Grain size characteristics of sediments(1)Sediment components and types: the contents of sand,silt and clay are 37 %,46 % and 17 % respectively,mainly composed of silt and sand.The sediment types are mainly composed of clayey silt,sandy silt and sand,and the clay silt content is the highest.(2)Grain size characteristics and grain size parameters of sediments: The distribution of median grain size of sediments showed a bimodal state,ranging from 6.2 ?m to 235.4 ?m with an average of 53.6 ?m.In general,the mean median grain size of the sediments in the North Branch and offshore area are both decrease from west to east.The sorting coefficient of sediments are between 0.6~2.9.On the whole,the sorting of sediments in the North Branch and offshore area are generally poor,while the North Branch channel inside the mouth is better than that in outer estuary of the North Branch;The skewness coefficient of sediments ranges from-0.17 to 0.8,and the grain size curve is generally positive skewness,with more coarse grain components.The skewness coefficient of sediments in the North Branch channel inside the mouth is larger than that in outer estuary of the North Branch.The skewness of the area with coarser grain size is relatively higher.The sediment kurtosis coefficient ranges from 0.68 to 3.68,and the grain size curve is sharp.The kurtosis inside the mouth of channel is larger than that in the outer estuary of the North Branch,and the kurtosis of the area with coarser grain size is relatively higher.(3)Grain shape characteristics of sediments: Sediment particles are close to the sphere.The surface of sediment particles is smooth,and the symmetry and width-length ratio of sediment particles are basically close.It indicates that the particle properties in the North Branch and outer estuary of the North Branch are relatively consistent.Strong currents and waves are the external dynamic environment that form the grain shape characteristics of sediments.Frequent exchange of sediment inside and outside the mouth of the North Branch is an important factor in the similarity of sediment particle shape parameters.2.Sedimentary Characteristics of the North Branch and offshore area of the Yangtze EstuarySedimentary characteristics of the North Branch are obviously segmented,and there are significant differences in sedimentary characteristics of different water depth areas in offshore area of the North Branch.(1)Sedimentary characteristics of the North Branch are obviously segmented: The sedimentary characteristics of the North Branch channel vary drastically and segment obviously.The reach from Chongtou to Lingdian harbor: the sediment type is mainly sand;grain size is coarse with a single peak shape and sharp kurtosis.The reach from Lingdian harbor to Sanhe harbor: the content of sand in sediment components decreases rapidly;the content of silt and clay components increases,the sorting becomes worse,the skewness and kurtosis decrease sharply,and the type of sediments transit rapidly from sand and silty sand to clayey silt.The reach from Sanhe harbor to Lianxing harbor: the sedimentary type is mainly clayey silt with fine particle size;median particle size is 13.5?m;poor sorting and medi?m kurtosis.(2)There are significant differences in sedimentary characteristics between different water depths in offshore area of the North Branch:(1)The shoal from 0 to 2 m isobaths is susceptible to wind wave.The sediment is coarse with a median grain size of 150 ?m and the sediments component is sand(93.4 %).(2)In the area between 2~5 m isobaths,the content of sand in sediment components decreased,and the content of clay and silt increased.The sediment type is still dominated by sand.(3)The area between 5~10 m isobaths is in the transitional stage.The sedimentary characteristics of the upper section are similar to those of the 2 m isobaths.In the lower section,the content of clay and silt of the sediment's component are further increased and the content of sand is further decreased to 40~50 %.(4)The 10 m isobaths is approximately the boundary between sand and silt.In the area between 0~10 m isobaths,the grain size is coarse.The sand content is high and the sedimentary type is mainly sand.The grain size characteristics change with the water depth.However,in the area outside the 10 m isobaths,the grain size is fine.The content of sand and clay is 20 % ~ 30 % respectively,and the content of silt is 50 % ~ 60 %.The variation of grain size changes little with the increase of water depth,and the spatial distribution of sedimentary characteristics is relatively stable.3.Changes in sedimentary characteristics of the North Branch and offshore area of the Yangtze Estuary in the past 30 yearsOn the whole,the sediments in the North Branch channel(including the south channel of the Guyuan sandbank)tend to be finer;the grain size in the shoal outside the mouth of the North Branch become coarser;which is relatively stable in the underwater area outside the mouth.(1)Compared with the grain size statistics of sediment samples collected in the 1980 s,the particle size range becomes larger,and the average median grain size decreases in 2015.In particular,the specific gravity of the coarse fraction have decreased,the specific gravity of the fine particle size have increased.The frequency distribution of each fraction is relatively uniform.(2)The characteristics of sedimentary change are shown as follow: The sediment of the North Branch channel has become finer,the grain size in the 0~10 m isobath area outside the mouth has become larger,which is relatively stable in the area outside the 10 m isobaths.The change of the sediment characteristics reflect the change of sedimentary environment characteristics: the grain size of the North Branch channel has decreased,which indicates the overall siltation environment in the channel.Due to the action of wind wave and insufficient sediment supply,the sediment in the 0~5 m isobaths area has coarsened.In the area outside the 10 m isobaths,the sediment grain size changed little,and the sedimentary environment is relatively stable because the wind waves are weakened in areas where water depth is large.(3)In general,from the perspective of time variation,the sediments in the North Branch channel tend to be finer;the sediment in the shoal outside the mouth(within 10 m isobaths)tend to be coarser;the underwater area outside the mouth(outside 10 m isobaths)is relatively stable.This is basically consistent with the trend of sedimentary characteristics change around 30 years.The period from 1990 s to early 21 st century was a relatively rapid overall refinement of sediments in the North Branch Channel.4.Mechanism analysis of sedimentary environment evolution in the North Branch and offshore area of the Yangtze Estuary under the natural conditions(1)The distribution pattern of the sedimentary environment in the North Branch and offshore area is affected by the source of sediments,and the change of it influences the adjustment and trend of sedimentary environment in the North Branch and offshore area.(2)The asymmetry of ebb/ flood tidal current is a direct factor affecting the difference of sedimentary environment in the North Branch.The asymmetry of ebb/ flood tidal current drives the sediment out of the mouth move to the North Branch channel inside mouth,resulting in the overall siltation and shrinkage of the channel.And sediment becoming finer.(3)The divergence between the flood and ebb current aggravated the local difference of the sedimentary environment in the North branch channel.The divergence between the flood and ebb current strengthens the sedimentary environment of erosion on the north bank and siltation in the southern shoal.The coarse grains in the middle of the river channel are easy to accumulate.The central bar,sand spit and tidal ridges have developed.Forming a sedimentary environment of thalweg in the north of channel,fine particles siltation in the south shoal and the typical landform development in the middle channel such as tide ridges.(4)Wind wave has the function of lifting sediments,and is the main driving force for the coarsening of the shoal sediments.Northerly and easterly wind wave prevail outside the mouth(3.1 m isobaths).The mean effective wave height is more than 1 m,and the northerly wave's is more than 1.2 m.The disturbance to the shoal outside the mouth is strong by the remarkable action of wind wave.In the area above 10 m isobaths outside the mouth,the sediment is coarse with high content of sand component due to the frequent wind wave effect.With the increase of water depth,the grain size and sand content decrease,while the silt and clay content increase.The effect of wind wave is weakened outside the 10 m isobaths,while the total sediment grain size is relatively fine and the spatial distribution of grain size characteristics is not much different.(5)The topographic erosion and accretion are not only the result of the change of sedimentary environment,but also the inducing factor affecting the change of sedimentary environment.It is of instructive significance to the change of sedimentary environment in the North Branch and offshore area.5.Evolution trend of sedimentary environment in the North Branch and offshore areaUnder the combined effect of natural evolution and human activities,the North Branch channel is generally in the trend of reduction sediment in the upstream and enhancement of the action of predominant flood current in the downstream.On the whole,the North Branch channel is in a silting environment,and the channel is shrinking.Due to the lack of sediment supply in the offshore area of the North Branch,the terrain changed from rapid acc?mulation to relatively stable and slightly eroded state.In the future,the evolution trend of sedimentary environment in the North Branch and offshore area will maintain,and the North Branch channel will shrink further.To improve the water and sediment conditions in the upper mouth of the North Branch,and carry out the regulation of the channel in the North Branch and control the reclamation project in the North branch are feasible measures to slow down the continuous shrinkage of the channel in the North branch.