| Sedimentary characteristics are the result of long-term action of many factors,such as geographical environment,hydrodynamic conditions and so on,which contain abundant information of sedimentary environment.The Sedimentary characteristics of the estuary sand island are complex and changeable due to the influences of river evolution,the interaction of runoff and tidal current,salt marsh vegetation and human activities.Chongming Island is a typical estuary sand island in the world,the sand island has been developing and extending since a large amount of sediment came into the Yangtze River estuary under the influences of water dispersion,mixture of fresh and salt water and the interaction of runoff and tidal current,which played a significant role in changing the sedimentary environment.In recent years,with the sharp change of sediment availability in the Yangtze River Basin and the construction of large-scale water conservancy projects,the sedimentary environment of Chongming Island and its surrounding areas has been profoundly affected.Therefore,under the dual effects of natural evolution and human activities,it is of great significance to study the sedimentary characterstics and the mechanism about the evolution of the sand island to have a better understanding of evolution of estuarine beach and enrich the theory of estuarine tidal flat sedimentation and protect estuarine resources and environment.Based on the surface sediment samples of 14 sections around Chongming Island from 2015 to 2018 and the flow-sediment data in the wet and dry season from 2013 to 2014 as well as the historical chart data of Chongming Island,we analysed the evolution of Chongming Island in recent years and studied the characteristics of tidal flat sedimentary environment around Chongming Island by laboratory experiments,data calculations and analysis.At the same time,the trend of Chongming Island beach sedimentary environment was discussed in combination with the process of human activities such as the sharp reduction of sediment from the upstream of the Yangtze River and the reclamation project construction.The results are shown as follows:1.The distribution of median grain size of beach sediments around Chongming Island showed a bimodal state,ranging from 9.1?m to 134.9?m with the average of 48.0 ?m.The Shepard classification results of sediments include 7 types,which are silt,silty sand,sand,sand-silt-clay,sandy silt and clay silt.The sandy silt forms the largest segment,accounting for 45.2% of all samples and widely distributes in the beaches around Chongming Islands.Silty sand accounting for 17.3%,and mainly located in the upper part of the north coast in Chongming Islands and the shoal of the middle section of the south coast.The proportion of clayey silt is 13.0% of the sample,which mainly located in the lower section of north coast in Chongming Islands and the intertidal zone on the north side of the Chongming East tidal flat.2.The tidal flat sediment around Chongming Island is dominated by sandy silt and silty sand.The average of the median size of sediment is 48 ?m,which displayed that the size sediments in south coast are largest while those in east coast are the smallest,and the sediment in north coast is between the above.In general,the south and north coasts have steep slope and narrow beach,and the slope of tidal flat on the north coast is slower than that of the south coast whose sediments become finer from west to east.The slope of the north coast is slow,and the area in the south coast between Chongtou and Miaozhen exists fine sediment.However,the east coast has slow slopes and wide beaches whose sediments become finer from the south to north.On the tidal flat profile,the content of clay components at high tidal flat is relatively high,and the sediments become finer with the water depth decreases.3.Influencing mechanisms of beach sedimentary environment around Chongming Islands under natural conditions include these bellow:1)The evolution of the southern and northern branches of the river regime contributes to the formation of the overall distribution pattern of the sedimentary environment of the beaches around Chongming Islands.(1)In recent years,the main channels of the south branch have been scoured and deepened,which makes the southern shore of Chongming Islands narrow,the slope steep and the sediment particles coarse.In some areas,such as Baimaosha North Channel,the sediment grains are slightly thinner than those of other beach sections along the south coast due to the recent siltation of the northern channel as a whole.(2)The northern branch channel is narrow and the deep channel becomes shallower,which makes the north coasts of Chongming Islands narrower and the slope slower than the south coasts.Because the northern branch is in the state of siltation as a whole,the sediment grains on the north coasts are thinner than those on the south coasts.(3)The continuous outward extension results in that the east tidal flat is wide and gentle,and sediment particles are relatively fine.The different characteristics of erosion and siltation along the north and south coasts cause the difference of sedimentary environment between the north and south sides of the Eastern Chongming Island.2)The effects of runoff and tidal current on the distribution characteristics of sedimentary environment on tidal flat profiles in Chongming Islands are obvious.The lower section of the north coast and the northern part of the east tidal flat of Chongming Islands are primarily controlled by tidal currents,and the sediments are mainly finegrained,with the tidal sediments being relatively fine.The middle part of the south coast and the upper part of the north coast are greatly affected by runoff,and the sediments are relatively coarse.3)The wind wave has a strong sorting effect on the sediments of the Eastern Chongming Island.SSE and SE are the main wind waves in spring and summer,while NNE and NE are the main wind waves in autumn and winter.On the one hand,the sediment size in the south of the Eastern Chongming Island is obviously larger than that of in the north owing to the strong tidal current and wind wave action.On the other hand,the sediments in the 0~2 m shoal of the Eastern Chongming Island are obviously sorted by wind wave,which leads to the obvious coarsening of the sediment in the shoal.4)Tidal vegetation mainly affects the distribution of fine sediments in the intertidal zone.The Common tidal vegetation around Chongming Island includes Phragmites australis,Scirpus trigonatus and Spartina alterniflora.The existence of tidal vegetation causes local turbulence in the water body,slows down the flow velocity and facilitates the deposition of fine-grained sediment.At the same time,the stem and leaves of vegetation have obvious adhesion to suspended sediment,which increases the sedimentation rate of fine-grained sediment on vegetation beach and promotes the refinement of beach sediment.4.The sedimentary environment of estuary islands is closely related to the water and sediment conditions in the basin.The evolution of estuary regime and hydrodynamic process have great significant impact on the sedimentary environment around the estuary islands.The reclamation of beaches around Chongming Island and the construction of deep-water waterway have resulted in the continuous changes of the sedimentary environment of beaches around Chongming Island.In recent years,Chongming Islands generally is still in a silting environment,especially in the east of island.With the sharp decreases of the upstream sediment and the intensification of human activities,the siltation rate of Chongming Island has slowed down slightly,while the underwater slope of some beach sections have been erosion.A large number of finegrained sediments in the high tide beach is encircled,which also leads to the overall coarsening of some beach sediments.Chongming Island is a typical estuary sand island in the world.With the practicable of “Chongming International Eco-Island” Program.Chongming Islands will remain the state that relatively stable on the south coast and gradually silting up on the north and East coasts. |
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