| Located at the hypersensitive zone of land-ocean interactions,estuarine tidal flats(ETF)not only are an important carrier for sediment transport and biogeochemical cycle,but also provide valuable land resources and potential space for urban development.However,ETF worldwide are facing new changing processes in recent years under the context of dramatic decline in fluvial sediment load and high intensive of land reclamation and water transportation engineering construction within the estuary.How to study the geomorphologic processes of the tidal flats through deposition dynamics at micro-scale is the key to understand the macroscopic and long-term tidal erosion and deposition mechanisms and patterns.South Nanhui tidal flat(SNTF),the largest continental margin tidal flat in the Changjiang(Yangtze)Estuary,serves as study object here.From April 2016 to May2017,surface sediments were collected monthly along the intertidal flat laterally during low spring tide.Meanwhile,high-resolution point cloud data were obtained simultaneously from a Light detection and ranging(LiDAR)system.By analyzing sediment components and grain size parameters,and constructing three-dimensional elevation model,monthly,seasonal and annual distribution characteristics of the surface sediment and evolution processes in different intertidal belts were studied.Moreover,the potential influencing factors of sedimentary dynamic process of SNTF and its development tendency were discussed.The main conclusions were summarized as follows:(1)SNTF presented obvious dominant current and sediment during a tidal cycle.Located at the edge of the estuarine turbidity maximum zone,SNTF indicated an effective wave height of 0.3 m near the mean tidal low waterline,an effective wave period of 3-4 s,and an average suspended sediment concentration of 9.74 kg/m~3 around the reach 20 cm above the bottom during a tidal cycle.The tidal current direction was roughly parallel to the shoreline.Both flow velocity and suspended sediment concentration of flood tide were greater than those of ebb tide,which facilitated the landward transport of sediment.Waves around SNTF were dominated by wind waves and swell.Wind in winter and summer were respectively offshore and onshore direction.Occasional events(typhoon and cold wave)can significantly increase the wave height and suspended sediment concentration near the bottom.Offshore flow conditions and sediment transport processes had been restored after the reclamation project.In the past half century,fluvial sediment from the Changjiang(Yangtze)River to the sea reduced greatly,with the sediment transport proportion during the dry season increasing gradually.However,suspended sediments concentration around the mouth bar and the adjacent coastal waters had no significant variation,still presenting higher values in dry season.(2)Monthly,seasonal,and intra-annual spatial-temporal differences existed in surface sediment components and grain size parameters.Sediment along the upper intertidal zone were coarser(dominated by sand with a percentage of 80%)while that of the middle-lower intertidal zone were finer(dominated by silt with a percentage of 70%).Sediment along the upper-middle intertidal zone were characterized by mixed features at the two sides with higher clay content,revealing a transformation of dominant sediment component.Monthly average sand and silt content in all sediment samples respectively showed increase and decrease trend from winter season to summer season,and as a result,sediments particles turned thicker in summer and finer in winter.Clay content in the sediment was relatively stable,with the high value occurring in water-logged area and adjacent upper-middle intertidal transition zone.Over the study period,the dominant component transition zone in the upper-middle intertidal zone moved gradually to the seawall,with coarse particles sampling sites decreasing continuously near the seawall.Sediments within the zone that is 0-100 m away from the seawall became finer while that within the zone 100-480 m away from the seawall turned coarser.According to the sedimentary dynamic environment zoning diagram,SNTF was characterized with strong hydrodynamics and relative strong interactions between tides and waves.(3)SNTF exhibited seasonal characteristics of weak erosion in Summer and Autumn,strong siltation in Winter and Spring.An erosion zone developed on the tidal flat that runs through the middle upper intertidal zone and western middle intertidal zone.The average flat elevation of SNTF fluctuated between 1.88 m and1.94 m from April 2016 to January 2017,followed by a sudden increase to 2.01 m in February 2017,and slightly declined thereafter.Over the study period,annual vertical deposition rate reached 10 cm.The upper flat accounted for a small area proportion and underwent shrinking.The area of middle flat decreased firstly,followed by a substantial increase and remained stable thereafter.The area changing tendency of the lower flat was contrary to that of the middle flat.The monthly variations of flat elevation,volume,and isobaths indicated weak erosion in Summer and Autumn over SNTF,and strong deposition in Winter and Spring,with the most remarkable geomorphological adjustment process occurring in August-September 2016 and January-February 2017.A strong erosion area(about 10-80 cm thick)was formed near the seawall,which runs through the middle upper intertidal zone and western middle intertidal zone.An obvious silting belt(5-30 cm)appeared in the upper flat along the western seawall,while a large-scale relative weaker silting zone(5-30 cm)occurred in the eastern middle tidal flat.In summary,transverse section that is 0-100 m away from the seawall showed erosion over the study period while that 100-480 m away from the seawall indicated deposition.Furthermore,the increase of tidal flat elevation was statistically positively and negatively correlated with the sand and silt content variation,respectively.(4)Stable tidal currents and suspended sediment transport modes provided favorable conditions for SNTF growth.Increased suspended sediment concentration,shoreward net sediment transport and relative low tide level in intertidal zone during the dry season caused significant deposition in Winter and Spring.Following the decrease of fluvial sediment discharge and the construction of deep-water channel project,water and sediment division of the South Passage increased gradually and“sedimentary bodies with shaking head”developed sustainably.Suspended sediment concentration in mouth bar area and turbidity maximum zone were basically stable due to seashore sediment supply and local sediment resuspension.After reclamation projects,SNTF still had a stable current and sediment circulation,residual sediment transport and dominant current and sediment,which contributed the sediment trapping in intertidal zone and promoted the accumulation of SNTF.During the dry season,high suspended sediment concentration,relative low tide level and weak sediment carry capacity synthetically facilitated siltation in the intertidal zone in Winter and Spring.However,Summer and Autumn season presented the opposite case.In general,the upper intertidal flat with vegetation showed higher deposition rate while the bare flat indicated relatively weak deposition rate.Meanwhile,sedimentary sand body gradually migrated westward.Strong erosion belt developed in the area where seawall has relative strong effects to the tidal current and the upper-middle intertidal transition zone with steeper slope.SNTF is still in a deposition state under the context of fluvial sediment reduction and large-scale of estuarine land reclamation,indicating that the sedimentary dynamic of SNTF was controlled by the flow and sediment dynamic environment of adjacent waters,but weakly related to the upstream sediment variation.It is urgent to understand the characteristics and mechanism of suspended sediment concentration variation in Changjiang Estuary against the background of fluvial sediment reduction. |
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