| The tidal flat in the Yangtze River Estuary is located in the transitional zone of the boundary of the sea and land.It has very high primary productivity and unique ecological system.It is an important part of the regional resources of the Yangtze River Estuary.Under this background,the comprehensive research on the tidal flat,vegetation and tidal creek in the Yangtze River Estuary is of great significance to the maintenance of the stability of the tidal flat in the Yangtze River Estuary.This paper focuses on the changes of tidal flat deposition in the Yangtze River Estuary,the dominant vegetation on the tidal flat and the development of tidal channels system and comprehensively carries out the remote sensing monitoring study on the variation law of tidal flat in the Yangtze River Estuary in the past 35 years.It is convenient to understand the temporal and spatial changes of tidal flat in the Yangtze River Estuary,which is not only helpful to explore the evolution factors of tidal flat and forecast the development direction of tidal flat in the Yangtze River Estuary,but also is of great significance to plan and manage the ecological development of tidal flat in Yangtze River Estuary,and maintain the stability of the whole wetland ecosystem and ensure the sustainable utilization of wetland resources in the future.In this paper,two typical tidal flat wetlands in Chongming East Beach and Jiuduansha are taken as the research objects and the temporal and spatial evolution of tidal flat in Yangtze River Estuary from 1984 to 2017 is explored in order to provide a theoretical basis for more efficient protection and utilization of tidal flat wetland resources in Yangtze River Estuary.In this paper,Jiuduansha and Chongming East Beach are taken as the main research objects,and the evolution of tidal flat area in the Yangtze River Estuary,the temporal and spatial distribution pattern of dominant vegetation of Spartina alterniflor,Scirpus triqueter and Reed in tidal flat and the development of tidal channels system in Yangtze River Estuary were studied comprehensively in the monitoring and analysis of tidal flat change in Yangtze River Estuary by using the landsat series of multi-temporal remote sensing images of seven age groups from 1984 to 2017.According to the main research results of this paper,the following significant conclusions are obtained:(1)In this paper,the object-oriented multi-scale segmentation method is applied to the tidal channels system extraction for the first time.Through global domain segmentation of multi-band data and NDWI local iterative segmentation,the best tidal channels extraction segmentation effect is achieved.Then set up the rule set to extract the Yangtze River Estuary tidal channels system.The accuracy,efficiency and applicability have obvious advantages over the traditional method based on pixel extraction by using this method.It`s not only using the spectral characteristics of the pixel,furthermore,but more features such as texture and spatial morphology of local objects are used to ensure the extraction accuracy of tidal channels information in the Yangtze River Estuary.(2)The normalized vegetation index(NDVI),the difference vegetation index(DVI),and the first three components of tassel transform(KT1,KT2,KT3)and tri-band ratio vegetation index(TRVI)are combined with the original bands of remote sensing images to generate new synthetic images,so as to enhance the image information.Then,three dominant vegetation communities,Reed,Spartina alterniflora and Ribes triqueter,were extracted by decision tree classification method,and the classification effect was good.The classification accuracy of vegetation in tidal flat was 93.31%.(3)After 35 years of development,there is the most mature and dense tidal channels system on the beach surface of Jiuduansha,and the development degree of tidal channels system in different sand bodies of Jiuduansha is distrinct.Generally speaking,the tidal channels system of the lower sand body is the most dense and developed,the middle sand and upper sand channelses are relatively narrow and short.Under the action of tide,Jiuduan tidal channels interactively grows with the tidal flat,and up to now,it has developed into a complicated network of tidal channels systems.(4)With the formation of new tidal flat and the change of hydrodynamics,the community of Scirpus triqueter first appeared in the tidal flat of Jiuduansha,and then was the community of Reed.As the tributaries of the fourth class tidal channels approach to the first class tidal channels,the dominant vegetation in tidal flat becomes Reed gradually.Spartina alterniflora community has the widest span and strong ecological adaptability,which mainly grows in middle sand and lower sand.It can be seen in different level tidal channels area,mainly distributed in the high tide zone of Jiuduansha.It also invades the Scirpus triqueter community in the nascent trench area of the intertidal zone.(5)During the period from 1984 to 2017,the natural silting tidal flat of Chongming East Beach has been continuously expanding toward the sea,and the vegetation area of the tidal beach outside the dike caused by human reclamation and embankment has shown a decreasing trend.From the direction of its siltation,the seaward position of the East Beach continuously shifts to the southeast,and the development direction of the north side of East Beach is significantly northward.Tidal flat vegetation plays an significant role in the development and extension of East Beach in Chongming.There are a lot of wetland vegetation in the east of East Beach near the East China Sea and the north side of East Beach,which can not only buffer tidal velocity and counteract wave impact,but also promote the formation of new tidal flat by suspended sediment deposition near tidal flat,and provide a new growth environment for tidal flat vegetation with the development and expansion of tidal flat.At the same time,the wetland vegetation also weakens the erosion of tidal current to the beach,and has the function of protecting the coast. |
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