Research The Characteristics Of Temporal And Spatial Evolution Of Tidal Flat Information In The Yangtze River Delta

The coastal zone is a transitional zone between land and sea which is affected by both the terrestrial and marine environments.The tidal flat is an important part of the coastal zone,called“intertidal zone”in geomorphology,and it refers to the tidal zone between the high tide level and the low tide level of the coastal tide.It is not only an important ecosystem,but also an important reserve land resource for urban development and spatial expansion.However,in recent years,the signs of widespread degradation of tidal flat resources have posed a serious threat to the tidal flat wetland ecosystem and regional socio-economic sustainable development and the contradiction between the protection and development of tidal flat resources has become increasingly prominent.It is urgent to carry out detailed investigation of tidal flat resources and the study of its evolution law.Due to the periodic submergence of tidal water,muddy beach,poor accessibility and frequent changes,the traditional survey methods are difficult to meet the requirements of monitoring change in this highly dynamic environment.Remote sensing technology has the outstanding advantages of macroscopic level,rapidity,high frequency observation and low cost,which provides a possibility for investigation of long-term and large-scale and timely monitoring.This paper takes the Yangtze River Delta as the study area,and uses Landsat MSS,TM,ETM+and OLI remote sensing images as data source.Based on remote sensing and GIS technology,the tidal flat data of the 1974-2017 Yangtze River Delta is extracted,and the temporal and spatial changes of tidal flat are analyzed from the two layers of the whole and sub-division.The reasons for the evolution of the tidal flat are analyzed.The main tasks are as follows:(1)Firstly,the normalized vegetation index(NDVI)of the remote sensing image and the improved normalized difference water body index(MNDWI)are calculated based on the lowest tide image available at the Sheshan station each year,and the threshold segmentation method is used to extract the vegetation line as the indicator of the high tide line and the water line as the indicator of the low tide line.The Initial information of tidal flats are obtained by vector subtraction.Taking into account the tidal spatial change,the images of other phases in the same year are supplemented and modified by visual interpretation to ensure that the tidal flats in each part of the study area are from the lowest tidal level in one year,and the annual tidal flat range data of the Yangtze River Delta from 1974 to 2017 is finally obtained.(2)The changes in the tidal flat area of the Yangtze River Delta during the period1974-2017 are analyzed.The overall tidal flat area shows a decreasing trend with a reduction rate of 17.5 km~2/a,but there are some difference in space.The tidal flats on the Shanghai Nanhui Beach,Chongming Dongtan and Subei coast north of the Yuantuojiao Point all showed a decreasing trend on the whole,but the decline rate and annual average increment have their own differences.The decline rate of the tidal flats along the northern part of Jiangsu shows an increasing trend with the largest area decrease,while the decline rate of Chongming Dongtan decreases.The tidal flats of Jiuduansha experience the rapid silting and rising stage and stable stage successively and show an increasing trend on the whole.(3)The edge detection method is used to extract the water boundary of the remote sensing images of Chongming Dongtan and Jiuduansha from 1974 to 2017 and generate the binary images of water and non-water.The submergence probability map is constructed by superimposing all binary images in 5 years.According to the lowest and highest tide levels of the images,the submergence probability is converted to elevation,and the DEM of the tidal flats in Chongming Dongtan and Jiuduansha is obtained.The scouring and silting evolution of the tidal flats in Chongming Dongtan and Jiuduansha is preliminary analyzed by comparing DEM of tidal flats in two adjacent time periods.The east and north coast in Chongming Dongtan are the main silting direction of the tidal flat,while the south coast is in a state of erosion.The construction of Deep-Water Channel Project is an important node for the transformation of the Jiuduansha scouring and silting state,which makes the overall shape,range and centroid movement of Jiuduansha have great changes.(4)In terms of driving force,the sediment flux into the Yangtze River,reclamation and estuary construction,sea level rise and land subsidence will affect the evolution of beach erosion and siltation in varying degrees.At the time scale of this paper,sediment coming from river basin and human activities such reclamation and estuary projects are the dominant factors affecting the evolution of tidal flats in the Yangtze River Delta.