Temporal And Spatial Evolution Analysis Of Sand Bodies In The Inner Edge Of The Radiant Sandbar In The South Yellow Sea Based On The Enhanced Remote Sensing Terrain Construction Method

The tidal flat is located at the intersection of sea and land,which has important ecological and economic value.Jiangsu Province is rich in muddy tidal flat resources,but with the increase of human activities,the environment of the tidal flat has changed significantly.Thus,it is necessary to monitor the dynamics of the tidal flat.The radial sand ridges(RSR)is an important part of the tidal flat in Jiangsu Province.Because of its special geographical location and complex survey environment,it is difficult to obtain basic geographic data such as topography.As an important method of tidal flat terrain inversion,the waterline method has been widely used in the study of muddy tidal flat terrain construction and change monitoring at home and abroad.However,when this method is applied to the construction of digital elevation model(DEM)in the areas with changeable topography such as RSR,there are many problems,such as the overlapping of water lines,etc.,which affect the inversion accuracy.Based on this,this paper proposes an enhanced terrain remote sensing construction method for the intertidal region based on the conventional waterline method: First,the waterlines with the correct trend of elevation change and spatial separation were selected through tidal level sorting.Second,the images group with similar waterlines position in the screening process were combined according to the mean of a sequence of the modified normalized difference water index(MNDWI)values in each pixel,and the boundary line of land-sea was obtained.These two methods worked together to construct the initial DEM of intertidal.Then,the DEM of tidal creeks was generated by using the boundary and centerline of the tidal creek in the low tide period.Through the mosaic processing,the DEM of tidal flat that can express the undulating microtopography of the beach surface was constructed.Through this method,we completed the terrain inversion of the intertidal zone of the Inner part of radial sandbanks(IPRS)for 2010,2014,and 2018,and then the spatial-temporal variation characteristics and influencing factors of the sandbank in the region were statistically analyzed from two aspects of the plane and three-dimensional morphology.The main conclusions are as follows:(1)Comparing with the measured terrain,the results in the IPRS showed that the mean absolute error(MAE)of the 4 measured sections was 0.43 m,the root mean square error(RMSE)was 0.54 m,and the average correlation(r)was 0.75.The topographical undulations of the two were consistent.The terrain was generally flat and open and was dominated by island-shaped topography with high middle and low surroundings.There were divided tidal creeks between the main sandbanks.The surface of the tidal flat had small dendritic or radial tidal creeks.Under the action of the undercutting of the tidal creeks,the surface of the sandbank had some small undulations.(2)The overall change in the area of the intertidal zone of the IPRS sandbanks from 2010 to 2018 was not large,which showed a trend of first decreasing and then slightly increasing in the three years of 2010,2014,and 2018.The volume showed a continuous decline,with an average annual decline of 2.33% from 2010 to 2018.Calculating the height change of the sandbank overlapping area from 2010 to 2014 and2014 to 2018,and comparing with the preset intertidal zone beach surface erosion and deposition intensity grading table,it was found that the overall balance of erosion and deposition in the overlapping area was dominant,and the area proportions were respectively reached 59.81% and 64.23%.The total area of the sandbanks showed the erosion of the northwest boundary,while the southeast boundary was silted to the east,and the overall trend was eastward.(3)In the main sandbanks,the area and volume of the Tiaozini sandbank showed a state of decreasing first and then increasing.The range of the sandbank obviously moved to the southeast,showing the characteristics of erosion on the northwest side and siltation on the southeast side.The area and volume of Jiangjiasha sandbank were continuously decreasing due to the influence of scouring and dividing tidal channels.The centroid moved slowly first and then fast along the southeast direction.The erosion and deposition conditions were balanced in the middle part and the southwest part was silted.The location of the Zhugensha sandbank was relatively independent and far away from the land,so the change was small.The range of the Gaoni sandbank was slightly reduced,and the northwest side gradually eroded and connected with the main sandbank.The centroid of sandbank continued to move to the northeast.Among the medium and small sandbanks,the change of Babahang and Baxianjiao-beixiaosha was small,the erosion of Sichuanhang was the most obvious,and the deposition of Zhugensha-nanxiaosha was the fastest.(4)The reclamation and tidal channel change were related to the sandbanks changes in the IPRS.Compared with 2010-2014,the newly reclaimed area of tidal flats in 2014-2018 decreased significantly,which was consistent with the trend that the change of IPRS which was first fast and then slow in the same period.The swing and bifurcation of large tidal channels were closely related to the change of sandbank morphology,and the change of Tiaozini,Jiangjiasha,and Zhugensha sandbank was significantly affected by the change of Xidagang and Dongdagang.