Recent Evolution of the Intertidal Sand Ridge Lines of the Dongsha Shoal in the Modern Radial Sand Ridges, East China

International Journal of Environmental Research and Public Health

Article Recent Evolution of the Intertidal Sand Ridge Lines of the Dongsha Shoal in the Modern Radial Sand Ridges, East China

Binglin Liu 1,2 , Haotian Wu 1, Zhenke Zhang 1,2,*, Guoen Wei 1 , Yue Wang 1,2, Jie Zheng 3, Xuepeng Ji 1 and Shengnan Jiang 1

1 The School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210023, China; [email protected] (B.L.); [email protected] (H.W.); [email protected] (G.W.); [email protected] (Y.W.); [email protected] (X.J.); [email protected] (S.J.) 2 The Collaborative Innovation Center of South China Sea Studies, Nanjing University, Nanjing 210093, China 3 The College of Landscape Architecture, Nanjing Forestry University, Nanjing 210037, China; [email protected] * Correspondence: [email protected]; Tel.: +86-25-8968-6694

Abstract: The Dongsha Shoal is one of the largest shoals in the South Yellow Sea and has important marine ecological value. The shoal extends in a south–north direction and is controlled by the regional dominating tidal currents. Recently, due to human activities and some natural factors, the geomorphic dynamics of the Dongsha Shoal has undergone drastic changes. However, few people have proposed quantitative research on the changes of tidal ﬂat morphology, let alone the long-term sequence analysis of sand ridge lines. Hence, we attempt to take the Dongsha Shoal in the Radial Sand Ridges as the research area, and analyze the trends of the long-term morphological evolution of the sand ridge lines over the period 1973–2016 based on a high-density time series of medium-resolution satellite images. The sand ridge line generally moves from southeast to northwest, and the position Citation: Liu, B.; Wu, H.; Zhang, Z.; distribution of the sand ridge line from north to south has gradually changed from compact to Wei, G.; Wang, Y.; Zheng, J.; Ji, X.; Jiang, S. Recent Evolution of the scattered. We also found that the geomorphological dynamics at different positions of the sand ridge Intertidal Sand Ridge Lines of the line are inconsistent. The north and south wings are eroded on the west side, while the central area is Dongsha Shoal in the Modern Radial eroded on the east side. Most of the sand ridge line is moving eastward. In addition, the change of Sand Ridges, East China. Int. J. sand ridge line is affected by multiple factors such as sediment supply, typhoon, reclamation and Environ. Res. Public Health 2021, 18, laver cultivation. 1573. https://doi.org/10.3390/ ijerph18041573 Keywords: intertidal sand ridge line; radial sand ridges; South Yellow Sea; remote sensing analysis; Dongsha Shoal Academic Editor: Elena Cristina Rada Received: 3 December 2020 Accepted: 2 February 2021 Published: 7 February 2021 1. Introduction

Publisher’s Note: MDPI stays neutral A coastal zone is a complex geographic unit of transition between terrestrial and ma- with regard to jurisdictional claims in rine ecosystems, and it is also the area responding fastest to global and land-sea changes [1]. published maps and institutional affil- Tidal flats, located between the average low tide and average high tide of the coastal iations. zone, are the most important landform system in the coastal zone. They can protect the biodiversity of the coastal zone, resist storm surges, and provide habitat for migrating birds, and provide reserve land resources for coastal economic development [2–5]. Therefore, the tidal flats have received extensive attention from all over the world [6–10]. Recently, due to global changes and intense human activities, coastal tidal flats have undergone Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. drastic changes, leading to a series of resource and environmental problems. Therefore, This article is an open access article it is necessary and urgent to study the dynamic trend of tidal flats and the response to distributed under the terms and increased human activities and natural factors. The sand ridge line is a watershed divide conditions of the Creative Commons on the large-scale low tide exposed tidal flat surface. It is the topographic boundary of Attribution (CC BY) license (https:// the adjacent tidal waterway control area, where the rising and falling tides on both sides creativecommons.org/licenses/by/ converge and disperse, and its position and shape change continuously with the regional 4.0/). tidal dynamics, which can reveal the regional dynamic geomorphological process [11].

Int. J. Environ. Res. Public Health 2021, 18, 1573. https://doi.org/10.3390/ijerph18041573 https://www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2021, 18, 1573 2 of 19

Along the west coast of the South Yellow Sea from the old Yellow River delta to the Yangtze River estuary in Jiangsu Province, these unique sand ridges have been developing since the late Quaternary [6]. Strong tidal currents, the highest observed tidal range being 9.39 m [12], are the main morphological forces driving the formation of the sand ridges, which cover an area of more than 20,000 square kilometers and are composed of dozens of large sand ridges with corresponding troughs between the ridges [13–15]. The tidal ridges are located not too far from the coastline and appear as large areas of silty tidal flats during the ebb tide. This area has huge ecological value for migratory birds given its importance as a coastal habitat. However, since the Yellow River returned to the Bohai Sea in 1855, the Radial Sand Ridges have lost a large amount of sediment supply [16]. Due to the needs of agriculture, aquaculture, wind power generation and port construction, a large number of coastal beaches in the area have been reclaimed [17]. In addition, global changes and overexploita- tion of land have caused sea levels to rise, the geomorphological dynamics conditions of the Radial Sand Ridges have changed drastically, and the sedimentation has become erosion [18]. Therefore, an accurate assessment of the evolution of the Radial Sand Ridges is critical to the protection of tidal flat resources and sustainable coastal development. Remote-sensing technology provides nearly continuous monitoring of tidal flats, which are commonly used in many coastal areas around the world [3,4,19,20]. The most repre- sentative methods are based on airborne light detection and ranging (LiDAR) and inter- ferometric synthetic aperture radar (InSAR) images [21–24]. However, due to the high cost of LiDAR-based data and aerial images, it is difficult to perform time series analysis on tidal flats [25,26]. Since the launch of the Landsat-1 satellite in 1972, many satellite images of research value have been archived [27,28]. As more Earth exploration satellites were launched, and other satellite data cooperation projects, the potential of intensive time series analysis has been greatly expanded, especially in tidal flats [29]. Researchers make extensive use of the time series of satellite images, using the waterline detection method (WDM) to construct a digital elevation model (DEM) of the tide plane [30,31]. This method is widely used in many intertidal tidal flats around the world, including the coast of England [32–35], the Wadden Sea tidal flats, Germany [36–39], the intertidal zone along the coast of Korea [40–42] and China coastal beach [43–46]. However, few people have proposed quantitative research on the changes of tidal flat morphology, let alone the long-term sequence analysis of sand ridge lines. In this study, we attempt to take the sand ridge lines of Dongsha Shoal as the research area, analyze the trends of the long-term morphological evolution of the sand ridge lines over the period 1973–2016 based on a high-density time series of medium-resolution satellite images. Our specific goals were (1) to study the long-term evolution of the location of the sand ridge lines, (2) to study the overall trend and change stage of the sand ridge lines (3) to determine the change trend of different positions of the sand ridge lines, and (4) to discuss global changes and the impact of human activities on the landform changes in Dongsha Shoal. This study will help evaluate changes in tidal flats in modern radial sand ridges in Jiangsu Province, provide support for the management of related sea areas, and provide scientific references for sustainable tidal flat development.

2. Study Area and Methods 2.1. Study Area Dongsha is the intertidal sandbank with the largest area and the highest mean altitude in this radial sand ridges (Figure1). The Dongsha Shoal is surrounded by a series of sand ridges and deep trenches. This area has a wide tidal range, broad tidal flats, typical tidal flat landforms, and unstable topography. Tidal flats in the study area are typically more than 25 km in width and have an average slope of approximately 0.18–0.19% [47]. In most periods, the top of the shoal cannot be submerged by tidal flow [48]. Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 3 of 20

Int. J. Environ. Res. Public Health 2021, 18, 1573 3 of 19 more than 25 km in width and have an average slope of approximately 0.18–0.19% [47]. In most periods, the top of the shoal cannot be submerged by tidal flow [48].

Figure 1. Study area and image of the Dongsha Shoal for a low tide condition. Maps of the Dongsha Figure 1. Study area and image of the Dongsha Shoal for a low tide condition. Maps of the Shoal (a) Mainland China, (b) the South Yellow Sea and (c) Dongsha Shoal. Dongsha Shoal (a) Mainland China, (b) the South Yellow Sea and (c) Dongsha Shoal. The Dongsha Shoal is a macro-tidal area with an average tidal range of 3.9–5.5 m and The Dongsha Shoal is a macro-tidal area with an average tidal range of 3.9–5.5 m and exhibits an irregular semi-diurnal tide [49]. The distribution of the tidal range has Jianggang exhibits an irregular semi-diurnal tide [49]. The distribution of the tidal range has Jiang- at the highest point with a gradual decrease towards the north and south sides [50]. The gang at the highest point with a gradual decrease towards the north and south sides [50]. coastal waters where the shoal is located have the largest tidal range, being more than The coastal waters where the shoal is located have the largest tidal range, being more than 3.9 m. The average tidal range of Changsha Port reaches 6.45 m, and the average tidal 3.9range m. The of the average Dongsha tidal Shoal range is of 5.44 Changsha m. The largest Port reaches tidal range 6.45 ofm, 9.39 and m the was average observed tidal on range17 October of the Dongsha 2012 at Xintiaoyugang Shoal is 5.44 m. Station The largest [12]. tidal range of 9.39 m was observed on 17 October 2012 at Xintiaoyugang Station [12]. 2.2. Research Methods 2.2.2.2.1. Research Selection Methods of Remote-Sensing Data and Data Preprocessing 2.2.1. SelectionThe sand of ridge Remote line- isSensing the watershed Data and divide Data Preprocessing of the tidal flat exposed on the tidal ridge at lowThe tide, sand and ridge the line return is the channel watershed water divide on both of sidesthe tidal of the flat tidal exposed flat enters on the different tidal ridge tidal atchannels. low tide, Theand tidalthe return range channel in the study water area on isboth very sides large, of andthe thetidal low flat tide enters exposed different tidal tidalflat channels. surface features The tidal obvious range topographicin the study features.area is very Through large, theand enhanced the low tide processing exposed of tidalremote-sensing flat surface features images andobvious combined topographic with the features. dynamic Through characteristics the enhanced of the tidalprocessing current ofchannel remote- onsensing both sidesimages of and the tidalcombined flat, the with relative the dynamic position characteristics of the tidal channel of the ends tidal on cur- the renttidal channel flat surface on both can besides visually of the distinguished, tidal flat, the andrelative then positi the sandon of ridge the linetidal can channel be extracted. ends onWe the consulted tidal flat thesurface tide can table, be usedvisually the distinguished, US Geological and Survey then Global the sand Visualization ridge line can Viewer be extracted.(GloVis, Reston,We consulted VA, USA), the andtide selected table, used the bestthe low-tideUS Geological middle-resolution Survey Global remote-sensing Visualiza- tionimages Viewer from (GloVis, 1973–2016 Reston, through VA, USA), comparison and selec andted screening. the best Alow total-tide of middle 33 preview-resolution satellite remoteimage-sensing frames images for the researchfrom 1973 area–2016 from through 1973 tocomparison 2016 were and subsequently screening. A selected. total of The33 previewspecific satellite parameters image are frames shown for in the Figure research2. These area images from 1973 were to acquired 2016 were by subsequently the following selected.sensors: The Landsat-1/2/3 specific parameters Multispectral are shown Scanner in Figure System 2. These (MSS) images (NASA, were Washington, acquired by DC, theUSA), following Landsat-4/5 sensors: Thematic Landsat-1/2/3 Mapper Multispectral (TM) (NASA, Scanner Washington, System (MSS) DC, USA),(NASA, Landsat-7 Wash- ington,Enhanced DC, ThematicUSA), Landsat Mapper-4/5 Plus Thematic (ETM+) Mapper (NASA, (TM) Washington, (NASA, DC,Washington, USA), Landsat-8 DC, USA), (OLI) Landsat(NASA,-7 Washington,Enhanced Thematic DC, USA). Mapper Plus (ETM+) (NASA, Washington, DC, USA), LandsatThe-8 (OLI) images (NASA, were pre-processedWashington, DC, using USA) ENVI. 5.0 (Environmental Systems Research Institute, Redlands, CA, USA) and ArcGIS 10.2 (Environmental Systems Research Institute, Redlands, CA, USA) with both projection transformation and geometric correction (pre- cision of less than one pixel). Meanwhile, image fusion was carried out according to the characteristics of the different sensors. Then interactive stretching in ENVI was used to enhance the images of the study area. The above operations were performed in ArcGIS10.2

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Int. J. Environ. Res. Public Health 2021, to18, facilitatex FOR PEER routine REVIEW visual interpretation and water body identiﬁcation, to vectorize4 of the 20 tidal ﬂats and the intertidal sand ridge line, to perform the superimposition analysis and to obtain numerical statistics for the vector elements of the different periods.

FigureFigure 2. 2. DateDate of of acquisition acquisition of of satellite satellite imagery imagery used used in inthis this study. study. (Landsat (Landsat multispectral multispectral scanner scan- (MSS);ner (MSS); Thematic Thematic Mapper Mapper (TM); (TM); Enhanced Enhanced Thematic Thematic Mapper Mapper (ETM+) (ETM+) and Operationa and Operationall Land Imager Land (OLI)).Imager (OLI)).

2.2.2.The Extraction images andwere Analysis pre-processed Method using of Sand ENVI Ridge 5.0 Line(Environmental Systems Research Institute,After Redlands, dealing with CA, theUSA series) and of ArcGIS 33 images 10.2 (1973(Environmental to 2016) for Systems the Dongsha Research Shoal, Insti- the tute,annual Redlands, and the CA, accumulated USA) with information both projection of the transformation sand ridge line and was geometric obtained. correction In order (precisionto study the of less overall than movement one pixel). trend Meanwhile, and movement image fusion phase was of carried the sand out ridge according line, we to theextracted characteristics the midpoint of the of different the sand sensors. ridge line Then and interactive summarized stretching it. in ENVI was used to enhanceIn order the to images understand of the the study movement area. The trends above of the operations sand ridge we line,re performed we have se-in ArcGIS10.2lected ﬁve typicalto facilitate sections, routine as visual shown interpretation in Figure3. The and positions water body of theidentification, sections are to Line vec- ◦ 0 00 ◦ 0 00 ◦ 0 00 ◦ 0 00 torize1 (33 13the30 tidalN), flats Line and 2 (33 the9 intertidal34 N), Line sand 3 (33ridge5 38line,N), to Lineperform 4 (33 the1 41superimpositionN) and Line ◦ 0 00 analysis5 (32 57 and45 toN), obtain which numerical traversed statistics the sand for ridge the vector line, respectively. elements of Thenthe different ﬁve sand periods. ridge points (intersection points) A, B, C, D, E and F, were obtained. We counted the locations of 2.2.2.sand ridgeExtraction points, and such Analysis a yearly Method sequence of Sand for the Ridge direction Line of movement and distance may be summarized and visualized with ArcGIS10.2, thus the spatial distribution characteristics After dealing with the series of 33 images (1973 to 2016) for the Dongsha Shoal, the of the intertidal sand ridge lines of the Dongsha Shoal may be assessed. annual and the accumulated information of the sand ridge line was obtained. In order to In order to study the current distribution and migration trend of the sand ridge line study the overall movement trend and movement phase of the sand ridge line, we ex- at each stage, we used the buffer analysis and superposition analysis function, taking the tracted the midpoint of the sand ridge line and summarized it. sand ridge line in 1973 as the baseline, setting up 1 km, 1–2 km, and 2–4 km buffer zones on theIn westorder side, to understand and at thesame the movement time setting trends up 1 km,of the 1–2 sand km, ridge 2–4 km, line, 4–8 we km have buffer selected zone fiveon the typical east side secti (Figureons, as4). shown Finally, in the Figure superposition 3. The analysispositions was of carriedthe sections out with are all Line sand 1 (33°13’30”ridge lines, N), and Line the proportion2 (33°9’34” ofN), the Line length 3 (33°5’38” of sand ridgeN), Line lines 4 in(33°1’41” each buffer N) and zone Line to the 5 (32°57’45”total length N), is calculated.which traversed the sand ridge line, respectively. Then five sand ridge pointsFinally, (intersection the superposition points) A, B, analysis C, D, E and was F, carried were obtained. out with We all sandcounted ridge the lines, locations and ofthe sand proportion ridge points, of the such length a yearly of sand sequence ridge linesfor the in direction each buffer of movement zone to the and total distance length mayis calculated. be summarized and visualized with ArcGIS10.2, thus the spatial distribution characteristics of the intertidal sand ridge lines of the Dongsha Shoal may be assessed. In order to study the current distribution and migration trend of the sand ridge line at each stage, we used the buffer analysis and superposition analysis function, taking the sand ridge line in 1973 as the baseline, setting up 1 km, 1–2 km, and 2–4 km buffer zones on the west side, and at the same time setting up 1 km, 1–2 km, 2–4 km, 4–8 km buffer zone on the east side (Figure 4). Finally, the superposition analysis was carried out with all sand ridge lines, and the proportion of the length of sand ridge lines in each buffer zone to the total length is calculated.

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Figure 3. The location of the five typical sections and the sand ridge points formed by their inter- Figure 3. TheThe locationlocation ofof thethe ﬁvefive typicaltypical sectionssections andand thethe sandsand ridge ridge points points formed formed by by their their intersec- inter- section with the sand ridge line. tionsection with with the the sand sand ridge ridge line. line.

Figure 4. Determination of Dongsha Shoal buffer zone. FigureFigure 4.4. DeterminationDetermination ofof DongshaDongsha ShoalShoal bufferbuffer zone.zone.

3. ResultsFinally, the superposition analysis was carried out with all sand ridge lines, and the proportionDongsha of the Shoal length is a large of sand coastal ridge sandbank, lines in each and buffer its degree zone of to evolution the total length is very is compli- calcu- cated.lated.lated. In this case, studying the morphological changes of the sand ridge line can deduce the displacement of the Dongsha Shoal, the erosion/deposition caused by the changes in the dynamic conditions of the east and west sides of the Dongsha Shoal, the changes

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3. Results Dongsha Shoal is a large coastal sandbank, and its degree of evolution is very com- plicated. In this case, studying the morphological changes of the sand ridge line can de- Int. J. Environ. Res. Public Health 2021duce, 18, 1573 the displacement of the Dongsha Shoal, the erosion/deposition caused by6 ofthe 19 changes in the dynamic conditions of the east and west sides of the Dongsha Shoal, the changes in appearance caused by the combination/separation of Dongsha Shoal and other smallin appearance sandbanks. caused Therefore, by the we combination/separation used various methods ofto Dongsha analyze the Shoal evolution and other trend small of thesandbanks. sand ridge Therefore, line of Dongsha we used Shoal. various methods to analyze the evolution trend of the sand ridge line of Dongsha Shoal. 3.1. Spatiotemporal Changes of Sand Ridge Lines 3.1. SpatiotemporalDongsha Shoal Changes is a well of- Sandpreserved Ridge core Lines geomorphic unit of the Radial Sand Ridges. The resultsDongsha from Shoal Figure is a 5 well-preserved show that the sand core geomorphicridge line of unitDongsha of the Shoal Radial is Sandgenerally Ridges. in theThe north results–south from direction, Figure5 show basically that theparallel sand to ridge the lineshoreline, of Dongsha and the Shoal tidal is flat generallys develop in unevenlythe north–south on the east direction, and west basically sides parallelof the sand to the ridge shoreline, line. A large and the area tidal of tidal flats flats develop de- velopedunevenly on on the the east east side, and with west irregular sides of shorelines the sand and ridge generally line. A wide large tidal area channels. of tidal flats By contrast,developed the on tidal the east flat side,areawith on the irregular west shorelinesside is smaller and generally and the wideshoreline tidal channels.is relatively By straight.contrast, the tidal flat area on the west side is smaller and the shoreline is relatively straight.

FigureFigure 5. 5. ChangesChanges in in the the sand sand ridgeline ridgeline of of Dongsha Dongsha Shoal Shoal during during 1973 1973–2016.–2016.

ItIt can can be be seen seen that that the the strike strike of of the the sand sand ridge ridge line line of of the Dongsha Shoal Shoal is is basically stable,stable, indicating indicating that that the the sedimentary sedimentary environment environment and t tidalidal current current field ﬁeld in the the core core area area ofof the the Radial Radial Sand Sand Ridges Ridges are are relatively stable. TheThe midpoint midpoint is is the the typical characteristic point point of of the sand ridge line. As shown in FigureFigure 66,, thethe change inin the position ofof the midpoint ofof the sand ridge line clearly expresses the overall movement trend of the sand ridge line. (1) In general, the overall movement the overall movement trend of the sand ridge line. (1) In general, the overall movement trend of the sand ridge line is from southwest to northeast, and the average movement trend of the sand ridge line is from southwest to northeast, and the average movement distance is 1.39 km. (2) The sand ridge line does not move in one direction, on the contrary distance is 1.39 km. (2) The sand ridge line does not move in one direction, on the contrary it is constantly oscillating, and with the passage of time, the frequency of the reverse oscillation has a tendency to increase. (3) According to the midpoint movement direction and speed, the sand ridge line movement can be divided into 5 stages. The period from 1973 to 1979 was stage 1, the sand ridge line moved from northeast to southwest with an

average speed of 1.25 km/year. It can be seen that there was erosion on the east side of Dongsha Shoal at this time. The period from 1979 to 1986 was stage 2, when the sand ridge line moved from south to north with an average speed of 1.11 km/year. 1986–1995 was Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 7 of 20

it is constantly oscillating, and with the passage of time, the frequency of the reverse oscillation has a tendency to increase. (3) According to the midpoint movement direction and speed, the sand ridge line movement can be divided into 5 stages. The period from 1973 to 1979 was stage 1, the sand ridge line moved from northeast to southwest with an Int. J. Environ. Res. Public Health 2021average, 18, 1573 speed of 1.25 km/year. It can be seen that there was erosion on the east side7 of of 19 Dongsha Shoal at this time. The period from 1979 to 1986 was stage 2, when the sand ridge line moved from south to north with an average speed of 1.11 km/year. 1986–1995 was stagestage 3, 3, the the sand sand ridge ridge line line moved moved from from north north to to south, south, but but it it began began to to swing swing in in opposite opposite directionsdirections twice, twice, with with an an average average speed speed of 1.31 of 1.31 km/year km/year.. Dongsha Dongsha Shoal Shoal merged merged with sev- with eralseveral small small sandbanks sandbanks on the on east the side, east side,resulting resulting in the in direction the direction of movement of movement of the ofsand the ridgesand line ridge to start line toto startbe unstable to be unstable.. The period The from period 1995 from to 2007 1995 was to stage 2007 4. was The stage sand 4. ridge The linesand moved ridge linefrom moved the southwest from the to southwest the northeast, to the northeast,with 9 reverse with swings, 9 reverse with swings, an average with an speedaverage of 1 speed.39 km of/year 1.39, km/year,indicating indicating that the acceleration that the acceleration of coastal of reclamation coastal reclamation in Jiangsu in ProvinceJiangsu Provincein the late in 1990s the late changed 1990s the changed marine the sedimentary marine sedimentary environment environment around Dongsha around Shoal.Dongsha The Shoal.erosion The of erosionDongsha of Shoal Dongsha has been Shoal intensified has been intensiﬁedwhile the scarcity while the of scarcitysediment of hassediment increased. has increased.At the same At time, the same the southern time, the end southern of the endXiyang of the tidal Xiyang channel tidal was channel also severelywas also eroded severely. The eroded. period Thefrom period 2007 to from 2016 2007 is stage to 2016 5. The is stagesand ridge 5. The line sand moves ridge from line southeastmoves from to southeastnorthwest, to and northwest, there are and 5 therereverse are swings, 5 reverse with swings, an average with an speed average of speed 1.72 kofm/year. 1.72 km/year. During this During period, this period,the Chinese the Chinese government government reclaimed reclaimed Tiaozini Tiaozini sandbank sandbank and Gaoniand Gaoni sandbank sandbank,, which which led to led the to thesediment sediment supply supply on onthe the south south side side of of Dongsha Dongsha Shoal Shoal beingbeing further further reduced reduced and and the the sand sand ridge ridge line line erod erodinging northward northward..

FigureFigure 6. 6. ChangesChanges in in the the midpoint midpoint of of the the sand sand ridgeline ridgeline of of Dongsha Dongsha Shoal Shoal during during 1973 1973–2016.–2016.

3.2.3.2. Variation Variation of of Sand Sand Ridge Ridge Lines Lines on on Typical Typical Section Section TheThe change change characteristics characteristics of of the the different different positions positions of of the the sand sand ridge ridge line line and and the the movementmovement speed speed reflect reflect in in detail detail the the scouring scouring and and silting silting situation situation in in the the Dongsha Dongsha Shoal, Shoal, andand reflect reflect the degreedegree and and trend trend of of dynamic dynamic geomorphology geomorphology changes changes in the in Dongsha the Dongsha Shoal. Shoal.The distribution The distribution of sand of ridgesand lineridge positions line positions in different in different periods periods is shown is shown in Figure in Figure7. The starting position for each point is the point for 1973; this point is located in the quadrant to the east of the origin; the point in the negative quadrant is the point west of the origin. We use five typical sections to analyze the changes in the spatial position of the sand ridge line. From the location distribution of the sand ridge points, most of the sand ridge points A, B, D and E are on the east side of the origin, and all points of the sand ridge points A are on the east side of the origin. Most of the sand ridge points C are on the west side of the origin. It can be inferred that the sand ridge line of most periods has a significant westward Int. J. Environ. Res. Public Health 2021, 18, 1573 8 of 19

bulge in the middle section. The farthest position of sand ridge points A on the east side is 3.1 km away from the origin. The farthest position of sand ridge points B on the east side is 2.05 km from the origin, and the farthest position on the west side is 2.14 km from the origin. The farthest position of sand ridge points C on the east side is 0.65 km from the origin, and the farthest position on the west side is 3.72 km from the origin. The movement distance of sand ridge points D and E is obviously longer than sand ridge points A, B and C. Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 8 of 20 The farthest position of sand ridge points D on the east side is 7.17 km from the origin, and the farthest position on the west side is 0.65 km from the origin. The farthest position of sand ridge points E on the east side is 6.25 km from the origin, and the farthest position on 7.the The west starting side is position 0.28 km fromfor each the origin.point is Unlike the point other for intersections, 1973; this point most sandis located ridge pointsin the quadrantC are located to the on east the westof the side origin; of the the origin. point Thisin the indicates negative that quadrant the position is the distribution point west of thethe origin. sand ridge line from north to south has gradually changed from compact to scattered.

Figure 7. LocationLocation distribution of sand ridge points in different periods.

WeWe use analyze five typical the movement sections speed to analyze of the the sand changes ridge in points. the spatial As shown position in Figure of the8 sand, the ridgeaverage line. speed of the sand ridge point of Dongsha Beach is 0.04 km/year (to the east), and the maximumFrom the location speed of distribution the sand ridge of the point sand is rid 2.70ge km/yearpoints, most (to theof the east). sand Among ridge points them, A,the B, fastest D and part E are of on the the sand east ridge side pointof the is origin, in the and south all (Section points 5of), the with sand an averageridge points speed A areof 1.66on the km/year east side (to of the the east). origin. The Most slowest of the part sand of ridge the sand points ridge C are point on isthe in west the middleside of (Section3), with an average speed of 0.23 km/year (to the west). the origin. It can be inferred that the sand ridge line of most periods has a significant westward bulge in the middle section. The farthest position of sand ridge points A on the east side is 3.1 km away from the origin. The farthest position of sand ridge points B on the east side is 2.05 km from the origin, and the farthest position on the west side is 2.14 km from the origin. The farthest position of sand ridge points C on the east side is 0.65 km from the origin, and the farthest position on the west side is 3.72 km from the origin. The movement distance of sand ridge points D and E is obviously longer than sand ridge points A, B and C. The farthest position of sand ridge points D on the east side is 7.17 km from the origin, and the farthest position on the west side is 0.65 km from the origin. The farthest position of sand ridge points E on the east side is 6.25 km from the origin, and the farthest position on the west side is 0.28 km from the origin. Unlike other intersections, most sand ridge points C are located on the west side of the origin. This indicates that the

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position distribution of the sand ridge line from north to south has gradually changed from compact to scattered. We analyze the movement speed of the sand ridge points. As shown in Figure 8, the average speed of the sand ridge point of Dongsha Beach is 0.04 km/year (to the east), and the maximum speed of the sand ridge point is 2.70 km/year (to the east). Among them, the fastest part of the sand ridge point is in the south (Section 5), with an average speed Int. J. Environ. Res. Public Health 2021of, 18 1.66, 1573 km/year (to the east). The slowest part of the sand ridge point is in the middle9 of 19 (Section 3), with an average speed of 0.23 km/year (to the west).

FigureFigure 8. 8. VelocityVelocity distribution distribution of of sand sand ridge ridge points points movement movement in in different different periods.

AsAs shown shown in in Figure Figure 99,, sincesince thethe velocityvelocity andand directiondirection ofof thethe sandsand ridgeridge pointspoints onon eacheach section section are are constantly constantly changing, changing, the the overall overall speed speed of ofthe the movement movement is represented is represented as aas low a low-speed-speed eastward eastward movement. movement. The movement The movement direction direction of the ofsand the ridge sand point ridge A point pre- sentsA presents the law the of laweast– ofwest east–west–east–west–east,–east–west–east, and the and degree the degreeof velocity of velocity fluctuation fluctuation graduallygradually increase. increase. The direction The direction of movement of movement of the sand of the ridge sand point ridge B presents point B presentsa law of west a law- eastof west-east-east-west-east,-east-west-east, and the amplitude and the amplitude of velocity offluctuations velocity fluctuations is first large is and first then large small. and Thethen moveme small. Thent direction movement of the direction sand ridge of the point sand C ridge presents point the C presents law of west the law–west of– west–east– eastwest–east–east–west,–west, and the speed and fluctuation the speed fluctuationrange is firstly range large is firstlyand then large small. and thenThe movement small. The movement direction of the sand ridge point C presents the law of west–east–east–east–west, direction of the sand ridge point C presents the law of west–east–east–east–west, and the and the velocity fluctuations are first small and then large. The direction of sand ridge point velocity fluctuations are first small and then large. The direction of sand ridge point C C movement on Section3 presents the law of west–west–east–east–west, and the degree movement on Section 3 presents the law of west–west–east–east–west, and the degree of of velocity fluctuation is first large and then small. The movement direction of the sand velocity fluctuation is first large and then small. The movement direction of the sand ridge ridge point D presents the law of west–east–east–east–west, and the velocity fluctuations are first small and then large. The movement direction of the sand ridge point E presents a regular eastward, and the speed and direction are relatively stable. It can be seen that the fluctuation degree of the speed of the sand ridge line gradually decreases from north to south. Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 10 of 20

point D presents the law of west–east–east–east–west, and the velocity fluctuations are first small and then large. The movement direction of the sand ridge point E presents a regular eastward, and the speed and direction are relatively stable. It can be seen that the Int. J. Environ. Res. Public Health 2021 18 fluctuation, , 1573 degree of the speed of the sand ridge line gradually decreases from north10 of 19 to south.

Figure 9. Movement of sand ridge line in typical sections. Figure 9. Movement of sand ridge line in typical sections. 3.3. Analysis of Sand Ridge Lines Migration Trend 3.3. AnalysisIn order of toSand study Ridge the Lines migration Migration trend Trend of the sand ridge line in the east–west direction, we tookIn order the to sand study ridge the line migr ination 1973 trend as the of baseline, the sand and ridge calculated line in the the east proportion–west direction of the , welength took ofthe the sand sand ridge ridge line line in in1973 each as buffer the baseline, zone at and each calculated time phase the to proportion the total length, of the lengthsummarized of the sand the data ridge to line obtain in Figureeach buffer 10. The zone results at each are time as follows: phase (1)to the Since total 1976, length, the summarizedwest side of Dongshathe data Shoalto obtain has beenFigure continuously 10. The results scoured. are Inas this follows: case, the(1) sandSince ridge 1976, line the westhas beenside movingof Dongsha eastward, Shoal andhas therebeen iscontinuously a tendency to scoured. move within In this the case, area 2–8the kmsand east ridge of the baseline. (2) In most time phases, the proportion of the length of the sand ridge line on line has been moving eastward, and there is a tendency to move within the area 2–8 km the east side of the baseline exceeds 50% and gradually increases. (3) The hydrodynamic east of the baseline. (2) In most time phases, the proportion of the length of the sand ridge force of the Xiyang Tidal Channel continues to increase. The east bank of Dongsha Shoal line on the east side of the baseline exceeds 50% and gradually increases. (3) The hydro- was gradually eroded, causing the sand ridge line to move eastward. After 2004, most of dynamic force of the Xiyang Tidal Channel continues to increase. The east bank of the sand ridge line has moved 1 km to 4 km west. Dongsha Shoal was gradually eroded, causing the sand ridge line to move eastward. After 2004, most of the sand ridge line has moved 1 km to 4 km west.

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FigureFigure 10. 10. OverlayOverlay analysis analysis of of sand sand ridgelines ridgelines buffer buffer zone.

4.4. Discussion Discussion 4.1.4.1. Influence Inﬂuence of of Natural Natural Factors Factors on on the the Change Change of of Sand Sand Ridge Ridge Line Line in in Dongsha Dongsha Shoal Shoal SeveralSeveral natural natural factors factors have have obvious obvious influence inﬂuence on on the the evolution evolution of of the the sand sand ridge ridge line. line.

4.1.1.4.1.1. Changes Changes in in Sediment Sediment Load Load InIn 1128, 1128, the the Yellow Yellow River River captured captured the the Huai Huai River River from from the the northern northern part part of of Ji Jiangsuangsu ProvinceProvince into into the the sea. sea. A A large large amount amount of of sediment sediment caused caused the the coasts coasts of of most most parts parts of JiangsuJiangsu to to move move rapidly rapidly toward toward the the sea sea [ [5151,,5252]].. Since Since the the Yellow Yellow River River moved moved to to the the Bohai Bohai SeaSea due due to to natural natural destruction destruction in in 1855 1855,, the the source source of of sediment sediment from from the the Yellow Yellow River into thethe South South Yellow Yellow Sea Sea has has been been greatly greatly reduced, reduced, and and the hydrodynamic conditions conditions of of the the DongshaDongsha Shoal Shoal have have changed changed [ [5353,,5454]].. As As a a result, result, the the radial radial sand sand ridge ridge has has entered entered an an unprecedented adjustment stage, causing severe ocean erosion and coastline retreat on the unprecedented adjustment stage, causing severe ocean erosion and coastline retreat on central coast of Jiangsu and the west side of the Dongsha Shoal [55,56]. This is also the the central coast of Jiangsu and the west side of the Dongsha Shoal [55,56]. This is also the driving force for the overall eastward migration of the sand ridge line. driving force for the overall eastward migration of the sand ridge line. 4.1.2. Changes in Sedimentary Dynamics 4.1.2. Changes in Sedimentary Dynamics Since 1855, the sedimentary dynamics of the radial sand ridge area has changed greatly.Since The 1855, impact the of sedimentary runoff and sediment dynamics load of onthe tidal radial flats sand gradually ridge weakened,area has changed and the greatly.leading The role impact of offshore of runoff tidal currentsand sediment gradually load increased.on tidal flats The gradually Xiyang tidal weakened, channel and is a thecoastal leading tidal role channel of offshore in the tidal northern currents flank gradually of the radiating increased. sandbar, The Xiyang extending tidal to channel the south, is acausing coastal thetidal low channel tidal flat in the to be northern eroded flank under of the the influence radiating of sandbar, nearshore extending hydrodynamic to the south,forces. causing Under thethe actionlow tidal of the flat coastal to be eroded currents under in Jiangsu the influence Province, of the nearshore erosion materialhydrody- is namictransported forces. southwardUnder the action [6]. In of addition, the coastal the currents inner area in ofJiangsu the radial Province, sand the ridge erosion is present ma- terialon the is strongtransported tidalcurrent southward coast. [6] When. In addition, the strong the tidalinner current area of hits the theradial concave sand ridge bank ofis presentthe tidal on flat, the this strong will causetidal current the erosion coast. of When the tidal the flat strong and thetidal repeated current swing hits the of theconcave sand bankridge of line. the tidal flat, this will cause the erosion of the tidal flat and the repeated swing of the sandGlobal ridge warming line. will accelerate sea level rise and increase the intensity and frequency of stormGlobal surges warming and will waves, accelerate which sea will level lead rise to and the increase possibility the of intensit increasedy and coastal frequency ero- ofsion. storm Historical surges and records waves, of tideswhich indicate will lead that to the possibility average rate of ofincreased sea level coastal rise along erosion. the Historicalthe Radial records Sand Ridge of tides is aboutindicate 5.26 that mm/year, the average which rate isof about sea level twice rise the along global the averagethe Ra- dial(2.43 Sand mm/year) Ridge [ 57is ].about In the 5.26 next mm/year, 30 years, thewhich average is about sea level twice along the theglobal Radial average Sand Ridge(2.43 mm/year)will rise by [57 about]. In the 85–150 next mm30 years, [28]. Thisthe average rise will sea lead level to aalong decrease the Radial in the Sand area ofRidge the tidalwill rise by about 85–150 mm [28]. This rise will lead to a decrease in the area of the tidal flat

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exposed at the lowest tide in the Dongsha Shoal, the expansion of the erosion area, and the ridge line of the sand will become unstable. flat exposed at the lowest tide in the Dongsha Shoal, the expansion of the erosion area, and the ridge line of the sand will become unstable. 4.1.3. Impacts of Typhoon 4.1.3.Tropical Impacts cyclones of Typhoon are an important driving factor for short-term catastrophes in the DongshaTropical Shoal cyclones [58]. The are antyphoon important information driving factor was forderived short-term from catastrophes the Tropical in Cyclone the Dongsha Best TrackShoal [Dataset58]. The issued typhoon by information the Shanghai was Typhoo derivedn fromInstitute the Tropical of the China Cyclone Meteorological Best Track Dataset Ad- ministrationissued by the [ Shanghai59]. Typhoon Institute of the China Meteorological Administration [59]. AfterAfter summarizing, summarizing, we we found found that that there there are mainly three typical paths of typhoons affectingaffecting the the coastal coastal areas areas of of Jiangsu. Jiangsu. The The typical typical paths paths are shown are shown in Figure in Figure 11. The 11 .storm The surgestorm levels surge levelscaused caused by typhoons by typhoons making making straight straight landfall, landfall, by typhoons by typhoons active active in the in off- the shoreoffshore area, area, and and by typhoons by typhoons moving moving northward northward after after landfall landfall were were high. high. In terms In terms of spa- of tialspatial distribution, distribution, the theradial radial sand sand ridge ridge area area was was seriously seriously affected affected by bythethe three three types types of typhoonof typhoon and and the thestorm storm surge surge was wasthe largest, the largest, with withthe highest the highest water water level of level 3.47 of m. 3.47 Stud- m. iesStudies have haveshown shown that before that before and after and afterthe passage the passage of a oftyphoon, a typhoon, the shape the shape of the of tidal the tidal flat changesflat changes greatly, greatly, mainly mainly manifested manifested by bythe the sudden sudden change change of of the the tidal tidal flat flat area area and and the aappearanceppearance of new tidal channel [ 60]]..

FigureFigure 11. TypicalTypical typhoon tracks along the Jiangsu coast.

AsAs shown inin FigureFigure 12 12,, several several typical typical examples examples show show the drasticthe drastic changes changes on the on south the southside of side the of Dongsha the Dongsha Shoal Shoal after after the typhoon the typhoon passed. passed. The The red red box box marks marks the the position position of ofdrastic drastic changes. changes. Among Among them, them, the the large large tidal tidal trench trench in in the the south south of of the the Dongsha Dongsha Shoal Shoal in in1985 1985 was was wider wider than than in in 1984. 1984. In In 1992, 1992, a a tidal tidal trough troughthat that runrun throughthrough thethe easteast and west banksbanks appear appeareded for for the the first ﬁrst time in the south of the Dongsha Shoal under the impact of typhoon.typhoon. In In 2008, 2008, under under the the influence inﬂuence of of the the transit transit typhoon, typhoon, two two tidal tidal troughs troughs connecting connecting thethe east east bank bank and and the the west west bank bank were were formed, formed, and and the tidal the tidal trough trough on the on south the southside grad- side gradually developed into a tidal channel. In 2015, the tidal trough on the north side ually developed into a tidal channel. In 2015, the tidal trough on the north side gradually gradually developed into a tidal trough. The shoal between the two tidal troughs was developed into a tidal trough. The shoal between the two tidal troughs was separated separated from the Dongsha Shoal, causing the south bank of the Dongsha shoal to move from the Dongsha Shoal, causing the south bank of the Dongsha shoal to move northward, northward, and the southern end of the sandy ridge line also moved north. and the southern end of the sandy ridge line also moved north.

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Figure 12. Several cases show the response of the Shaling Line to the typhoon. Figure 12. Several cases show the response of the Shaling Line to the typhoon. 4.2. The Impact of Human Activities 4.2.1. Port Construction4.2. The Impact of Human Activities Port construction4.2.1. Port along Construction the Jiangsu coast has also accelerated since the 1980s and stimulated the reclamationPort construction process of the along tidal the ﬂats. Jiangsu Taking coast the Yancheng has also coastalaccelerated wetland since as the 1980s and an example, in thestimulated past decade the reclamation the ports of Binhai,process Sheyang of the tidal and flats. Dafeng Taking have the expanded. Yancheng the coastal wetland changes of wateras dynamican example, environment in the past due decade to the the massive ports of coastal Binhai, engineering Sheyang and contributes Dafeng have expanded. to the worseningthe erosion changes at of the water western dynamic tidal ﬂatenvironm of Dongshaent due Shoal to the [61 ].massive coastal engineering contributes to the worsening erosion at the western tidal flat of Dongsha Shoal [61]. 4.2.2. Reclamation Activities

Tidal ﬂats are an important part of coastal wetlands in Jiangsu Province and one of the most important natural resources. In order to alleviate the situation of large popula- tion and small land in Jiangsu Province, coastal areas have been carrying out long-term

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4.2.2. Reclamation Activities Int. J. Environ. Res. Public Health 2021, 18, 1573Tidal flats are an important part of coastal wetlands in Jiangsu Province and one14 of of 19 the most important natural resources. In order to alleviate the situation of large popula- tion and small land in Jiangsu Province, coastal areas have been carrying out long-term tidaltidal flat flat reclamation projects. Historically, Historically, the reclamationreclamation area along beaches in Jiangsu 6 2 Province has reached 22 ×× 10106 hmhm2 [[6262]].. Since Since 1949, 1949, large large-scale-scale reclamation reclamation in Jiangsu has not only occu occupiedpied a large amount of tidal flat flat resources, but also changed the offshore sedimentary environment [6] [6].. InIn recent reclamation projects, a number of long dykes has cut off thethe connectionconnection between the inner tidal flats flats and the outer seawater. The dyke construction has changed radically not not only the estuarine tidal system inside the dyke, but also the coastal marine environment outside the dyke (Figure 1212).). When a seawall is built, the rich supply of sediment will provide more sediment to thethe new beach, so that the beach graduallygradually becomes silted and thenthen gradually extends outward (Figure 13). The result result is that that the the construction construction of the seawall seawall narrows the western tidaltidal cross cross section, u undernder the premise of constant tidal flux, flux, tidal range will increase increase.. After thethe completioncompletion ofof the the first first and and second second stage’s stage’s reclamations, reclamations, the the peak peak velocity velocity and unitand tidalunit tidaldischarge discharge in Xiyang in Xiyang tidal tidal inlet increasedinlet increased about about 20% and20% 5~10% and 5~10% [63]. [63].

Figure 13. TheThe history history and distribution of tidal flat ﬂat reclamation in Jiangsu Province.

As shown in Figure 14,, when the flow flow rate increases at the same time, the flowflow rate inin the the waterway waterway will will inevitably inevitably become become larger, larger, resulting resulting in inerosion erosion by bythe the watercourse watercourse on bothon both sides sides of ofthe the channel channel such such that that the the water water channel channel is is widened. widened. In In this this respect the Dongsha Shoal isis aa typicaltypical example. example. A A large large amount amount of of sediment sediment in in the the embankment embankment before be- forethe seawallthe seawall gradually gradually becomes becomes silted silted hence hence the sediment the sediment content content of the of western the western seawater sea- watergradually gradually decreases decreases [14]. Thus,[14]. Thus, the increase the increase in the in tidal the tidal current current and and the decreasethe decrease in the in sediment content in seawater have led to an increase in the erosion of the west bank of the the sediment content in seawater have led to an increase in the erosion of the west bank Dongsha Shoal. of the Dongsha Shoal. In order to quantitatively analyze the response of sand ridge points movement of In order to quantitatively analyze the response of sand ridge points movement of Dongsha Shoal to the reclamation, we searched for the data of Jiangsu coastline reclamation Dongsha Shoal to the reclamation, we searched for the data of Jiangsu coastline reclama- since modern times, and calculated the cumulative reclamation of the Yancheng coast in tion since modern times, and calculated the cumulative reclamation of the Yancheng coast the 1970s, 1980s, 1990s, 2000s and 2010s area [64]. Then the relationship between sand in the 1970s, 1980s, 1990s, 2000s and 2010s area [64]. Then the relationship between sand ridge point movement and reclamation was analyzed, and the result is shown in Figure 15. ridge point movement and reclamation was analyzed, and the result is shown in Figure The beach reclamation activities on the coast of Yancheng have a greater impact on the 15. The beach reclamation activities on the coast of Yancheng have a greater impact on the movement of the sand ridge line. According to the linear relationship between cumulative movement of the sand ridge line. According to the linear relationship between cumulative movement distance and cumulative reclamation area of sand ridge points in different periods, the influence process can be expressed as follows: With the increase of tidal flat reclamation area, the cumulative movement distances of sand ridge points A, D, and E show an increasing trend. By contrast, the cumulative moving distance of sand ridge points B and C shows a decreasing trend. It can be seen that the tidal flats speed up the movement Int. J. Environ. Res. Public Health 2021,, 18,, xx FORFOR PEERPEER REVIEWREVIEW 15 of 20

movement distance and cumulative reclamation area of sand ridge points in different periods, the influence process can be expressed as follows: With the increase of tidal flat Int. J. Environ. Res. Public Health 2021, 18, 1573 15 of 19 reclamation area, the cumulative movement distances of sand ridge points A, D, and E show an increasing trend. By contrast, the cumulative moving distance of sand ridge points B and C shows a decreasing trend. It can be seen that the tidal flats speed up the movementof the northern of the northern and southern and partssouthern of the parts sand of ridge the sand line, ridge while line, the movement while the speedmovement of the speedcentral of the part central slows part down. slows down.

FigureFigure 14. 14.RepresentationRepresentation of the of thewater water flow ﬂow cross cross sections sections of the of theXiyang Xiyang tidal tidal channel channel before before and and afterafter reclamation. reclamation.

FigureFigure 15. 15.The Theresponse response of sand of sandridge ridge points points movement movement of Dongsha of Dongsha Shoal to Shoal the reclamation to the reclamation in Yanchengin Yancheng.. 4.2.3. Laver Cultivation Human activities such as the rapid development of marine aquaculture in the central

Jiangsu coast have had a marked impact on the tidal ﬂat morphology. The Dongsha Shoal Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 16 of 20

4.2.3. Laver Cultivation Int. J. Environ. Res. Public Health 2021, 18, 1573 16 of 19 Human activities such as the rapid development of marine aquaculture in the central Jiangsu coast have had a marked impact on the tidal flat morphology. The Dongsha Shoal has unique resource advantages such as good light conditions and no pollution, and has excellenthas unique conditions resource advantagesfor breeding such Porphyra as good yezoensis light conditions. and no pollution, and has excellentAs shown conditions in Figure for breeding 16, existingPorphyra studies yezoensis have .shown that Porphyra yezoensis breeding areasAs began shown to appear in Figure in 16 the, existing Dongsha studies Shoal have at the shown beginning that Porphyra of this century. yezoensis Inbreeding 2007, the areaareas was began 75.82 to appearhectares, in theand Dongsha in 2015 it Shoal further at the increased beginning to 954.0 of this8 century.hectares In[65 2007,]. The the east sidearea of was the 75.82 Dongsha hectares, Shoal and faces in 2015 the open it further sea. Under increased the toaction 954.08 of hectaresnortheast [65 wave]. The and east east side of the Dongsha Shoal faces the open sea. Under the action of northeast wave and east wave, the sediment moves west and southwest with the high tide water, and accumulates wave, the sediment moves west and southwest with the high tide water, and accumulates on the beach surface, making the east side of the Dongsha Shoal continue to grow. In this on the beach surface, making the east side of the Dongsha Shoal continue to grow. In century, humans began to cultivate laver at the low-tide beach in the northeastern part of this century, humans began to cultivate laver at the low-tide beach in the northeastern the Dongsha Shoal. The large area of laver cultivation nets has slowed down the northeast part of the Dongsha Shoal. The large area of laver cultivation nets has slowed down the annortheastd eastward and eastwardwaves to wavesa certain to aextent certain and extent blocked and blockedpart of the part sediment of the sediment from going from to thegoing middle to the and middle high andtide highbeaches. tide The beaches. advancement The advancement of the shoal of slowed the shoal down slowed the downsiltation speedthe siltation of the speedhigh tide of the beach high and tide the beach middle and thetide middle beach, tideand beach,the sand and ridge the sandline als ridgeo ap- pearedline also to appeared move to tothe move northeast to the as northeast a whole. as a whole.

Figure 16. Representation of the water flowflow cross sections of the Xiyang tidal channel beforebefore andand after reclamation. 4.3. Ecological Value and Protection Strategy of the Dongsha Shoal 4.3. Ecological Value and Protection Strategy of the Dongsha Shoal The Dongsha Shoal has great value in the support of wetland function and biodiversity. The Dongsha Shoal has great value in the support of wetland function and biodiver- With reference to coastal development, the government has given much attention to the sity. With reference to coastal development, the government has given much attention to promotion of coastal development and the ensuing economic benefits but, at the same time, the promotion of coastal development and the ensuing economic benefits but, at the same has neglected the need to protect the Dongsha Shoal. The development activities in the time,Yancheng has neglected coastal area the have need caused to protect some the disturbance Dongsha to Shoal. the intertidal The development natural environment. activities in theReclamation Yancheng projects coastal can area cause have a caused sizeable some and adverse disturbance threat to to the the intertidal ecology of natural a region, environ- and ment.the government Reclamation has projects now realized can cause that land a sizeable reclamation and adverse must be supportedthreat to the by aecology rigorous of a region,environmental and the impactgovernment assessment has now and realized sound that scientific land reclamation and technological must be support. supported The by aMigratory rigorous Birdenvironmental Sanctuaries impact along the assessment Coast of the and Yellow sound Sea–Bohai scientific Gulf and of technological China (Phase sup- I) port.was selectedThe Migratory as as part Bird of theSanctuaries World Heritage along the List Coast in 2019 of andthe Yellow the Radial Sea Sand–Bohai Ridges Gulf of Chinais the core(Phase protection I) was selected area of as this as heritage part of the site, World which Heritage means that List thein 2019 protection and the of Radial the SandDongsha Ridges Shoal is willthe core receive protection more support area of from this the heritage government. site, which For now, means the that construction the protec- tionof artificial of the islandsDongsha and Shoal land will reclamation receive hasmore basically support stopped from the and government. protection measures For now, for the cothenstruction Dongsha Shoalof artificial wetlands islands are inand place land and reclamation overseen by has the basically government. stopped and protection measures for the Dongsha Shoal wetlands are in place and overseen by the govern- 5. Conclusions ment. Scientific analysis and demonstration of the evolution and the factors that influence 5.the Conclusions sand ridge line of the Dongsha Shoal will aid in understanding the changes in the tidal flat in modern radial sand ridges in Jiangsu Province and achieve sustainable use of the Scientific analysis and demonstration of the evolution and the factors that influence tidal flat. the sand ridge line of the Dongsha Shoal will aid in understanding the changes in the tidal In this study, based on a novel remote-sensing method, we explored the detailed temporal and spatial dynamics of the modern Dongsha Shoal landform evolution through the location of sand ridge lines obtained from 33 scenes of Landsat satellite images of the

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study area from 1973 to 2016. Our results show that in the past 33 years, the strike of the sand ridge Line has been basically stable, and the overall movement trend of the sand ridge line is from the southwest to the northeast. From the change of the sand ridge line it can be inferred that the dynamics of the geomorphology are inconsistent. The north and south wings are eroded on the west side, while the central area is eroded on the east side. Most of the sand ridge line is moving eastward. In addition, the change of sand ridge line is affected by multiple factors such as sediment supply, hydrodynamics and reclamation. This research has been based on medium-resolution remote-sensing satellite image analysis and the discussion has focused on the dynamic changes of the sand ridge line of the Dongsha Shoal. Limitations of the study relate to the satellite repeat cycle and the level of cloud cover; yet to solve some problems, such as the monitoring of the sand ridge line of the Dongsha Shoal, seasonal changes for the region, which are currently lacking, are needed. Due to the limitations of this research, this paper does not draw clear conclusions on these issues. In the future, higher-resolution satellite images will be used to conduct in-depth research on the above issues in combination with a seasonal biodiversity survey so that countermeasures and suggestions for in situ conservation can be put forward, in order to achieve sustainable development of the tidal ﬂat of Jiangsu Province.

Author Contributions: B.L. and Z.Z. conceived and designed the research topic; B.L., G.W., and H.W. carried out the method and processed the data; B.L. prepared and wrote the original draft; G.W., Y.W., and J.Z. reviewed and edited the paper. writing—original draft preparation, B.L.; writing review and editing, B.L., X.J., and S.J.; software, G.W., H.W., and J.Z.; supervision, X.J. and S.J. All authors have read and agreed to the published version of the manuscript. Funding: This study is funded by the National Key Research and Development Program of China. No. 2018YFE0105900-41, National Natural Science Foundation of China (No. 41371024). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Publicly available datasets were analyzed in this study. This data can be found here: http://glovis.usgs.gov/, http://tcdata.typhoon.org.cn/zjljsjj_zlhq.html. Acknowledgments: The authors are grateful to the United States Geological Survey (USGS) for providing all Landsat 1-8 MSS/TM/ETM+/OLI images. Please note that any errors or shortcomings in the paper are the responsibility of the authors. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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