Journal of Marine Science and Engineering

Article Satellite-Based Monitoring of Annual Coastal Reclamation in and since the 21st Century: A Comparative Study

Tanxin Feng 1 and Nan Xu 2,*

1 College of Earth Science, Chengdu University of Technology, Chengdu 610051, China; [email protected] 2 College of Marine Science and Engineering, Nanjing Normal University, Nanjing 210023, China * Correspondence: [email protected]; Tel.: +86-15380508199

Abstract: To solve the shortage of land resources, many coastal cities have implemented coastal reclamation projects over the past few decades. Coastal reclamation can promote the economic development of coastal cities and improve human well-being in coastal zones. However, it can inevitably cause a series of ecological and environmental issues, such as coastal water pollution, ecosystem destruction, habitat loss, and land subsidence. Shenzhen and Hong Kong are two large neighboring Chinese coastal cities in southern China with different systems. As densely populated and economically developed cities, they face similar land shortage issues. However, recent coastal reclamation changes in Shenzhen and Hong Kong are unclear under different social and political systems. To fill this gap, this study aimed to monitor and compare recent annual coastal reclamation in Shenzhen and Hong Kong with totally different systems using free and open satellite products. Then, to compare the results of coastal reclamation between Shenzhen and Hong Kong. Large- scale coastal reclamation in Shenzhen and Hong Kong from 2000 to 2018 can be observed. The 2  total area of coastal reclamation was 4140.7 hm , of which the total coastal reclamation area in  Shenzhen was 3409.8 hm2 and the total coastal reclamation area in Hong Kong was 730.9 hm2. Coastal Citation: Feng, T.; Xu, N. reclamation in Shenzhen showed a temporal characteristic of “increasing slightly and then decreasing 2 Satellite-Based Monitoring of Annual sharply”. Before and after 2010, the area of accumulated coastal reclamation were 3202.9 hm and Coastal Reclamation in Shenzhen and 206.9 hm2. Comparatively, coastal reclamation in Hong Kong exhibited a temporal characteristic of Hong Kong since the 21st Century: A “first decreasing and then increasing and then decreasing”. In 2002 and 2014, the scale of coastal Comparative Study. J. Mar. Sci. Eng. reclamation was relatively large, with a total area of 501.8 hm2, accounting for 69.19% of its total area 2021, 9, 48. https://doi.org/10.3390/ of coastal reclamation. The comparison between our produced coastal reclamation and the official jmse9010048 area on coastal reclamation exhibited a good agreement based on correlation analysis (r = 0.99) and RMSE (Root Mean Square Error) (0.72). Since the 21st century, Shenzhen and Hong Kong have shown Received: 30 November 2020 different, even opposite, policies on coastal reclamation. It will be necessary to continuously monitor Accepted: 31 December 2020 Published: 5 January 2021 future coastal reclamation driven by policies for better conducting sustainable coastal development in the Guangdong-Hong Kong-Macao Greater Bay Area.

Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- Keywords: coastal reclamation; Shenzhen; Hong Kong; satellite; remote sensing; landsat ms in published maps and institutio- nal affiliations.

1. Introduction In recent years, with the continuous development of urbanization and industrializa- Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. tion, the demand for land resources has increased over time [1]. To alleviate the shortage This article is an open access article of land resources, many coastal countries around the world, such as the Netherlands [2], distributed under the terms and con- Italy [3], Japan [4], and Singapore [5], have gained a large amount of land from coastal recla- ditions of the Creative Commons At- mation activities. China began coastal reclamation activities in the 1950s and 1960s, and 2 tribution (CC BY) license (https:// by the end of the last century, the area of coastal reclamation in China reached 12,000 km , 2 creativecommons.org/licenses/by/ with an average of 230 to 240 km of reclamation per year, and over 30% of tidal land 4.0/). reclaimed [6]. There are many purposes for coastal reclamation, such as population living,

J. Mar. Sci. Eng. 2021, 9, 48. https://doi.org/10.3390/jmse9010048 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, 48 2 of 15

freight transportation, industrial production, agricultural planting, and recreation, which have brought huge benefits to urban development and people’s lives [7]. However, coastal reclamation has also brought many potential threats to coastal zones, such as coastal water pollution [8], habitat loss, ecosystem destruction [9], and land subsidence [10], which have threatened human well-being and economic development [11]. Therefore, spatial coverage and temporal changes of coastal reclamation can serve as fundamental data for policy-making on biological conservation, environmental protection, and sustainable development in coastal zones. Shenzhen is the first special economic zone in China and its economy has grown rapidly since the reform and opening up, 40 years ago. To meet the land resource demand for rapid urbanization, Shenzhen has launched many coastal reclamation projects since the establishment of the Special Economic Zone in 1980 [12]. In particular, it was a period of rapid growth in Shenzhen coastal reclamation from the mid-1990s to the end of the 1990s with an annual average reclamation of 337.16 hm2 [13]. The history of coastal reclamation in Hong Kong is longer when compared with Shenzhen. Since the mid-19th century, many coastal reclamation projects have been conducted [14]. Over the past 100 years, Hong Kong has intermittently conducted many coastal reclamation activities dozens of times, reaching the peak of coastal reclamation in the 1990s. By 2014, coastal reclamation in Hong Kong has exceeded 67 km2, accounting for more than 6% of the total land area of Hong Kong [15]. From the 21st century to the present, to promote environmental protection, the coastal reclamation in Shenzhen and Hong Kong has entered a stage of rational growth and the rate of coastal reclamation has declined significantly [13]. However, after 2018, to promote economic development and solve the housing tension problem, the Hong Kong government re-planned coastal reclamation, which means that the area of coastal reclamation in Hong Kong may increase largely in the future. In many previous studies, remotely sensed data have been widely used to study coastal reclamation in Shenzhen and Hong Kong. Hong, et al. used Landsat images to interpret the dynamics of reclamation in Shenzhen from 1979 to 2014 at 10-year intervals and analyzed the influencing factors [16]. Wu used Landsat images to extract Shenzhen coastline data from 1988 to 2015 at 6-year intervals [17]. Fung used Landsat MSS and SPOT High Resolution Visible (HRV) data to produce a map of Hong Kong’s expansion from 1979 to 1987 [18]. These studies used several remote sensing images to monitor coastal reclamation in Shenzhen and Hong Kong at an interval of several years. However, none of these studies obtained detailed information on coastal reclamation in Shenzhen and Hong Kong on an annual scale, which is essential for tracking coastal reclamation with massive and rapid expansion [19–22]. It is necessary to monitor coastal reclamation changes at an annual scale to obtain specific years of coastal reclamation projects, which are lost in previous studies. In this study, Landsat data and the Joint Research Centre Global Surface Water (GSW) dataset [23] from 1999 to 2018 were combined to track annual coastal reclamation changes in Shenzhen and Hong Kong under totally different social and political systems since the 21st century for a comparative study. Then, we analyzed and compared the spatial distribution and temporal changes of coastal reclamation in Shenzhen and Hong Kong from 2000 to 2018. Google Maps in 2019 was also used to help manually interpret the types and utilization of coastal reclamation in our study area. Moreover, we validated our result by comparing it with the statistics of coastal reclamation provided by Hong Kong officials (detailed in Section 2.2 Data Source).

2. Study Area and Data Sources 2.1. Study Area Shenzhen and Hong Kong were selected as our study area. Shenzhen and Hong Kong are located in the Pearl River Delta, belonging to the Guangdong-Hong Kong-Macao Greater Bay Area (GBA). Shenzhen is located on the seashore of Guangdong Province of southern China. It faces Daya Bay and Dapeng Bay in the east and borders Pearl River J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 3 of 14

Shenzhen and Hong Kong were selected as our study area. Shenzhen and Hong Kong J. Mar. Sci. Eng. 2021, 9, 48 are located in the Pearl River Delta, belonging to the Guangdong-Hong Kong-Macao3 of 15 Greater Bay Area (GBA). Shenzhen is located on the seashore of Guangdong Province of southern China. It faces Daya Bay and Dapeng Bay in the east and borders Pearl River EstuaryEstuary and and Lingding Lingding Channel Channel in in the the west. west. ItIt connectsconnects HongHong Kong Kong by by the the ShenzhenShenzhen River River toto the the south south and and borders borders Dongguan Dongguan and and Huizhou Huizhou in the in north.the north. Although Although they arethey two are large two neighboringlarge neighboring Chinese Chinese coastal coastal cities, theycities, have they totally have totally different different systems. systems. Specifically, Specifically, Shen- zhenShenzhen has a has socialist a socialist system system while while Hong Hong Kong Ko hasng a has capitalist a capitalist system. system. After After the reform the reform and openingand opening up, as up, a special as a special economic economic zone, Shenzhen’s zone, Shenzhen’s economy economy has been has developing been developing rapidly sincerapidly the since 1980s. the Comparatively, 1980s. Comparatively, Hong Kong Hong developed Kong asdeveloped a warehousing as a warehousing and distribution and centerdistribution for U.K. center trade withfor U.K. southern trade China with sincesouthern 1842 afterChina the since Treaty 1842 of Nanking.after the AlthoughTreaty of Shenzhen’sNanking. Although social and Shenzhen’s economic development social and ec isonomic far behind development that of Hong is far Kong, behind Shenzhen that of exhibitsHong Kong, a higher Shenzhen development exhibits speed. a higher According development to the speed. Shenzhen According China websiteto the Shenzhen [24], by theChina end website of 2018, [24], Shenzhen by the had end a of total 2018, land Shenzhen area of 1997had a km total2, a land total area sea areaof 1997 of 1145 km2 km, a total2, a coastlinesea area lengthof 1145 of km 2602, a km, coastline and a totallength population of 260 km, of and 13 million. a total population Shenzhen isof dominated 13 million. byShenzhen mountainous is dominated areas with by few mountainous plains. Since areas the reformwith few and plains. opening Since up, rapidthe reform economic and developmentopening up, rapid has led economic to a significant development problem has ofled land to a scarcity.significant The problem limitation of land of land scarcity. has becomeThe limitation an important of land factor has become that restricts an important the development factor that of restricts Shenzhen the [25 development]. Hong Kong of isShenzhen on the south [25]. sideHong of ShenzhenKong is on and the it south is composed side of Shenzhen of the Hong and Kong it is Island,composed of the Peninsula,Hong Kong the Island, inland Kowloon area of thePeninsula, , the inland and area 262 of smallthe New islands. Territories, According and 262 to thesmall Hong islands. Kong According Lands Department to the Hong [26] Kong and the Lands Hong Department Kong Census [26] and and Statistics the Hong Depart- Kong mentCensus [27 and], by Statistics the end ofDepartment 2018, Hong [27], Kong by hadthe end a total of land2018, areaHong of Kong 1106 kmhad2, a a total total land sea areaarea of of 1648.7 1106 km km22, ,a a total coastline sea area length of 1648.7 of about km 3602, a km, coastline and a totallength population of about 360 of 7.5 km, million. and a Hongtotal population Kong has one of of7.5 the million. highest Hong construction Kong has density one worldwideof the highest due construction to large-scale density urban developmentworldwide due and to limited large-scale land urban resources development [28]. Figure and1 showslimited the land map resources of our study[28]. Figure area (i.e.,1 shows Shenzhen the map and of Hong our study Kong). area (i.e., Shenzhen and Hong Kong).

FigureFigure 1. 1.Map Map ofof thethe studystudy areaarea (i.e.,(i.e., ShenzhenShenzhen andand HongHong Kong).Kong). InIn thethe inset,inset, thethe redred boxbox indicatesindicates thethe location location of of the the study study area. area.

2.2.2.2. DataData SourcesSources InIn thisthis study,study, three three data data sources sources(i.e., (i.e., LandsatLandsat imagery,imagery, GSWGSW datasetdataset (Table(Table1 ),1), and and thethe HongHong KongKong ReclamationReclamation Map)Map) werewere usedused toto monitor monitor annual annual coastal coastal reclamation reclamation in in ShenzhenShenzhen andand HongHong KongKong sincesince thethe 21st21st century.century. LandsatLandsat imageryimagery waswas usedused forfor visual visual interpretationinterpretation to to extract extract coastlines, coastlines, the GSW the datasetGSW wasdataset used forwas land-ocean used for classification, land-ocean andclassification, these two and data these were two used data to obtainwere used the coastal to obtain reclamation the coastal results. reclamation The Hong results. Kong The Reclamation Map was used to produce the accuracy of the results of reclamation.

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Table 1. Detailed information on Landsat imagery and Global Surface Water (GSW) dataset used in this study.

Data Landsat Imagery GSW Dataset Source Temporal 1999–2012 2013–2018 1999–2018 span (year) Landsat-5 Landsat-8 Sensor Landsat-5, 7, and 8 TM OLI Number of 56 24 20 images Type of Raster data Spatial 30 m resolution Temporal 16 d resolution Value 0 1 2 3 seasonal permanent Description no data not water water water Note: TM (Thematic Mapper); OLI (Operational Land Imager).

To be specific, the first data source is the Landsat imagery provided by the United States Geological Survey (USGS) (https://earthexplorer.usgs.gov/). Four Landsat scenes (path/row: 121/44, 121/45, 122/44, and 122/45) can cover the entire study area. In this study, 80 Landsat-5 Thematic Mapper (TM) and Landsat-8 Operational Land Imager (OLI) images over 1999–2018 (with 4 images per year) were used. Among them, 56 Landsat-5 TM images were acquired from 1999 to 2012, and 24 Landsat-8 OLI images were acquired from 2013 to 2018. The spatial resolution of the Landsat imagery is 30 m and the temporal resolution is 16 days. To better extract artificial coastlines with coastal reclamation (e.g., dikes, ports, aquaculture ponds, and entertainment), cloud-free Landsat images were collected. The used Landsat images in this study had been pre-processed to L1T level (i.e., Standard Terrain Correction). The geo-registration is consistent and within prescribed tolerances (i.e., georeferencing error < 12 m) (https://lta.cr.usgs.gov/LETMP). The second data source is the GSW dataset, which was available in the Google Earth Engine (GEE) cloud computing platform (https://earthengine.google.com/). Specifically, the annual water classification historical images between 1999 and 2018 across our study area were collected (one image per year, a total of 20 images) from the GSW dataset. These annual water classification maps derived from 30-m Landsat-5, 7, and 8 data have four values: 0 (i.e., no data), 1 (i.e., not water), 2 (i.e., seasonal water), and 3 (i.e., permanent water). Here, permanent water was extracted from the annual images to derive the land-ocean classification map for each year. The third data source is the Hong Kong Reclamation Map with a vector format, which is based on the coastal reclamation data provided by the Hong Kong Lands Department. The map was digitized by using Google Maps Engine for visualization and interactivity online. Thus, the Hong Kong Reclamation Map is reliable for comparison with our coastal reclamation result. This map illustrates the coastal reclamation projects on both sides of the Hong Kong Victoria Harbor and their areas between 1840 and 2018 at 10-year intervals. Here, coastal reclamation data over 2000–2009 and 2010–2018 were used to evaluate our produced coastal reclamation result.

3. Methods 3.1. Land-Ocean Classification In this study, GEE was used to extract the permanent water layers from the GSW dataset for deriving land-water maps. However, owing to many inland water bodies J. Mar. Sci. Eng. 2021, 9, 48 5 of 15

(e.g., coastal wetlands and aquaculture ponds) in coastal zones, it is difficult to accurately distinguish land from the ocean (Figure2b) using land-water maps derived from the J. Mar. Sci. Eng. 2021, 9, x FOR PEERpermanent REVIEW water layer. To solve this problem, an automatic coastline extraction method5 of 14 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 5 of 14 was adopted [29]. For the land-water maps, functions “imclose” and “imfill” available in MATLAB were applied to generate land-ocean maps. In Figure2, a Landsat TM im- age, and a land-ocean map before and after 2003 in the Shekou Peninsula in Shenzhen 113°57◦ ′0 E, 22°26◦ 0′–22°33◦′ N)0 were◦ used0 to demonstrate the step of generating a land-ocean (113°57map.113 50 Specifically,′ E,–113 22°2657′–22°33E the, 22 reclaimed′ 26N)–22 were33 usedlakeN) onto were thedemonstrate usedright side to demonstrate the of the step Shekou of generating the Peninsula step ofa land-ocean generatingis an inland a land-oceanmap.water Specifically, body, map. which Specifically, the reclaimedwas regarded the lake reclaimed onas thea waterright lake onside body the of rightthein Shekouthe side original of Peninsula the Shekou land-water is an Peninsula inland map. is anwaterHowever, inland body, water it belongswhich body, was to which the regarded land was regardedarea as aftera water as automatic a waterbody body incoastline the in original the extr originalaction land-water land-water processing. map. map. In However,However,this study, itit belongsifbelongs a water to to the bodythe land land near area area the after after coastl automatic automaticine is coastlinenot coastline directly extraction extr connectedaction processing. processing. to the ocean In In this study,thisaccording study, if a water toif satellitea bodywater near imagery,body the near coastline this the water coastl is notboinedy directly isbelongs not connecteddirectly to inland connected to water the ocean ratherto the according thanocean the to satelliteaccordingocean. imagery, to satellite this waterimagery, body this belongs water bo tody inland belongs water to ratherinland thanwater the rather ocean. than the ocean.

Figure 2. An example of generating a land-ocean map using Landsat data. (a) The Landsat image FigureFigure 2.2. An example of generating a a land land-ocean-ocean map map using using Landsat Landsat data. data. (a ()a The) The Landsat Landsat image image of of Shekou Peninsula in Shenzhen in 2003; (b) the land-ocean map before automatic coastline Shekouof Shekou Peninsula Peninsula in Shenzhenin Shenzhen in in 2003; 2003; (b )(b the) the land-ocean land-ocean map map before before automatic automatic coastline coastline extraction processing;extraction processing;processing; and (c) the andand land-ocean ( (cc)) the the land-ocean land-ocean map after map automaticmap after after automati automati coastlinec ccoastline extraction coastline extraction extraction processing. processing. processing.

3.2.3.2. Post-ProcessingPost-Processing of of Land-Ocean Land-OceanLand-Ocean Classification ClassificationClassification InInIn thisthisthis section,section,section, the the post-processing post-processingpost-processing of of la land-oceanland-oceannd-ocean classification classification classification was was was conducted conducted conducted to to to removeremove errorserrorserrors inin ourour produced produced land-ocean land-oceanland-ocean maps maps induced induced by by by the the the complex complex complex landscape landscape landscape in in in coastalcoastal zones zoneszones (e.g., (e.g.,(e.g., coastal coastalcoastal wetlands, wetlands,wetlands, aquaculture aquacult aquacultureure ponds, ponds, ponds, and and and ports). ports). ports). The The The specific specific specific steps steps steps were detailedwere detailed as follows asas followsfollows (Figure 3(Figure(Figure). First, 3). coastal3). First,First, land coastalcoastal expansions land land expansions expansions and coastal and and land coastal coastal fluctuations land land canfluctuations be grouped cancan based bebe groupedgrouped on their based temporalbased onon thei changestheir rtemporal temporal based onchanges changes our produced based based on on annual our our produced land-oceanproduced mapsannual over land-oceanland-ocean 1999–2018. mapsmaps Second, overover 1999–2018.1999–2018. these coastal Seco Seco fluctuationsnd,nd, these these coastal coastal induced fluctuations fluctuations by coastal induced induced reclamation by by forcoastal lakes reclamationreclamation and shoals forfor with lakes lakes low and and distinction shoals shoals with with were low low manuallydistinction distinction checkedwere were manually manually and refined checked checked to obtainand and land-oceanrefined toto obtainobtain maps land-ocean betweenland-ocean 1999 maps maps and between between 2018 with 1999 1999 the and and help 2018 2018 of thewith with reference the the help help land-oceanof of the the reference reference map in 1999land-ocean using Google mapmap inin Earth 19991999 using software.using Google Google Note Earth Earth that so thesoftware.ftware. manual Note Note checking that that the the canmanual manual ensure checking checking the precision can can ofensure land-ocean thethe precisionprecision maps withofof land-oceanland-ocean a working maps maps resolution with with a a working (i.e.,working scale) resolution resolution of about (i.e., 1:60,000.(i.e., scale) scale) Finally,of of about about for each1:60,000. annual Finally,Finally, land-ocean forfor eacheach map annu annu afteralal land-ocean land-ocean 1999, we subtractedmap map after after 1999, 1999, the landwe we subtracted areasubtracted of last the the year land land from area area the landof last area year of fromfrom that the yearthe landland to produce areaarea of of that that the year annualyear to to produce produce coastal the land the annual annual expansion coastal coastal map land land between expansion expansion 2000 andmap 2018. between 20002000 andand 2018. 2018.

FigureFigure 3.3. CoastalCoastal land land expansionsexpansions and and and fl fluctuations fluctuationsuctuations in in inthree three three typical typical typical co coastal coastalastal areas areas areas in in the inthe study the study study area. area. area. ((aa)) West WestWest Coast CoastCoast of ofof Bao’an Bao’anBao’an District,District, District, Shenzhen; Shenzhen; ( (b(bb)) )Shekou ShekouShekou Peninsula, Peninsula,Peninsula, Shenzhen; Shenzhen;Shenzhen; and andand (c ()(c cChek)) ChekChek Lap LapLap Kok, Kok, Hong Kong. HongKok, Hong Kong. Kong.

3.3. Extraction ofof ththee CoastalCoastal Reclamation Reclamation InIn thethe studystudy area,area, both both natural natural factors factors (e.g (e.g.,., sediments sediments from from upstream upstream rivers rivers and and sea- sea- level rises)rises) andand coastalcoastal reclamationreclamation (e.g.,(e.g., dikes,dikes, ports,ports, aquacultureaquaculture ponds,ponds, and and

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J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 3.3. Extraction of the Coastal Reclamation 6 of 14

In the study area, both natural factors (e.g., sediments from upstream rivers and sea-level rises) and coastal reclamation (e.g., dikes, ports, aquaculture ponds, and enter- entertainment)tainment) can result can resultin coastal in coastal land landexpansions expansions [30]. [ 30Fortunately,]. Fortunately, it itis iseasy easy to to distinguish distinguish coastalcoastal reclamation reclamation from from remote remote sensing sensing images images by by their their regular regular shapes. shapes. The The “Raster to “Raster to Polygon”Polygon” function function in inArcGIS ArcGIS was was applied applied to toobtain obtain original original annual annual coastal coastal land land expansion expansion mapsmaps with with a avector vector format. format. Specifically, Specifically, by by comparing comparing the the original original result result with with the Landsat the Landsat imageimage forfor allall years, years, natural natural expansion expansion areas areas were were removed removed to to obtain obtain the the refined annual refined annualcoastal coastal reclamation reclamation maps maps in ArcGISin ArcGIS with with a working a working resolution resolution (i.e., (i.e., scale) scale) of about 1:60,000. of about 1:60,000.Here, Here, the extraction the extraction of coastal of co reclamationastal reclamation in the in Shekou the Shekou Peninsula, Peninsula, Shenzhen in 2003 ◦ 0 ◦ 0 ◦ 0 ◦ 0 Shenzhen in (2003113 56 (113°56–113 ′57–113°57E, 22′ 29E, 22°29–22 31′–22°31N) was′ N) presented was presented as an exampleas an example in Figure in4 . To be spe- Figure 4. To becific, specific, the coastline the coastline map refined map refined in 2002 in and 2002 the and coastal the coastal reclamation reclamation map in map 2003 (Figure4b ) in 2003 (Figurewere 4b) stacked were stacked on the 2003on the Landsat 2003 Landsat image and image the and latter the was latter refined was manuallyrefined in ArcGIS manually in (FigureArcGIS4 (Figurec). Then, 4c). we Then, combined we combined the refined the coastal refined reclamation coastal reclamation map with map the coastline in with the coastline2002 toin be2002 the to reference be the reference coastline coastline in 2004 (Figurein 20044 (Figured). 4d).

Figure 4. An exampleFigure 4. ofAn extracting example coastalof extracting reclamation coastal of reclamation one year. ( aof) Mapone year. of the (a coastline) Map of aroundthe coastline the Shekou around Peninsula in Shenzhen inthe 2002; Shekou (b) mapPeninsula of the in coastline Shenzhen around in 2002; the ( Shekoub) map of Peninsula the coastline in Shenzhen around the in 2003; Shekou (c) originalPeninsula coastal land expansion mapin Shenzhen in 2003; and in (2003;d) refined (c) original coastal coastal land expansionland expansion map inmap 2003. in 2003; For each and subfigure,(d) refined the coastal basemap land is the true expansion map in 2003. For each subfigure, the basemap is the true color composition of Landsat color composition of Landsat imagery in the corresponding year. imagery in the corresponding year. 3.4. Assessment of the Coastal Reclamation 3.4. Assessment of the Coastal Reclamation We assessed the accuracy of our produced coastal reclamation dataset using the coastal We assessed the accuracy of our produced coastal reclamation dataset using the reclamation data provided by the Hong Kong Lands Department [31] (i.e., Coastal Recla- coastal reclamation data provided by the Hong Kong Lands Department [31] (i.e., Coastal mation Maps on both sides of Victoria Harbor, Hong Kong, 1840–2018). To be specific, the Reclamation Maps on both sides of Victoria Harbor, Hong Kong, 1840–2018). To be Root Mean Square Error (RMSE) and correlation coefficient (r) between coastal reclamation specific, the Root Mean Square Error (RMSE) and correlation coefficient (r) between areas from our result and official data were calculated by the correlation analysis. RMSE coastal reclamation areas from our result and official data were calculated by the and r were calculated according to Equations (1) and (2). Moreover, the chi-square test (x2) correlation analysis. RMSE and r were calculated according to Equations (1) and (2). was performed according to Equation (3) to evaluate the agreement of coastal reclamation Moreover, the chi-square test (x2) was performed according to Equation (3) to evaluate the area results between our result and official data. The Root Mean Square Error (RMSE), agreement of coastal reclamation area results between our result and official data. The correlation coefficient (r), and chi-square test (x2) were calculated as follows. Root Mean Square Error (RMSE), correlation coefficient (r), and chi-square test (x2) were v calculated as follows. u n u 2 u ∑ (Xi − Yi) n t i=1 RMSE =− 2 (1) ()X iiY n = (1) RMSE = i 1 where X is the coastal reclamation arean from our result; Y is the official coastal reclamation area; and n is the number of areas of coastal reclamation projects used for calculation. In this where X is the coastal reclamation area from our result; Y is the official coastal reclamation area; and n is the number of areas of coastal reclamation projects used for calculation. In this study, four coastal reclamation projects were considered and were detailed in Section 5.2 (i.e., n = 4).

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study, four coastal reclamation projects were considered and were detailed in Section 5.2 (i.e., n = 4). n ∑ (Xi − X)(Yi − Y) i=1 r = s s (2) n 2 n 2 ∑ (Xi − X) ∑ (Yi − Y) i=1 i=1 where X is the coastal reclamation area from our result; Y is the official coastal reclamation area; and n is the number of areas of coastal reclamation projects used for calculation.

n (X − Y )2 x2 = ∑ i i (3) i=1 Yi

where X is the coastal reclamation area from our result; Y is the official coastal reclamation area; and n is the number of areas of coastal reclamation projects used for calculation.

4. Results 4.1. Spatial Distribution of Coastal Reclamation Figure5 shows the spatial distribution of the coastal reclamation in the study area (i.e., Shenzhen and Hong Kong) from 2000 to 2018 at a 30-m spatial resolution derived from Landsat imagery and GSW dataset. Over the study period, many coastal reclamation activities were conducted in our study area, with a total area of 4140.7 hm2. Coastal reclamation in the study area showed a large spatial difference. The amount of coastal reclamation in Shenzhen was much larger than that in Hong Kong. Specifically, the total coastal reclamation area in Shenzhen was 3409.8 hm2 and the total coastal reclamation area in Hong Kong was 730.9 hm2, about 1/5 of that in Shenzhen. Six typical areas of coastal reclamation in the study area were enlarged and displayed in Figure5 to demonstrate the details of the spatial distribution of coastal reclamation. Note that almost all the coastal reclamation projects were included in the six areas. Specifically, the sum of coastal reclamation in the six areas was 4035.4 km2, accounting for 97.5% of the total result of coastal reclamation in the study area. Then, land use information [25] and Google Maps in 2019 across this study area were integrated to interpret the types of coastal reclamation. The online Google maps can provide the name and attribution of each landscape patch, so they can be used to characterize the types of coastal reclamation. Among six areas, three areas (Figure5a,b,f) are located in Shenzhen. Area a is located on the west coast of Baoan District, which is Shenzhen Baoan International Airport. Area b is located on the southern coast of the Bao’an District and the Shekou Peninsula in the Nanshan District, which consist of harbors and residential land. Area f is located on the east coast of Yantian District, which is the Yantian Port. The other three areas (Figure5c–e) are located in Hong Kong. Area c is located in , which is an artificial island at the Hong Kong-Zhuhai-Macao Bridge. Area d is located on the northeast coast of and Island, which is Hong Kong Disneyland and Container Terminal No. 9. Area e is located on both sides of and is a green space of Cultural Plaza and Hong Kong West Kowloon Station. Over the past 19 years, the largest coastal reclamation project in Shenzhen was the Shekou Peninsula Reclamation Project, with an area of 1119.4 hm2. The largest coastal reclamation project in Hong Kong was the Hong Kong Disneyland construction, with an area of 263.7 hm2. J. Mar. Sci. Eng. 2021, 9, 48 8 of 15 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 8 of 14

Figure 5. Spatial distribution of the coastal reclamation in Shenzhen and Hong Kong over 2000–2018. Figure 5. Spatial distribution of the coastal reclamation in Shenzhen and Hong Kong over 2000– Different2018. Different colors colors indicate indicate coastal coastal reclamation reclamation in different in different years. Six years. typical Six areastypical of areas coastal of reclamation coastal werereclamation enlarged were to demonstrate enlarged to demonstrate the detailedinformation the detailedof information coastal reclamation. of coastal (reclamation.a) The western (a) coastThe ofwestern Baoan coast District of Baoan in Shenzhen; District (inb )Shenzhen; the southern (b) the coast southern of Shenzhen coast of Baoan Shenzhen District Baoan and District the Shekou and Peninsulathe Shekou in Peninsula Nanshan in District; Nanshan (c) District; the Chek (c Lap) the Kok; Chek ( dLap) the Kok; northeast (d) the coast northeast of Lantau coast Islandof Lantau and TsingIsland Yi and Island; Tsing (e Yi) Hong Island; Kong (e) Hong Victoria Kong Harbour; Victoria and Harbour; (f) Shenzhen and (f Yantian) Shenzhen District Yantian East District Coast. East Coast. 4.2. Temporal Changes of Coastal Reclamation 4.2. TemporalFigure6 showsChanges the of Coastal time series Reclamation of the coastal reclamation areas and the total results in ShenzhenFigure 6 andshows Hong the time Kong series during of the 2000–2018. coastal reclamation Over the pastareas 19 and years, the total the temporalresults in characteristicsShenzhen and of Hong the coastal Kong reclamation during 2000–2018. in Shenzhen Over and the Hong past Kong 19 years, exhibited the significant temporal differences.characteristics The of total the areacoastal of the reclamation coastal reclamation in Shenzhen in Shenzhen and Hong and HongKong Kongexhibited was 2 2 4140.7significant hm ,differences. with an average The annualtotal area coastal of th reclamatione coastal reclamation of 217.9 hm in. TheShenzhen maximum and coastal Hong 2 2 2 reclamationKong was 4140.7 area of hm 700.42, with hm anoccurred average in annual 2003 (684.1 coastal hm reclamationin Shenzhen of and217.916.3 hm hm2. Thein 2 Hongmaximum Kong). coastal The reclamation minimum coastal area of reclamation 700.4 hm2 occurred area of 2.8 in hm2003 occurred(684.1 hm in2 in 2017 Shenzhen (all in Shenzhen).and 16.3 hm Apart2 in Hong from Kong). 2002 and The 2014, minimum the coastal coastal reclamation reclamation area area in Shenzhen of 2.8 hm was2 occurred larger thanin 2017 that (all in in Hong Shenzhen). Kong. BeforeApart 2010,from coastal2002 and reclamation 2014, the activitiescoastal reclamation in Shenzhen area were in relatively frequent, with a cumulative coastal reclamation area of 3202.9 hm2, accounting Shenzhen was larger than that in Hong Kong. Before 2010, coastal reclamation activities2 forin Shenzhen 93.9% of were the total relatively area, frequent, with an with average a cumulative annual coastal coastal reclamation reclamation ofarea 320.3 of 3202.9 hm . After 2010, coastal reclamation activities in Shenzhen decreased sharply. Specifically, the hm2, accounting for 93.9% of the total area, with an average annual coastal reclamation of

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J. Mar. Sci. Eng. 2021, 9, 48 9 of 15 320.3 hm2. After 2010, coastal reclamation activities in Shenzhen decreased sharply. Specifically, the cumulative coastal reclamation area reached 206.9 hm2, accounting for only 6.1% of its total area, with an average annual coastal reclamation of 23.0 hm2. In 2002 cumulative coastal reclamation area reached 206.9 hm2, accounting for only 6.1% of its and 2014, Hong Kong had large-scale coastal reclamation. The coastal reclamation area in total area, with an average annual coastal reclamation of 23.0 hm2. In 2002 and 2014, Hong 2 2 2 2002Kong and had 2014 large-scale were 300.6 coastal hm reclamation. and 201.2 Thehm coastal, respectively, reclamation totaling area in501.8 2002 hm and, 2014accounting forwere 68.7%300.6 of hm the2 andtotal 201.2 coastal hm 2reclamation, respectively, area totaling in Hong 501.8 Kong hm2, accountingover the past for 68.7%19 years. of From Figurethe total 6, coastalwe can reclamation find that areathe incoastal Hong reclamation Kong over the in past Shenzhen 19 years. exhibited From Figure a temporal6, characteristicwe can find that of the“increasing coastal reclamation slightly inand Shenzhen then decreasing exhibited a temporalsharply” characteristic while the coastal reclamationof “increasing in slightlyHong Kong and then exhibited decreasing a temporal sharply” characteristic while the coastal of “first reclamation decreasing in and thenHong increasing Kong exhibited and then a temporal decreasing”. characteristic To be sp ofecific, “first decreasingduring the and first then 10 years, increasing the coastal reclamationand then decreasing”. activities Toin beShenzhen specific, duringwere relatively the first 10 frequent, years, the with coastal an reclamation average annual activities in Shenzhen were relatively frequent, with an average annual coastal reclamation coastal reclamation of 320.3 hm2, which decreased sharply in the following 9 years, with of 320.3 hm2, which decreased sharply in the following 9 years, with an average annual 2 ancoastal average reclamation annual coastal of 23.0 reclamation hm2. By comparison, of 23.0 hm the. coastalBy comparison, reclamation the in coastal Hong Kongreclamation 2 2 inonly Hong occurred Kong in only 2002 (300.6occurred hm2 )in and 2002 2014 (300.6 (201.2 hm hm2)). and In other 2014 years, (201.2 coastal hm reclamation). In other years, coastalactivities reclamation were relatively activities few, with were an relatively average of few, 13.5 hmwith2 per an year.average of 13.5 hm2 per year.

Figure 6. Time series of the coastal reclamation areas in Shenzhen and Hong Kong. The horizontal Figureaxis is 6. the Time year, series the left of vertical the coastal axis is reclamation the annual coastal areas reclamation in Shenzhen area, and and Hong the right Kong. vertical The axishorizontal axisis the is cumulativethe year, the coastal left vertical reclamation axis area. is the The an barnual graphs coastal are reclamation the annual coastal area, reclamationand the right area vertical in axisShenzhen is the cumulative and Hong Kong, coastal and reclamation the curves are area. the cumulativeThe bar graphs coastal are reclamation the annual area coastal and the reclamation total areacoastal in Shenzhen reclamation and area Hong in Shenzhen Kong, and and Hongthe curves Kong. are the cumulative coastal reclamation area and the total coastal reclamation area in Shenzhen and Hong Kong. To demonstrate more details of temporal changes, Table 3 and Figure7 were presented to illustrateTo demonstrate coastal reclamation more details results ofof Shenzhen temporal and changes, Hong Kong Table over 3 different and Figure periods 7 were presentedfrom 2000 toto 2018.illustrate For Shenzhen coastal andreclamation Hong Kong, results this Table of Shenzhen presents their and respective Hong Kong and over differenttotal reclaimed periods area from over 2000 the past to 192018. years, For their Shen respectivezhen and and Hong total reclaimed Kong, this area Table of each presents year, and their total coastal reclamation area every five years. From 2000 to 2018, the total their respective and total reclaimed area over the past 19 years, their respective and total coastal reclamation area in Shenzhen and Hong Kong was 4140.7 hm2, with an average reclaimedannual coastal area reclamationof each year, of and 217.9 their hm 2total. The coastal total coastal reclamation reclamation area area every in Shenzhenfive years. From 2000was to 3409.8 2018, hm the2, withtotal an coastal annual reclamation average coastal area reclamation in Shenzhen of 179.5 and hmHong2, and Kong the totalwas 4140.7 hmcoastal2, with reclamation an average area inannual Hong Kongcoastal was reclamation 730.9 hm2, with of an217.9 annual hm average2. The coastaltotal coastal reclamationreclamation ofarea 38.5 in hm Shenzhen2, about 1/5was of 3409.8 that of hm Shenzhen.2, with an As annual shown inaverage Table2 andcoastalFigure reclamation7 , ofShenzhen 179.5 hm and2, and Hong the Kongtotal havecoastal experienced reclamation obvious area in coastal Hong reclamation Kong was declines 730.9 hm over2, with an annualthe past average 19 years. coastal The coastal reclamation reclamation of 38.5 areas hm in2, 2000–2004, about 1/5 2005–2009,of that of Shenzhen. 2010–2014, andAs shown 2 2 2 2 in2015–2018 Table 2 wereand Figure 2022.8 hm7, Shenzhen, 1681.3 hm and, 421.0 Hong hm Kong, and 15.6have hm experienced, respectively. obvious Coastal coastal reclamation in Shenzhen showed a temporal characteristic of “increasing slightly and then reclamation declines over the past 19 years. The coastal reclamation areas in 2000–2004, decreasing sharply,” and the areas of coastal reclamation during the four periods were 2 2 2 2005–2009,1581.2 hm2 ,2010–2014, 1621.8 hm2, 192.4and hm2015–20182, and 14.5 were hm2 ,2022.8 respectively. hm , Coastal1681.3 reclamationhm , 421.0 inhm Hong, and 15.6 hm2, respectively. Coastal reclamation in Shenzhen showed a temporal characteristic of “increasing slightly and then decreasing sharply,” and the areas of coastal reclamation during the four periods were 1581.2 hm2, 1621.8 hm2, 192.4 hm2, and 14.5 hm2,

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respectively. Coastal reclamation in Hong Kong showed a temporal characteristic of “first decreasing and then increasing and then decreasing” and the areas of coastal reclamation during the four periods were 441.6 hm2, 59.5 hm2, 228.6 hm2, and 1.2 hm2, respectively.

Table 2. Statistics for coastal reclamation areas in Shenzhen and Hong Kong.

Area of Reclamation/hm2 Regions 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Total Shenzhen 197.1 372.8 154.6 684.1 172.7 428.3 359.1 173.4 51.5 609.5 106.9 5.1 43.7 31.1 5.6 3.8 3.4 2.8 4.5 3409.8 Hong 11.1 76.9 300.6 16.3 36.7 0 59.5 0 0 0 19.8 7.3 0 0.3 201.2 1.2 0 0 0 730.9 Kong J. Mar. Sci. Eng. 2021, 9, 48 10 of 15 208.2 449.7 455.1 700.4 209.4 428.3 418.6 173.4 51.5 609.5 126.7 12.4 43.7 31.3 206.8 4.9 3.4 2.8 4.5 2022.8 1681.3 421.0 15.6 Total Kong showed a temporal characteristic of “first decreasing and then increasing and then 4140.7 2022.8 decreasing” and the areas1681.3 of coastal reclamation during the421.0 four periods were 441.6 hm2,15.6 59.5 hm2, 228.6 hm2, and 1.2 hm2, respectively.

Figure 7. Accumulated areas of the coastal reclamation in Shenzhen and Hong Kong within different Figure 7. Accumulated areas of the coastal reclamation in Shenzhen and Hong Kong within five-year intervals. different five-year intervals. Table 2. Statistics for coastal reclamation areas in Shenzhen and Hong Kong.

5. Discussion Area of Reclamation/hm2 Regions 2000 2001 2002 20035.1. 2004Economy 2005 2006 and 2007 Urbanization 2008 2009 2010 Drivers 2011 2012 2013 2014 2015 2016 2017 2018 Total Shenzhen 197.1 372.8 154.6 684.1 172.7 428.3 359.1 173.4 51.5 609.5 106.9 5.1 43.7 31.1 5.6 3.8 3.4 2.8 4.5 3409.8 Hong 11.1 76.9 300.6 16.3 36.7It is 0 necessary 59.5 0 to 0 investigate 0 19.8 7.3 the 0relationship 0.3 201.2 1.2 between 0 0 coastal 0 730.9 reclamation and the Kong 208.2 449.7 455.1 700.4local 209.4 economy 428.3 418.6 because 173.4 51.5 609.5coastal 126.7 reclamation 12.4 43.7 31.3 acti 206.8vities 4.9 can 3.4 2.8affect 4.5 the coastal environment Total 2022.8[7,32]. Specifically, 1681.3 coastal reclamation 421.0 is strongly 15.6associated4140.7 with socioeconomic 2022.8variables, such 1681.3as GDP, population, and 421.0 urbanization level. 15.6 From the Shenzhen Statistical Yearbook 2019 [33] and the Hong Kong Census and Statistics Department [34], between 20005. Discussion and 2018, both Shenzhen and Hong Kong experienced rapid economic and social 5.1. Economy and Urbanization Drivers development. In 2000, the GDP of Shenzhen was ¥218.7 billion, while the GDP of Hong It is necessary to investigate the relationship between coastal reclamation and the local Kongeconomy was because$172.7 coastal billion. reclamation In 2018, activities the GDP can of affect Shenzhen the coastal was environment ¥2422.2[ 7billion,,32]. while the GDP of Specifically,Hong Kong coastal was reclamation $362.7 isbillion. strongly The associated growth with socioeconomicrates are 1007.5% variables, and such 110.0% as respectively. Besides,GDP, population, in 2000, and the urbanization population level. Fromof Shenzhen the Shenzhen was Statistical 7.0 million, Yearbook 2019while [33] the population of and the Hong Kong Census and Statistics Department [34], between 2000 and 2018, both HongShenzhen Kong and was Hong 6.8 Kong million. experienced In 2018, rapid the economic popu andlation social of development. Shenzhen In was 2000, 13.0 million, while thethe population GDP of Shenzhen of Hong was ¥218.7 Kong billion, was while 7.5 the mill GDPion. of Hong The Kong growth was $172.7 rates billion. are 85.7% and 10.3% respectively.In 2018, the GDP We ofcan Shenzhen confirm was that¥2422.2 there billion, is a while strong the GDPpositive of Hong relationship Kong was between coastal $362.7 billion. The growth rates are 1007.5% and 110.0% respectively. Besides, in 2000, reclamation,the population the of Shenzhenlocal economy was 7.0 million, (i.e., whileGDP), the and population the population. of Hong Kong Apart was from GDP and population,6.8 million. In urbanization 2018, the population level of Shenzhencan also was be 13.0 associated million, while with the populationcoastal reclamation. of In this study,Hong the Kong urbanization was 7.5 million. rate The growthwas used rates to are meas 85.7%ure and 10.3%the urbanization respectively. We level can of Shenzhen and confirm that there is a strong positive relationship between coastal reclamation, the local Hongeconomy Kong. (i.e., From GDP), andthe the website population. of the Apart Guangdong from GDP and Statistics population, Bureau urbanization [35] and World Bank [36],level Hong can also Kong be associated and Shenzhen with coastal are reclamation. cities with In this high study, urbanization the urbanization rates. rate The urbanization

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was used to measure the urbanization level of Shenzhen and Hong Kong. From the website of the Guangdong Statistics Bureau [35] and World Bank [36], Hong Kong and Shenzhen are cities with high urbanization rates. The urbanization rate of Shenzhen was 92.46% in 2000 and reached 100% in 2010. For Hong Kong, as early as 1993, its urbanization rate has reached 100%. Owing to the high urbanization rates, rapidly growing economy, and population, the Shenzhen and Hong Kong governments obtained land from the ocean for increasing industrial lands, commercial lands, construction lands, and residential lands, which resulted in an increasing demand for land area through coastal reclamation. In summary, GDP, population, and urbanization level are associated with coastal reclamation. From Table3, compared with Hong Kong, Shenzhen exhibited a greater increase in both economy, population, and urbanization rate from 2000 to 2018. In the future, the potential correlation between coastal reclamation events and economic-social development in coastal zones can be further examined.

Table 3. GDP, population, and urbanization rate of Shenzhen and Hong Kong in 2000 and 2018, and coastal reclamation area over 2000–2018.

City Shenzhen Hong Kong Growth Growth Year (a) 2000 2018 2000 2018 rate rate GDP (¥) 218.7 B 2422.2 B 1007.5% 172.7 B 362.7 B 110.0% Population 7.0 M 13.0 M 85.7% 6.8 M 7.5 M 10.3% Urbanization 92.46% 100% 7.54% 100% 100% 0 rate Area of reclama- 3409.8 hm2 730.9 hm2 tion Note: In Table3, B represents billion and M represents million. In this study, the urbanization rate was defined as the proportion of the population of a country (region) resident in urban areas to the total population of the country (region) [37].

5.2. Comparison with Other Data Sources Figure8 shows a comparison between the coastal reclamation data obtained in this study and the Hong Kong reclamation map in the areas on both sides of Victoria Harbor from 2000 to 2018. According to our calculations, coastal reclamation areas from our study and official data are 35.4 hm2 and 37.1 hm2, which differ by 4.6%. From Figure8, areas of four coastal reclamation projects from our calculations are 8.3 hm2, 19.8 hm2, 4.3 hm2, and 3.0 hm2, and the area of four coastal reclamation projects from official data are 8.6 hm2, 21.1 hm2, 4.5 hm2, and 2.9 hm2. We can find a good agreement (RMSE = 0.72, r = 0.99) between coastal reclamation areas from our result and official data by correlation analysis. According to the chi-square test, our coastal reclamation data and official data exhibit a good agreement at the 0.05 significance level. It can also be seen that the spatial distribution of coastal reclamation from our study is consistent with the official data, indicating that our coastal reclamation data have high accuracy. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 11 of 14

rate of Shenzhen was 92.46% in 2000 and reached 100% in 2010. For Hong Kong, as early as 1993, its urbanization rate has reached 100%. Owing to the high urbanization rates, rapidly growing economy, and population, the Shenzhen and Hong Kong governments obtained land from the ocean for increasing industrial lands, commercial lands, construction lands, and residential lands, which resulted in an increasing demand for land area through coastal reclamation. In summary, GDP, population, and urbanization level are associated with coastal reclamation. From Table 3, compared with Hong Kong, Shenzhen exhibited a greater increase in both economy, population, and urbanization rate from 2000 to 2018. In the future, the potential correlation between coastal reclamation events and economic-social development in coastal zones can be further examined.

Table 3. GDP, population, and urbanization rate of Shenzhen and Hong Kong in 2000 and 2018, and coastal reclamation area over 2000–2018.

City Shenzhen Hong Kong Year (a) 2000 2018 Growth rate 2000 2018 Growth rate GDP (¥) 218.7 B 2422.2 B 1007.5% 172.7 B 362.7 B 110.0% Population 7.0 M 13.0 M 85.7% 6.8 M 7.5 M 10.3% Urbanization rate 92.46% 100% 7.54% 100% 100% 0 Area of reclamation 3409.8 hm2 730.9 hm2 Note: In Table 3, B represents billion and M represents million. In this study, the urbanization rate was defined as the proportion of the population of a country (region) resident in urban areas to the total population of the country (region) [37].

5.2. Comparison with Other Data Sources Figure 8 shows a comparison between the coastal reclamation data obtained in this study and the Hong Kong reclamation map in the areas on both sides of Victoria Harbor from 2000 to 2018. According to our calculations, coastal reclamation areas from our study and official data are 35.4 hm2 and 37.1 hm2, which differ by 4.6%. From Figure 8, areas of four coastal reclamation projects from our calculations are 8.3 hm2, 19.8 hm2, 4.3 hm2, and 3.0 hm2, and the area of four coastal reclamation projects from official data are 8.6 hm2, 21.1 hm2, 4.5 hm2, and 2.9 hm2. We can find a good agreement (RMSE = 0.72, r = 0.99) between coastal reclamation areas from our result and official data by correlation analysis. According to the chi-square test, our coastal reclamation data and official data exhibit a good agreement at the 0.05 significance level. It can also be seen that the spatial J. Mar. Sci. Eng. 2021, 9, 48 12 of 15 distribution of coastal reclamation from our study is consistent with the official data, indicating that our coastal reclamation data have high accuracy.

Figure 8. 8.Comparisons Comparisons between between (a) our(a) our produced produced coastal coastal reclamation reclamation result and result (b) theand Hong (b) the Kong Hong reclamationKong reclamation map from map official from data. official Figure data.8a showsFigure the 8a selectedshows the four selected areas of four coastal areas reclamation of coastal withreclamation the basemap with ofthe a Landsatbasemap image of a Landsat in 2018. Theimag coastale in 2018. reclamation The coastal area onreclamation the north coastarea on of the Victorianorth coast Harbor of isVictoria Yau Ma Harbor Tei; the coastalis Yau reclamationMa Tei; the area coastal on the reclamation south coast ofarea Victoria on the is Admiralty,south coast of WanVictoria Chai, is andAdmiralty, North Point Wan from Chai, west and to North east. In Point Figure from8a, thewest yellow to east. and In the Figure red represent 8a, the yellow the and coastalthe red reclamation represent duringthe coastal 2000–2009 reclamation and 2010–2018. during In 2000–2009 Figure8b, theand pink 2010–2018. and the light In Figure pink are 8b, the the pink coastal reclamation projects from the Hong Kong Reclamation Map during 2000–2009 and 2010–2018.

5.3. Future Reclamation Planning in the Study Area In general, we can find that national policy plays an essential role in promoting coastal reclamation activities in Shenzhen and Hong Kong. According to the Shenzhen Coastal Zone Comprehensive Protection and Utilization Plan (2018–2035) [38], from 2018 to 2035, the coastal reclamation area in Shenzhen will be concentrated on the west coast of Dakonggang New City, Bao’an District, with a total reclamation area of 1724.9 hm2, mainly used for port constructions. Compared with 2000–2018, the scale of the coastal reclamation activities in Shenzhen will greatly decline, with more emphasis on coastline restoration and coastal zone development. By contrast, from 2018, Hong Kong has planned many coastal reclamation projects, such as airport city construction in the north of Hong Kong International Airport (650 hm2), New Town Expansion (130 hm2), Lantau Island Xinao Reclamation Project (80 hm2), New Territories Longgutan Reclamation Project (220 hm2), and Lantau Tomorrow Vision project (1700 hm2). According to the Hong Kong Sustainable Lantau Office [39], compared with 2000–2018, the scale of the coastal reclamation activities in Hong Kong will increase significantly, mainly concentrated on the less developed Lantau and surrounding islands. At the end of the 20th century, coastal reclamation projects in Hong Kong caused dissatisfaction among environmentalists, who believed that coastal reclamation projects were unnecessary and can destroy the coastal environment. To protect the feature of Victoria Harbor as a natural and cultural heritage and the marine ecological environment, some councilors started a series of campaigns [40]. In 1997, the “Protection of the Harbor Act Ordinance” was performed to restrict the coastal reclamation projects dominated by the Hong Kong government. In 2003, some people overturned the “Wan Chai Reclamation Plan” in the Hong Kong Court of Final Appeal. Since then, these campaigns have directly affected the future planning of coastal reclamation in Hong Kong. As a result, more than 600 hm2 of planned coastal reclamation in Hong Kong has been “deleted” [40], resulting in fewer coastal reclamation projects than Shenzhen in the 21st century. After 2018, to promote economic development and solve the housing tension problem, the Hong Kong government has put forward a series of coastal reclamation plans under pressure from protests. These plans will greatly increase the amount of coastal reclamation in Hong Kong in the future. The policy of the Shenzhen government is different from Hong Kong. According to the China Academy of Urban Planning and Design Shenzhen [41], Shenzhen released the “Shenzhen City Master Plan (2010–2020)” to protect the coastal environment in Shenzhen in 2010, and there were few planned coastal reclamation projects. The shift of planning agreed well with the decline of coastal reclamation in Shenzhen before and after 2010 (Table3 and Figure7). J. Mar. Sci. Eng. 2021, 9, 48 13 of 15

In addition, it is important to understand the future reclamation development policies to better monitor the future reclamation development of Shenzhen and Hong Kong using satellite data. According to the Shenzhen Coastal Zone Comprehensive Protection and Uti- lization Plan (2018–2035), Shenzhen will pay more attention to strengthening the protection and restoration of habitats and improving the ecological service function after 2018. As a consequence, the coastal reclamation area will be further reduced in the future. Overall, under the current and future coastal reclamation planning in Shenzhen and Hong Kong, the area of coastal reclamation in Hong Kong will exceed that in Shenzhen. Therefore, in the coming decades, it will be necessary to continuously monitor the coastal reclama- tion in Shenzhen and Hong Kong to provide an accurate and updated dataset for better understanding and evaluating the role of coastal reclamation related policies in coastal environmental changes.

6. Conclusions In this study, we monitored annual coastal reclamation in Shenzhen and Hong Kong since the 21st century and obtained several conclusions. First, the method based on Landsat imagery and GSW dataset in this study could be applied to monitor coastal reclamation at the annual scale. Second, according to our results derived from our method, a large-scale coastal reclamation in Shenzhen and Hong Kong with a total coastal reclamation area of 4140.7 hm2 over 2000–2018 could be observed. Shenzhen and Hong Kong have exhibited different temporal characteristics in coastal reclamation since the 21st century. The area of coastal reclamation in Shenzhen (i.e., 3409.8 hm2) was much larger than that in Hong Kong (i.e., 730.9 hm2). In addition, coastal reclamation in Shenzhen exhibited a significant declining trend. Third, as coastal reclamation is dominated by policies, it is necessary to produce an accurate and updated dataset on coastal reclamation from satellite data to further investigate coastal reclamation related environmental issues and feedback to policy-making in the future. For example, coastal reclamation may result in coastal land subsidence, which can increase the vulnerability of coastal zones especially in the context of rising sea levels. An accurate and updated dataset on coastal reclamation could be used to guide the coastal land use planning to avoid the coastal area with a higher vulnerability. Moreover, by benefiting from the global coverage and free access to Landsat and GSW datasets, this method can be extended to the globe in the future. Furthermore, our method could be extended to an earlier year to provide a longer history of coastal reclamation.

Author Contributions: T.F. contributed to the data collecting, analyzing and writing of this study. N.X. contributed to the overall idea and editing of this study. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Nanjing Normal University Startup Research Foundation (184080H202B254) and the National Key Research and Development Plan (2017-YFC-1405500). Institutional Review Board Statement: “Not applicable” for studies not involving humans or animals. Informed Consent Statement: “Not applicable” for studies not involving humans. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors thank the data distribution agencies who provided the publicly released data used in this study. The Landsat imagery provided by the United States Geological Survey (USGS). The GSW dataset was obtained from Google Earth Engine (GEE) cloud computing platform. The Hong Kong Reclamation Map based on the coastal reclamation data was provided by the Hong Kong Lands Department. Conflicts of Interest: The authors declare no conflict of interest. J. Mar. Sci. Eng. 2021, 9, 48 14 of 15

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