Monitoring Temporal Change of River Islands in the Yangtze River by Remotely Sensed Data

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Article Monitoring Temporal Change of River Islands in the Yangtze River by Remotely Sensed Data

Jinyan Sun 1,2, Lei Ding 3, Jiaze Li 4, Haiming Qian 1,5, Mengting Huang 1,2 and Nan Xu 6,*

1 Anhui & Huaihe River Institute of Hydraulic Research, Hefei 230088, China; [email protected] (J.S.); [email protected] (H.Q.); [email protected] (M.H.) 2 Anhui Province Key Laboratory of Water Conservancy and Water Resources, Hefei 230088, China 3 Nanjing Hydraulic Research Institute, Key Laboratory of Port, Waterway and Sedimentation Engineering of the Ministry of Transport, Nanjing 210029, China; [email protected] 4 Saint-Venant Laboratory for Hydraulics, EDF-Recherche et Développement, Laboratoire Saint-Venant (EDF R&D, CETMEF, Ecole des Ponts ParisTech), Chatou 78401, France; [email protected] 5 Anhui Dayu Project Construction Supervision and Consult Co., Ltd., Hefei 230088, China 6 Department of Earth System Science, Tsinghua University, Beijing 100084, China * Correspondence: [email protected]; Tel.: +86-150-1096-3820

Received: 8 September 2018; Accepted: 18 October 2018; Published: 21 October 2018

Abstract: The spatial extent and area of river islands are always changing due to the impact of hydrodynamic conditions, sediment supply and human activities. A catastrophic flood disaster was driven by sustained and heavy rainfall around the middle and lower Yangtze River in 18 June to 21 July 2016. The flood resulted in the most serious social-economic loss since 1954 and caused a larger-scale inundation for a short time. It is essential to continuously monitor the dynamics changes of river islands because this can avoid frequent field measurements in river islands before and after flood disasters, which are helpful for flood warning. This paper focuses on the temporal change of three river islands called Fenghuangzhou, Changshazhou, and one uninhabited island in the Yangtze River in 2016. In this study, GF-1 (GaoFen-1) WFV (wide field view) data was used for our study owing to its fine spatial and temporal resolution. A simple NDWI (Normalized Difference Water Index) method was used for the river island mapping. Human checking was then performed to ensure mapping accuracy. We estimated the relationship between the area of river islands and measured water levels using four models. Furthermore, we mapped the spatial pattern of inundation risk of river islands. The results indicate a good ability of the GF-1 WFV data with a 16-m spatial resolution to characterize the variation of river islands and to study the association between flood disaster and river islands. A significantly negative but nonlinear relationship between the water level and the area of the river island was observed. We also found that the cubic function fits best among three models (R2 > 0.8, P < 0.001). The maximum of the inundated area at the river island appeared in the rainy season on 8 July 2016 and the minimum occurred in the dry season on 28 December 2016, which is consistent with the water level measured by the hydrological station. Our results derived from GF-1 data can provide a useful reference for decision-making of flood warning, disaster assessment, and post-disaster reconstruction.

Keywords: river island; temporal change; water level; the Yangtze River

1. Introduction The middle and lower reaches of the Yangtze River have developed a certain number of river islands, which have an important inﬂuence on the stability of the Yangtze River [1,2]. More attention should be paid to these islands because they make the ﬂood control problem of the island reach more

Water 2018, 10, 1484; doi:10.3390/w10101484 www.mdpi.com/journal/water Water 2018, 10, 1484 2 of 17 complicated. River islands exhibit various shapes and different surface areas. They are molded river sediment formed by continuous scouring and silting. Generally, flow action can result in the erosion of the head of a river island. Sediment deposition occurs at its tail where shoals are born. The island divides a river into multiple channels and forms the connection of interrelation and interaction between two channels [3,4]. Therefore, river islands can provide information on active processes and the sediment regime at the reach scale and are employed for geomorphic and sediment assessments for rivers worldwide [5–7]. They are also an important component of the morphodynamics of a river and are widely used to investigate the dynamics of flow regime [8–12]. The formation and development of river islands are important for better understanding of fluvial processes and their controlling factors. Thus, the study of temporal variation of river islands is a significant body of work. River floods are considered to be the dominant force in shaping island morphology, as their power and kinetic energy are very high [13]. This is especially so for the monsoon-affected Yangtze River, where fluvial geomorphic work is largely carried out during monsoon floods [14]. Affected by long-term river erosion and sediment deposition, the rapidly evolving river island can significantly affect the waterway and flood control safety and threatens the production and life of people on both sides of the Yangtze River [15–20]. To maintain shipping conditions and ensure the safety of the people, the Chinese government has carried out many projects to strengthen river embankments and rectify river courses [21–23]. Limited by flood control and drainage capacity, a catastrophic flood disaster caused by sustained and heavy rainfall happened in the middle and lower Yangtze River in 2016 [24]. This river flood caused huge social-economic losses to Anhui province, second only to the 1954 flood [25] (pp. 29, 30, and 71), which also had a potential impact on river islands [26]. Therefore, it is necessary to grasp the changes of river islands before and after flood disasters and provide a scientific basis for river embankment reinforcement and river regulation. Remote sensing is a powerful tool for continuously monitoring dynamic changes of the river island, because it can provide large-scale, high spatial-temporal images [27–29]. Some scholars studied the area changes of river islands. Shi et al. [30] examined the long-term evolution of seven bars in the lower Yangtze River and found that the total area of bars decreased over time, with sediment and river discharge. Yang et al. [31] found the variations of low-water level have a relationship with river bed evolution. Wang et al. [32] analyzed four river islands in the middle Yangtze River from the Landsat data (30 m) in the flooding season of 2002. They found that two of the islands tended to be stable, while the other two experienced severe erosion during the flood. Those islands can also be large sediment sources rather than depositional areas during the flood. The decrease of sediment load in the middle of the Yangtze River was found to be responsible for the dramatic morphodynamics of river islands, which could last for a long period of time, depending on the operation of the Three Gorges Dam, which opened in 2003. Lou et al. [33] explored mid-channel bars evolution in the middle and lower reach of the Yangtze River and their responses to the Three Gorges Dam, the world’s largest hydrological engineering project. Chen et al. [34] extracted water information of Shangri-la County’s wetland from ETM+ (Enhanced Thematic Mapper) images using the threshold value method, interpolation method and spectral relationship method and found that the threshold value method cannot distinguish the mountain shadow and water easily. The use of remote sensing technology to obtain accurate and timely spatial range of river islands helps to avoid frequent on-site measurements of river islands before and after flood disasters. Bates et al. [35] studied a two-dimensional finite element model applied to the Missouri River, Nebraska and compared with a synchronous Landsat TM (Thematic Mapper) image of flood inundation extent. Godoy et al. [36] mapped fluvial islands using images from the Landsat 5 between 1988 and 2010 showed that the existing islands in the estuary suffered great changes during this period. Most of the above work was carried out either through experimental and theoretical studies or was focused on long-term investigations. The spatial resolution of remotely sensed data in the above studies ranged from 30 m to 250 m. To date, few studies have detected the abrupt change of river islands in response to a single flood event. Moreover, it is essential to uncover the relationship between the area of river islands and the water level, and the response of river islands to a major river flood. Water 2018, 10, 1484 3 of 17

This paper focuses on the temporal change of three river islands within the lower Yangtze River in 2016. Compared with other satellites, Chinese GF-1 (GaoFen-1) satellite has a short revisit period (only 4 days) and a high spatial resolution (16-m). Therefore, this study mainly uses GF-1 data, Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES } supplemented by other satellite data, such as Sentinel-2, Chinese GF-4 (GaoFen-4) data. The paper is organized(only 4 days) as follows. and a The high study spatial area, resolution remotely (16-m sensed). Theref dataore, and this hydrological study mainly data uses description GF-1 data, are presentedsupplemented in Section by2. other Methods satellite is described data, such in as Section Sentinel3-,2, results Chinese and GF analysis-4 (GaoFen are-4) presented data. The inpaper Section is 4. The conclusionorganized as is follows. given in The Section study5. area, remotely sensed data and hydrological data description are presented in Section 2. Methods is described in Section 3, results and analysis are presented in Section 2. Data4. The conclusion is given in Section 5.

2.1. Study2. Data Area

The2.1. Study Yangtze Area River (also known as the Changjiang River) is the world’s third longest river and the longest one in Asia. The Yangtze River basin is a typical monsoon climate zone, where rainfall The Yangtze River (also known as the Changjiang River) is the world’s third longest river and is concentratedthe longest one in thein Asia. summer The Yangtze and autumn. River basin The is averagea typical monsoon annual rainfall climate iszone, about where 1100 rainfall mm is in the Yangtzeconcentrated River basin, in the and summer more and than autumn. 70% of The the average annual annual rainfall rainfall occurred is about from 1100 April mm toin October. the The riverYangtze island River is abundantbasin, and more in the than Yangtze 70% of River. the annual Among rainfall them, occurred the larger from river April island to October. generally The has ﬁxedriver villages island and is abundant townships, in the such Yangtze as FenghuangzhouRiver. Among them, and the larger Changshazhou. river island generally This paper has fixed selected threevillages river islandsand townships, Fenghuangzhou, such as Fenghuangzhou Changshazhou and Changshazhou. and the uninhabited This paper islandselected as three an river example (Figureislands1). These Fenghuangzhou, river islands Changshaz are locatedhou and in the the uninhabited Anhui reach island which as an atexample the lower (Figure reaches 1). These of the Yangtzeriver River. islands Fenghuangzhou are located in (alsothe Anhui known reach as Fengyizhou) which at the has lower formed reaches a shoal of the in Yangtze the Song River. Dynasty (960 A.D.~1279Fenghuangzhou A.D. ).(also The known embankment as Fengyizhou) was built has formed on the a early shoal Qingin the DynastySong Dynasty(1650 (960 A.D.~1670 A.D.~1279 A.D.) . A.D.). The embankment was built on the early Qing Dynasty (1650 A.D.~1670 A.D.). There are more There are more than 8000 people living on Fenghuangzhou, with planting cotton, corn, and vegetables, than 8,000 people living on Fenghuangzhou, with planting cotton, corn, and vegetables, and breeding and breeding[37]. Near [ 37the]. Fenghuangzhou, Near the Fenghuangzhou, Changshazhou Changshazhou has more than hassix tho moreusand than people, six thousand who plant people, cotton, who plantcorn cotton,, and corn, poplars and for poplars a living for [38]. a living [38].

FigureFigure 1. Map 1. Map of the of the study study site site in in the the Yangtze Yangtze River, River, showing showing the the river river islands islands Fenghuangzhou, Fenghuangzhou, ChangshazhouChangshazhou and and the the uninhabited uninhabited island island by by GoogleGoogle Earth image image acquired acquired on onJanuary 21 January 21, 2017 2017..

At AnhuiAt Anhui reach, reach, the the formation formation of of the the flood flood waswas mainly caused caused by by channeling channeling of the of strong the strong runoff runoff (come(come from from the upperthe upper and and middle middle reaches reaches of of the the YangtzeYangtze River), River), and and the the rainstorm rainstorm flood flood (come (come from from lakes, Chuanjiang River, and the Han River basin). The influence of the tributaries in the Anhui lakes, Chuanjiang River, and the Han River basin). The influence of the tributaries in the Anhui province is small, and the transit flood is the main source of flood. In Anhui province, the flood season province is small, and the transit flood is the main source of flood. In Anhui province, the flood season of the Yangtze River is from May to October. The period from June to August is the high-water period. of theThe Yangtze highest River water is level from in May each to hydrological October. The station period is mainly from June in this to period; August the is thedry high-waterseason is from period. The highestDecember water to February. level in The each lowest hydrological water level station of each is station mainly over in thisthe years period; has themainly dry occurred season isin from Decemberthe dry to season February. [39,40]. The From lowest the “2016 water China level Flood of each and Drought station overDisaster the Bulletin years has” promulgated mainly occurred by

Water 2018, 10, 1484 4 of 17 in the dry season [39,40]. From the “2016 China Flood and Drought Disaster Bulletin” promulgated by the Ministry of Water Resources of the People’s Republic of China, we know that, the rainfall in Anhui province during the flood season was 887 mm, which was 27% more than the same period in the normal year. In the flood season of 2016, a major flood broke out in the Anhui reach of the Yangtze River, and the flood level was second only to 1954.

2.2. Remotely Sensed Data In this study, Chinese GF-1 (GaoFen-1) WFV (wide ﬁeld view) data were collected for our study owing to its high temporal frequency and high spatial resolution. GF-1 satellite was successfully launched by the Chinese Jiuquan satellite launch center at 26 April 2013. It is equipped with four multispectral scanners with 16-m resolution. Multispectral data obtained by GF-1 contain four bands with center wavelengths of 503 nm, 576 nm, 680 nm, and 810 nm, respectively. 16-m spatial resolution of GF-1 data can be a good way to capture the details of river island space, while their high frequency revisit time is only four days [41]. This paper collected the GF-1 data with a low cloud cover and better quality in the study area as the main data source, to accurately track the temporal changes of the river island. The Sentinel-2 data and Chinese GF-4 (GaoFen-4) VNIR (visible light near infrared) data acquired in October 2016, was collected for supplementary. In addition, GF-2 (GaoFen-2) PMS (pan and multi-attenuation sensor) data with 1-m spatial resolution was collected for the validation. The spatial resolution of Sentinel-2 data is 10 m, and the GF-4 data is 16-m. GF-1, GF-2 and GF-4 data are downloaded from the Terrestrial Observing Satellite Data Service Platform [42] and the acquisition time is presented in Table1. GF-1, GF-4 and Sentinel-2 daily images were mapped to a cylindrical equidistant projection at 16-m spatial resolution (~0.00016◦).

Table 1. Experimental satellite data acquisition time.

Data Acquisition Time (Beijing Time) 2016.01.26 2016.02.03 2016.02.16 2016.02.20 2016.02.28 2016.03.11 2016.03.27 2016.03.28 2016.04.01 2016.04.13 2016.04.21 2016.04.29 2016.04.30 2016.05.03 2016.05.04 2016.05.11 2016.05.12 2016.05.16 GF-1 2016.06.05 2016.06.13 2016.06.14 2016.07.08 2016.07.24 2016.07.25 2016.07.28 2016.07.29 2016.08.10 2016.08.14 2016.08.18 2016.08.31 2016.09.12 2016.09.20 2016.11.04 2016.11.11 2016.11.28 2016.12.02 2016.12.15{2} 2016.12.28 2016.12.31 (11:00:00~11:54:28) GF-4 2016.10.09 (09:35:12) Sentinel-2 2016.10.04 (10:59:05) GF-2 2016.08.11{2} (11:09:22,11:09:25) Note that {2} represents two images in the same acquisition time (i.e., the same day).

2.3. Hydrological Data In our analysis, water level data, monthly mean instantaneous runoff data, monthly mean suspended sediment concentration (SSC) data near the study area was used to explore the association between hydrological parameters and river islands. Daily water level, daily runoff and rainfall information about three station of the Datong hydrological station (117.62◦ E, 30.76◦ N), Anqing gauging station (117.05◦ E, 30.50◦ N) and Baidang gauging station (117.42◦ E, 30.76◦ N) of the study area was collected from the Anhui hydrological telemetry information network [43]. The water level is based on the Wusong elevation system. Datong hydrological station is located near the study area. It is a comprehensive hydrological station which is the nearest one to the estuary of the Yangtze River without being affected by tides. It is an important hydrological station as it is helpful for protecting water resources and controlling runoff from the Yangtze River to the sea. The water and sediment from upstream are abundant at Anhui reach. Annual average runoff (1950~2015) of Datong hydrological station is 8931 billion m3, and the average annual sediment discharge (1951~2015) is 3.68 billion tons. At Datong hydrological station, Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES }

3 Waterm /s 2018(August, 10, 1484 1, 1954) and the maximum flood-crest stage was 16.64 m (August 1954). In 2016,5 ofthe 17 annual runoff of Datong hydrological station was 10,450 billion m3, the maximum instantaneous flow was 70,700 m3/s (July 11, 2016) and the flood-crest stage was 15.66 m (July 8, 2016). The hourly water 3 thelevel maximum and flow annual information runoff isof 13,600 the Datong billion h mydrological(1954), the station maximum are collected. instantaneous According flow value to the is 3 92,600acquisition m /s time (1 August of remotely 1954) sensed and the data, maximum the average flood-crest of water stage level was values 16.64 collected m (August at 11:00 1954). and In 12:00 2016, 3 theis taken annual as runoffthe water of Datong level of hydrological the satellite transit station instant was 10,450 (Figure billion 2). In m addition,, the maximum the monthly instantaneous average 3 flowof instantaneous was 70,700 mrunoff/s (11 values July 2016)and monthly and the mean flood-crest SSC data stage were was collected 15.66 m (8to Julystudy 2016). the relationship The hourly waterbetween level the and runoff, flow SSC, information and the area of the of Datongthe islands. hydrological station are collected. According to the acquisitionAnqing time gauging of remotely station sensed is located data, 4 thekm averageupstream of waterof the levelstudy values area. Baidang collected gauging at 11:00 andstation 12:00 is islocated taken onas the the water tributary level of of the the study satellite area, transit 2.5 instant km from (Figure the intersection2). In addition, of the the tributarymonthly average and the ofYangtze instantaneous River (Figure runoff 1). values Water and level monthly data from mean Anqing SSC datagauging were station collected and toBaidang study thegauging relationship station betweenwere collected the runoff, in the SSC, same and way the as area Datong of the hydrological islands. station.

(a) The water level at remotely sensed data acquisition time.

(b) The monthly mean suspend sediment concentration (SSC).

Figure 2. Cont.

Water 2018, 10, 1484 6 of 17 Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES }

(d) The cumulative monthly rainfall.

FigureFigure 2. VariationVariation of the of water the water level level(a), SSC (a ),(b), SSC runoff (b), (c runoff) and rainfall (c) and (d) rainfall at Datong (d) hydrological at Datong hydrologicalstation. station.

3. MethodsAnqing gauging station is located 4 km upstream of the study area. Baidang gauging station is located on the tributary of the study area, 2.5 km from the intersection of the tributary and the Yangtze Three mathematical models were constructed to estimate the relationship between the land area River (Figure1). Water level data from Anqing gauging station and Baidang gauging station were of the river island and measured water level. Three principal steps in this process are described in collected in the same way as Datong hydrological station. this section: Image processing for eliminating terrain effects or deformation, land-water classification 3.for Methods river islands mapping, defining the index for estimation of the risk in the river islands. High- resolution mapping results of GF-2 data are collected as the reference data to measure the accuracy Three mathematical models were constructed to estimate the relationship between the land area of the results derived from GF-1 data. Human checking was then conducted to ensure the mapping of the river island and measured water level. Three principal steps in this process are described in this quality. section: Image processing for eliminating terrain effects or deformation, land-water classification for river3.1. Image islands Processing mapping, defining the index for estimation of the risk in the river islands. High-resolution mapping results of GF-2 data are collected as the reference data to measure the accuracy of the results derivedWe from used GF-1ENVI5.2 data. software Human to checking carry out was RPC then (Rational conducted polynomial to ensure coefficients the mapping) orthorectification quality. without control points for GF-2, GF-1 and GF-4 data to eliminate terrain effects or deformation caused 3.1.by cameraImage Processing orientation. The results of the orthorectification of the GF-2 data were obtained by implementing the GS transform (Gram-Schmidt Pansharping, GS), fusing the full-color band (0.8 m We used ENVI5.2 software to carry out RPC (Rational polynomial coefficients) orthorectification resolution) and the multispectral band (4-m resolution) Information (1-m resolution) of multispectral without control points for GF-2, GF-1 and GF-4 data to eliminate terrain effects or deformation images. GF-4 data and Sentinel-2 data were resampled to a 16-m resolution data based on the bilinear caused by camera orientation. The results of the orthorectification of the GF-2 data were obtained by interpolation method. All the data are processed into Geotiff format and projected to implementing the GS transform (Gram-Schmidt Pansharping, GS), fusing the full-color band (0.8 m WGS_1984_UTM_zone_50N coordinates. resolution) and the multispectral band (4-m resolution) Information (1-m resolution) of multispectral images. GF-4 data and Sentinel-2 data were resampled to a 16-m resolution data based on the 3.2. Water Extraction bilinear interpolation method. All the data are processed into Geotiff format and projected to WGS_1984_UTM_zone_50NClear water is highly absorptive coordinates. in the infrared (IR) and slightly more reflective in the visible range [44]. Both vegetation and bare soil are more reflectively in the IR than in the visible. Therefore, 3.2.the Water Normalized Extraction different water index（NDWI）proposed by McFeeters [45] is designed to: (1) maximizeClear the water reflectance is highly of absorptive the water inbody the in infrared the green (IR) band; and slightly(2) minimize more reflectivethe reflectance in the of visible water rangebody in [44 the]. BothNIR vegetationband. To enhance and bare water soil information are more reflectively and suppress in the non IR- thanwater in information, the visible. Therefore,the NDWI thefor NormalizedGF-1, GF-4, differentSentinel-2 water and GF index-2 data (NDWI) is directly proposed calculated by McFeeters as: [45] is designed to: (1) maximize the reflectance of the water body in the green band;Green (2) minimize− NIR the reflectance of water body in the NDWI = (1) NIR band. To enhance water information and suppressGreen + non-waterNIR information, the NDWI for GF-1, GF-4,where Sentinel-2 Green and and NIR GF-2 are data the israw directly Digital calculated Numbers as: (DN) value of the Green and NIR bands. The resulting NDWI image values varied between Green−1 and− 1.NIR Since the NDWI threshold can affect the NDWI = (1) accuracy of water extraction [46], a different, independentlyGreen + NIR NDWI threshold was derived by raster color slice and histogram manually for each scene image. This method can capture the optimal threshold for each satellite image. The produced binary images were built with 0 means land and 1 means water.

Water 2018, 10, 1484 7 of 17 where Green and NIR are the raw Digital Numbers (DN) value of the Green and NIR bands. The resulting NDWI image values varied between −1 and 1. Since the NDWI threshold can affect the accuracy of water extraction [46], a different, independently NDWI threshold was derived by raster color slice and histogram manually for each scene image. This method can capture the optimal threshold for each satellite image. The produced binary images were built with 0 means land and 1 means water.

3.3. Estimation Risk in the River Island To evaluate the risk of river islands with the variation of water level, the risk index (RI) of river islands was constructed based on a ratio between the river flooding times and the total effective observation times, ( ∑M SN 1, Water RI = i=1 i , SN = (2) M i 0, Land where M is the total effective observation time, SN is value of pixels with 1 indicates flood area, 0 indicates unflooded area. The resulting RI values varied between 0 and 1. The closer the RI value is to 0, the safer it is; the closer the RI value is to 1, the more dangerous it is.

3.4. Evaluation of Mapping Accuracy In this study, Overall Accuracy (Equation (3)), User Accuracy (Equation (4)), Producer Accuracy (Equation (5)) and Kappa Coefﬁcient (Equation (6)) were used for the accuracy assessment of mapping from the GF-1 data. The series of accuracy metrics can be derived from a confusion matrix which is the most frequently used method in accuracy assessment [47].

N Overall Accuracy = 100 × r ii (3) ∑ i=1 i N User Accuracy = ii (4) Ni+ N Producer Accuracy = ii (5) N+i N × r N − r N × N = ∑i=1 ii ∑i=1 i+ +i Kappa Coefﬁcient 2 r (6) N − ∑i=1 Ni+ × N+i where Nii is the number of correctly samples of class i. r is the number of classes; N is the number of training samples; N+i is the number of training samples of class i. Ni+ is the number of classiﬁed samples of class i.

4. Results and Discussion

4.1. Accuracy Evaluation To evaluate the mapping results from GF-1 data (16-m), GF-2 data with a higher spatial resolution (1-m) was used. The acquisition time of the data used for evaluation are very close, while GF-2 data is 11 August 2016 and GF-1 data is August 10, 2016. The confusion matrix between the mapping results of GF-2 data and the GF-1 data has been shown in Table2, which is used for accurate evaluation. Among the 320,672 pixels of the study area, 116,592 pixels were successfully identified as river island pixels, and 187,217 pixels were successfully identified as water body pixels, with a total accuracy of 94.74% and a Kappa coefficient of 88.95%. Therefore, the accuracy of adopting the NDWI threshold method to mapping river island is higher, which can meet our needs for river mapping. Water 2018, 10, 1484 8 of 17

Table 2. Confusion matrix obtained by NDWI methods.

GF-2 Category Total User Accuracy Island Water Island 116,592 4962 121,554 95.92% GF-1 water 11,901 187,217 199,118 94.02% total 128,493 192,179 320,672 Producer accuracy 90.74% 97.42% Overall Accuracy = 94.74% Kappa Coefﬁcient = 0.8895 Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES }

4.2.4.2. TemporalTemporal ChangeChange ofof RiverRiver IslandIsland AreaArea andandWater Water Level Level FigureFigure3 3shows shows the the temporal temporal change change of of the the river river islands islands in in the the study study area. area. The The area area of of the the river river islandisland exhibited exhibited obviousobviousseasonal seasonal variationsvariations andand certaincertain short-termshort-term fluctuations.fluctuations. Similarly,Similarly, the the waterwater levellevel also also exhibited exhibited a a significantsignificant seasonalseasonal variation variation and and certaincertain short-termshort-term fluctuations. fluctuations. TheThe differencedifference betweenbetween thethe maximummaximum andand minimumminimum landland areasareas ofof thethe riverriver islandisland isis defineddefined asas thethe intra-annualintra-annual fluctuationfluctuation ofof thethe riverriver islandisland land.land. WeWe foundfound thatthat thethe annualannual areaarea fluctuationfluctuation ofof Fenghuangzhou,Fenghuangzhou, 2 2 2 Changshazhou,Changshazhou, and and the the uninhabited uninhabited island island is 21.04 is 21.04 km , 4.4 km km2, 4.4, 0.83 km km2, 0.83respectively, km2 respectively, during January during 2016January to December 2016 to December 2016. The 2016. water The level water in the level Datong in the hydrological Datong hydrological station was station fluctuated was fluctuated between 5.72between m and 5.72 15.63 m m. and The 15.63 water m. level The average water levelof Datong average hydrological of Datong station, hydrological Baidang station, gauging Baidang station andgauging Anqing station gauging and stationAnqing are gauging 10.14 m, station 11.69 m,are and10.14 11.79 m, 11.69 m respectively, m, and 11.79 and m the respectively, standard deviation and the isstandard 2.96 m, 1.90deviation m, and is 3.282.96 m,m, respectively.1.90 m, and 3.28 m, respectively.

FigureFigure 3.3.River River islandsislands changechange withwith thethe waterwater level.level.

TheThe landland areasareas ofof thethe riverriver islandsislands areare highlyhighly affectedaffected byby thethe drydry seasonseason andand thethe floodflood season,season, showingshowing aa significantsignificant intra-annualintra-annual variationvariation patternpattern ofof firstfirst shrinkingshrinking andand thenthen expandingexpanding (Figure(Figure2 2).). TheThe changechange areaarea ofof thethe riverriver islandsislands atat thethe beginningbeginning andand endend ofof 20162016 isis defineddefined asas thethe netnet changechange duringduring thisthis year.year. The change change of of the the water water level level at at the the beginning beginning and and end end of of2016 2016 is defined is defined as the as thenet netchange change of water of water level level during during this this year. year. The The calculation calculation shows shows that that the the net net change change in land ofof threethree islandsislands Fenghuangzhou, Fenghuangzhou, Changshazhou Changshazhou and and the the uninhabited uninhabited island island studied studied are are− 1.06−1.06 km km2,2−, −0.340.34 kmkm22,, +0.09+0.09 kmkm22,, respectively. respectively. The The net net change change in inthe the upstream upstream water water level level (Anqing (Anqing gauging gauging station) station) is +1.45 is +1.45m and m andthe net the netchange change in the in thenearby nearby water water level level (Datong (Datong hydrological hydrological station) station) is is +1.14 m. TheThe netnet changechange ofof thethe tributarytributary water level (Baidang (Baidang gauging gauging station) station) is is −−0.740.74 m. m. The The net net change change around around the theriver river island island is much is much smaller smaller than than its fluctuation its fluctuation during during the theyear, year, and and the theconclusion conclusion is similar is similar for forthe thewater water level. level. The The net netchange change of area of area of river of river islands islands are mainly are mainly caused caused by the by net the change net change of water of water level. level.We estimated We estimated the relationship the relationship between between land area land of area river of islands river islands and water and waterlevel and level found and founda strong a strongnegative negative correlation correlation (R2 > ( R 0.8,2 > 0.8,P < P0.001).< 0.001). The The calculations calculations were were conducted conducted for for Fenghuangzhou, ChangshazhouChangshazhou andand oneone uninhabiteduninhabited island, island, and and similar similar conclusions conclusions were were obtained. obtained. As shown in Figure 2a, from April 30 to August 31, 2016, water levels of the three hydrological stations are observed during the high water level period throughout the year. This period is the high water level period of the Yangtze River in the flood season, and the land area of the river island is relatively small. The land area of the river island decreased with the increase of water level and increased with the decrease of water level. In addition to the obvious seasonal patterns, we found that the land area of the river island showed certain short-term variations, such as February 3, June 5, and December 15. The water level at the corresponding moment also shows a certain variability. This shows that the water level changes can affect the change of the land area of the river island, from the season scale to a shorter time scale. Together with the measured rainfall information of the hydrological station and the forecast of the meteorological department, the middle and lower reaches of the Yangtze River continued to rain, and heavy rain occurred in some areas, from June 16 to July

Water 2018, 10, 1484 9 of 17

As shown in Figure2a, from 30 April to 31 August 2016, water levels of the three hydrological stations are observed during the high water level period throughout the year. This period is the high water level period of the Yangtze River in the flood season, and the land area of the river island is relatively small. The land area of the river island decreased with the increase of water level and increased with the decrease of water level. In addition to the obvious seasonal patterns, we found that the land area of the river island showed certain short-term variations, such as 3 February, 5 June, and 15 December. The water level at the corresponding moment also shows a certain variability. This shows that the water level changes can affect the change of the land area of the river island, from the season scale to a shorter time scale. Together with the measured rainfall information of the Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES } hydrological station and the forecast of the meteorological department, the middle and lower reaches 6.of The the minimum Yangtze River land continuedarea of total to rain,three andriver heavy islands rain is occurred26.90 km2 in, on some Julyareas, 8, 2016 from (Figure 16 June 4b). toAt 6 that July. 2 time,The minimumwater levels land in areathree of hydrological total three riverstation islandss were is all 26.90 the kmhighest, on water 8 July level 2016 (17.66 (Figure m4 b).in Anqing At that gaugingtime, water station, levels 15.23 in threem in Baidang hydrological gauging stations station were and all 15.63 the highestm in Datong water hydrological level (17.66 s mtation), in Anqing and abovegauging the station, warning 15.23 water m inlevel. Baidang The gaugingmaximum station land andarea 15.63 of three m in river Datong islands hydrological appeared station), in the dry and seasoaboven theon December warning water 28, 2016, level. which The maximumwas 53.09 km land2 (Figure area of 4a). three At riverthe same islands time, appeared Anqing in gauging the dry 2 stationseason and on 28Datong December hydrological 2016, which station was were 53.09 the kmlowest(Figure water4 a).levels, At the6.86 same m, and time, 5.72 Anqing m, respectively. gauging stationThe and embankment Datong hydrological is the guarantee station were for the the lowest life and water property levels, 6.86 safety, m, and social 5.72 and m, respectively. economic developmentThe embankment of the residents is the of guarantee the river for island. the life In the and flood property season safety, in 2016, social both and Fenghuang economic embankmentdevelopment and of the Changsha residents embankment of the river were island. not In broken, the flood and season land areas in 2016, surrounded both Fenghuang by the embankmentembankment were and about Changsha 11 km embankment2, 10 km2, respectively. were not On broken, July 8, andthe land land area areas of Changshazhou surrounded by and the 2 2, Fenghuangzhouembankment were was about about 11 11.33 km ,km 102 km and respectively.15.45 km2, respectively. On 8 July, theAt landthe same area time, of Changshazhou the uninhabited and 2 2, islandFenghuangzhou without embankment was about 11.33 protection km and was 15.45 largely km submerged,respectively. and At theonly same about time, 0.12 the km uninhabited2 were not 2 submerged.island without Moreover, embankment we continuously protection wasmonitor largely the submerged, area of the andriver only and aboutcan capture 0.12 km shortwere-term not changes.submerged. During Moreover, 29 April weand continuously 4 May, the water monitor level thefirst area increased of the from river 11.83 and canm to capture 11.85 m, short-term and then droppedchanges. back During to 11.75 29 April m, the and total 4 May, area the of waterriver islands level first also increased changed from synchronous 11.83 m toly. 11.85 m, and then dropped back to 11.75 m, the total area of river islands also changed synchronously.

(a) (b)

Figure 4. The maximum (a) and minimum (b) area of the river island. Figure 4. The maximum (a) and minimum (b) area of the river island. 4.3. Relationship between River Island Area and Water Level 4.3. Relationship between River Island Area and Water Level To explore the relationship between land area change and the water level change, we collected waterTo level explore data the near relationship the river island. between Three land models area change of primary and function, the water quadratic level change, function, we collected and cubic waterfunction level are data used near to characterizethe river island. the relationship Three models between of primary the land function, area of quadratic the river islandfunction, and and the cubicwater function level at theare correspondingused to characterize imaging the timerelationship (Figure5 between). the land area of the river island and the waterThe calculationslevel at the corresponding demonstrate that imaging all these time three (Figure models 5). indicate signiﬁcant correlations between the landThe area calculations of river islands demonstrate and the that measured all these water three level. models This conclusion indicate significant is similar correlations to the water betweenlevel data the of land the three area waterof river stations: islands the andR 2theof measured the ﬁtted model water islevel. higher This than conclusion 0.55, and is the similarP values to the are water level data of the three water stations: the R2 of the fitted model is higher than 0.55, and the P values are less than 0.001. However, we can discover some differences among the three models. The correlation between the water level measured by Datong hydrological station and the area of Fenghuangzhou, Changshazhou is higher than the uninhabited island. Since the unmanned island is too small, it is greatly affected by the error caused by the satellite image resolution. The correlation between the water level measured by Datong hydrological station and the area of river islands is higher than that the other two hydrological station, due to the location of the hydrological station and its distance from the study area. Anqing gauging station is located 4 km upstream of the study area, while Datong is the nearby hydrological station. Baidang gauging station is located on the tributary of the study area, 2.5 km from the intersection of the tributary and the Yangtze River. Baidanghu sluice is a drainage station, water level of Baidang gauging station is higher than the actual

Water 2018, 10, 1484 10 of 17 less than 0.001. However, we can discover some differences among the three models. The correlation between the water level measured by Datong hydrological station and the area of Fenghuangzhou, Changshazhou is higher than the uninhabited island. Since the unmanned island is too small, it is greatly affected by the error caused by the satellite image resolution. The correlation between the water level measured by Datong hydrological station and the area of river islands is higher than that the other two hydrological station, due to the location of the hydrological station and its distance from the study area. Anqing gauging station is located 4 km upstream of the study area, while Datong is the nearby hydrological station. Baidang gauging station is located on the tributary of the study area, 2.5 km from the intersection of the tributary and the Yangtze River. Baidanghu sluice is a drainage Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES } station, water level of Baidang gauging station is higher than the actual water level from the Yangtze River,water and level it cannot from the accurately Yangtze River, represent and it the cannot water accurately level information represent the of water the Yangtze level information River near of the riverthe island. Yangtze River near the river island.

Fenghuang- zhou

Changsha- zhou

The uninhabited island

(a) primary function (b) quadratic function (c) cubic function

FigureFigure 5. 5. RelationshipsRelationships between between the thearea areaof river of islands river and islands the water and level the of water Datong level hydrological of Datong hydrologicalstations. stations.

Additionally,Additionally these, these three three fitted fitted models models showed showed somesome differences in in the the fitting. fitting. Among Among them, them, the the 2 cubiccubic function function performs performs the the best best in in the the fitting fitting ( R(R2 >> 0.85, P < 0.001).0.001). This This also also reflects reflects the the complex complex nonlinear relationship between the land area of the river island and the water level in the Yangtze nonlinear relationship between the land area of the river island and the water level in the Yangtze River, which is mainly controlled by the topography of the river island tidal flat. River, which is mainly controlled by the topography of the river island tidal flat. 4.4. Relationship between River Island Area and SSC 4.4. Relationship between River Island Area and SSC As shown in Table 3, compared with the multi-year average water and sand characterization As shown in Table3, compared with the multi-year average water and sand characterization values between 2007~2016, the annual runoff and the annual sediment discharge of Datong valueshydrological between s 2007~2016,tation in 2016 theare 20 annual% and 17% runoff larger, and respectively. the annual Compared sediment with discharge annual water of Datongand hydrologicalsand characterization station in 2016values are in 2015, 20% the and annual 17% larger,runoff and respectively. the annual sediment Compared discharge with annual of Datong water andh sandydrological characterization station in 2016 values are in 14% 2015, and the 31% annual higher, runoff respectively. and the Moreover annual sediment, annual runoff discharge and of Datongannual hydrological sediment discharge stationin in 2016Datong are h 14%ydrological and 31% station higher, in 2016 respectively. were mainly Moreover, concentrated annual in May runoff- andOctober, annual sediment with the annual discharge runoff in and Datong annual hydrological sediment discharge station inaccounting 2016 were for mainly 66% and concentrated 72% of the in May–October,year, respectively. with the annual runoff and annual sediment discharge accounting for 66% and 72% of the year, respectively. Table 3. Interannual comparison of water and sediment characteristic of Datong hydrological station.

Annual Runoff Annual Sediment Annual Average Sediment Characteristics (108 m3) Discharge (108 T) Concentration (kg/m3) Multi-year 8712 1.3 0.414 average （2007–2016）（2007–2016）（1951–2015） 2015 9139 1.16 0.127 2016 10450 1.52 0.145

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Table 3. Interannual comparison of water and sediment characteristic of Datong hydrological station.

Annual Runoff Annual Sediment Annual Average Sediment Characteristics (108 m3) Discharge (108 T) Concentration (kg/m3) 8712 1.3 0.414 Multi-year average (2007–2016) (2007–2016) (1951–2015) 2015 9139 1.16 0.127 2016 10,450 1.52 0.145

Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES } To explore the association between the area of river islands and SSC, the primary function model To explore the association between the area of river islands and SSC, the primary function model was used to describe the relationship between monthly mean SSC of Datong hydrological station was used to describe the relationship between monthly mean SSC of Datong hydrological station and and monthlymonthly average area area of of river river island islandss (Figure (Figure 6.).6 ).The The calculations calculations show show that that there there is a issignificant a signiﬁcant correlationcorrelation between between the the area area of of river river islands islandsand and SSCSSC ( P << 0.05). 0.05).

(a) Fenghuangzhou (b) Changshazhou.

FigureFigure 6. Relationships 6. Relationships between between monthly monthly areaarea ofof river islands islands and and the the monthly monthly mean mean SSC SSCof Datong of Datong hydrologicalhydrological station. station.

Also,Also from, from the the long-term long-term monitoring monitoring results, results, the the annual annual sediment sediment discharge discharge of Datong of Datong hydrologicalhydrological station station exhibited exhibited a generala general decreasingdecreasing trend trend (Figure (Figure 7),7 ),for for three three reasons: reasons: 1. Since 1. Since 1989, 1989, China has implemented soil and water conservation projects in the upper reaches of the Yangtze China has implemented soil and water conservation projects in the upper reaches of the Yangtze River River to restore vegetation; 2. The scouring and silting changes of the Yangtze River, lakes and to restoretributaries vegetation; have a certain 2. The adjustment scouring andeffect silting on the changessediment discharge of the Yangtze of Datong River, hydrological lakes and station; tributaries have3. a certainDue to the adjustment large number effect of on water the conservancy sediment discharge projects constructed of Datong hydrologicalin the upper reaches station; of 3. the Due to the largeYangtze number River of (such water as conservancythe Gezhouba projectsWater Conservancy constructed Project, in the upperthe Three reaches Gorges of Dam), the Yangtze a large River (suchamount as the Gezhoubaof sediment Water is deposited Conservancy in the reservoir. Project, However, the Three under Gorges the Dam), influence a large of flood amount discharge of sediment in is depositedthe flood in season the reservoir. in 2016, the However, flow is turbulent, under theand influence the sediment of floodfrom the discharge middle and in the upper flood reaches season in 2016,of the the flow Yangtze is turbulent, River and and its tribut the sedimentaries significantly from the increased middle andcompared upper to reaches 2015. With of the the Yangtze changesRiver and itsof tributariesrunoff and sediment significantly volume increased in this period, compared river toislands 2015. also With exhibited the changes some variations of runoff. and sediment volume in this period, river islands also exhibited some variations.

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FigureFigure 7. 7.Changes Changes in runoff runoff and and sediment sediment discharge discharge in Datong in hydrographic Datong hydrographic station over the station past decades over. the pastFigure decades. 7. Changes in runoff and sediment discharge in Datong hydrographic station over the past decades. 4.5. Relationship between River Island, Runoff and Rainfall 4.5. Relationship between River Island, Runoff and Rainfall Apart from water level and SSC, some other potential factors (such as runoff, rainfall) can also contributeApart from to waterthe temporal level and change SSC,SSC of, somesome river otherotherislands. potentialpotential To quantify factors the (suchassociation as runoff, between rainfall)rainfall the area) cancan of alsoalso contributeriver islands to the, runoff temporal and changerainfall , ofthe river primary islands.islands. function T Too quantifymodel was the used association to describe between the relationship the area of riverbetween islands,islands ,monthly runoff andmean rainfall,rainfall instantaneous, thethe primaryprimary runoff function(Figure 8.) model, monthly was cumulative used to describedescribe rainfall (Figure thethe relationshiprelationship 9.) and betweenmonthly monthly average mean area instantaneous of river islands runoff. runoff The calculation (Figure (Figure 88.) ),show,monthly monthlys that the cumulativecumulative runoff and rainfallrainfall the river (Figure(Figure island 9 area9.)) and and monthlyhave averagestrong correlation area of riverriver for each islands.island ones. The(Fenghuangz calculationhou: showsshow R2 = s0 .91, that P the< 0.001 runoff; Changshazhou: and the river R island2 = 0.92, area 2 2 2 2 haveP strong < 0.001 correlationcorrelation; the uninhabited forfor eacheach island: oneone (Fenghuangzhou:R(Fenghuangz = 0.68, P < 0.001hou:) R andR2 == the 0.91,0.91, total PP < <(R 0.001;0.001 = 0.91,; Changshazhou:Changshazhou: P < 0.001). Moreover, RR2 = 0.92, 2 2 PP << 0.001;0.001there; is thethe a significant uninhabiteduninhabited correlation island:island: betweenR 2 = 0.68, the P area < 0.001 0.001) of river) and islands the total and the (R2 cumulative = 0.91, P < monthly 0.001 0.001).). Moreover, rainfall for each one (Fenghuangzhou: R2 = 0.49, P < 0.05; Changshazhou: R2 = 0.53, P < 0.01; the uninhabited therethere isis a signiﬁcantsignificant correlation betweenbetween thethe areaarea ofof river islands and the cumulative monthly rainfall island: R2 = 0.37, P < 0.05) an2d the total (R2 = 0.49, P < 0.05). Additionally,2 a significant positive for each one (Fenghuangzhou: R 2 = 0.49, P < 0.05; Changshazhou: R 2 = 0.53, P < 0.01; the uninhabited for each one (Fenghuangzhou: R = 0.49, P < 0.05; Changshazhou:2 R = 0.53, P < 0.01; the uninhabited relationship2 was found between runoff and2 rainfall (R > 0.52, P < 0.01). island:island: R2 = 0.37, 0.37, P < 0.05 0.05)) anandd the total ((RR2 = 0.49, P << 0.05).0.05). A Additionally,dditionally, a signiﬁcant significant positive positive 2 relationship waswas foundfound betweenbetween runoffrunoff andand rainfallrainfall ((RR2 > 0.52, P < 0.01 0.01).).

(a) Fenghuangzhou (b) Changshazhou.

Figure 8. Relationships between monthly area of river islands and the monthly mean instantaneous runoff of Datong hydrological station.

FigureFigure 8. Relationships 8. Relationships between between monthly monthly area of area river of islands river islands and the and monthly the monthly mean instantaneous mean instantaneous runoff of Datongrunoff hydrological of Datong hydrological station. station.

Water 2018, 10, 1484 13 of 17 Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES }

(a) Fenghuangzhou (b) Changshazhou.

Figure 9. Relationships between monthly area of river islands and the cumulative monthly rainfall of Datong hydrological station.station.

4.6. Flooding Risk of River Islands We usedused thethe frequencyfrequency ofof riverriver islands’islands’ inundation to reflectreflect the risk of floodingflooding of those river islands.islands. FromFrom FigureFigure 10 10,, we we can can see see that that the the spatial spatial distribution distribution of the of riskthe ofrisk submergence of submergence in the in study the area.study Redarea. represents Red represents the highest the highest value value of 1 and of 1 green and green represents represents the lowest the lowest value value of 0. Pleaseof 0. Please note thatnote athat larger a larger value value indicates indicates a higher a higher flooding flooding risk. risk According. According to the to RIs,the RIs we, definewe define areas area withs with a RIs a lessRIs less than than 0.1 as 0.1 safe as safe areas, area ands, areasand area withs with a RI greatera RI greater than 0.5than as 0.5 dangerous as dangerous areas. area Calculationss. Calculations show 2 thatshow the that studied the studied river island river island owns 29.37owns km 29.37safe km areas,2 safe accountingareas, accounting for 52.39% for 52.39% of the total of the land total area land of 2 2 riverarea of islands; river 26.69islands; km 26.69are vulnerable km2 are vulnerable to floods, to of whichfloods, 8.45 of which km are 8.45 dangerous km2 are areas, dangerous accounting areas, foraccounting 15.07% offor the 15.07% total landof the area total of riverland islands.area of river These islands. areas are These mainly areas located are mainly on the located banks of on river the islandsbanks of and river on theislands banks and of on the the rivers. banks Moreover, of the rivers. during Moreov the paster, decades, during the runoffpast decades, and SSC the exhibited runoff aand dramatic SSC exhibited decrease a after dramatic the impoundment decrease after of the Three impoundment Gorges Dam of in Three June 2003Gorges [48 ].Dam To better in June predict 2003 the[48]. risk To ofbetter river predict islands, the data risk on of runoff river islands, and sediment data on at runoff a long-term and sediment scale was at required. a long-term scale was required.

Water 2018, 10, 1484 14 of 17 Water 2018, 10, x FOR PEER REVIEW { PAGE } of { NUMPAGES }

(a) Risk Index (RI) mapping of river islands. 0.9～1.0 3.51 0.8～0.9 1.39 0.7～0.8 1.04 0.6～0.7 1.04 0.5～0.6 1.47 RI 0.4～0.5 2.49 0.3～0.4 1.81 0.2～0.3 3.31 0.1～0.2 10.64 0～0.1 29.37 0 5 10 15 20 25 30 Area (×106 m2)

(b) Interval statistical results of RI.

FigureFigure 10 10.. TheThe river river island island Inundation Inundation Risk Risk Index. Index.

5.5. Conclusions Conclusions OurOur results results confirmed confirmed the the availability availability of ofGF GF-1-1 data data in temporal in temporal monitoring monitoring of river of riverislands. islands. This paperThis paper takes takesthree threeriver riverislands islands at the at Yangtze the Yangtze River River as an asexample, an example, using using Chinese Chinese GF-1 GF-1satellite satellite data asdata the as main the main data data source, source, together together with with methods methods of of water water index index and and threshold threshold segmentation, segmentation, to to monitormonitor t thehe temporal temporal change change of of river river islands. islands. GF GF-2-2 data data with with higher higher spatial spatial resolution resolution were were used used for for validationvalidation with with the the accuracy accuracy of of 94.74%. 94.74%. Using Using the the proposed proposed method method in in this this paper, paper, seasonal seasonal variations variations andand certain short-termshort-term fluctuationsfluctuations ofofland land areas areas of of three three typical typical river river islands islands in thein the Yangtze Yangtze River River can canbe observed.be observed Based. Based on theon hydrologicalthe hydrological station station data, data, we exploredwe explored the correlationthe correlation between between theland the landarea ofarea the of river the r islandiver island and the and water the water level of level the Yangtzeof the Yangtze River. TheRiver. cubic The function cubic function can better can describe better 2 describethe relationship the relat betweenionship land between area and land water area level and ( waterR > 0.8, levelP < ( 0.001).R2 > 0.8, The P temporal < 0.001). variation The temporal of the variationriver island of the was river mainly island controlled was mainly by the controlled variation by of the the variation water level of the during water the level year. during Moreover, the year. we Moreover,can capture we the can abrupt capture change the abrupt in river change islands in induced river islands by flooding-driven induced by flooding water level-driven variations. water level variationIn addition,s. we generated a spatial distribution map of river islands inundation risk. We found that areasIn addition, with high we riskgenerated are mainly a spatial located distribution in the boundaries map of river of the islands river island.inundation This providesrisk. We afound basis thatfor floodareas forecastingwith high risk and are post-disaster mainly located reconstruction in the boundaries and can of be the applied river island. to other This similar provides river islands.a basis for flood forecasting and post-disaster reconstruction and can be applied to other similar river islands. Author Contributions: J.S. and N.X. designed the broad research strategy for the work, collected the data, executed the analysis, and finished the manuscript. L.D. and J.L. contributed to the result analysis. H.Q. and M.H. Authorrevised theContributions: manuscript. J.S. and N.X. designed the broad research strategy for the work, collected the data, executed the analysis, and finished the manuscript. L.D. and J.L. contributed to the result analysis. H.Q. and M.H. revised the manuscript.

Water 2018, 10, 1484 15 of 17

Funding: This research was partially supported by the Youth Science and Technology Innovation Fund Project (No. KY201703), which provided by Anhui Province Key Laboratory of Water Conservancy and Water Resources. Acknowledgments: The authors gratefully acknowledge the anonymous reviewers and editors for their careful reviews and suggestions, which significantly contributed to the manuscript improvement. Conflicts of Interest: The authors declare no conflict of interest.

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