Variation of River Islands Around a Large City Along the Yangtze River from Satellite Remote Sensing Images

sensors

Article Variation of River Islands around a Large City along the Yangtze River from Satellite Remote Sensing Images

Haiyun Shi 1, Chao Gao 2, Changming Dong 1,3,*, Changshui Xia 4,5 and Guanglai Xu 6

1 School of Marine Sciences, Nanjing University of Information Science & Technology, Nanjing 210044, China; [email protected] 2 Department of Geography & Spatial Information Techniques, Ningbo University, Ningbo 315211, China; [email protected] 3 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA 4 Key Lab of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China; xiacs@ﬁo.org.cn 5 Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China 6 College of Territorial Resources and Tourism, Anhui Normal University, Wuhu 241000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-025-5869-5733

Received: 3 August 2017; Accepted: 21 September 2017; Published: 27 September 2017

Abstract: River islands are sandbars formed by scouring and silting. Their evolution is affected by several factors, among which are runoff and sediment discharge. The spatial-temporal evolution of seven river islands in the Nanjing Section of the Yangtze River of China was examined using TM (Thematic Mapper) and ETM (Enhanced Thematic Mapper)+ images from 1985 to 2015 at five year intervals. The following approaches were applied in this study: the threshold value method, binarization model, image registration, image cropping, convolution and cluster analysis. Annual runoff and sediment discharge data as measured at the Datong hydrological station upstream of Nanjing section were also used to determine the roles and impacts of various factors. The results indicated that: (1) TM/ETM+ images met the criteria of information extraction of river islands; (2) generally, the total area of these islands in this section and their changing rate decreased over time; (3) sediment and river discharge were the most significant factors in island evolution. They directly affect river islands through silting or erosion. Additionally, anthropocentric influences could play increasingly important roles.

Keywords: ﬂow and sediment; Nanjing; river islands; remote sensing; soil erosion; sensor; the Yangtze River

1. Introduction River islands are structural sediment deposits which extend above the water level in braided streams. They divide a river into multiple channels and form the connection of interrelation and interaction between two channels [1,2]. Riverine islands have various shapes and wildly different surface areas. They are more stable than central bars and shoals. Generally, the head of a river island is greatly affected by wave action resulting in erosion. Sediment deposition occurs at its tail where shoals are born. Consequently, the island continuously crawls along the river channel, becoming longer and narrower in the process. This process is dependent on sediment ﬂow and boundary conditions.

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River island evolution has an important influence on the river channel, the river bed, embankments, ports, and ecosystems, biological communities, human activities on the island itself. Thus, the study of river islands has important real-world applications [3] Previous research on the shifting, collapse and erosion of river islands found that these phenomena significantly affect river channels, island bodies and banks [4–8]. Some scholars studied the area changes of river islands. Li et al. [9] analyzed river islands in the middle and lower reaches of Yangtze River quantitatively from Landsat and Google Earth images and found that the area of these islands decreased due to the reduction of sediment availability after the Three Gorges Reservoir began to fill. Gao [10] analyzed eight river islands in Mawutong section of Yangtze River and found that their areas firstly increased then decreased over 30 years. The extraction methods of surface features were also been discussed. Chen et al. [11] extracted water information of Shangri-la County’s wetland from ETM+ images using threshold value method, interpolation method and spectral relationship method and found that threshold value method cannot distinguish the mountain shadow and water easily. Some other scholars analyzed the influencing factors including sediment in the rivers. Wang et al. [12] discussed sediment yield, transport and deposition by the sediment budget method, finding that 10% of fine sediment transport by the Yangtze River estuary is done by tidal flow into the sea. Sediment supply is a significant determinant of channel formation and maintenance [13]. Park and Latrubesse [14] demonstrated their MODIS (Moderate Resolution Imaging Spectroradiometer)-based model in capturing the spatial and temporal variability of surface sediments in the Amazon River Basin, the largest river system on Earth. With the development of remote sensing (RS) technology, the temporal and spatial resolution of RS images have continually improved. More remotely sensed data (MODIS, TM (Thematic Mapper), ETM (Enhanced Thematic Mapper)+, Quickbird, radar, aerial photo etc.) are being applied in various geoscience research field, including, but not limited to, land resources planning and environmental monitoring [15,16]. The boundary between land and water can be easily extracted using remote sensing techniques because of its macroscopic characteristics [17]. Based on time span and spatial resolution requirements, this study utilized and analyzed Landsat5 TM and Landsat7 ETM+ derived images of the study area. However, few studies on the quantitative information extraction and influencing factor analysis of river islands have been conducted [9,10]. In this paper, the evolution of seven typical river islands in study area was quantitatively evaluated based on remotely sensed TM and ETM+ data. The primary factors were identified by combining annual runoff and sediment discharge data as measured at the hydrological station. The primary objective was to reveal the spatial and temporal variation of river islands, and to evaluate the factors influencing this variation.

2. Study Area and Data

2.1. Study Area The Yangtze River (also known as the Changjiang River) is the world’s third longest river and the longest one in Asia. It flows from Anhui Province to Jiangsu Province. The Nanjing section is located in its lower reaches, between 118◦220–119◦140 and 31◦140–32◦370, for a total river length section of 95 km. The Yangtze River flows through central Nanjing from the southwest to northeast. Nanjing, which features a northern, subtropical monsoonal climate, lies in southeast of Jiangsu Province and the mean annual air temperature is 15.7 ◦C. Annual precipitation is 1106.5 mm [18]. The Qinhuai River and Chuhe River located south and north of Yangtze River, respectively, are two primary tributaries which flow through Nanjing. Most of the land along the river is classified as plains, specifically valley plains and lakeshore plains, where the agro type is dominated by yellow brown soil. The lower reaches of the Yangtze River in the Nanjing region, play host to many riverine islands. Because of sediment transport and deposition, tidal backwater and river tides, more than a dozen river islands with a variety of Sensors 2017, 17, 2213 3 of 20

shapesSensors 2017 and, 17 sizes, 2213 appear in Nanjing section of Yangtze River (hereinafter referred to as NYR). They3 of are 20 shoals, washland, shallows and river islands, and their average elevation is four to seven meters. The seven selected river islands, from south to north are Xinshengzhou, Xinjizhou, Zihuizhou, Zimuzhou, Jiangxinzhou, Qianzhou and Baguazhou (Figure 11).). TheThe firstfirst sixsix islandsislands areare longlong andand narrow, whilewhile Baguazhou Baguazhou island island is locatedis located at theat river’sthe river’s inflexion inflexion point, point, where where the flow the of flow the Yangtze of the RiverYangtze turns River east turns from east the from northeast. the northeast. Baguazhou Baguazh is theou largestis the largest river island river island of the of group, the group, and third and largestthird largest within within the river. the It river. plays hostIt plays to advanced host to agricultureadvanced andagriculture a population and ofa approximatelypopulation of thirty-threeapproximately thousand thirty-three and is shapedthousand like and a cattail is leafshaped fan. Xinjizhoulike a cattail and Xinshengzhouleaf fan. Xinjizhou islands wereand originallyXinshengzhou linked, islands but became were segmentedoriginally afterlinked, a flood but became in 1998. Thesegmented changes after in these a flood seven in river 1998. islands The arechanges expected in these to reflect seven changes river islands in other are islands expected in theto reflect Nanjing changes section in and other even islands in the in lower the Nanjing reaches ofsection Yangtze and River even effectivelyin the lower and reaches provide of basis Yangtz to supporte River effectively healthy managerial and provide practices basis into thesupport river channelhealthy managerial and bank. practices in the river channel and bank.

Figure 1. Map of the Nanjing Section of the Yangtze River, showing the river islands Xinshengzhou, Xinjizhou, Zihuizhou,Zihuizhou, Zimuzhou, Zimuzhou Jiangxinzhou,, Jiangxinzhou, Qianzhou Qianzhou and Baguazhouand Baguazhou by Landsat7 by Landsat7 ETM+ acquired ETM+ onacquired 12 October on 12 2015. October 2015.

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2.2. Data

2.2.1. Remotely Sensed Data To study the spatial-temporal characteristics of these islands, Landsat TM (Thematic Mapper) and ETM+ (Enhanced Thematic Mapper) data from 1985 to 2015, was acquired at ﬁve-year intervals (Table1). The dataset was provided by Geospatial Data Cloud at the Computer Network Information Center, Chinese Academy of Sciences. The satellite track of path 120 and row 38 gives full coverage of the study area. In order to reduce the inﬂuence of clouds on information extraction, relatively cloud-free images were chosen. Further, to minimize the impact water has on the river islands, only images during the dry season were used.

Table 1. Remote sensing data with the resolution of 30 m used in the study area.

Date Satellite/Sensor Center Latitude (◦) Center Longitude (◦) 18/November/1985 Landsat5/TM 31.75 118.80 4/December/1990 Landsat5/TM 31.76 118.80 30/January/1995 Landsat5/TM 31.75 118.89 10/October/2000 Landsat5/TM 31.73 118.89 19/December/2005 Landsat7/ETM+ 31.73 118.89 17/December/2010 Landsat7/ETM+ 31.74 118.86 12/October/2015 Landsat7/ETM+ 31.74 118.86 TM: Thematic Mapper; ETM: Enhanced Thematic Mapper.

2.2.2. Hydrological Data The Datong hydrological station, which lies upstream of the study area in the stem stream of Yangtze River, is located in Chizhou City, Anhui. It is an important hydrological station as it is responsible for controlling water ﬂow from the Yangtze River to the sea and protecting water resources. Datong Station has measured water levels, annual runoff, sediment discharge, average sediment concentration, among other parameters, since 1950. Based on statistics from the Jiangsu Province Water Resources Bulletin [19], China River Sediment Bulletin [20] and Changjiang Sediment Bulletin [21], the measured average annual runoff for 66 years from 1950 to 2015 is 893.1 billion m3, the average annual sediment discharge for 65 years from 1951 to 2015 is 0.368 billion tons, and the annual average sediment concentration for 65 years from 1951 to 2015 is 0.414 kg/m3.

2.3. Image Preprocessing It is important to note that the Scan Lines Corrector (SLC) of Landsat7 ETM+ on 31 May 2003 suffered a sudden malfunction, which resulted in some images being covered by black stripes over 25%. The SLC-off model was used to rework these images by revising the single band stripe (Figure2). This paper used the function “tm_destripe” in the ENVI (The Environment for Visualizing Images) image processing software package for this. The basis of tm_destripe is the Multiple Image Local Adaptive Regression Analysis Model or Multiple Image Fixed Window Regression Analysis Model, which can be used to revise stripes. This method can more accurately repair images and hence, can recover surface feature information. It facilitates the interpretation and extraction of information from images and meets present research demands. Sensors 2017, 17, 2213 5 of 20

can meet the demands of this research, the fourth band of TM and ETM+ were used in this paper. Geometric registration was applied to the near-infrared images of seven years by Ground Control SensorsPoints2017 (GCPs), 17, 2213 and using ENVI 5.1 software (ITT Visual Information Solutions Company, Boulder,5 of 20 CO, USA). These images were cropped to the same ROI.

(a) Baguazhou (original) (b) Baguazhou (reconstructed)

Figure 2. Sample Landsat 7 ETM+ SLC-off model correction effectiveness of 19 December 2005, Figure 2. Sample Landsat 7 ETM+ SLC-off model correction effectiveness of 19 December 2005, images images over (a,b) Baguazhou island and (c,d) Qianzhou and Jiangxinzhou islands. over (a,b) Baguazhou island and (c,d) Qianzhou and Jiangxinzhou islands. 3. Methods The selected TM and ETM+ data in this paper are L1T standard topographic correction products. The3.1. products’ River IslandArea geodetic Extraction correction Methods relies on precision ground control points and high-precision Digital ElevationTM Model and ETM+ (DEM) images data. are The gray-scale map of the images. study Each area pixel can beof derivedgray-scale using images the is band described combination by a andquantized Region ofdiscrete Interest gray (ROI) value subset. [22]. Gray The value data’s is original also called projection, digital number UTM (Universal (DN). DN Transversevalue is Mercator)-WGSbetween 0 and (World 255 because Geodetic of eight-bit System) quantization 84 Antarctica. The Polar DN Projection, value of 0was shows transformed black and intothat anof 255 equal areashows projection, white. i.e.,The thereflectivity Sample of Lambert rivers, lakes Azimuthal and other Equal water Area bodies Projection. are lower The than near-infrared other surface band wavelengthfeatures, and of TM their and DN ETM+ values is between are different. 0.76 and Then 0.90 aµ m,binary which model can bewas used used to effectivelyto distinguish distinguish river waterislands bodies from from the Yangtze vegetation River cover. water. According Building abinarization to ﬁeld investigations, model in ENVI, the main ground object type on the islands is vegetation. Since the near-infrared band can meet the demands of this research, the fourth band of TM and ETM+ were used in this paper. Geometric registration was applied to the near-infrared images of seven years by Ground Control Points (GCPs) and using ENVI 5.1 software (ITT Visual Information Solutions Company, Boulder, CO, USA). These images were cropped to the same ROI. Sensors 2017, 17, 2213 6 of 20

3. Methods

3.1. River IslandArea Extraction Methods TM and ETM+ images are gray-scale images. Each pixel of gray-scale images is described by a quantized discrete gray value [22]. Gray value is also called digital number (DN). DN value is between 0 and 255 because of eight-bit quantization. The DN value of 0 shows black and that of 255 shows white. The reflectivity of rivers, lakes and other water bodies are lower than other surface Sensors 2017, 17, 2213 6 of 20 features, and their DN values are different. Then a binary model was used to distinguish river islands from the Yangtze River water. Building abinarization model in ENVI, ≥ λ 1( if DN μ = 1 i f DN ≥ λ (1) µ = (1) 0 0 otherwiseotherwise where,where, μµ is is the the image image after after binarization, binarization, DN DN is isthe the graygray valuevalue ofof everyevery pixelpixel inin thethe image,image, λλis theis thethreshold threshold value value of islandof island and and the the Yangtze Yangtze River. River. ToTo ensure ensure the the value value of of λλ,, thisthis study used the the threshold threshold value value method, method, which which applied applied the the differencedifference in in the the reflectivity reflectivity of of some some bands bands between between a asurface surface feature feature and and other other background background surface surface features.features. According According to toa priori a priori knowledge knowledge of visual of visual image image interpretation interpretation and andfield fieldconfirmation, confirmation, we randomlywe randomly and uniformly and uniformly selected selected 100 points 100 points from fromone image one image and andjudged judged whether whether they theyare land are land or not.or not.Then Then we obtained we obtained an optimal an optimal threshold threshold value value of 45, of which 45, which is the isDN the value DN valueof water of waterand land and demarcationland demarcation (Figure (Figure 3). The3). Thethreshold threshold value value method method has has a high a high effectiveness effectiveness to to extract extract water water informationinformation based based on on the the significant significant difference difference in in the the reflectivity reflectivity between between water water and and other other objects. objects.

FigureFigure 3. 3.TheThe value value of of the the demarcation demarcation between between river river isla islandsnds and and the the Yangtze Yangtze River. River. X-axis X-axis represents represents pointpoint numbers numbers and and y-axis y-axis represents represents digital digital numb numberer (DN) (DN) value value at this at this point point in the inthe image. image. In the process of acquisition and transmission, the image influenced by sensors and atmosphere In the process of acquisition and transmission, the image influenced by sensors and produces noise. Low-pass filtering (LPF) can filter out high-frequency components and noise, achieving atmosphere produces noise. Low-pass filtering (LPF) can filter out high-frequency components and sharp noise removal and image smoothing. Mean value smoothing is a common method of LPF. noise, achieving sharp noise removal and image smoothing. Mean value smoothing is a common When the area is given by M × N, the formula is method of LPF. When the area is given by M × N , the formula is M N r(i, j)M = ∑ N∑ φ(m, n)t(m, n) (2) m= n= r(i, j) = 1 1φ(m,n)t(m,n) (2)   m=1 n=1 where, M × N is the size of the assumed template, r(i, j) is the image after the process of LPF, t(m,n) is the template, φ(m,n) is the window which is as the same size as template, and the DN values of pixels in the window are chosen from the image to multiply t(m,n) t(m,n) is actually a convolution kernel. Conduct convolution using a 3 × 3 template gives

Sensors 2017, 17, 2213 7 of 20 where, M × N is the size of the assumed template, r(i, j) is the image after the process of LPF, t(m, n) is the template, φ(m, n) is the window which is as the same size as template, and the DN values of pixels in the window are chosen from the image to multiply t(m, n) t(m, n) is actually a convolution kernel. Conduct convolution using a 3 × 3 template gives

 1 1 1  8 8 8 ( ) =  1 1  t m, n  8 0 8  (3) 1 1 1 8 8 8

To filter out noise, 3 × 3 template is enough and the templates bigger than 3 × 3 will cause the loss of information. The usage of t(m, n) puts first data point at center pixel eroding edges slightly. Unsupervised classification was conducted by the K-means method. The basis of K-means is to minimize the distance square sum between multi-mode points and the category center point and move various category center points successively by iteration until it comes out with an optimal result. After classifying, the images were discontinuous in space and spots or empty areas were formed in the classification regions. It was therefore necessary to conduct reclassification for these small spots. Further cluster analysis was carried out on the results of classification by using the Clump Class function in ENVI and by merging the adjacent similar classification regions using mathematical morphological operators. This process is different from LPF, because the classification information of images after smoothing with LPF was disturbed by adjacent classification encoding. However, with cluster analysis this deleterious effect was not produced. Then continuous images were obtained for further calculation. At last, vectorization of the images was employed by ENVI, and calculation of the aforementioned seven island areas was done using the Geometry Calculator function in ArcGIS software.

3.2. River Island Area Trend Methods The shape and area information of selected river islands during 1985–2015 were determined through a series of remotely sensed image processing. To understand the area change trend clearly, the anomaly of each island’s area and the total area of seven islands respectively was calculated using the formula: ∆Aik = Aik − Ak (4) where, k refers to a particular island or islands, i refers to a particular year, ∆Aik is the area anomaly of a particular island in a particular year, Aik is the area of a particular island in a particular year, Ak is the 30-year average area of the particular island or islands. Based on the areas of 1985, the formula of average area change rates derived as:

A2 − A1 η12 = × 1000%0 (5) 5 × A1 where, η12 is the annual average area change rate in a time period and the dimension of η is 1/year, A1 is the island’s former size and A2 is the island’s latter size, “5”in the denominator is 5 years. When η12 is greater than zero it indicates area increase, zero indicates no change, and negative indicates area decrease. The annual average area change rate during the 30 years is the mean value of η12 in the six-time periods. Sensors 2017, 17, 2213 8 of 20

3.3. Analytical Methods of Influencing Factors The correlation between the area of all seven river islands sand influencing factors including annual runoff and sediment discharge was derived using Pearson’s correlation coefficient, based on pixel space method with the unit of year. The formula is:

n ∑ [(xi − x)(yi − y)] i=1 Rxy = (6) s n n 2 2 ∑ (xi − x) ∑ (yi − y) i=1 i=1 where, Rxy is the correlation coefficient, xi is the area of river islands in a given year and yi is the influencing factor in a given year, x is the 30-year average area of river islands, y is the 30-year average value of influencing factors and i is the particular year. Rxy can reflect the close relations of area changes and influencing factors. If Rxy is greater than zero, the correlation is positive, otherwise correlation is negative. The closer the value of Rxy is to 1, the higher the correlation. In general, the degree of correlation is divided into four classes as follows (Table2):

Table 2. Correlation Classiﬁcation.

Rxy Rxy < 0.3 0.3 ≤ Rxy < 0.5 0.5 ≤ Rxy < 0.8 0.8 ≤ Rxy < 1 Degree of correlation weak low moderate high

4. Results

4.1. River Island Area Extraction The images of river islands obtained after classifying and clustering are shown in Figure4. Figure5 is the calculation of these islands’ areas during 1985–2000. The largest island, Baguazhou, and the smallest island, Zihuizhou, have an average size of 5485 hm2 and 96.71 hm2 (1 hm2 = 10,000 m2), respectively. The islands listed in order of size, from largest to smallest were: Baguazhou, Xinjizhou (Xinshengzhou), Jiangxinzhou, Zimuzhou, Qianzhou and Zihuizhou. It should be noted that Xinjizhou and Xinshengzhou were linked together but divided into two river islands in 1998. So the areas of these two islands were grouped together for calculations. Sensors 2017, 17, 2213 9 of 20 Sensors 2017, 17, 2213 9 of 20

(a) 1985 (b) 1990

Figure 4. Cont.

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(e) 2005 (f) 2010

(g) 2015

Figure 4. River island changes in NYR during 1985–2015 (a–g). Figure 4. River island changes in NYR during 1985–2015 (a–g).

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Figure 5. Area changes of river islands in NYR (Nanjing section of the Yangtze River) during Figure1985–2015 5. Area (hm 2changes). of river islands in NYR (Nanjing section of the Yangtze River) during 1985–2015 (hm2). 4.2. River Island Area Evolution Trend 4.2. River Island Area Evolution Trend The trend of all islands’ area and each island’s area were calculated by Formula (4) respectively. TheThe total trend area of theall islands’ river islands area and in NYR each clearlyisland’s decreased area were during calculated the 30by yearsFormula (Figure (4) respectively.6). The area Thedecreased total area faster of the during river 1985–2000 islands in andNYR slower clearly from decreased 2000 to during 2015. These the 30 islands years (Figure presented 6). aThe general area decreaseddecrease with faster a 30-yearduring 1985–2000 annual average and slower area of from 8975.14 2000 hm to2. 2015. The totalThese area isla declinednds presented by an averagea general of 2 decrease19.97 hm with2 a year. a 30-year Every annual island’s average area change area of can 8975.14 be seen hm clearly. The intotal Figure area7 declined. The areas by of an Baguazhou, average of 2 19.97Qianzhou, hm a Jiangxinzhouyear. Every island’s and Xinjizhou area change (Xinshengzhou) can be seen islandsclearly decreased,in Figure 7. and The that areas of of Zimuzhou Baguazhou, and Qianzhou,Zihuizhou Jiangxinzhou islands increased and duringXinjizhou the 30(Xinshengzho years. u) islands decreased, and that of Zimuzhou and Zihuizhou islands increased during the 30 years. The annual average area change rate during the six-time periods are shown in Figure 8 by Formula 5. The largest magnitude of rate of area change occurred with Zihuizhou, which increased by 30.7‰ per year. The smallest island, Baguazhou, decreased by 0.66‰ per year. Listing the islands

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fromSensors largest 2017, 17 to, 2213 smallest, in order of the absolute value of each island area change rate12 ofwere: 20 Zihuizhou, Zimuzhou, Qianzhou, Xinjizhou (Xinshengzhou), Jiangxinzhou and Baguazhou. Disregardingfrom largest Zihuizho to smallest,u, each in orderisland ofarea the change absolute rate value tend edof eachtowards island zero, area with change time, ratesuggesting were: a trendZihuizhou, towards Zimuzhou,stability. Qianzhou, Xinjizhou (Xinshengzhou), Jiangxinzhou and Baguazhou. Disregarding Zihuizhou, each island area change rate tended towards zero, with time, suggesting a Sensorstrend2017 ,towards17, 2213 stability. 12 of 20 500 y = -318.9ln(x) + 388.39 400 R2 = 0.9483 500 y = -318.9ln(x) + 388.39 ) 2 300400 R2 = 0.9483 )

2 200300 100200 1000 -1000 Area anomaly(hm Area -200-100 Area anomaly(hm Area -300-200 -300 1985 1990 1995 2000 2005 2010 2015 1985 1990 1995 2000Year 2005 2010 2015 Year

FigureFigure 6. 6.Area Area anomalyanomaly of the seven rive riverr islands islands in in the the study study area area (hm (hm2). 2). Figure 6. Area anomaly of the seven river islands in the study area (hm2).

2 FigureFigure 7. 7.Area Area anomaly anomaly of each each island island in in the the study study area area (hm (hm). 2). Figure 7. Area anomaly of each island in the study area (hm2).

The annual average area change rate during the six-time periods are shown in Figure8 by Equation (5). The largest magnitude of rate of area change occurred with Zihuizhou, which increased by 30.7 per year. The smallest island, Baguazhou, decreased by 0.66 per year. Listing the islandsh from largest to smallest, in order of the absolute value of eachh island area change rate were: Zihuizhou, Zimuzhou, Qianzhou, Xinjizhou (Xinshengzhou), Jiangxinzhou and Baguazhou. Disregarding Zihuizhou, each island area change rate tended towards zero, with time, suggesting a trend towards stability. Sensors 2017, 17, 2213 13 of 20

250 Sensors 2017, 17, 2213 13 of 20 Sensors 2017, 17, 2213 200 13 of 20

150 250

200100

15050

100 0

50-50

Average annual areachange rate/‰ 0 -100 -501985-1990 1990-1995 1995-2000 2000-2005 2005-2010 2010-2015

Average annual areachange rate/‰ -100 Baguazhou Qianzhou 1985-1990 1990-1995Jiangxinzhou 1995-2000 2000-2005 Zimuzhou 2005-2010 2010-2015 BaguazhouZihuizhou Qianzhou Xinjizhou(Xinshengzhou) Jiangxinzhou Zimuzhou Zihuizhou Xinjizhou(Xinshengzhou) Figure 8. Average annual area change rate of the islands in the study area (‰). Figure 8. Average annual area change rate of the islands in the study area ( ). Figure 8. Average annual area change rate of the islands in the study area (‰). 4.3.CorrelationAnalysisof Influencing Factors h 4.3.CorrelationAnalysisof4.3.CorrelationAnalysisofThe results of Influencingthe Influencing correlation Factors Factors analysis conducted on 7 selected years’ runoff and sediment discharge data are displayed in Figures 9 and 10. The results indicate that river island areas are TheThe results results of of the the correlation correlation analysisanalysis conduc conductedted on on 7 7selected selected years’ years’ runoff runoff and andsediment sediment dischargedischargeinversely data data areproportional displayedare displayed into Figures annualin Figures9 andrunoff 9 10and. and The 10. resultsTheproportional results indicate indicate to that annual riverthat river islandsediment island areas areasdischarge, are inverselyare with a proportionalinverselycorrelation proportional to coefficient annual runoff to of annual −0.68 and runoff proportionaland 0.79, and respectively.proportional to annual toWithin sediment annual a sedimentcertain discharge, range, discharge, with the a areawith correlation exhibiteda a coefficientcorrelationsmall ofdecrease− coefficient0.68 andwith 0.79,of the −0.68 respectively. increase and 0.79, in respectively. Withinrunoff aand certain Within a larger range, a certain decreased the arearange, exhibited thewith area the aexhibited smalldrop-in decrease a sediment withsmalldischarge. the increase decrease The in with runoffchange the and inincrease area a larger was in decreased runoffnoticeable and with adue larger the to drop-inthreedecreased great sediment withfloods the discharge. which drop-in occurred sediment The change during in that discharge. The change in area was noticeable due to three great floods which occurred during that areaperiod. was noticeable Consequently, due to three the greatarea floodswas more which affected occurred by during sediment that period. discharge Consequently, than runoff. the area Drastic period. Consequently, the area was more affected by sediment discharge than runoff. Drastic was morereduction affected of bysediment sediment discharge discharge preceded than runoff. the Drasticlack of reduction sediment of particles sediment which discharge are precededimportant to reductionisland’s growth. of sediment Especially, discharge since preceded many the big lack dams of sediment and reservoirs particles werewhich built are importantin the upper-stream, to the lackisland’s of sediment growth. Especially, particles which since aremany important big dams to and island’s reservoirs growth. were Especially, built in the since upper-stream, many big dams there was a direct and drastic reduction of sediment discharge and concentration. On one hand was andthere reservoirs was a weredirect builtand drastic in the reduction upper-stream, of sedime therent discharge was a direct and concentration. and drastic reduction On one hand of sedimentwas the continual washing of relatively clean water, and on the other hand was no net import of dischargethe continual and concentration. washing of Onrelatively one hand clean was water, the continualand on the washing other hand of relatively was no cleannet import water, of and on the othersediment, hand which which was noled led netto toa importnegative a negative of growth sediment, growth of the of which islands. the islands. led to a negative growth of the islands.

Figure 9. Changes of annual runoff and area. Figure 9. Changes of annual runoff and area.

Figure 9. Changes of annual runoff and area.

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Figure 10. Changes of annual sediment discharge and area. Figure 10. Changes of annual sediment discharge and area. 5. Discussion 5. Discussion 5.1. Description of the Islands and Analysis of River Island Area Changes 5.1. Description of the Islands and Analysis of River Island Area Changes Neglecting Xinjizhou and Xinshengzhou, which were originally only one island, the largest islandsNeglecting were Baguazhou Xinjizhou andand Jiangxinzhou.Xinshengzhou, Concerningwhich were their originally own areas,only one very island, little changethe largest was islandsobserved were during Baguazhou the past and 30 years. Jiangxinzhou. The area ofConcer thesening islands their decreased own areas, slightly, very butlittle were change relatively, was observedremained during stable. the past 30 years. The area of these islands decreased slightly, but were relatively, remainedBaguazhou, stable. located in the northwest of Qixia District, Nanjing City, has a plain-type sandbank classicallyBaguazhou, formed located by scoured in the and northw accumulatedest of Qixia silt. District, The island Nanjing is low City, and has flat a and plain-type its northwest sandbank part classicallyis a slightly formed higher by than scoured its southeast and accumulated area. Its groundsilt. The elevation island is low is between and flat 5.2 and and its 7.7northwest m (Wusong part isElevation) a slightly but higher mostly than under its 6.5southeast m. The area. part aboveIts ground 6.5 m iselevation found along is between Xiaojiang 5.2 River and in7.7 the m northwest.(Wusong Elevation)Established but islands mostly lie under at a higher 6.5 m. elevation The part than above building 6.5 m and is found pioneer along islands Xiaojiang [23]. Upon River reaching in the northwest.Baguazhou, Established the Yangtze islands River lie split at a into higher two elevation channels, than south building and north. and pioneer The south islands channel, [23]. Upon being reachingthe main Baguazhou, current stream, the Yangtze had a maximum River split depth into oftwo 35 channels, m. The northern south and channel, north. whichThe south is a tributary,channel, beinghad a the maximum main current depth ofstream, 10 m. had A primary a maximum and secondary depth of flood35 m. banksThe northern were built channel, around which the island. is a tributary,Most of the had secondary a maximum flood depth bank ofwas 10 m. along A primary the periphery and secondary and protected flood banks the island were frombuilt around general theflooding. island. TheMost top of markthe secondary of the primary flood bank flood was bank’s along anti-flood the periphery wall reached and protected up to 12.5 the misland and itfrom has generalthe ability flooding. to control The thetop worstmark floodingof the primary in a century. flood bank’s Hence, anti-flood flood banks wall play reached important up to 12.5 roles m in and the itstability has the of ability Baguazhou. to control the worst flooding in a century. Hence, flood banks play important roles in theJiangxinzhou, stability of Baguazhou. located to the west of Jianye District, Nanjing City, runs in a southwest to northeast direction.Jiangxinzhou, From north located to south, to the Jiangxinzhou west of Jianye has a District, length of Nanjing 12 km,from City, east runs to west,in a southwest an average to of northeast1.2 km and direction. it had an From average north area to ofsouth, 1406.86 Jiangxinzh hm2 overou thehas past a length 30 years. of 12 It km, was from formed east by to sedimentwest, an averageerosion andof 1.2 deposition km and it with had its an current average contour area of having 1406.86 taken hm2 form overduring the past the 30 Song years. Dynasty. It was formed Also called by sedimentPlum Island erosion because and of deposition its lanky plum with looking its current shape, contour Jiangxinzhou having holdstaken anform important duringposition the Song in Dynasty.development Also alongcalled andPlum across Island the because Yangtze of River.its lanky Since plum the looking Qing Dynastyshape, Jiangxinzhou when the island holds was an importantopened up position for development, in development Jiangxinzhou along hasand nowacro anss the established Yangtze agriculture River. Since and the tourism Qing Dynasty industry. whenConsequently, the island its was shore opened experienced up for development, a high degree Ji ofangxinzhou development, has factors now an which established greatly agriculture affected its andsurface tourism area. industry. Consequently, its shore experienced a high degree of development, factors whichQianzhou greatly affected is to the its east surface of Yangtze area. River’s main channel in Jianye District. It has a 30-year average areaQianzhou of 111.57 hm is 2towhich the east decreased of Yangtze on average River’s by main 13.37 channelper year. in Most Jianye area District. of the islandIt has is a still 30-year in its averagenatural state,area of covered 111.57 inhm fields2 which of wilddecreased reedand on average desolateh by beaches. 13.37‰ The per land year. above Most waterarea of level the island shows is still in its natural state, covered in fields of wild reed and desolate beaches. The land above water level shows distinct seasonal impacts. Since Qianzhou is off the beaten track, anthropogenic influences on the island have been minimal, with little change in area.

Sensors 2017, 17, 2213 15 of 20 distinct seasonal impacts. Since Qianzhou is off the beaten track, anthropogenic influences on the Sensors 2017, 17, 2213 15 of 20 island have been minimal, with little change in area. Xinjizhou (Xinshengzhou) has an average area of 1599.86 hm2 and it has shrunk by 448 hm2 in the Xinjizhou (Xinshengzhou) has an average area of 1599.86 hm2 and it has shrunk by 448 hm2 in the past 30 years, decreasing on on average average by by 8.62‰ 8.62 aa year.year. XinjizhouXinjizhou andand Xinshengzhou are located in the upstream of the Jiangsu section along the Yangtze Yangtzeh Rive River.r. A typical bottomland island with its southern bank belonging to Jiangning District and the northe northernrn bank belonging to Pukou District, Xinjizhou and Xinshengzhou were were once once linked together. During During the the early 80s when Yangtze River water potential changed, a a ditch of more than ten meters wide was dug, which resulted in the splitting of one island intointo two. two. After After a a catastrophic catastrophic flood flood within within the the Yang Yangtzetze Basin in 1998, 1998, people people on on the the island island were were forced forced to withdraw withdraw from from the the area. area. The The flood flood water water was was so so great great that that the the ditch ditch expanded expanded to toa mid-channel a mid-channel of moreof more than than 2000 2000 m long, m long, 200 200m wide, m wide, and and 10 to 10 14 to m 14 deep. m deep. The Thechange change of the of mid-channel the mid-channel before before and afterand afterthe flood the flood can canbeen been clearly clearly seen seen in inthe the images images of of 1998 1998 and and 1999 1999 by automatic contour-linecontour-line extraction method (Figure 11).11). At At present, present, the mid-ch mid-channelannel is still evolving and its width is increasing every year. The stability of of the the islands islands and and the the dikes dikes along along the the banks banks of of Yangtze Yangtze River River will will be be affected. affected. The security of the main channel of Yangtze RiverRiver will bebe threatened.threatened. Collapse and washing are the primary causes of these islands’ area decrease. A fe feww remediation projects over the years have slowed down the rate rate of of erosion erosion and and even even led led to to some some measure measure of of recovery. recovery.

(a) (b)

Figure 11. Before-and-after images of mid-channel between Xinjizhou and Xinshengzhou ((a) 7 Figure 11. Before-and-after images of mid-channel between Xinjizhou and Xinshengzhou February1998; (b) 19 December1999). ((a) 7 February1998; (b) 19 December1999). 5.2. Analysis of River Island Shape Changes 5.2. Analysis of River Island Shape Changes By comparing TM and ETM+ images at different times, changes on the islands’ shapes were By comparing TM and ETM+ images at different times, changes on the islands’ shapes were distinctly visible. The shape changes of the larger islands like Jiangxinzhou and Baguazhou were not distinctly visible. The shape changes of the larger islands like Jiangxinzhou and Baguazhou were very obvious. On the hand, smaller islands like Zimuzhou and Qianzhou displayed large not very obvious. On the hand, smaller islands like Zimuzhou and Qianzhou displayed large transformation as their heads eroded and vanished and their ends grew due to sediment deposits, transformation as their heads eroded and vanished and their ends grew due to sediment deposits, thus thus moving the entire body downstream. Due to the inflexion point around Baguazhou, the river moving the entire body downstream. Due to the inflexion point around Baguazhou, the river current current was weaker which resulted in a weakened scouring effect. The remaining six islands were all was weaker which resulted in a weakened scouring effect. The remaining six islands were all long long and narrow, and the protuberant banks trended toward smoothness because of long-term and narrow, and the protuberant banks trended toward smoothness because of long-term washing. washing. Most islands are located in the east of the channel (Figure 12). The split ratio of the western Most islands are located in the east of the channel (Figure 12). The split ratio of the western channel is channel is greater than the eastern channel, allowing more water to flow on the west side. Ships greater than the eastern channel, allowing more water to flow on the west side. Ships traveling along traveling along the west side caused waves lapping against the shoal and intensified the erosion of the west side caused waves lapping against the shoal and intensified the erosion of western banks. western banks. This led to these islands’ shapes to shift more on the western bank than on the This led to these islands’ shapes to shift more on the western bank than on the eastern banks. eastern banks.

Sensors 2017, 17, 2213 16 of 20

5.3. Factors Influencing of River Island Changes

5.3.1. Natural Factors Rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads [24]. This study processed the annual runoff and sediment discharge data during the period 1985–2015 as measured by the Datong hydrological station (Figures 13 and 14). The average annual runoff in the 30 years was measured at 8857.29 × 108 m3 with a small decreased rate of 10.384 × 108 m3 per year. The maximum runoff of 10,354.77 × 108 m3 and minimum runoff of 6671 × 108 m3 were recorded in 1999 and 2011, respectively. The average annual sediment discharge in the 30 years was 2.62 × 108 T and decreased drastically by 0.1108 × 108 T per year. The maximum sediment discharge of 4.19 × 108 T was recorded in 1990, whereas the minimum sediment discharge of 0.718 × 108 T was measured in 2011. Associating with correlation analysis of Sensorsinfluencing2017, 17 factors,, 2213 sediment discharge is the main cause for the area decrease of river islands.16 of 20

Figure 12. Map of the channel in NYR by band 4, Landsat 7 ETM+ acquired on 10 October 2000.

5.3. Factors Inﬂuencing of River Island Changes

5.3.1. Natural Factors Rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads [24]. This study processed the annual runoff and sediment discharge data during the period 1985–2015 as measured by the Datong hydrological station (Figures 13 and 14). The average annual runoff in the 30 years was measured at 8857.29 × 108 m3 with a small decreased rate of 10.384 × 108 m3 per year. The maximum runoff of 10,354.77 × 108 m3 and minimum runoff of 6671 × 108 m3 were recorded in 1999 and 2011, respectively. The average annual sediment discharge in the 30 years was 2.62 × 108 T and decreased drastically by 0.1108 × 108 T per year. The maximum sediment discharge of 4.19 × 108 T was recorded in 1990, whereas the minimum sediment discharge of 0.718 × 108 T was measured in 2011. Associating with correlation analysis of inﬂuencing factors, sediment discharge is the main cause for the area decrease of river islands. Sensors 2017, 17, 2213 17 of 20 Sensors 2017, 17, 2213 17 of 20 Sensors 2017, 17, 2213 17 of 20

Figure 13. Annual runoff of Datong hydrological station (108 m3). Figure 13. Annual runoff of Datong hydrological station (108 m3). Figure 13. Annual runoff of Datong hydrological station (108 m3). 4.5 4.54 y = -0.1108x + 4.3502 y = -0.1108x2 + 4.3502 3.54 R = 0.7845 R2 = 0.7845 3.53 2.53 2.52 1.52 1.51 0.51 Annual sediment discharge(10*8T) sediment Annual 0.50 Annual sediment discharge(10*8T) sediment Annual 5 7 9 1 3 5 7 9 1 3 5 7 9 1 3 5 0 8 8 8 9 9 9 9 9 0 0 0 0 0 1 1 1 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 5 7 9 1 3 5 7 9 1 3 5 7 9 1 3 5 8 8 8 9 9 9 9 9 0 0 0 0 0 1 1 1 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 Figure 14. Annual sediment discharge of Datong hydrological station (108 T). Figure 14. Annual sediment discharge of Datong hydrological station (108 T). 8 Apart fromFigure flow-sediment 14. Annual conditions, sediment discharge meteorological of Datong factors hydrological such as precipitationstation (10 T). [25–27], natural Apart from flow-sediment conditions, meteorological factors such as precipitation [25–27], factors such as vegetation coverage and vegetation type [28,29] and climatic factors such as the naturalApart factors from such flow-sediment as vegetation conditions, coverage and meteorological vegetation type factors [28,29] such and as climatic precipitation factors [25–27],such as monsoon, also influenced the evolution of river islands to a considerable degree. During times of thenatural monsoon, factors also such influenced as vegetation the evolution coverage of and river vege islandstation to type a considerable [28,29] and degree.climatic During factors times such ofas heavy precipitation, water in the lakes of an island will spill over and flow into Yangtze River, scouring heavythe monsoon, precipitation, also influenced water in the the evolution lakes of ofan river island islands will tospill a considerable over and flow degree. into DuringYangtze times River, of as it goes, forcing the loss of soil and shoals. However, as is apparent, vegetation serves a remarkable scouringheavy precipitation, as it goes, forcing water thein theloss lakes of soil of and an shoaislandls. willHowever, spill over as is andapparent, flow intovegetation Yangtze serves River, a function of water and soil retention. Comparatively, arbor is better than shrubbery and sod, mixed remarkablescouring as functionit goes, forcing of water the and loss soil of retention.soil and shoa Comparatively,ls. However, arboras is apparent, is better than vegetation shrubbery serves and a forest is better than pure forest in this respect. Good vegetation type and coverage can promote the sod,remarkable mixed forestfunction is betterof water than and pure soil forest retention. in th isComparatively, respect. Good arborvegetation is better type than and shrubbery coverage andcan stability of river islands. promotesod, mixed the forest stability is better of river than islands. pure forest in this respect. Good vegetation type and coverage can 5.3.2.promote Human the stability Factors of river islands. 5.3.2. Human Factors 5.3.2.With Human socio-economic Factors development, anthropogenic influence plays an increasingly significant role. BanksWith collapse socio-economic caused by over-exploitation development, anthropogenic and unreasonable influence land use plays always an happens.increasingly It threatens significant the safetyrole. WithBanks of the socio-economic collapse channel andcaused influences development, by over-exploitation the evolution anthropogenic of and river unreasonable influence islands. For plays example,land an useincreasingly since always 2011, happens. significant the Elwha It Riverthreatensrole. Banks dam the in collapse Washingtonsafety of caused the State channel by which over-exploitation and stored influences about 21thande million evolution unreasonable m3 ofof sedimentriver land islands. use was always removedFor example, happens. gradually, since It 3 and2011,threatens geomorphic the Elwhathe safety River evolution, of damthe channel ecosystemin Washington and and influences environmentState which the evolution stored along about the of riverriver 21 weremillionislands. deeply m For of affectedexample, sediment by since was the removed2011, the Elwhagradually, River and dam geomorph in Washingtonic evolution, State whichecosystem stored and about environment 21 million along m3 of the sediment river were was removed gradually, and geomorphic evolution, ecosystem and environment along the river were

Sensors 2017, 17, 2213 18 of 20 project [30]. More than 5000 reservoirs have been constructed along the Yangtze River basin since the 1950s [31], and the biggest key water control project in human history, the Three Gorges Reservoir, started to store water in 2003 when the water level in front of the dam was 135 m. In October 2010, the level reached 175 m for the ﬁrst time, which was the normal water level. According to the Changjiang Sediment Bulletin, sediment deposited in the Three Gorges Reservoir reached 0.449 × 108 T in 2014. The sediment delivery ratio of the reservoir was 19.0%. Thus, a strong link between the Three Gorges Reservoir and the great decrease of sediment discharge in the lower reaches of Yangtze River can be found. In addition, river channel sand mining activities in the middle and lower reaches also affected sediment transport [32,33]. A number of soil and water conservation projects have been conducted in Jiangsu Province and upstream Anhui Province in recent years. In 2013, NDRC (National Development and Reform Commission) approved a total investment of 7.21 million USD for the regulation project of Xinjizhou in NYR. The project is aimed at plugging the mid-channel between Xinjizhou and Xinshengzhou, thus curbing the trend of south channel’s increasing split ratio. It will weaken scouring in the mid-channel, and protect river channels and dikes along the two banks.

6. Conclusions Applying TM and ETM+ images of Nanjing City, this study takes advantage of threshold value method, binarization model and a series of image processing techniques to extract information about the areal changes of seven riverine islands in the lower reach of the Yangtze River over 30 years, split into seven 5 year time periods. The characteristics of their evolution and influencing factors were then analyzed. The conclusions are as follows: (1) Landsat TM and ETM+ images can meet the requirements to study ground features at certain time scales and spatial resolutions. They have good adaptability in extracting information from water and land area such as river islands. ETM+ images reconstructed by the SLC-off model can also meet research criteria. (2) The total area of the seven river islands in NYR exhibited a continuous decrease of 19.97 hm2 per year. However, a more rapid decrease of 25.8 hm2 per year was observed during the period 1985–2000, while a slower decrease of 4.13 hm2 per year was noted during the 2000–2015 period. The largest absolute value of the rate of area change was at Zihuizhou, which increased by 30.7 per year whereas the smallest recorded value was at Baguazhou with a recorded decrease of 0.66h per year. (3) Although all seven islandsh had different degrees of atrophy, the shape changes of the largest islands, Baguazhou and Jiangxinzhou, were not obvious. On the contrary, smaller islands like Zimuzhou and Qianzhou displayed significant changes. The heads of the river islands eroded away, vanishing as their tails grew, resulting in downstream migration. With the exception of Baguazhou, the remaining islands were all long and narrow because of the long-term washing. Their bank lines were all smoothed and the longitudinal axis direction of their bodies laid all along the Yangtze River’s direction. The erosion of the west bank was observed to be more intense than that of the east. (4) According to correlationanalysis, the island areas are inversely proportional to annual runoff with acorrelationcoefficient of −0.68, and proportional to annual sediment discharge with a correlationcoefficient of 0.79, that is, the area is more affected by sediment discharge than runoff. The evolution of river islands is simultaneously affected by both natural and anthropogenic factors. In particular, human activities changed the imports of exogenous sediment particles and are thus playing increasingly important roles in the evolution of these riverine islands.

Acknowledgments: C.D. and H.S. appreciate the supports from the National Natural Science Foundation of China (NSFC: 41476022, 41490643), Startup Foundation for Introducing Talent of Nanjing University of Information Science and Technology (2013r121, 2014r072), Program for Innovation Research and Entrepreneurship team in Jiangsu Province, and National Programme on Global Change and Air-Sea Interaction (No. GASI-03-IPOVAI-05). Sensors 2017, 17, 2213 19 of 20

C.G. and G.X. appreciate the support from NSFC (41571018). C.X. appreciates the support from The National Basic Research Program (973 Program) of China under contract No. 2014CB745004 and The National Key Research and Development Program of China (2016YFC0503602, 2016YFB0201103, 2017YFA0604101, 2017YFA0604104). Author Contributions: Haiyun Shi did literature search, collected data, processed data and wrote the paper. Chao Gao designed the study and participated in data processing. Changming Dong proposed the original idea and analyzed the processing results. Changshui Xia and Guanglai Xu participated in analyzing the processing results and contributed in the revision of the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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