Marine Geology 363 (2015) 202–219

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Marine Geology

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Morphological change in the Delta,

Wei Zhang a,b,c,⁎, Yang Xu b, A.J.F. Hoitink c,M.G.Sassid,JinhaiZhenga,b, Xiaowen Chen e, Chi Zhang a,b a State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China b College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, China c Hydrology and Quantitative Water Management Group, Department of Environmental Sciences, Wageningen University, Wageningen, Netherlands d Royal Netherlands Institute for Sea Research, NIOZ, Den Burg, Netherlands e Xijiang River Administration, , 519090, China article info abstract

Article history: Morphological changes in the (PRD) have been investigated using bathymetric charts, underwa- Received 2 July 2014 ter Digital Elevation Models, remote sensing data and Geographic Information Systems. Water depths were ex- Received in revised form 16 February 2015 tracted from digitized charts to explore the accretion–erosion characteristics of three estuarine environments, Accepted 21 February 2015 and to provide quantitative estimates of changes in sediment volumes. Multi-temporal satellite images have Available online 26 February 2015 been used, in combination with topographical data, to analyze the coastline changes. PRD has gained an abundant amount of sediment of almost 9.45 × 105 km3 above the 10-m isobath in the period roughly between 1970 and Keywords: 4 3 Morphological change 2010; the average sedimentation rate was 3.15 × 10 km /yr. Between 1976 and 2006, the coastline extended Sedimentation seaward by 579.2 m on average, with a mean net extension rate of 19.3 m/yr. The results suggest that the PRD Land reclamation experienced a major phase of accretion, with net erosion only in some local zones. Coastline extension, associated Coastline with major morphological changes, has accelerated in recent decades. Changes in boundary conditions, such as Pearl River Estuary sea-level rise, seem to have relatively minor impacts on the dramatic changes in the morphology of the estuaries. The seaward extension of the coastlines shows an increasing trend whereas the sediment supply from the delta displays a decreasing trend. A detailed comparative analysis demonstrates that land reclamation in the PRD is the most significant factor that progressively alters the delta morphology, overwhelming the effects of subsidence and sediment supply. © 2015 Elsevier B.V. All rights reserved.

1. Introduction system (Jabaloy-Sánchez et al., 2010) and the Estuary (Cui and Li, 2011). Syvitski and Saito (2007) selected a consistent database of The morphological changes in the hydrodynamical and hydrological fifty-one deltas, which covered the global parameter range of rivers enter- regime of rivers for estuarine zones are considered to be the cause of a ing all major coastal seas and oceans, to characterize key environmental major interference for estuarine zones, especially in morpho- and sedi- factors known to control delta morphology. The feedback of morphologi- ment-dynamics. The likely long-term impacts of physical factors con- cal changes may alter the morphodynamic regime. For instance, a delta trolling the morphological changes include sea-level rise (Ferla et al., may initially develop as a tide-dominated system, but gradually become 2007), wave fields (Pratolongo et al., 2010), tidal circulation (Wang a wave-dominated system as it progrades onto the open, continental and Townend, 2012), river flow (Inglis and Allen, 1957), sediment dis- shelf (Ta et al., 2002, 2005). Therefore, understanding the long-term mor- charge (Jiang et al., 2013) and storm surges (Riddin and Adams, phological changes in estuarine zones is of great value, not only because 2010). Intensive anthropogenic activities, such as channel dredging the knowledge provides insights to the historical characteristics of estuar- (Pinter et al., 2004), dam construction (Yang et al., 2006), sand excava- ies and facilitates the predication of future estuarine evolution, but also tion (Luo et al., 2007), and land reclamation (Chen et al., 2011)arealso because it enables researchers to assess the impact of natural changes significant causes for morphological changes. These factors were subject and anthropogenic activities on the coastal system. to study in a wide variety of delta and estuarine systems, including the Hydrodynamic models combined with sediment transport and a Senegal River Estuary (Barusseau et al., 1998), the Haringvliet Estuary morphodynamic module (‘bottom–up’ models) are widely used to pre- (Tönis et al., 2002), the Mersey Estuary (Blott et al., 2006; Thomas dict the short-term (hours to days) morphological changes in estuaries et al., 2002), the Ribble Estuary (van der Wal et al., 2002), the (Friedrichs and Aubrey, 1996; Green et al., 2000; Whitehouse, 2002; Estuary (Chu et al., 2013; Wang et al., 2008, 2013), the Adra River deltaic Hibma et al., 2004; Townend, 2005; Blott et al., 2006; Karunarathna et al., 2008). Geological and geomorphological evolution models (some- times referred to as ‘top–down’ models), such as the Historical Trend ⁎ Corresponding author at: State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China. Analysis (HTA) and the Expert Geomorphological Assessment (EGA), E-mail address: [email protected] (W. Zhang). are considered to be a more effective approach to estimate long-term

http://dx.doi.org/10.1016/j.margeo.2015.02.012 0025-3227/© 2015 Elsevier B.V. All rights reserved. W. Zhang et al. / Marine Geology 363 (2015) 202–219 203 morphological changes ranging from decades to a few hundred years et al., 2011; Duong et al., 2014; Fanget et al., 2014; Maselli et al., 2014; (Dennis et al., 2000; Tönis et al., 2002; Whitehouse, 2002; Hibma Samaras and Koutitas, 2014; Stollhofen et al., 2014; Tanabe et al., et al., 2004; Townend, 2005; Karunarathna and Reeve, 2008; 2003; Tessier et al., 2012). However, the booming economic develop- Karunarathna et al., 2008). However, top–down models have inherent ments have disrupted the natural balance of a large number of delta sys- limitations due to the lack of detailed physics (Prandle, 2004, 2006; tems worldwide and have caused the excessive exploitation of natural Townend, 2005; Blott et al., 2006; Wang et al., 2013). Therefore, the resources, e.g. Pearl River Delta (PRD). The PRD is located at the north- analyses of historical bathymetric charts, remote sensing data and GIS ern margin of the (Fig. 1). With an area of no more (RS-GIS) integration, field measurements and topographical surveys than 0.5% of China's territory, this densely populated delta has a mature are still indispensable when analyzing the morphological changes economy that produces approximately 20% of national GDP. The re- (Thomas et al., 2002; van der Wal et al., 2002; Blott et al., 2006; Jiang markable achievement doesn't come without cost. The morphology of et al., 2012; Wang et al., 2013). These approaches are particularly suit- the PRD has changed greatly due to intensive human interventions, able for the study of the long-term morphologic evolution in real- such as the building of coastal defense structures, sand mining and world delta regions (van der Wal et al., 2002; Prandle, 2004; Blott land reclamation. Zhang et al. (2012) report that the sediment et al., 2006; Wang et al., 2008, 2013; Li et al., 2011). transported from the upper river has decreased significantly due to Global delta areas are inhabited by almost two thirds of the world's the upper rivers' dam construction, which is very likely to cause coast- population. There is no doubt that the evolution of delta morphology is line recession. To satisfy the need of land, large-scale land reclamation closely intertwined with socio-economic development. Delta morphol- took place between the 1970s and the 2000s (Chen et al., 2011). This ogy evolves as the combined result of both natural- and human-factors. in turn led to coastline extension. The ongoing construction of the Most researches on delta morphology focused on the response of delta –Zhuhai–Macao Bridge will undoubtedly exert a significant morphology evolution to global changes in climate, continent-scale up- impact on the hydrodynamics, and change the geomorphologic evolu- lift and sea level (Amsler et al., 2005; Anderson et al., 2014; Bruneau tion in this region. Therefore, as a research paradigm, the PRD offers

Fig. 1. Geographic information of the study area with isobaths in colors including the following: (1) the map of China, which shows the location of the Pearl River; and (2) the major to- pographic features of the Pearl River Delta. The names of the three major rivers and eight outlets are shown on the map. Blue circles represent four metropolitan cities. 204 W. Zhang et al. / Marine Geology 363 (2015) 202–219 the opportunity to study morphological changes in delta regions under changes in this region. Details of 55 admiralty charts and 15 topographic rapid economic development around the world. The morphological maps are given in Tables 1 and 2 respectively. The surveys were mainly changes in the PRD increasingly attract substantial research efforts. In carried out during four periods: 1960s–1970s, 1980s, 1990s and 2000s. the Lingding Bay, the largest estuary of the PRD, coastline changes These periods represent minor intervention, strong intervention, mod- (Li and Damen, 2010), coastal landscape changes (Chen et al., 2005), erate intervention and regulation, respectively. The charts for each peri- morphologic evolution and hydrodynamic variation (Deng et al., od were a composition of several surveys. 2011) have been subjects of morphology research. These studies have Due to inconsistencies in the datum levels in relation to tides, there demonstrated that the position of the coastlines experienced large var- are several uncertainties and errors associated with the use of historical iations (Li and Damen, 2010), the waterways were getting narrower bathymetric charts of estuaries (van der Wal et al., 2002; van Der Wal (Chen et al., 2005) and the shallow sub-aqueous areas have expanded and Pye, 2003). To account for this, the water depths used in this to occupy deeper sub-aqueous area (Deng et al., 2011) due to land study were referenced to the Lowest Astronomical Tide (LAT). Using a reclamation. As a response of these changes, the tidal dynamics have similar approach, van der Wal et al. (2002) suggested that elevation changed significantly, which in turn led to the reduction of tidal power change greater than ±0.5 m between subsequent bathymetric surveys and to periodical changes in the direction of tidal current (Deng et al., can be considered significant. 2011). In the Huangmao Bay, which is located towards the southwestern The admiralty charts were digitized and analyzed using ArcInfo in a side of the PRD, researches showed that the estuary has transformed Geographical Information System (GIS). Data from the admiralty charts from a depositional basin into an eroding area from 1994 on (Gong and the topographic maps were also recorded, stored and analyzed in a et al., 2014; Jia et al., 2012), principally as a result of human activities. PC-based MapInfo Professional Geographical Information System to The triggering factors for the transition are sand mining along the tidal build an underwater Digital Elevation Model (DEM). Using the lati- river, dredging of the navigation channel and land reclamation along tude/longitude information, contours, spot heights, spot depths and es- the peripheral of the estuary (Jia et al., 2012). tuary outlines were geo-referenced into the UTM-WGS84 coordinates Previous studies in the PRD were limited to specific regions of the of China on the charts. Each set of spot heights and depths was interpo- PRD, i.e. Lingding Bay and Huangmao Bay as stated above. These studies lated to a grid DEM with 50 × 50 m cells. A Kriging approach with a lin- have mostly focused on temporal changes in tidal range variation ear variogram and best-fit slope was applied to interpolate the sparse (Zhang et al., 2010), water level fluctuation (Zhang et al., 2009), sedi- data (Burrough et al., 1998; Webster and Oliver, 2001). The DEM of ment transport (Hu et al., 2011), and morphological changes in the the PRD was built for the quantitative analysis of the isobaths evolution, upper parts of the river channel network caused by sand excavation scour and silting developments. Different periods of bathymetric (Luo et al., 2007). This is not the case in our study. Using the latest datasets were compared to calculate volumetric changes with the aid data, different regions of the PRD are taken into consideration in order of ArcGIS software through a cut-fill process. to gain a clear insight into the comprehensive long-term morphological changes in this region. The objectives of this paper are as follows: (1) to 3.2. Collection and processing of image data elucidate quantitatively the morphological changes in sediment vol- umes and coastlines in recent decades; and (2) to identify the main fac- Additional information on coastline changes was obtained from tors affecting the observed geomorphological changes. multi-temporal remote sensing data of satellite images from 1972 to 2008 for the entire PRD region (Table 3). The spatial resolutions are 2. Study area 30 m for ETM and TM images, and 80 m for MSS images. Using coordinate transformations, image mosaics and false color The PRD extends from 21°N to 24°′N, 112°E to 115°E, occupying an compositions, images were made better interpretable for analysis. A area of approximately 5.6 × 104 km2. The PRD contains four sub- second order polynomial was used to transform the column and line lo- estuaries, Lingding Bay, Modaomen, Jitimen and Huangmao Bay sub- cations of pixels in the satellite images' longitude and latitude locations, estuaries. The four estuaries receive discharge and sediment from the with the purpose of referring to the same geographical coordinates of West River, the North River and the East River through eight outlets. UTM-WGS84 of China. Then the test of different band false color combi- Four outlets (Yamen, Hutiaomen, Jitimen, and Modaomen) are located nations indicated that bands 3, 2, 1 for RGB of MSS images, bands 5, 4, 3 in the west, while the remaining four outlets (Hengmen, Hongqimen, for RGB of TM images and band 5, 4, 3 for RGB of ETM images can be ef- Jiaomen, and ) are located in the east. fective in identifying coastlines, river banks and vegetation. According The PRD is subject to a subtropical climate, with seasonal variations to existing knowledge about morphological features, and sediment or in temperature and precipitation due to large air–sea net heat flux ex- vegetation characteristics (color, water content and sediment type), changes. The annual precipitation of the PRD region varies between the mean high tide line as the coastlines were delineated manually by 1500 and 2000 mm and annual temperature is between 22 and the same person at the same scale to ensure accuracy from each com- 22.5 °C. The tidal regime in the PRD is mainly mixed semi-diurnal and bined MSS 321, TM 543 and ETM 543 pseudo-color image in this the tides propagate from offshore towards the estuary with a mean study. ENVI software was used to extract coastlines from the pre- tidal range between 1.0 and 1.7 m (Mao et al., 2004; Hu et al., 2011; processed satellite images. Then a haze reduction method was applied Zhang et al., 2013). The wet season mean circulation in the PRD estuar- as a means of atmospheric correction. The method of overlaying, ies is characterized by a two-layer estuarine circulation in the vertical, referencing and comparing coastlines relative to the initial 1960s– with a sharp salinity front near the mouth of the estuary (Ji et al., 1970s coastlines enable the precise calculation of recession or 2011a). The upper-layer circulation in the deep water outside the PRD progradation. The change rate of a coastline was calculated as the ratio can be considered barotropic during the dry season (Ji et al., 2011b). of enclosed area to the average coastline lengths.

3. Data and method 4. Result

3.1. Collection and processing of admiralty charts 4.1. Variations of geomorphology in the PRD

As a result of frequent offshore projects, bathymetric surveys in the Fig. 2 shows the bathymetric charts in the PRD, here used to estimate PRD have been carried out many times throughout the latest decades. the subaqueous morphological changes in the region. The coastlines The historical time series of admiralty charts and topographic maps based on measurements taken in the 2000s are used as a reference to are collected in this study to quantitatively estimate the morphological compare the variation of geomorphology in different decades. The W. Zhang et al. / Marine Geology 363 (2015) 202–219 205

Table 1 Table 2 Admiralty charts used in the research. Topographic maps used in this study.

No. Title Scale Published Surveyed No. Names Scale Year Datum plane

1 Jishuimen to Neilingding Island 1:25,000 1996 1989 1 Modaomen 1:10,000 1977 Zhujiang datum plane 2 Aizhou Islands to 1:30,000 2004 1995–2004 2 Dahengqin Island 1:25,000 1977 3 Port 1:10,000 1996 1964–1991 approaches 4 Macau Port and approaches 1:30,000 1996 1969–1991 3 Lingding Bay 1980s 1:10,000 1985–1986 5 Macau Port to Zhuhai Port 1:75,000 2004 1964–2003 4 Huangmao Bay 1:25,000 1989 6 Chisha Channel 1:10,000 1999 1991–1996 5 Xiaohengqin Island 1:10,000 1977 7 Dachan Basin 1:20,000 1994 1989–1991 approaches 8 Dachan Island and approaches 1:30,000 2004 1998–2003 6 Modaomen 1980s 1:10,000 1983 9 Dawanshan Island and approaches 1:25,000 1993 1964–1990 7 Macau to Hong Kong 1:75,000 1984–1986 Macao datum plane 10 Dangan Island to Sanzao Island 1:120,000 2004 1995–2004 8 Macau 2004 11 Dongao Island to Jiuzhou Islands 1:30,000 2004 1998–2003 9 Lingding Bay 1990s 1:10,000 1998 1985 National height 12 Fanshi Channel and approaches 1:30,000 2004 1998–2003 10 West Shoal 1:10,000 2000 datum 13 Gaolan Islands 1:30,000 1984 1963, 1964, 11 , Hong Kong 2004 1977 and Macau Bridge 14 Gaolan Islands and approaches 1:60,000 1970 1963–1964 12 Jitimen 2000 1:10,000 2000 15 Guishan Island and approaches 1:25,000 1995 1964–1991 13 Modaomen 1:10,000 2000 16 Guishan Island to Neilingding Island 1:50,000 1996 1964–1992 14 Yamen outfall waterway 1:60,000 2003 Theoretical bathymetrical 17 Guishan Island to Neilingding Island 1:50,000 1985 1962–1977 datum 18 Guishan Island to Shajiao 1:75,000 2003 1998–2002 15 Jitimen 1990 1:10,000 1990 Yellow Sea height datum 19 Hengmen and approaches 1:25,000 1994 1974–1989 20 Humen and approaches 1:12,500 1997 1989–1994 21 Humen Channel 1:10,000 2001 2001 Jiaomen (JM) (Fig. 3a). The morphology of the West Shoal changed as 22 Huangpu to 1:30,000 2004 2000–2001 23 Huangpu to Nizhoutou 1:25,000 1999 1962–1996 a result of deposition of about 1 m during the pre-1990s period and 24 Jiuzhou Port channel (2) 1:10,000 1993 1990 strong scour area during the post-1990s period (Fig. 3a and b). In gener- 25 Jiuzhou Port and approaches 1:40,000 1997 1964–1991 al, the depth of the West Shoal decreased between the 1960s and the 26 Mawan to Shekou 1:15,000 2001 2001 2000s, except for the eastern area of the shoal (Fig. 3c). The West Chan- 27 Mayoushi to Neilingding Island 1:30,000 2004 1996–2003 28 Mayoushi to Xiaochan Island 1:40,000 2004 1996–2003 nel was deepened, with erosion of over 1 m in the upper and middle 29 Neilingding Island 1:10,000 1994 1964–1989 parts, and relatively minor erosion in the lower part of the channel dur- 30 Neilingding Island to Humen 1:50,000 1996 1975–1993 ing the pre-1990s (Fig. 3a). The West Channel was deepened further in 31 Neilingding to Shajiao 1:50,000 1989 1989 the upper and lower parts during the post-1990s period (Fig. 3b). Par- 32 Neiningding to Gulf 1:25,000 1989 1989 ticularly, erosion during this period took place at the top of the upper 33 Nizhou Channel 1:10,000 2001 2000–2004 34 Nizhoutou to Dahaozhou 1:30,000 2004 1997–2003 part, where the Chuanbi Waterway is located. From the 1960s to the 35 Nizhoutou to Dahushan 1:12,500 1997 1978–1991 2000s, the West Channel experienced sustainable erosion (Fig. 3c). 36 Nizhoutou to Shanbanzhou 1:25,000 1998 1978–1994 The Middle shoal displayed a depositional feature during the pre- 37 Qi'ao Island to Xiangzhou Gulf 1:25,000 1993 1964, 1977, 1990s period (Fig. 3a). Remarkable deposition over 2 m took place in 1989 38 Yamenwaikou 1:30,000 1982 1977 the southeastern region, and erosion occurred simultaneously in the 39 Sanzao Island and approaches 1:30,000 1985 1963, 1964, western area of the Middle Shoal, as well as in the eastern area of the 1977 Neilingding Island. The sediment volume continued to accumulate in 40 Shanbanzhou approaches 1:10,000 1997 1989–1994 the southeastern area during the post-1990s (Fig. 3b). The Middle 41 Shanbanzhou to Dahu Island 1:30,000 2004 1997–2003 42 Shekou Port to Mawan Port 1:15,000 1999 1994–1995 43 1:12,500 1999 1996 Table 3 44 Wanshan Islands and approaches 1:60,000 1970 1964–1965 List of remote sensing images. xiaoputai Data product Image type Acquisition time Acquisition data 45 Hong Kong to Shangchuan Island 1:150,000 1971 1963–1970 46 Xiangzhou Gulf to Qi'ao Island 1:30,000 2004 1998 International Scientific Data Service Platform 47 Xiaojin Island to Huangchengshan 1:120,000 1992 1961–1987 GLS1975 MSS 1972–1987 p130/r044_19790930 48 Xiaojin Island to Mangzhou 1:75,000 1992 1961–1987 p130/r045_19731031 49 Xiaoputai Island to Xiaojin Island 1:75,000 1996 1963–1991 p131/r044_19791019 50 Xiaoputai to Shanbanzhou 1:75,000 1968 1964–1965 p131/r045_19731225 51 Yamen Channel 1:30,000 1998 1988 p132/r044_19791020 52 Yantian Port to Shekou Port 1:75,000 2003 1995–2003 p132/r045_19791020 53 Zhuhai Port 1:20,000 2001 2001 GLS1990 TM 1984–1997 p121/r044_19911009 54 Pearl River Estuary 1:150,000 2004 1995–2003 p121/r045_19891120 55 Pearl River Estuary and approaches 1:150,000 1997 1964–1991 p122/r044_19901013 p122/r045_19951230 p123/r044_19910921 p123/r045_19900902 analyses of the bathymetric changes over the eastern region (E), the GLS2000 ETM+ 1999–2003 p121/r044_20011231 middle region (M) and the western region (W) of the PRD were per- p121/r045_20011231 formed separately (Figs.3,4and5). p122/r044_20000914 In the eastern region (Fig. 3), the subaqueous topography of p122/r045_19991115 p123/r044_19991224 Lingding Bay essentially maintains a three-shoal and two-channel con- p123/r045_19991224 figuration in recent decades: West shoal (WS), West Channel (WC), Middle Shoal (MS), East Channel (EC) and East Shoal (ES) from the GLCF of Maryland University GLS2005 TM, ETM+ 2003–2008 p121/r044_20041012 west to the east, although some parts of shoals were eroded and chan- p121/r045_20041020 nels deepened. The West Shoal experienced both erosion and deposi- p122/r044_20051123 tion in its internal areas, causing the differentiation of secondary p122/r045_20070918 shoals and channels. Two secondary channels (SC) were formed to the p123/r044_20061219 p123/r045_20061219 southeastern ends of the Hengmen (HM) and the south branch of the 206 W. Zhang et al. / Marine Geology 363 (2015) 202–219

Fig. 2. Selected bathymetry charts of the Pearl River Estuary from 1960s to 2000s: (a) 1960s and 1970s, (b) 1980s, (c) 1990s, and (d) 2000s. Selected 3 regions for geomorphology study including the following: E (the eastern region), M (the middle region), and W (the western region). In the other three charts, the position of selected regions is same as (a).

Fig. 3. Bathymetric changes (depth of later time minus that of the previous time) in the Eastern area in the following periods: (a) 1960s–1990s, (b) 1990s–2000s, and (c) 1960s–2000s. Note the significant changes of the eastern region in the 2000s when compared to the 1960s and 1970s depths. The names of the channels and shoals are shown in the chart of (a). The area subenvironments are indicated: West Shoal (WS), West Channel (WC), Middle Shoal (MS), East Channel (EC), East Shoal (ES), and Secondary Channel (SC),ChuanbiWaterway(CB).The names of outlets and islands are as follows: Hengmen (HM), Hongqimen (HQM), Jiaomen (JM), Qi'ao Island (QA), Neilingding Island (NLD), and Lantau Island (LT). Contours are in 0.1 m intervals in figures. W. Zhang et al. / Marine Geology 363 (2015) 202–219 207

Fig. 4. Bathymetric changes (depth of later time minus that of the previous time) in the Middle area in the following periods: (a) 1960s–1990s, (b) 1990s–2000s, and (c) 1960s–2000s. Note the significant changes of the middle region in the 2000s when compared to the 1960s and 1970s depths. The names of the channels and shoals are shown in the chart of (a). The area subenvironments are indicated: MW (Modaomen Waterway), BW (Bailong Waterway). Contours are in 0.1 m intervals in figures.

Shoal grew larger and expanded to the southeast between the 1960s In the western region, the West Shoal of the Huangmao Bay and the 2000s (Fig. 3c). In the East Channel, deepening took place in aggraded during the pre-1990s (Fig. 5a), while erosion of over 1 m oc- the middle and lower parts, and minor deposition occurred in the curred at the top of the shoal in the post-1990s (Fig. 5b). The West upper part of the channel during the pre-1990s period (Fig. 3a). The de- Shoal continued to expand to the West Channel, and the channel position could be detected in the middle part, and minor erosion of less narrowed as a result. Notable deposition of over 2 m took place in the than 1 m appeared in the upper part of the East Channel during the upper part of the Main Channel in the Huangmao Bay and minor erosion post-1990s period (Fig. 3b). Considerable erosion and deposition oc- occurred in other parts during the pre-1990s (Fig. 5a). Erosion exceed- curred simultaneously in the lower part, featuring changes exceeding ing 1 m occurred in the upper part of the channel during the post-1990s 2 m. The East Channel was deepened, with deposition in the upper period (Fig. 5b). The depth of the upper part of the Main Channel de- and middle-upper parts of the channel, and deposition and erosion oc- creased and erosion continued to occur in the middle and lower parts curred in the lower part during the period between the 1960s and the between the 1960s and the 2000s (Fig. 5c). The East Shoal experienced 2000s (Fig. 3c). The East Shoal was largely in equilibrium (Fig. 3). notable deposition during the pre-1990s, especially at the north edge of In the middle region, there was erosion of over 1 m in the the shoal where the accumulations reached 5 m (Fig. 5a). There were Modaomen Waterway and deposition of over 2 m in the Bailong River detectable depositions in the northern part and slight erosion in the Waterway during the pre-1990s period (Fig. 4a). The river-mouth bar southern part of the East Shoal during the post-1990s (Fig. 5b). Conse- expanded seaward, showing large-scale deposition. There was deposi- quently, the East Shoal continued to maintain deposition of over 5 m tion exceeding 1 m in the shoal at the eastern Bailong Waterway, during the whole period (Fig. 5c). There was deposition of less than which occurred mainly during the post-1990s (Fig. 4b). During this pe- 1 m in the Dahuanhai Shoal between the 1960s and the 2000s (Fig. 5). riod, the Modaomen Waterway continued to deepen, but deposition oc- Deposition took place in the East Channel during the pre-1990s period, curred in some local areas of the waterway. The Modaomen Waterway and erosion occurred during the post-1990s. The morphodynamic re- deepened whereas the Bailong River Waterway became shallower, and gime around islands in the western region changed from erosion during sediment accumulated over 2 m in the western regime of the middle the pre-1990s to deposition during the post-1990s. In the Jitimen area, area between the 1960s and the 2000s (Fig. 4c). At the same time, the large-scale deposition occurred during the post-1990s period (Fig. 5). river-mouth bar experienced slight deposition and expanded outward The Main Channel in this region deepened during the pre-1990s, but in different directions. showed deposition of over 2 m in the post-1990s.

Fig. 5. Bathymetric changes (depth of later time minus that of the previous time) in the Western area in the following periods: (a) 1960s–1990s, (b) 1990s–2000s, and (c) 1960s–2000s. Note the significant changes of the western region in the 2000s when compared to the 1960s and 1970s depths. The names of the channels and shoals are shown in the chart of (a). The area subenvironments are indicated: West Shoal (WS), West Channel (WC), Main Channel (MC), and East Shoal (ES), Dahuanhai Shoal (DS), and East Channel (EC). The names of outlets and islands are as follows: DJ (Dajin Island), DM (Damang Island), HB (Hebao Island) and GL (Gaolan Island). Contours are in 0.1 m intervals in figures. 208 W. Zhang et al. / Marine Geology 363 (2015) 202–219

4.2. Variations in sediment volume can be used to estimate the extension and recession of the shallow coastal plain. The 5-m isobaths were located along the channels and at Since nearly all 2-m isobaths were located along the coastlines and the border of shoals and channels (Fig. 6b), which reflect the interaction the edges of the coastal shoal (Fig. 6a), variations of the 2-m isobaths between shoals and channels. The 10-m isobaths were mainly located

Fig. 6. Distribution of isobaths in the Pearl River Estuary of 1960s–1970s, 1990s and 2000s including (a) 2-m isobaths and (b) 5-m isobaths. 2 m isobaths in figure (a) are divided into 5 subregions: the Lingding Bay (L), Modaomen (M), Jitimen (J), Huangmao Bay (H), and Open-sea (O). 5-m isobaths in figure (b) are divided into 3 subregions: the Lingding Bay (L), Western area (W), and Open-sea (O). The legend in each figure describes the colored lines used to differentiate study periods. W. Zhang et al. / Marine Geology 363 (2015) 202–219 209

Fig. 7. Chart (a) shows sediment volume changes of the Pearl River Estuary above 2-m, 5-m and 10-m isobath from 1960s to 2000s. Chart (b) shows erosion (negativevalues)anddepo- sition (positive value) rates of the Pearl River Estuary above 2-m, 5-m and 10-m isobath from 1960s to 2000s. The names of 5 subregions are as follows: the Huangmao Bay (H), Jitimen (J), Modaomen (M), Lingding Bay (L), and Open-sea (O). T means the changes in the overall area. The error bar is ±1SD (standard deviation). 210 W. Zhang et al. / Marine Geology 363 (2015) 202–219

Fig. 8. Coastline evolution in the Pearl River Estuary for the period 1970s to 2000s. Colored lines represent coastline changes in different periods. Four selected submaps are given in Figs. 11–14. along the deeper channels at the bay mouth. Variations of the 10-m 5 m, and 2 m can be seen in Fig. 7. Changes in the sediment volume isobaths represent the extension and recession of the entire estuary. show that the entire area above the 10-m isobath received a net amount Thus the extension and recession of the shoals and the shifts of the of 9.45 × 105 km3 sediment over the whole period (1970s–2000s), with channels in the PRD can be synoptically revealed through the changes an average sedimentation rate of 3.15 × 104 km3/yr. The amount of sed- in isobaths of 2 m, 5 m and 10 m. imentation was 8.38 × 105 km3 over the area between 5-m and 10-m To quantify the sediment volume, the PRD region in the study was di- isobaths with an average sedimentation rate of 2.79 × 104 km3/yr. The vided into five subregions (Fig. 6): Lingding Bay (L), Modaomen (M), sedimentation rate displayed an increasing trend from the 1970s to Jitimen (J), Huangmao Bay (H), and open-sea (O). Sediment volumes of the 2000s. The amount of sedimentation in the area between the 2-m the subregions and that of the total region were calculated based on the and 5-m isobaths was small in comparison to areas above the 5-m DEMs to illustrate the erosion or sedimentation in the selected areas. and 10-m isobaths, but still reached 5.35 × 105 km3 with an average The net changes in the sediment volume between the isobaths of 10 m, sedimentation rate of 1.78 × 104 km3/yr.

Table 4 Coastline changes of the overall estuary.

Offshore Length (m) Length change (m) Growth rate (m/yr) Area change (m2) Distance of extension (m) Extension rate (m/yr)

1976 1.076 × 106 1993 1.124 × 106 4.758 × 104 2.799 × 103 3.801 × 108 344.7 20.3 2000 1.172 × 106 4.874 × 104 6.963 × 103 1.763 × 108 153.4 21.9 2006 1.182 × 106 0.991 × 104 1.652 × 103 0.955 × 108 81.1 13.5 Total 10.623 × 104 3.541 × 103 6.519 × 108 579.2 19.3

Open sea Length (m) Length change (m) Growth rate (m/yr) Area change (m2) Distance of extension (m) Extension rate (m/yr)

1976 3.876 × 105 1993 4.204 × 105 3.275 × 104 1.927 × 103 8.269 × 106 20.4 1.2 2000 4.105 × 105 −0.981 × 104 −1.402 × 103 0.749 × 106 1.8 0.3 2006 4.051 × 105 −0.540 × 104 −0.899 × 103 −0.264 × 106 −0.6 −0.1 Total 1.754 × 104 0.585 × 103 8.754 × 106 21.6 0.7 W. Zhang et al. / Marine Geology 363 (2015) 202–219 211

Fig. 9. Coastline changes in the Lingding Bay during the 1979 to 2005 period. The most distinct changes in coastline can be observed along the Jibaosha, Wanqingsha and Hengmen.

There are some differences in the sediment volume changes in indi- indicated that deposition occurred in the area between the 5-m vidual subregions. Fig. 7 shows that subregion L experienced deposition and 10-m isobaths. During the period 1990 to 2010, the zone above above the 2-m, 5-m and 10-m isobaths over the study period. The sed- the 5-m isobath experienced slight accumulation, implying erosion iment volume above the 10-m isobath was smaller than that above the between the 10-m and the 5-m isobaths and accumulation between 5-m isobath, which indicates that erosion occurred in the area between the 5-m and 2-m isobaths. the 5-m and 10-m isobaths. In contrast, the area between the 2-m and The accretion rate maintained a high value above the 10-m and the 5-m isobaths experienced steady accumulation, as the sediment volume 5-m isobaths in subregion J between the 1970s and the 2000s above the 5-m isobath was larger than that above the 2-m isobath. The (Fig. 7b). It is worth mentioning that the sediment volume above the sedimentation rate above the 2-m, 5-m and 10-m isobaths reduced over 2-m isobath was larger than that above the 5-m and the 10-m isobaths time (Fig. 7b). between the 1990s and the 2000s. This indicates erosion in the area be- Changes in the sediment volume in subregion M showed an erosion tween the 10-m and the 2-m isobaths. The average sedimentation rate trend above the 2-m and 10-m isobaths during the study period. The of subregion J above the 10-m and the 5-m isobaths reduced over sediment volume above the 5-m isobath, however, fluctuated with re- time, while the sedimentation rate above the 2-m isobath increased cession and extension alternating in different periods. The calculated during the study period. During the whole period in subregion H erosion volume in the zone above the 10-m isobath turned out to above the 10-m and the 5-m isobaths, the volume of sediment in- be smaller than that above the 5-m isobath before the 1990s, which creased. The zone above the 2-m isobath, however, experienced steady

Table 5 Coastline changes in the Lingding Bay water area.

Offshore Length (m) Length change (m) Growth rate (m/yr) Area change (m2) Distance of extension (m) Extension rate (m/yr)

1979 6.360 × 105 1990 6.796 × 105 4.352 × 104 3.956 × 103 1.217 × 108 185.0 16.8 2000 7.242 × 105 4.464 × 104 4.464 × 103 1.172 × 108 167.0 16.7 2005 7.321 × 105 0.790 × 104 1.580 × 103 0.610 × 108 83.8 16.8 Total 9.605 × 104 3.694 × 103 3.000 × 108 436.8 16.8 212 W. Zhang et al. / Marine Geology 363 (2015) 202–219

Fig. 10. Coastline changes of Modaomen during the 1973 to 2007 period. The most distinct changes in coastline can be observed at the Southern Hezhou. accumulation before the 1990s and slight erosion afterwards. There was stable extension rate of 16.8 m/yr over the study period (Table 5), less significant accumulation in the area between the 2-m and the 5-m than the average extension rate of the whole PRD region. The length isobaths after the 1990s. of coastline, however, showed dramatic changes. The growth rate of the length of coastline was 4.46 m/yr during the period from 1990 to 4.3. Changes in the coastlines 2000, but decreased to 1.58 m/yr from 2000 to 2005. Assuming that the initial area of every subregime is 0, the result of the analysis of the Fig. 8 shows variations of the coastlines of the PRD during the period Lingding Bay subregime showed a positive relationship between the from 1976 to 2007. The coastlines were classified as offshore coastlines length of coastline and the extension area during the study period or open-sea island coastlines. The offshore coastlines of the PRE extend- (Fig. 13a). It indicates that the land area increment increased as coast- ed by 579 m seaward with a net extension rate of 19.3 m/yr, and lines increased. the length of coastlines increased by about 10.62 km accordingly In subregion M, dramatic coastline changes occurred mainly at (Table 4). The average extension rates of the offshore coastlines in Southern Hezhou (Fig. 10), caused by land reclamation taken place be- different periods are similar, 20.3 m/yr between 1976 and 1993, tween 1973 and 1995. The region adjacent to the eastern part of 21.9 m/yr between 1993 and 2000, and 13.5 m/yr between 2000 and Island was originally two small islands, which were united in 2006. The open-sea island coastlines as a whole extended by 22.2 m sea- 1973 due to reclamation activities and sedimentation processes. The ex- ward from 1973 to 2000, and retreated by an insignificant distance of tension distance of subregion M was almost 816.5 m in total during the 0.6 m in the following period, until 2006. study period, with a mean extension rate of 24 m/yr. This figure is much The coastlines of four subregions, i.e. Lingding Bay (L), Modaomen larger than the average rate of the whole region between 1979 and 2005 (M), Jitimen (J) and Huangmao Bay (H), were analyzed separately to (Table 6). In the period from 1973 to 1995, the coastline extended sea- detect local changes in coastlines. In subregion L, the most distinct ward most intensely by 807.3 m with an average rate of 36.7 m/yr. The changes in coastlines can be observed along Jibaosha, Wanqingsha and extension distance amounted to 52.6 m for the period between 1995 Hengmen (Fig. 9). The coastline of this region moved seaward with a and 1999 and 36.5 m between 1999 and 2007, with growth rates of

Table 6 Coastline changes in the Modaomen water area.

Offshore Length (m) Length change (m) Growth rate (m/yr) Area change (m2) Distance of extension (m) Extension rate (m/yr)

1973 1.348 × 105 1995 1.735 × 105 3.873 × 104 1.760 × 103 12.446 × 107 807.3 36.7 1999 1.833 × 105 0.982 × 104 2.454 × 103 0.940 × 107 52.6 13.2 2007 1.844 × 105 0.102 × 104 0.128 × 103 0.671 × 107 36.5 4.6 Total 4.957 × 104 1.458 × 103 14.056 × 107 816.5 24.0 W. Zhang et al. / Marine Geology 363 (2015) 202–219 213

Table 7 Coastline changes in the Jitimen water area.

Offshore Length (m) Length change (m) Growth rate (m/yr) Area change (m2) Distance of extension (m) Extension rate (m/yr)

1973 1.005 × 105 1995 0.758 × 105 −2.463 × 104 −1.120 × 103 4.457 × 107 505.7 23.0 1999 0.728 × 105 −0.304 × 104 −0.759 × 103 0.578 × 107 77.9 19.5 2007 0.761 × 105 0.329 × 104 0.411 × 103 0.855 × 107 114.9 14.4 Total −2.438 × 104 −0.717 × 103 5.891 × 107 746.1 21.9

13.2 m/yr and 4.6 m/yr, respectively. The changes of coastline length the rising sea level can flood more land and erode the coast, causing re- showed similar trends as the extension distances. Fig. 13b shows the treat. Contrary to this, most coastlines of the PR display a seaward ex- positive relationship between the coastline and the areas of the tending tendency. The calculation indicates that the coastline of Modaomen subregime. When the length of coastline increased, the ex- tension area increased. Regarding subregion J, the coastline extended seaward by 746.1 m in total, with a mean net extension rate of 21.9 m/yr (Table 7). It is worth noting that the coastline changes were concentrated in the land reclama- tion area near the northern Sanzao Island (Fig. 11). Sanzao Island, Nanshui Island and Gaolan Island were originally separate islands, and were merged with the main land as a result of land reclamation in the 1970s. During the periods from 1973 to 1995, 1995 to 1999 and 1999 to 2007, the coastline of this region extended seaward by 505.7 m, 19.5 m and 114.9 m, corresponding to 23 m/yr, 19.5 m/yr and 14.4 m/yr, respective- ly. A negative relationship between the coastline and the land area of the Jitimen subregime was found (Fig. 13c). The length of the coastline did not increase with the coastline's dramatic seaward extension. The length of the coastline decreased by 27.67 km during the most distinct seaward extension period (1973 to 1999), as the curvy coastlines be- came straighter due to land reclamation. In subregion H, the coastline changes were concentrated in the vi- cinity of Naishui Island and Yamen (Fig. 12). Coastlines in this region ex- perienced continuous extension by 676.9 m in total over the 34 years (1973–2007), implying a net accretion rate of 19.9 m/yr (Table 8). The largest extension rate occurred in the period between 1995 and 1999, reaching peaks of 51.5 m/yr. This is three times more than the rate be- tween 1973 and 1995 and five times more than that between 1999 and 2007. The coastlines of open-sea islands (Shangchuan Island) expe- rienced extension and recession before and after 1995, corresponding to mean rates of 1.1 m/yr and −0.5 m/yr, respectively. The negative rela- tionship between the coastline and the land area of the Huangmao Bay subregime is presented in Fig. 13d. This means that the area of land became larger as the length of coastline became shorter. The length of the coastline of subregion H continued to decrease between 1973 and 2007, as the coastlines became straighter.

5. Discussion

5.1. Sea-level rise

Based on a recent global analysis, Syvitski et al. (2009) demonstrated that rates at which river deltas are sinking generally exceed rates of sea- level rise. They classify the Pearl River Delta as a delta in great peril, with virtually no aggradation and/or very high accelerated compaction. This is based on a reported early-twentieth-century aggradation rate of 3 mm/yr, a twentieth century aggradation rate of 0.5 mm/yr, and a rel- ative sea-level rise of 7.5 mm/yr. This figure for sea-level rise seems large in comparison to that of China, which is given in a 2012 report on China's sea-level compiled by the State Oceanic Administration (SOA). This report suggests that the sea-level along China's coastlines has risen by 2.9 mm per year over the past three decades. Focusing on the South China Sea coast, the mean sea level exhibited a fluctuating in- creasing tendency and the value in 2012 was about 136 mm higher than the sea-level averaged over the period 1975 – 1986. SOA also predicted that the mean sea levels along the coastlines of South China would prob- Fig. 11. Coastline changes of the Jitimen during the 1973 to 2007 period. The most distinct ably rise by 60–130 mm in the next 30 years. Under natural conditions, changes in coastline can be observed at the northern Sanzao Island. 214 W. Zhang et al. / Marine Geology 363 (2015) 202–219

Pearl River has experienced two stages (Zhang et al, 2012). The sedi- ment supply displayed an increasing trend in the pre-1980s. The trend, however, began to decrease in the 1980s and dropped signifi- cantly since the 1990s, due to the dramatic increase of large dam constructions. The sediment supply from the upper Pearl River was merely 40.70 × 106 ton/yr up till 2000s, which was about half of that in the 1980s (Zhang et al., 2011). The changes in sediment volume below the 10-m isobath in the subregions of the Lingding Bay, the Modaomen, the Jitimen, the Huangmao Bay, and open-sea are 1.56 × 108 m3, 1.20 × 108 m3, −0.78 × 108 m3,5.73×108m3, 1.75 × 108 m3 and 9.46 × 108 m3. It can be noticed that sediment supply in most subregions increased with the exception of the Modaomen, be- cause the water area of this region decreased. The coastlines of the PRD of the same period extended seaward obviously. The increment in sed- iment volume and extending coastlines of the subregions don't agree with what could be expected from a reduced sediment supply from the upper Pearl River, indicating that the sediment supply is not the main factor controlling the long-term morphological changes of the PRE. Again, the human control on sediment budgets is dominant.

5.3. Land reclamation

The large-scale land reclamation projects in the PRD, as revealed in the previous section, may be seen as an activity increasing the risk of flooding of the delta, as the total length of coastline that is to be protected would increase. The analysis carried out in the Jitimen subre- gion shows that this is not systematically the case, because in this sub- region, land reclamation did not increase the length of the coastline. Around 59 km2 of land was realized while reducing the length of the coastline by 25 km. Given that a new coastal defense may be designed such that it outperforms the historical coastal defense in terms of pro- tection against floods, the geomorphological trends as described cannot be easily translated into an increased flood risk. Land reclamation changes the length of the coastline and the shape of land or an island in a direct manner. It extends the river mouth Fig. 12. Coastline changes of the Huangmao Bay during the 1973 to 2007 period. The most while decreasing the river gradient and the associated sand and silt- distinct changes in coastline can be observed along the Naishui Island and the Yamen carrying capacity of the channel flow. Consequently, it may be expected outlet. that more sediment would be deposited in the estuaries, depending on extension rates. Over the study period, the mean net seaward extension rates of the coastline of the PRE were 20 m/yr, 22 m/yr, 24 m/yr, 17 m/yr subregions Lingding Bay, Modaomen, Jitimen and Huangmao Bay ex- in subregions Huangmao Bay, Jitimen, Modaomen, and Lingding Bay tended by 436.8 m, 816.5 m, 746.1 m and 676.9 m respectively. In the respectively. It is obvious that they are much faster than extension PRD, human influence thus overwhelms the natural coastal response rates by natural aggradation processes. From the analysis of Table 9, to sea-level rise. during 1978–2003 5.6 × 104 hm2 of land was reclaimed, including 2.6 × 104 hm2 in Lingding Bay, 1.5 × 104 hm2 in the Modaomen water 5.2. Sediment supply from upstream area, and 1.5 × 104 hm2 in the Jitimen water area and Huangmao Bay (Chen et al., 2011). Consequently, the coastline extended by 579.2 m Sediment supply plays an important role in changing estuarine geo- seaward during 1976–2006 (Table 4), which may significantly affect morphology. Over the latest decades, sediment supply from the upper channel hydraulic properties.

Table 8 Coastline changes in the Huangmao Bay water area.

Offshore Length (m) Length change (m) Growth rate (m/yr) Area change (m2) Distance of extension (m) Extension rate (m/yr)

1973 2.310 × 105 1995 2.205 × 105 −1.046 × 104 −475.6 8.419 × 107 373.0 17.0 1999 2.164 × 105 −0.411 × 104 −1027.6 4.499 × 107 205.9 51.5 2007 2.122 × 105 −0.419 × 104 −524.3 1.940 × 107 90.5 11.3 Total −1.877 × 104 −552.0 14.858 × 107 676.9 19.9

Open sea Length (m) Length change (m) Growth rate (m/yr) Area change (m2) Distance of extension (m) Extension rate (m/yr)

1973 1.512 × 105 1995 1.529 × 105 1.684 × 103 76.5 3.532 × 106 23.2 1.1 1999 1.492 × 105 −3.686 × 103 −921.5 −0.316 × 106 −2.1 −0.5 2007 1.534 × 105 4.178 × 103 522.3 −0.034 × 106 −0.2 0.0 Total 2.176 × 103 64.0 3.181 × 106 20.9 0.6 W. Zhang et al. / Marine Geology 363 (2015) 202–219 215

Fig. 13. Relation between area and length of four subregion coastlines in the PRD: (a) positive relationship between area and length in the Lingding Bay subregion (1979–2005); (b) positive relation between area and length of coastline in the Modaomen subregion (1973–2007); (c) negative relation between area and length of coastline in the Jitimen subregion (1973–2007); and (d) negative relation between area and length of coastline in the Huangmao Bay subregion (1973–2007).

The spatial expansion of reclaimed land in the PRD can also be clear- northwest of Nanshui Island, continued between 1973 and 2007 ly identified from satellite images (Figs. 14 and 15). Compared with the (Fig. 15). Significant variation of the coastlines took place exactly in other three subregions (M, J and H), the land reclamation activities are the area where notable reclamation occurred. By analyzing the area the largest in subregion L. The movement of coastlines in this subregion of land reclamation from 1978 to 2003 (Table 9), it can be observed is clearly displayed in Fig. 11, showing a 16.8-m annual average exten- that the area increased by 4.532 × 104 hm2 (which accounts for over sion over 26 years (Table 5). By comparing the images, it is clear that 80% of the total reclamation area in the whole period) with a the west of the Lingding Bay has expanded seaward and large new 0.267 × 104 hm2 annual average increase for the first 17 years. In the lands were formed outside of Jiaomen and Hengmen, which include last 6 years, the area increase was 1.593 × 104 hm2, implying a Wanqingsha and Jibaosha (Fig. 14). 0.135 × 104 hm2 annual average expansion rate. The rapid increasing The temporal satellite images of subregions M, J and H taken in 1973, rate of reclamation area was consistent with the coastline extension in 1995, 1999 and 2007 show that coastal developments have a far- subregions M and J (Tables 6 and 7). reaching influence on the delta planform. In subregion M, Sanzao Island Human interventions other than land reclamation have also played a was connected to the mainland and the area of Northern Hezhou as well role in the morphological changes as described. In subregion L, a num- as Hengqin Island expanded dramatically from 1973 to 1995 (Fig. 15a ber of port engineering constructions caused the reduction of water and b). Hezhou was extrapolated to the south from 1973 to 1999 areas and deposition at the foreshore. Owing to the Lingding Bay river (Fig. 15a, b and c) and the inner sea of subregion M no longer existed. mouth regulation guideline scheme in the 1990s (Fig. 14b), the Lingding In subregion J, notable land reclamation occurred to the east of Nanshui Bay has a regular bell shape. Its eastern coastlines are smooth and Island from 1973 to 1995 (Fig. 15a and b) and the island areas southeast straight and its west coastline has multiple prolonged outlines of estuar- of Sanzao were joined together in 1999 (Fig. 15a, b and c). A levee ies (Fig. 14d). In subregion M, the continuous projects to dredge the connecting Gaoland and Nanshui Islands gradually appeared in the pe- channel caused seaward extension of the channel, outside the estuary, riod between 1973 and 1995 (Fig. 15a and b). Subsequently, the north- forming a Modaomen submerged delta (Fig. 15b, c and d; Jia et al., ern part of the levee was occupied by artificial reclamation between 2009). In subregion J, the closure of the entrance waterway in 1990 1999 and 2007 (Fig. 15c and d). In subregion H, land reclamation led to deposition near the local zone (Jiang et al., 1993). As for subregion along both sides of Huangmao Bay, especially at the west outlet and H, the breakwater structure not only changed the coastlines directly, but

Table 9 Land reclamation area in recent years.

Subregions Reclamation areas (×104 hm2) Rates (×104 hm2/yr)

1978–1992 1992–1995 1995–2003 1978–1995 1978–2003 1978–1992 1992–1995 1995–2003 1978–1995 1978–2003

Lingding Bay 1.313 0.677 0.607 1.99 2.597 0.094 0.226 0.076 0.117 0.104 Modaomen 0.796 0.609 0.096 1.405 1.501 0.057 0.203 0.012 0.083 0.060 Jitimen 0.122 0.067 0.088 0.189 0.277 0.009 0.022 0.011 0.011 0.011 Huangmao Bay 0.708 0.24 0.285 0.948 1.233 0.051 0.08 0.036 0.056 0.049 Total 2.939 1.593 1.076 4.532 5.608 0.210 0.531 0.135 0.267 0.224 216 .Zage l aieGooy33(05 202 (2015) 363 Geology Marine / al. et Zhang W. – 219

Fig. 14. Comparison of the subregion of Lingding Bay (L) in 4 times: (a): 1979; (b): 1990; (c): 2000; and (d): 2005. With the use of multi-temporal Landsat images, morphological changes were clearly shown. The names of outlets in the d are as follows: Jiaomen (JM) and Hengmen (HM). W. Zhang et al. / Marine Geology 363 (2015) 202–219 217

Fig. 15. Comparison of subregions of Huangmao Bay (H), Jitimen (J) and Modaomen (M) outlets in 4 times: (a): 1973; (b): 1995; (c): 1999; and (d): 2007. The names shown on the c are as follows: Nanshui Island (NS I.), Gaolan Island (GL I.), Sanzao Island (SZ I.) and Hengqin Island (HQ I.). also led to deposition near the breakwater in the 1990s (Fig. 15b; Jia (Yang et al., 2011). The elevation points distributed in the study area et al., 2012). The deep channel project in Yamen deepened the channel have a horizontal resolution of 90–200 m and a vertical resolution of in the mid-1990s. 500 m–1500 m. This is believed to be sufficient for the spatial analysis It is obvious that the intensive human interferences, especially land of estuarine geomorphology as changes in bathymetry. reclamation, cause dramatic morphological evolution in the PRD. Errors of remote sensing data stem from the spatial resolution of sat- These intensive human activities are still ongoing for two reasons. ellite imagery inaccuracies, which in turn came from coastline extrac- First, land is needed to speed up the process of modernization and ur- tion, variations in sea level rise and other errors. The error is small in banization; secondly, local authorities have an interest in land reclama- comparison to the magnitude of the overall changes in coastline varia- tion from the sea as it brings huge profit(Weng, 2002). Although the tions of the PRD at intervals of 10 years (Chu et al., 2013). coastline changes can be quantified and analyzed in great detail, the ac- curacy of the bathymetric maps is insufficient to reveal the detailed sys- 6. Conclusion tem response of the delta front to the human perturbations. A yearly monitoring based on multibeam sampling may offer insight into the Although sea-level rise, subsidence and reduced sediment supply morphological development towards a new equilibrium. The under- are present in the PRD, they are not the main causes for the observed standing of the long-term morphological changes in the PRD is relevant morphological changes. The comprehensive morphological develop- not only because the knowledge provides insights into the historical ments during the period from the 1960s to the 2000s are primarily evolution, it also facilitates the predication of future estuarine evolution caused by the intensive anthropogenic activities such as land reclama- by numerical modeling. tion. The variations of estuarine geomorphology, sediment volumes and coastline developments were quantified and analyzed based on 5.4. Error analysis of used data bathymetric data, RS-GIS integration technology and a Digital Elevation Model. Admiralty charts and satellite imagery only provide crude assess- Geomorphological change analysis, based on 40 years' historical ments of the changes of delta bathymetry and topography. Sediment bathymetric data of the PRD, indicates that the eastern region main- volumetric changes and the recession and progradation of coastlines tained an underwater topography characterized by a “three shoals and were widely analyzed based on those data sources, to monitor changes two troughs” pattern in the Lingding Bay. The middle region experi- in the Ribble Estuary (van der Wal et al., 2002), the Mersey Estuary enced substantial erosion in the Modaomen Waterway and predomi- (Blott et al., 2006), the Yangtze Estuary (Wang et al., 2008, 2013) and nant deposition in the Bailong Waterway. In the western area, the Yellow River Delta (Chu et al., 2013; Cui and Li, 2011; Jiang et al., deposition continuously occurred in shoals and channels throughout 2012). In this paper, errors of bathymetry data were mainly caused by the entire domain. The changes in sediment volume indicate that the measurement inaccuracy, the limited density of survey points and inter- shores became shallower and larger, while deep channels became polation artifacts. Measuring errors relate to the degree in which varia- deeper and narrower. The coastlines continuously extended seaward tion of wave and tidal levels are accounted for, boat posture and squat at a rapidly increasing rate, which dramatically affected the length of and human mistakes (Tönis et al., 2002; Wang et al., 2013). The error as- the coastline. In most regions, length of the coastline increased as a con- sociated with the morphological analysis based on bathymetry and the sequence of land reclamation. However, in Jitimen subregion, around Kriging interpolation approach largely depends on the number and den- 59 km2 area of land was realized while the length of the coastline was sity of elevation points along traverse lines used for map construction reduced by 25 km. Human factors, especially land reclamation, are the 218 W. Zhang et al. / Marine Geology 363 (2015) 202–219 main processes affecting the observed morphological changes in the Jia, L.W., Ren, J., Xu, Z.Z., Tan, C., 2009. Morphological evolution in recent years and water- way regulation of the sandbar area in the Modaomen Estuary [J]. Ocean Eng. 3, 011. PRE. Jia, L.W., Luo, J., Ren, J., 2012. The analysis of evolution of a sand bar and its formation in the Huangmao Bay of the Zhujiang Estuary [J]. Acta Oceanol. Sin. 34 (5), 120–127 (in Chinese). Acknowledgments Jiang, P.L., Wang, T.H., Shen, J.C., Duan, W.R., 1993. Opinions on the improvement align- ments for Huangmaohai Sea and Jitimen Outlet [J]. Pearl River. 3, 6–9 (in Chinese). Jiang, C.J., Li, J.F., De Swart, H.E., 2012. Effects of navigational works on morphological This research was financially supported by “the Natural Science Foun- changes in the bar area of the Yangtze Estuary. Geomorphology 139–140, 205–219. dation of China” (NSFC, Project Number: 41376094, 41006046), the “Joint Jiang, X.Z., Lu, B., He, Y.H., 2013. Response of the turbidity maximum zone to fluctuations Research Projects NSFC-NWO” (Project Number: 51061130545), the in sediment discharge from river to estuary in the Changjiang Estuary (China). Estuar. Coast. Shelf Sci. 131, 24–30. “Commonweal Program of Chinese Ministry of Water Resources” (Project Karunarathna, H., Reeve, D., 2008. A boolean approach to prediction of long-term evolu- Number: 201301072) and the “Fundamental Research Funds for the Cen- tion of estuary morphology. J. Coast. Res. 51–61. tral Universities” (Project Number: 2014B06214). Karunarathna, H., Reeve, D., Spivack, M., 2008. 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