Recent Geomorphic Change in Lingding Bay, China, in Response to Economic and Urban Growth on the Pearl River Delta, Southern China

Global and Planetary Change 123 (2014) 1–12

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Global and Planetary Change

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Recent geomorphic change in LingDing Bay, China, in response to economic and urban growth on the Pearl River Delta, Southern China

Ziyin Wu a,⁎, John D. Milliman b,⁎, Dineng Zhao c, Jieqiong Zhou a,CaihuaYaod a Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, SOA, Hangzhou 310012, China b Virginia Institute of Marine Science, College of William and Mary, Gloucester Pt., VA 23062, USA c Department of Earth Sciences, Zhejiang University, Hangzhou 310028, China d East China Mineral Exploration and Development Bureau, Nanjing 210000, China article info abstract

Article history: Decreased fluvial water and sediment discharge, increasing land reclamation, changing climate, and rising sea Received 2 September 2014 level have had an ever-increasing impact on river deltas, particularly those deltas bordering Southeast Asia. Uti- Accepted 14 October 2014 lizing 100 years of navigational and bathymetric data, together with more than 50 years of fluvial discharge data, Available online 23 October 2014 we here document the impact of human activities on the Pearl River Delta and its estuary at LingDing Bay, China, the site of recent rapid economic expansion and urbanization. Between 1906 and 1988, approximately 80 km2, Keywords: ~1 km2/yr, of land was reclaimed from the Bay, almost entirely for agriculture. During the ensuing 20 years, Pearl River – 2 2 LingDing Bay 1988 2008, nearly 200 km of land was reclaimed (10 km /yr), mostly to be used for manufacturing and ship- China ping facilities; even some of the previously reclaimed agricultural land was converted to industrial use. Most of Economic expansion the reclaimed land came from a corresponding loss of intertidal and shallow depths: between 1988 and 2008, Land reclamation the area occupied by supratidal and subtidal flats (−2 to 5 m water depths) declined by ~180 km2. Channel dredging to accommodate ever larger ships, in contrast, increased areas with depths greater than 10 m by about 36 km2. Although water volume in the northern bay has decreased by 20% since 1906, decreasing bay area has resulted an overall 15% deepening of the bay. With continued economic expansion in the Pearl River Delta, LingDing Bay should continue to shrink in both area and water volume, although the bay should continue to deepen. © 2014 Elsevier B.V. All rights reserved.

1. Introduction the global ocean (Milliman and Meade, 1983; Saito et al., 2001; Syvitski et al., 2005; Milliman and Farnsworth, 2011). Moreover, Asian Deltas and their estuaries act as important continent-ocean conduits rivers and their deltas serve as the sites for many of the world's largest that convey water and suspended and dissolved solids to the coastal population and commercial centers; Shanghai (Yangtze River), Jakarta ocean. As such, delta fronts act as both “drivers” and “recorders” of nat- (Ciliwung River), Ho Chi Minh City (Mekong River), Haiphong (Red ural and anthropogenic environmental change (Syvitski and Saito, River), Bangkok (Chao Phyra River), Dhaka (Brahmaputra River) Calcutta 2007; Bianchi and Allison, 2009). Throughout recorded history, human (Ganges River) and Karachi (Indus River) are obvious examples. Any activities, particularly deforestation and farming, have led to increased change to fluvial discharge or delta modification, therefore, can have con- land erosion and therefore increased sediment discharge (Milliman siderable impact on both the delta and its inhabitants. et al., 1987; Kao and Liu, 2002; C.J. et al., 2005; Milliman and Because their historic record is so long, Chinese rivers are of particu- Farnsworth, 2011), resulting in accelerated delta growth (e.g., Syvitski lar interest in documenting and understanding natural and anthropo- et al., 2005, 2009; Maselli and Trincardi, 2013, and references therein). genic changes to the watershed and their downstream effects on the Over the past century, however, damming and irrigation have tended coastal zone. In recent years these changes and effects have been partic- to decrease both water and sediment fluxes from many rivers and ularly well documented for China's two largest rivers, the Yangtze thus to the deltas into which they discharge (e.g., Yang et al., 2004; (Changjiang) and Yellow (Huanghe) rivers, which prior to large-scale Yang et al., 2006; Syvitski et al., 2005; Milliman and Farnsworth, 2011). dam construction collectively discharged 1500 million tons per year Nowhere are deltas more important than in Asia and adjacent (mt/yr) of sediment to the coastal ocean (Milliman et al., 1987; Saito Oceania, where rivers discharge, respectively, about 30%, 60% and et al., 2001; Wang et al., 2007; Yang et al., 2011), nearly 10% of the global 40% of the water, suspended sediments and dissolved solids reaching total. The effects of human activities on China's 3rd largest river, the Pearl ⁎ Corresponding author. River (Zhujiang) in southern China (Fig. 1), have been less rigorously E-mail addresses: [email protected] (Z. Wu), [email protected] (J.D. Milliman). documented even though, in terms of population (N50 million) and

Fig. 1. Pearl River (Zhujiang) drainage basin, showing West, North and East rivers. Pearl River delta and LingDing Bay are outlined by small box to the lower right. recent economic development, the Pearl River Delta (PRD) (Fig. 2) export shipping to and from Guangzhou, Dongguan and (increasingly) (total land area greater than 50,000 km2) is one of the most econom- Shenzhen has risen by nearly 500% since the late 1990s (Fig. 5), ically important and urbanized deltas in southern Asia, in large Because of its protected nature and relatively deep channels, LingDing part due to an increasing amount of foreign investment (Seto and Bay (LDB) has long served as a major port for southern China, for the past Kaufmann, 2003). The Pearl River Delta, alone among other southeast 150 years providing access to and from Guangzhou (Canton). In recent Asian deltas, is the site of two megacities (N10 million population), years, since the inception of the Pearl River Delta Economic Zone, former- Shenzhen and Guangzhou, two slightly smaller cites, Dongguan (8 mil- ly small rural and ﬁshing villages such as Dongguan and (notably) lion) and Foshan (7 million), as well as nearby Hong Kong-Kwaloon (7 Shenzhen have also become major ports. As of 2011, these three cites million). collectively annually shipped nearly 750 million tons of (mostly) manufactured goods out of the Pearl River, Dongguan and Shenzhen together accounting for about 300 mt/yr (Fig. 5). 1.1. Recent economic and urban growth in the Pearl River Delta In this paper we discuss changes in both the shoreline and bathymetry of LDB, the Pearl River's main estuary (Fig. 2). Given the relatively Following the creation of the Pearl River Delta Economic Zone in the long history of shipping activity in this area, we have been able to utilize early 1980s, demographics in the delta changed rapidly (Fig. 3). In 1980 navigational charts extending back to 1906 to document these changes. the PRD's population was primarily agricultural and rural, only about In recent years there have been a number of published studies on 16% of the population being considered urban. Between 1989 and changes in the water and sediment ﬂux from the Pearl River drainage 1997, in response to the economic expansion, cropland decreased by basin as well as the estuary's sedimentary environment (e.g., Jia and 48% in area, matched by a similar increase of urban area (Weng, Peng, 2003; Jia et al., 2005; Luo et al., 2007; Zhang et al., 2008). Much 2007). At present, about 80% of the 55 million people inhabiting the of the recent literature, however, has emphasized sedimentation and delta (www.ucer.camcom.it) are considered urban, and the number of pollution within the estuary (Lin et al., 1998; Cao et al., 2004; Liu cities on the delta has increased from 3 to 23 (Shen et al., 2006). et al., 2005; Owen, 2005; Ma et al., 2002; Fu et al., 2003; Xing et al., Shenzhen's population, for example, increased from ~350,000 in the 2005; Dai et al., 2008; Zhang et al., 2009; Liu et al., 2010; Wu et al., early 80s to more than 11 million at present (Fig. 4(a)). This rapid ur- 2010). The relatively few papers dealing with changes in the coastline banization in the PRD has been both a cause and result of the explosive and bathymetry in the Pearl River (Lin et al., 1995; Seto et al., 2002; economic growth, annual GDP increasing from about 19 billion Yuan (6 Chen et al., 2005: Chen et al., 2006; Zhang et al., 2008; Zong et al., Yuan ~1 US dollar) in 1978 to 5800 BY in 2012 (Fig. 4(b)). Accordingly, 2009; Li and Damen, 2010; Deng and Bao, 2011; Xia et al., 2013; Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12 3

Fig. 2. Pearl River Delta, showing LingDing Bay (LDB, red box) as well as the location of the West, North and East rivers gauging stations (Gaoyao, Shijiao and Boluo), the four major distributaries (Humen, Jiaomen, Hongqimen and Hengmen) to the west, the East River, and the major urban centers of Guangzhou, Shenzhen, Dongguan, Foshan, Hong Kong and Macao. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Wang et al., 2013) have dealt primarily with post-1980 changes. We are of 75–80 mt/yr, however, varied considerably; some years being as unaware of any published study dealing with longer-term changes in low as 40 mt/yr, and some years greater than 120 mt (Fig. 6). According the Pearl River estuary's geomorphology or bathymetry, the topic of to Zhang et al. (2008), as of 2005, 387 medium and large dams had been this paper. constructed within the Pearl River's watershed; combined storage capacity was 46.7 km3. With accelerated dam construction in the early 1.2. Study area 1990s, and particularly after completion of the Yantan reservoir (see Fig. 1) in 1994, annual sediment discharge declined to its present levels With a catchment area of 415,000 km2 (Zong et al., 2009), the Pearl of 20–30 mt/yr (Fig. 6). River is the largest river in southern China; and within all of China its The Pearl River has eight distributaries that flow directly or indirectly 50,000 km2 delta is 2nd in area only to the Yangtze River delta. Although into the South China Sea (Fig. 2), four of which (the Humen, Jiaomen, its annual discharge of 260 km3/yr ranks 2nd, 6th and 18th in terms Hongqimen and Hengmen rivers) discharge into LingDing Bay. Until of Chinese, Asian and global (respectively) rivers (Milliman and the early 1990s, these four distributaries discharged an estimated Farnsworth, 2011), it often has been relegated to an almost after- 35 mt/yr of sediment into LDB (Luo et al., 2007,theirFig. 2). Increased thought compared to the much greater international interest in the mining of river sands in the Sixianjiao Channel in the early 1990s in- Yangtze and Yellow rivers. The most distinctive feature of the Pearl creased the flow from the West River into the North River, thus increas- River is the complex configuration of its many branches and distributar- ing flow into LDB by ~5% (Luo et al., 2007, their Table 3). This sand ies. The Pearl River actually consists of three main branches, the West removal, however, has undoubtedly decreased even further the com- (Xijiang), North (Beijiang) and East (Dongjiang) rivers (Fig. 1). Of the bined sediment discharge from the North and West rivers. We estimate three, the West River is by far the largest, draining an area of present-day sediment discharge into LDB to be ~10 mt/yr. 350,000 km2; the North River drains ~38,000 km2 and the East River Surrounding the eight distributaries is the Pearl River Delta, approxi- ~25,000 km2 (see Dai et al., 2008). The Pearl River's total discharge mately 8600 km2 in area, of which more than 3700 km2 lies at elevations (260 km3/yr) is estimated as the sum of discharges past the seaward- less than 2 m above sea level (Syvitski et al., 2009). The seaward-most most gauging stations on the West (at Gaoyao), North (at Shijiao) and 30–60 km of the delta have accreted over the last several millennia East (at Boluo) rivers. Mean annual runoff is ~530 mm/yr (Milliman due to falling sea level (Fyfe et al., 1997; Fyfe et al., 1999; Hanebuth and Farnsworth, 2011). et al., 2000), and, during the past 1000 years, due to land reclamation Because of its greater basin area and because it drains western during the Tang, Song and Qing dynasties (Zheng et al., 2004; Zong mountains, prior to dam construction in the early 1990s the West et al., 2009, their Fig. 5). Land subsidence in and around LDB averages River discharged annually an average of 68 million tons (mt) of sedi- about 1.5 mm/yr (Huang et al., 2004). ment, as measured at the Gaoyao gauging station (see location in LingDing Bay itself consists of two relatively deep troughs, the East Fig. 1). In comparison, the North and East rivers discharged about 5.5 Trough (Fanshi Fairway) and West Trough (LingDing Fairway), and 2.5 mt/yr, as measured at the Shijiao and Boluo gauging stations, boarded by three shoals, East Shoal, Middle Shoal and West Shoal, respectively (Dai et al., 2008, their Table 1). The cumulative discharge each shallower than 5 m. The troughs merge up river (northward) into 4 Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12

Fig. 3. Infrared-enhanced satellite images of the Pearl River Delta, 1979, 1990, 2000 and 2013, illustrating the increased urbanization of the delta. In these images vegetation is portrayed in red, water in blue, and urbanized land in gray. Narrowing of LingDing Bay, particularly after 1990, the result of land reclamation, as shown in greater detail in Figs. 9 and 12. Boxes in the 2013 image indicate locations of detailed satellite time-series images shown in Fig. 10 (Longxue Island: left box) and 11 (Shenzhen: right box). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Longxue Channel (Fig. 7). Maximum present-day depths in Longxue declined considerably (Fig. 6), perhaps somewhat offset by a change Channel exceed 20 m, mostly the result of channel dredging (see in upstream divergence of flow (Luo et al., 2007). Fig. 7 lower). There are several potential problems in comparing navigational The northern extent of the bay is ~15 km wide at the junction of the charts taken over a long period of time, including the type and density Humen and Jiaomen rivers; width of the southern end of LDB is ~40 km. of soundings as well as navigational accuracy. Because of the shallow Tidal range is, on average, 1.6 m, although it can exceed 3 m during as- nature of the bay, generally less than 10 m, soundings obtained by tronomical tides (Huang et al., 2004). leadline in 1906, we assume, approximate those taken in later years by echo-sounding. Notations on the 1906 chart show 3025 soundings 2. Methods in LDB spaced over a subtidal area of 490 km2, equating to ~7–8sound- ings/km2. While this sounding density is a far less than soundings Our main source of data used in this paper has been a series of achieved by continuous echo-sounding, the gradual bottom gradients Chinese navigational charts printed in 1959 (based primarily on data within the bay suggest that the 1906 sounding density was sufficient collected in 1955), 1990 (using mainly data from 1988), and 2008; as to accurately represent the bathymetry. such, we refer to the data on the former charts as representing the The second potential problem in comparing 100 years of navigation- years 1955, 1988 and 2008. We also accessed a 1918 Chinese chart al data lies in the differences of navigational accuracy. Landforms in the whose navigation and soundings were based primarily on British data 2008 chart, for instance, are based on GPS, whereas the 1959 chart collected in 1906 that extended south to 23° 31′N, encompassing the (using primarily 1955 data) was based on radar, and the 1906 chart area that in this paper we refer to as northern LingDing Bay; southern was based on compass and visual observations. Close proximity to LingDing Bay refers to the area from 22°22′Nto23°31′N, and is included land—and thus to navigational fixes—however, suggests that the in the 1955, 1988 and 2008 navigational charts. Use of the 1906 data accuracy of these visual observations may compare favorably with gives us a 102-year time span (1906–2008) with which to compare more modern observations. The shape and locations of high-standing changes bathymetry and configuration of the adjacent coast in northern islands in the 1906 chart, in fact, are similar to those delineated in LingDing Bay. Conveniently, as will be discussed below, the timing of later charts, although they often displaced 50 to 100 m to the north these charts provides us with very different temporal perspectives. (Fig. 8). As our study area is roughly 50 km in length, this 50–100 m dis- Between 1906 and 1955 there was relatively little change in fluvial displacement can be considered to represent a relatively minor error. It is charge, little dredging of the bay, and relatively little land reclamation. interesting to note, however, that the shapes of some islands defined The period between 1955 and 1988 saw slow economic growth, with in 1906 and 1959 are distinctly different in configuration that the some increase in land reclamation; river discharge remained more or present-day islands. We can assume that these differences reflect less constant. The years 1988–2008 saw the rapid economic expansion, both natural coastline evolution and, more dramatically, land recla- as described above, and a corresponding marked increase in land reclamation. While the present location and shape of the northern edge mation and dredging; as mentioned previously, sediment discharge of Shanghengdang Island remain roughly the same as it was in 1906, Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12 5

Fig. 5. Shipping in LingDing Bay, 1999–2011. Note the marked increase in shipping to and from Shenzhen.

Fig. 4. (a) Pearl River Delta Economic Zone population, 1980–2012, during which total population has increased from ~16 million to 55 million, the most rapid population growth occurring in Shenzhen (350,000 to 10 million) and Dongguan. Collectively, the ﬁve largest cities in the PRDEZ (including Hong Kong) account for nearly 40 million people, the remainder of which, although occupying a far greater land area, account for only about 18 million people, many of which can be considered rural. (b) Exponential growth of the Gross Domestic Product (GDP) in the PRDEZ, 1980 (~25 billion Chinese Yuan/yr) to 2012 (5800 billion Chinese Yuan/yr). the southern edge of the island has been extended to serve as a base for a cross-bay bridge (Fig. 8 upper). An even more dramatic change in island conﬁguration is seen with Longxue island (Fig. 8 lower), where, by 2008, the island shown in the 1906 and 1959 charts had been incorporated within a broad zone of reclaimed land; the eastern edge of the 1906 island now lies more than 600 m from the bay (Fig. 8 lower).

2.1. Morphologic Changes in LingDing Bay, 1906–2008

2.1.1. Reclaimed Land Fig. 6. (a) Estimated water (left axis) and sediment (right axis) discharges from the Pearl River, 1955–2010, the summation of discharges monitored at the Gaoyao (West River), Even a quick glance at Fig. 9 shows several obvious long-term Shijiao (North River) and Bolou (East River) gauging stations. Note the dramatic decrease changes in the general morphology of LingDing Bay that occurred in sediment discharge after closing of the Yantan Dam (located in Fig. 1) in 1994. between 1906 and 2008. Perhaps most apparent is the changing con- (b) Cumulative sediment and water discharge since 1955. Note the increase in sediment figuration of the coastline and the ever-growing loss of intertidal area discharge from the mid-80s to early 90s, reflecting, we assume, increased landuse, particularly in the West River watershed. By 1995, due to dam construction, cumulative sedi- (labeled in green in Fig. 9). Reclaimed-land area surrounding LingDing ment discharge began to decline relative to water discharge. The decline in water 2 2 Bay increased by nearly 270 km , ~220 km in northern LingDing Bay discharge, we assume reflects both increased water storage and use and well as, presum- (Table 1). It is also interesting to note—and critical to our thesis that ably, decreased precipitation. 6 Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12

Fig. 7. Right: General bathymetry of LingDing Bay, showing the four distributary rivers (Humen, Jiaomen, Hongqimen and Hengmen) to the west, as well as the East and West troughs and intervening shoals. Left: site of the Sixianjiao Channel deepening, the result of sand mining, which led to increased water and sediment discharge into LingDing Bay (after Luo et al., 2007). economic growth was the main trigger for recent morphologic changes to 50 km2 increase after 1988 (Table 1). The rate of land reclamation in LDB—that the rate of land reclamation increased dramatically after in LDB thus increased from ~1 km2/yr before 1988 to ~10 km2/yr after 1988. Between 1906 and 1988, land area in northern LDB increased by 1988. 75 km2 in 80 years. During the following 20 years, 1988–2008, land The type and use of reclaimed land also changed signiﬁcantly. As area in northern LDB increased by 146 km2. Despite having a much shown in Fig. 10, reclaimed land in the vicinity of Longxue Island in shorter record, a somewhat similar pattern is seen in southern LDB: northwestern LDB (see Figs. 3 and 8 for its location) was mostly agricul- 5km2 increase in reclaimed land between 1955 and 1988, compared tural in nature, as indicated by the infrared-induced red colors in 1986 Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12 7

Table 1 Calculated areas for areas of land and various depths in northern LingDing Bay (1906, 1955, 1988 and 2008) and for southern LingDing Bay (1955, 1988 and 2008).

North LingDing Bay (1906–2008)

(km2) 1906 1955 1988 2008 Land area 391 438 466 612 Tidal ﬂat 168 157 171 68 0–2 m depth 121 149 134 103 2–5 m 188 142 138 119 5–10 m 161 143 125 112 N10m 25262041

South LingDing Bay (1955–2008)

1955 1988 2008 Land area 39 44 94 Tidal ﬂat 58 98 93 0–2 m depth 128 97 91 2–5 m 255 277 254 5–10 m 151 115 88 N10 m 26 25 40

for industrial and shipping purposes, reﬂecting its rapid economic and demographic growth (see Figs. 4 and 5).

2.1.2. LingDing Bay bathymetry The most obvious change in LingDing Bay's bathymetry, not surprisingly, has been the loss of supratidal and subtidal area (due to land reclamation; see Figs. 12, 13) and the deepening of the Western Trough (due to channel maintenance; see Figs. 13, 14). Total bay area declined from 1248 km2 in 1955 to 990 km2 in 2008 (20% loss), of which 51 km2 of intertidal area were lost (Table 1). Changes are most obvious in northern LDB, particularly after 1988 (Fig. 13), primarily due to land Fig. 8. Comparison of Shanghengdang and Xiahengdang Islands (top) and Longxue (bot- reclamation. tom) Island locations from 1906 and 1955 charts, as well as present-day satellite observa- Comparison of the net changes in the northern section of LingDing tions. Locations of islands also shown in Fig. 5. Note the south and west displacements of Bay (Figs. 12(d), 13(a)), shows considerable shoaling in water depths both islands in 1955 relative to the locations noted in the 1906 chart, which we infer reflects less accurate navigation in 1906. By 2013 the configuration of both islands had shallower than 5 m and as much as 10 m deepening of the troughs. In been markedly altered by land reclamation, Longxue Island now being incorporated into the 102-yr period, supratidal area in the northern part of LDB declined amuchlargerisland. by 221 km2 (35% loss), and the intertidal area by 100 km2 (60% loss). Between 1906 and 1988, however, northern LingDing Bay showed relatively little shoaling or deepening (Table 1): other than a 2–3m and 1990. In contrast, much of the post-1990 reclaimed land was used deepening along the western areas between 1906 and 1955, bathymet- for industrial and shipping purposes, as noted by the gray colors and ric change generally was less than 1 m (Fig. 12(a)). Overall, with the more geometric shoreline configurations. In fact, by 2013 some of the exception of the dredged channels, northern LingDing Bay showed previously agricultural reclaimed land shown in the 2000 image a general shoaling between 1906 and 2008 (Fig. 13(d)). Equally appears to have been converted to industrial use. Land reclamation in important was the fact that the total bay lost 276 km2 of area, the re- the port area of Shenzhen (Fig. 11), in contrast, has been almost entirely sult of land reclamation. In terms of water volume, the northern part of

Fig. 9. LingDing Bay shoreline and bathymetry based on navigation and soundings from 1906 (a), 1955 (b), 1988 (c) and 2008 (d). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 8 Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12

Fig. 10. Tine series aerial photographs of the Longxue Island and Nansha port area, 1986, 1990, 2000 and 2013. The red color in the infrared-enhanced images reﬂects land vegetation, whereas gray colors indicate non-vegetation or, in the December 2013 image, wintertime vegetation. Note that between 1986 and 1990, most reclaimed land was used for agriculture, whereas the subsequent rapid land expansion was almost entirely for manufacturing and shipping. An enlarged image of the area around Longxue Island isshowninFig. 8.(Forinterpre- tation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

the bay contained in about 432 km3 less water in 2008 than it did in altered—sometimes drastically—its natural environment and thus, 1906. ultimately, its geology. Although the initiation of the Anthropocene has been debated, the human-modiﬁed environment in Asia can be 2.2. Concluding remarks traced back at least several thousand years. The Indus, Tigris–Eu- phrates, and Yellow rivers, for example, have been heavily impacted Increasingly the term “Anthropocene” has been used to describe by human occupation for millennia, where as the New World's Mis- the Earth's recent history in response to human activities that have sissippi River has only been signiﬁcantly impacted over the last Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12 9

Fig. 11. Time series aerial photographs of the Shenzhen port area in southern LingDing Bay, 1986, 2000, 2006 and 2013. The rapidly expanded land area seen during the 25 years was almost entirely used for the siting of manufacturing and shipping activities, as indicated by the gray colors of these infrared-enhanced images.

Fig. 12. Bathymetric changes in LingDing Bay between 1906 and 1955 (a), 1955 and 1988 (b), 1998 and 2008 (c), and the entire 102-year period between 1906 and 2008 (d). The north– south and east–west bathymetric proﬁles shown below each chart show that bathymetry changed over these periods. Elevations of reclaimed land based on DEM data. 10 Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12

Fig. 13. Comparison of water depth areas from (a) northern LingDing Bay as noted in 1906, 1955, 1988 and 2008 (left), and (b) southern LingDing Bay as noted in 1955, 1988 and 2008 (right). several hundred years. We can think of few river systems in which The 102-yr record of morphological change in (northern) LingDing the physical environment has been as altered by human activities Bay records some rather parallel changes—not surprisingly—to the eco- as long as the Pearl River, its delta (PRD), and its estuary, LingDing nomic development of the area. Between 1906 and 2008, land area in Bay. northern LDB increased by ~230 km2 and tidal ﬂat and intertidal area Genesis of the modern PRD can be envisioned as beginning about declined by 100 km2 (Table 1). In fact, subtidal depths also decreased 7000 years ago, when the mid-Holocene sea level was about 3–4m almost as much: In 1955 these depths accounted for 968 km2 in area, above present level (Hanebuth et al., 2000; Liu et al., 2004) and the whereas in 2008, these depths accounted for 767 km2, a decrease of delta consisted of small islands separated from one another by water ~20%(Table 1). In contrast depths greater than 10 m actually increased depths 3–6 m. Subsequent sea-level regression to its present level in area (Figs. 12, 13) because of continual channel dredging, as shown in resulted in a gradual emergence of new land. As the sea-level regression Fig. 14. slackened, about 2000 years ago (Zong et al., 2009), increased defores- It is not surprising that total water volume in the northern part of the tation and expanding agriculture, resulting from the southward human bay declined over the 102-yr period (2250 to 1800 km3)(Fig. 15), migration, increased the Pearl River's sediment discharge, thus provid- although increasing slightly after 1988 due to channel dredging ing more “building material” for and ever-shoaling and expanding (Fig. 14). Because of the decreased bay area, mean water depth has delta. Early land reclamation, beginning in the Tang Dynasty (about gradually increased since 1955: from 4 m (1955) to 4.1 m (1988) to 1300 years ago; Parham, 2010)) led to the coalescence of individual 4.5 m (2008). islands into an increasingly broad, low-level landmass, which could be Although predicting future changes in geomorphology and ba- drained by canals and subsequently farmed; this new land could then thymetry are problematic, we suspect that as the Pearl River Delta's be protected from ﬂoods and waves by the construction of levees and economic expansion and demographics continue to develop, the dikes (see review by Weng, 2007). bay will continue to narrow, as more of the bay's shoal areas are

Fig. 14. CHIRP proﬁles (obtained in 2013) in LingDing Bay showing altered seaﬂoor depressions (a) and maintained channels (b, c) due to recent dredging. Z. Wu et al. / Global and Planetary Change 123 (2014) 1–12 11

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