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Delta changes in the Pearl River estuary and its response to human activities (1954–2008)
Quaternary International xxx (2015) 1e8
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Delta changes in the Pearl River estuary and its response to human activities (1954e2008) Chuang Shou Wu a, b, *, Shilun Yang c, Shichang Huang a, b, Jinbin Mu a, b a
Zhejiang Institute of Hydraulic & Estuary, Hangzhou 030020, China Key Laboratory of Estuarine and Coastal Research in Zhejiang Province, Hangzhou 030020, China c State key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
This paper is concerned with delta evolution in response to human activities in the Pearl River estuary. Utilizing more than 50 years data bathymetric data, together with simulation fluvial discharge data, the net accretion rate in the Lingdingyang subaqueous delta decreased from 16.6 mm/y over the period 1955 e1964 to 1.6 mm/y for the period 1998e2008 because there was a dramatic downward trend in the sediment load. In particular, the closure of the Longtan Dam (LTD) in 2006 resulted in the sediment load entering the Pearl River falling from 75 Mt/y (annual average: 1954e2006) to 25.2 Mt/y, a decrease of ~67%. Another reason may be sand mining in the Lingdingyang estuary. Based on the linear relationship between sediment load and accumulation rate, an annual average sediment load of 21.7 Mt/y carried into the Lingdingyang estuary is the critical threshold value that separates delta progradation from recession. However, the outer subaqueous delta in the Modaomen estuary underwent deposition in the same period, because there is no obvious change in sediment load into the area during 1954e2008. In the coming decades, ongoing human interferences will reduce the fluvial sediment load entering the sea, which probably will result in a phase of general delta recession replacing delta progradation. © 2015 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Delta Sediment load Human activities Pearl River
1. Introduction Many of the world's river deltas currently face serious problems due to the combined pressure of human development and climate change (Day and Giosan, 2008). Throughout recorded history, dams and irrigation, as well as improved soil and water conservation practices, have reduced the sediment loads carried by many rivers. Consequently, the accretion/erosion response of deltas to this decline in the fluvial sediment supply has become a topic of global concern (Milliman, 1997; Syvitski et al., 2009; Edmonds, 2012). For example, an expanse of coastal land the size of 0.05 km2 disappears from the Mississippi River delta every hour due to the decreasing sediment load and sea level rise (Edmonds, 2012). The sediment load in the river Nile in Egypt was once between 100 106 and 124 106 t, but today there is almost no net annual sediment input to the Nile delta due to the Aswan High Dam, which began operating in 1964, and the delta is degrading with a shoreline erosion
* Corresponding author. Zhejiang Institute of Hydraulic & Estuary, Hangzhou 030020, China. E-mail address: [email protected] (C.S. Wu).
rate of 143e160 m/y (Stanley and Warne, 1993, 1998; Fanos, 1995; Frihy et al., 2003). The sediment load of the river Ebro located in northeastern Span was reduced from ~1.0 107 Mt/y to 0.3 106 Mt/y, after the construction of the RibarrojaeMequinenza dam in the lower Ebro at the end of the 1960s. Currently, the load ranges from 0.1 106 to 0.2 105 Mt/y. More than 99% of the sediment load has been trapped in the reservoirs, which was the n ~ ez et al., 1996; major cause of recession in the river mouth area (Iba nchez-Arcilla et al., 1998). The Colorado River, one of the most Sa highly regulated rivers in the word due to the numerous dams, once supplied more than 150 106 Mt/y of sediment to the Gulf of California. At present, the dams and course diversions effectively trap most of the sediment, and this has led to coastal erosion nchez, 1999; Huh et al., 2007). In China, the (Carriquiry and Sa Yangtze River, one of the largest rivers in the world, carried a sediment load of around 490 Mt/y in the 1950s and 1960s, but this fell to 150 Mt/y after the closure of the Three Gorges Dam (TGD). In response to this drastic decrease in sediment supply, the river channel has changed from net accretion to net erosion, and the deltaic coast has shifted from progradation to recession (Yang et al., 2003, 2005, 2011; Luo et al., 2012; Dai and Liu, 2013; Dai and Lu, 2014). The Yellow River (Huanghe) once carried the second
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Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
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C.S. Wu et al. / Quaternary International xxx (2015) 1e8
largest sediment load in the world (Milliman et al., 1987), but now carries only 14% of the widely cited estimate of the peak load of 1.08 Gt/y, due to both natural processes and human activities over the past 56 years (Wang et al., 2007). This dramatic decrease in sediment load has caused the whole delta to switch from a state of accretion to erosion (Peng et al., 2010). As the second largest river China, and the 25th largest river in the world in terms of water discharge, the Pearl River is considered one of the world's most complicated fluvial networks. It is a compound river system, comprising three major tributaries (the West, North, and East rivers), and some other small rivers draining into the Pearl River delta (Fig. 1), which occupies an area of ~17,200 km2. The Pearl River delta is one of the most developed areas in China owing to implementation of China's open-door and reform policies since the 1980s, and at least 8636 reservoirs had been constructed in the watershed by the late 1990s. As a consequence, the hydrological regime of the Pearl River basin has been substantially altered by this intensive human activity, and the Pearl River has become one of the most highly impacted rivers in the world (Nilsson et al., 2005). Over recent years, previous studies have focused on various aspects of the changing water and sediment load of the Pearl River
(e.g., Dai et al., 2008; Zhang et al., 2008, 2009; Wu et al., 2012; Wu et al., 2012; Zhang et al., 2012; Liu et al., 2014) and channel changes (Lu et al., 2007). These papers indicate that human activities, in particular dam construction and soil conservation, have altered the natural sediment transport regime. However, to the best of our knowledge, no previous work has demonstrated the collective impact of changes to the subaqueous delta of the Pearl River caused by the changing sediment supply. Therefore, in this paper, we investigate the evolution of the subaqueous delta in response to human activities, based on hydrological data and bathymetric maps covering the period from 1950s to 2008. 2. Physical setting The Pearl River originates on the Yunnan Plateau and crosses low-terrain hills and even mountains as it runs 2400 km eastwards to South China Sea (Wu and Zhou, 2001). Due to its long-term average annual water discharge of 3260 108 m3/y, it plays an important role in fresh water supply to the large cities in the Pearl River delta region, such as Macau, Hong Kong, Zhuhai, and Guangzhou. The annual suspended sediment load of the river is
Fig. 1. (a) The Pearl River basin in South China. (b) The Pearl River chorographic map, including the Longtan Dam (LTD) and the Gaoyao, Shijiao, and Boluo gauging stations. (c) the Pearl River subaqueous delta, showing the Lingding Bay and Modaomen areas (dashed boxes). Numbers on the map represent eight “gates”: 1, Humen; 2, Jiaomen; 3, Hongqili; 4, Hengmen; 5, Modaomen; 6, Jitimen; 7, Hutiaomen; 8, Yamen.
Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
C.S. Wu et al. / Quaternary International xxx (2015) 1e8
88.7 Mt/y. The Pearl River is comprised of three major tributaries (the West, North, and East rivers), and some other small rivers draining into the Pearl River delta (Fig. 1). The West River, as the largest branch, (Xijiang), has a total length of 2214 km and a drainage area of 35.5 104 km2; the North River (Beijiang) has a total length of 468 km and a drainage area of 4.68 104 km2; while the East River (Dongjiang) has a total length of 562 km and a drainage area of 3.24 104 km2. The annual average water and suspended sediment discharges via the West River are 2281 108 m3/y and 6567 104 t/y, respectively, while for the North River they are 449 108 m3/y and 864 104 t/y, respectively, and for the East River are 234 108 m3/y and 236 104 t/y, respectively. The total water and sediment discharges of these three major rivers account for more than 80% and 95%, respectively, of the load entering the sea, which causes the seaward extension of the mouth region of the Pearl River at a rate of 40 m per year (Gong and Chen, 1964). The basin is located on the Tropic of Cancer, and has a subtropical and tropical monsoon climate with a dry season from October to March and a rainy season from April to September. The mean annual temperature is 14 e22 C and annual precipitation is 1470 mm (Dai et al., 2008). After entering the delta plain, the river bifurcates continuously and forms a complex network of distributaries that eventually discharge into estuarine bays. The Pearl River delta, the third largest in China (after the Yangtze and Yellow river deltas), is located between 21400 and 23 N, and 112 and 113 E (Fig. 1b). Sediment load from the rivers plays an important role during the delta forming process (Li et al., 2001; Wu et al., 2010). Eventually water discharge and sediment load enter a complex network of distributaries, with a channel density of 0.68 km/km2e1.07 km/km2, and flow out through the eight “gates”, Humen, Jiaomen, Hongqili, Hengmen, Modaomen, Jitimen, and Hutiaomen, into estuarine bays (Chen and Chen, 2002) (Fig. 1c). The delta was created by the deposition of river sediments into the receiving basin after the end of the Holocene transgression over the past 6000 years (Li et al., 2001), and consists of the West, North, and East river sub-deltas. 3. Materials and methods An annual record of water discharge and sediment load from the Pearl River basin for the period 1954e2011 was obtained from the Ministry of Water Resource of China (MWRC). These measurements were taken at the Gaoyao, Shijiao, and Boluo stations, the lowermost hydrological stations in the drainage basins (Fig. 1c), which serve as the controlling stations for monitoring water discharge and sediment load from the Pearl River to the delta. In order to calculate the temporal and spatial changes in the Pearl River subaqueous delta, bathymetric maps at scales of 1:25,000, 1:50,000, and 1:75,000, surveyed in 1955, 1964, 1977, 1989, 1998, and 2008, were provided by the Guangzhou Maritime Safety Administration and China People's Liberation Army Navy Command Assurance Department of Navigation. Due to the size and complexity of the delta, no single map covered the entire area. Therefore, the Lingdingyang and Modaomen deltas are the focus of this paper. The largest possible area of these two deltas was selected to ensure that the data are representative, and the accretion/erosion rates were calculated over an interval of two decades. Each set of depths and isobaths from the maps was processed and a digital elevation model (DEM) constructed using ArcGIS software, then each set of depth soundings was interpolated onto a grid of 50 50 m cells using Kriging interpolation. Digitized maps were used to calculate the vertical accretion/erosion rates and to delineate accretion/erosion areas. For each grid, deduction of the later depth from the earlier depth gave the thickness of accretion (positive) or erosion (negative). The total volumes of accretion and
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erosion, and the difference between them, were then calculated. The rate of annual accretion or erosion could then be calculated from the net volume divided by the area and time period (years). 4. Results and discussion 4.1. Changes in water discharge and sediment load Annual water discharge and sediment load have been analyzed for the Pearl River in the different periods, taking account of peak values and human effect (Fig. 2). Fig. 2 shows that water discharge has remained relatively constant, except for occasional unusually dry or wet years, since the 1950s, averaging ~282 km3/y as calculated from the stations at Gaoyao, Shijiao, and Boluo. The variations in annual water discharge are closely related to precipitation changes in the region (Zhang et al., 2009; Wu et al., 2012). However, the Pearl River to the estuary began to show a dramatically decreasing trend after 1994 (Fig. 2). Before the 1990s, the sediment load in the river basin was mainly determined by hydro-climatic factors, i.e., fluctuations in sediment load were synchronous with fluctuations in water discharge. This is confirmed by the statistically significant correlation coefficients between water discharge and sediment load of 0.56e0.88 (Fig. 3). Fig. 3 also shows that sediment load transport reached its peak during the period 1983e1994. This upward trend in sediment load was probably related to the increasing rocky desertification of the surrounding karst caused by the flow regime in the upper reaches of the Pearl River basin (Zhang et al., 2012). After 1994, much of the river's sediment load was trapped by the reservoirs (Wu et al., 2012). For example, during 1994e2006, sediment load fell from 88.0 to 57.9 Mt/y due to the completion of the Yantan Dam (YTD) in 1992. Longtan Dam (LTD), as the second largest dam in China, was completed in the Pearl River. After operating in 2006, the sediment load to fall from about 85.9 Mt/y in the 1980s and 1990s, to 29.2 Mt/ y from 2006 to 2011, and approximately 66% of the sediment load is now deposited in the reservoirs. Many reservoirs have been constructed along the Pearl River for flood control and power generation since the 1990s, and the total storage capacity had reached 621 km3 by 2006 (Fig. 4). These data indicate that human activities in the Pearl River basin, especially construction of the LTD, have become the dominant control on sediment delivery from the Pearl River to the South China Sea. 4.2. Morphological changes on the Pearl River delta Unlike the Yangtze and Yellow river deltas, the Pearl River delta is filled by a large amount of silt in the mouths of the West, North and East rivers. At present, there are numerous islands along the Pearl River delta coast which act as nuclei for the deposition of sediment carried by the rivers. It remains unclear how the subaqueous delta of the Pearl River estuary has responded to changes in river flow. In an attempt to identify temporal trends in the Pearl River delta bathymetry and morphology, we compared bathymetric data to analyze the delta evolution from 1950s to 2000s. 4.2.1. Lingding Bay delta The delta of Lingding Bay, located in the northeast of the Pearl River estuary, was initially part of a coastal embayment during the mid-Holocene (Wang et al., 2007), but was filled by the large amount of silt provided by the Humen, Jiaomen, Hongqimen, and Hengmen rivers, known as the ‘four east men’, which accounted for ~50% of the total sediment load from the Pearl River (Table 1) (Luo et al., 2007). To determine the temporal and spatial trends associated with the development of the Lingding Bay subaqueous delta, we compared data from 1955, 1977, 1989, 1998, and 2008 over an
Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
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C.S. Wu et al. / Quaternary International xxx (2015) 1e8
Fig. 2. Annual water discharge and sediment load for the Pearl River basin from 1954 to 2010 as measured at the Gaoyao, Shijiao, and Boluo stations.
area of 862 km2, which accounts for ~84% of the whole Lingdingyang area. For the period between 1955 and 1977, the bathymetric data indicate that most of the Lingding subaqueous delta was accumulating sediments over an area of 600 km2 (Table 2), with sediment being supplied from the Pearl River into Lingding bay at an annual average of 39 Mt/y. The accumulation rate was between 0 and 36 mm/y. The net deposition rate was approximately 1.66 cm/y (Table 2). The total amount of sediment deposited was 428 Mt, which is equivalent to ~80% of the total sediment load (546 Mt) over this period (1.23 g/cm3). The results indicated that most sediment load from ‘four east men’ is deposited in Lingdingyang bay. The other 20% of sediment load may be deposited in the channel or enters the South China Sea. Between 1977 and 1989, the area of the Lingding subaqueous delta continued in a depositional state. The depositional area decreased to 534 km2, and the erosional area increasing during this period, especially in the northern section where the scour depth exceeded 1 m due to commercial sand extraction. The urbanization process accelerated at this time to meet the needs of economic development, and there was considerable development activity in the Lingdingyang estuary. According to incomplete statistics, the average dredging amount was ~10 106 m3/y (Yao, 2007). In addition, the Lingdingyang channel has been regularly dredged to 8.6e9.0 m water depth to facilitate navigation since 1979. Therefore, the total dredged amount reached 205 106 m3 from channel
maintenance and sand extraction. If we ignore sediment removal by sand mining, the accumulation rate is 11.1 mm/y, which represents a decrease of 33% over this period. However, the amount removed by dredging had a significant impact on delta evolution. Therefore, the actual accumulation rate was up to 29.4 mm/y (Fig. 5), while the sediment load entering the Lingdingyang was 44.2 Mt/y. Between 1989 and 1998, the area under erosion continued to expand, to about 374 km2, more than 50% of the total study area. The east shoal of the study area was eroding as in 1977e1989 (Fig. 5). According to the calculation results of the bathymetric maps between 1989 and 1998, the Lingdingyang subaqueous delta was transitioning from deposition to erosion, with an erosion rate of 3.1 mm/y (Fig. 5; Table 2). However, during this period, there were many human activities in the Lingdingyang estuary, such as sand mining and channel dredging. If we want to know the evolution of the subaqueous delta responding to sediment supply, we cannot ignore this volume, which can give rise to delta erosion. According to incomplete statistics, since the 1990s, the average sand removal for dredging and navigation maintenance accounts for ~19.53 106 m3 (Luo et al., 2007; Yao, 2007). If the account of sediment is added into the former volume (0.027 km3) (Table 2), as a consequence, the delta would actually be accumulating sediment, and the accumulation rate was 16.5 mm/y if there were no sand mining and channel dredging activities. Between 1998 and 2008, the erosional area became increasingly large. The calculated erosion rate is 2.5 mm/y, but the deepening of the Lingding channel which was from 9 to 11.5 m in 2000 played
Fig. 3. Relationship between water discharge and sediment load in Pearl River during 1954e1963, 1963e1983, 1983e1994, 1994e2006 and 2006e2010.
Fig. 4. Total storage capacity of the reservoirs on the Pearl River during the period 1954e2010.
Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
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Table 1 The ratios of water discharge and sediment load in the eight “gates” of Pearl River estuaries (unit: %).
Water discharge Sediment load
Humen
Jiaomen
Hongqimen
Hengmen
Modaomen
Jidimen
Hutiaomen
Yamen
18.5 9.3
17.3 18.2
6.4 7.3
11.2 13.0
28.3 33.0
6.0 7.0
6.2 7.1
6.0 5.1
Table 2 Decrease in accretion rate response to decline in riverine sediment supply in Lingdingyang bay during different periods(The bold represent sediment rate without sand mining and channel dredging activities). Time interval
Accretion 2
1955e1977 1977e1989 1989e1998 1998e2008
Erosion 3
Net accretion 2
3
3
Sediment supply (Mt/yr)
Area (km )
Vol (km )
Rate (cm/yr)
Area (km )
Vol (km )
Rate (cm/yr)
Vol (km )
Rate (mm/yr)
600 534 488 468
0.439 0.339 0.321 0.520
3.05 4.90 6.58 10.01
262 328 374 394
0.096 0.215 0.348 0.545
1.53 5.04 9.28 12.53
0.342 0.125 0.027 0.025
16.6 11.1/29.4 3.1/16.5 2.6/1.6
an important role in the evolution of the Lingdingyang. The total dredged amount was about 40.0 106 m3 (Yao, 2007). If we add this amount to the results between 1998 and 2008, the subaqueous delta in Lingdingyang would also be undergoing accumulation, but the accumulation rate is only 1.6 mm/y. Compared to the period from 1989 to 1998, the sedimentation rate dropped dramatically mainly due to the decreased sediment supply from the Pearl River. Sediment supply to Lingding Bay was 20.9 Mt/y, which is a decrease of 44% compared with the period 1989e1998. This decline was caused by the construction of many large reservoirs in the Pearl River basin. The total storage capacity of these reservoirs was 455 km3, which equates to 167% of the storage capacity during the period 1958e1998 (Fig. 4).
39.0 44.2 37.1 20.9
Based on the bathymetric changing analysis in the Lingdingyang over the past 50 years, the results indicate that the evolution of the subaqueous delta in the study area has undergone two stages of rapid growth and rapid decline due to sediment supply from the rivers. There is a good correlation between the accumulation rate and sediment load as measured in the Pearl River (R2 ¼ 0.92) (Fig. 6). The linear relationship suggests that erosion of the subaqueous delta will occur if the sediment load from the river is less than ~21.7 Mt/y. 4.2.2. Evidence from the Modaomen delta The Modaomen delta is located in the southern part of the Pearl River delta. As the main waterway of the West River, it is much
Fig. 5. A) Accumulation/erosion in the subaqueous delta of the internal LingDing Bay (negative value represents erosion; positive value represents accretion) during the periods 1955e77, 1977e89, 1989e1998 and 1998e2008. B) Comparison of sediment load and sediment accumulation/erosion rate into Lingdingyang estuary. Circle grid represents natural accumulation/erosion condition based on no sand mining and channel dredging hypothesis.
Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
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Fig. 6. Comparison of subaqueous delta accumulation/erosion between various bathymetric surveys and mean sediment load in Lingdingyang estuary during the periods 1954e77, 1977e89, 1989e1998 and 1998e2008 (R: correlative coefficient; P: level of significance).
more strongly influenced by river runoff than by tides, with the ratio of mean annual runoff to tidal discharge being 5.77 (Jia et al., 2009). The annual average river sediment load is ~23.4 Mt/y, which
is ~30.4% of the total for the whole area (Luo et al., 2007; Wu et al., 2010). Over the past 50 years, the delta has been widely affected by human activity and highly utilized. Previous studies of the delta have concentrated on hydraulics, and sand bar and morphological evolution (Wu et al., 2010; Mo et al., 2011; Jia et al., 2009). To assess the response of the subaqueous delta to changing sediment load, bathymetric data for 1955, 1964, 1977, and 2005 were obtained from the Guangzhou Maritime Bureau. The study area, which covers ~682 km2, is shown in Fig. 1c. Between 1955 and 1964, the common study area covered 394 km2, only about 58% of that analyzed in the two later periods, due to the bathymetric data being unavailable for 1955. The accumulation rate over this restricted area was 9 mm/y. We used the least squares method to calculate that the accumulation rate for the whole area was ~1.6 mm/y, based on the Pearl River discharging an average sediment load of 23.5 Mt/y into the Modaomen Estuary (Table 3). From 1964 to 1977, the outer subaqueous delta of the Pearl River continued to prograde due to the increased sediment discharge (Fig. 7). During this 14-year period, 44% of the area shoaled between 0 and 0.5 m, and 36% eroded generally between 0 and e0.5 m in the north of the area. The calculated net shoaling for the entire study area increased to 2.4 mm/y as the sediment load increased to 28.2 Mt/y. From 1977 to 2005, the erosion area in the north of the study area increased in size. Although the river sediment load decreased
Table 3 Decrease in accretion rate response to decline in riverine sediment supply in Modaomen estuary during different periods(Rates basically represent natural evolution without human intervention). Time interval
1955e1964 1964e1977 1977e2005
Accretion
Erosion
Net accretion
Sediment supply (Mt/yr)
Area (km2)
Vol (km3)
Rate (mm/yr)
Area (km2)
Vol (km3)
Rate (mm/yr)
Vol (km3)
Rate (mm/yr)
210 378 270
0.155 0.114 0.121
66.84 21.46 15.49
183 328 412
0.151 0.091 0.051
74.99 21.37 4.26
0.003 0.023 0.071
0.75 2.56 3.56
23.9 28.2 25.9
Fig. 7. A) Accumulation/erosion in the outer subaqueous delta in the Modaomen estuary (negative value represents erosion; positive value represents accretion) during the periods 1955e77, 1977e89, 1989e1998 and 1998e2008. B) Comparison of sediment load and sediment accumulation/erosion rate into Lingdingyang estuary.
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from 28.2 to 25.9 Mt/y during this period, the whole subaqueous delta continued to prograde, but at only 1.5 mm/y. Compared with the former period, the accumulation rate decreased by 38%, whereas the sediment load decreased by only 8%. After the closure of the LTD in 2006, the sediment load flowing into the Modaomen Estuary decreased sharply to 9.36 Mt/y. As a consequence, the outer delta is now experiencing sediment starvation, so we must focus on the evolution of the Modaomen delta in the next decades. 5. Conclusions The sediment load from the Pearl River basin, as measured at Gaoyao, Shijiao, and Boluo, carried into the South China Sea, increased from ~69.8 Mt/y (1954e1963) to 88.0 Mt/y (1984e1993), and then decreased to 29.2 Mt/y after the closure of the LTD in 2006. In contrast, water discharge from 1954 to 2008 remained relatively stable. These results indicate that the sediment load carried into the sea has been significantly disturbed by human activities, and primarily trapped behind dams. In response to the dramatic decrease in sediment supply, the subaqueous delta in Lingdingyang has experienced sediment starvation, and the accretion rate has decreased from 17 to 1.6 mm/y. This decrease has been more rapid than the decline in river sediment load over the same period. Thus, net erosion is likely to occur first in Lingdingyang as the decline in sediment load continues. The conversion of the delta from accretion to erosion appears to have occurred when the fluvial sediment input to Lingdingyang fell below 22 Mt/y. However, the accumulation rate of the Modaomen outer subaqueous delta has increased from 1.3 mm/y to 3.5 mm/y in the past 50 years due to sediment supply. The account of sediment supply during 1980e2005 is balanced with the period of 1954e1964. In the coming decades, the LTD's operation and other human activities were initiated in the watershed from 2006 to 2008, the sediment load carried into the Modaomen Estuary decreased to 9.6 Mt/y, which is ~40% of the sediment load during the period 1980e2005. Therefore, the outer subaqueous delta in Modaomen may undergo significant changes in the future. Based on the Comprehensive Treatment Plan in Pearl River Estuary approved by the China State Council in 2010, the river sediment load carried into the sea has been increasingly disturbed by human activities and is likely to continue to decline. Consequently, erosion of the subaqueous delta is likely to continue over coming decades. Acknowledgements This work was supported by the Natural Science Foundation of China (41021064) and the Ministry of Science and Technology of China (2010CB951202). The Natural Science Foundation of China (41021064) and the Ministry of Science and Technology of China (2010CB951202), China Postdoctoral Fund (2014M551773), the Science and Technology Plan for Zhejiang Province (2012F20049), and the Natural Science Foundation of China (LY13E090001). References nchez, A., 1999. Sedimentation in the Colorado River delta and Carriquiry, J.D., Sa Upper Gulf of California after nearly a century of discharge loss. Marine Geology 158, 125e145. Chen, X.H., Chen, Y.D., 2002. Hydrologicla change and its causes in the river network of the Pearl River Delta. Acta Geographica Sinica 57 (4), 430e436 (in Chinese). Day, J.W., Giosan, L., 2008. Survive or subside? Nature Geoscience 1, 156e157.
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Delta changes in the Pearl River estuary and its response to human activities (1954e2008) Chuang Shou Wu a, b, *, Shilun Yang c, Shichang Huang a, b, Jinbin Mu a, b a
Zhejiang Institute of Hydraulic & Estuary, Hangzhou 030020, China Key Laboratory of Estuarine and Coastal Research in Zhejiang Province, Hangzhou 030020, China c State key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
This paper is concerned with delta evolution in response to human activities in the Pearl River estuary. Utilizing more than 50 years data bathymetric data, together with simulation fluvial discharge data, the net accretion rate in the Lingdingyang subaqueous delta decreased from 16.6 mm/y over the period 1955 e1964 to 1.6 mm/y for the period 1998e2008 because there was a dramatic downward trend in the sediment load. In particular, the closure of the Longtan Dam (LTD) in 2006 resulted in the sediment load entering the Pearl River falling from 75 Mt/y (annual average: 1954e2006) to 25.2 Mt/y, a decrease of ~67%. Another reason may be sand mining in the Lingdingyang estuary. Based on the linear relationship between sediment load and accumulation rate, an annual average sediment load of 21.7 Mt/y carried into the Lingdingyang estuary is the critical threshold value that separates delta progradation from recession. However, the outer subaqueous delta in the Modaomen estuary underwent deposition in the same period, because there is no obvious change in sediment load into the area during 1954e2008. In the coming decades, ongoing human interferences will reduce the fluvial sediment load entering the sea, which probably will result in a phase of general delta recession replacing delta progradation. © 2015 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Delta Sediment load Human activities Pearl River
1. Introduction Many of the world's river deltas currently face serious problems due to the combined pressure of human development and climate change (Day and Giosan, 2008). Throughout recorded history, dams and irrigation, as well as improved soil and water conservation practices, have reduced the sediment loads carried by many rivers. Consequently, the accretion/erosion response of deltas to this decline in the fluvial sediment supply has become a topic of global concern (Milliman, 1997; Syvitski et al., 2009; Edmonds, 2012). For example, an expanse of coastal land the size of 0.05 km2 disappears from the Mississippi River delta every hour due to the decreasing sediment load and sea level rise (Edmonds, 2012). The sediment load in the river Nile in Egypt was once between 100 106 and 124 106 t, but today there is almost no net annual sediment input to the Nile delta due to the Aswan High Dam, which began operating in 1964, and the delta is degrading with a shoreline erosion
* Corresponding author. Zhejiang Institute of Hydraulic & Estuary, Hangzhou 030020, China. E-mail address: [email protected] (C.S. Wu).
rate of 143e160 m/y (Stanley and Warne, 1993, 1998; Fanos, 1995; Frihy et al., 2003). The sediment load of the river Ebro located in northeastern Span was reduced from ~1.0 107 Mt/y to 0.3 106 Mt/y, after the construction of the RibarrojaeMequinenza dam in the lower Ebro at the end of the 1960s. Currently, the load ranges from 0.1 106 to 0.2 105 Mt/y. More than 99% of the sediment load has been trapped in the reservoirs, which was the n ~ ez et al., 1996; major cause of recession in the river mouth area (Iba nchez-Arcilla et al., 1998). The Colorado River, one of the most Sa highly regulated rivers in the word due to the numerous dams, once supplied more than 150 106 Mt/y of sediment to the Gulf of California. At present, the dams and course diversions effectively trap most of the sediment, and this has led to coastal erosion nchez, 1999; Huh et al., 2007). In China, the (Carriquiry and Sa Yangtze River, one of the largest rivers in the world, carried a sediment load of around 490 Mt/y in the 1950s and 1960s, but this fell to 150 Mt/y after the closure of the Three Gorges Dam (TGD). In response to this drastic decrease in sediment supply, the river channel has changed from net accretion to net erosion, and the deltaic coast has shifted from progradation to recession (Yang et al., 2003, 2005, 2011; Luo et al., 2012; Dai and Liu, 2013; Dai and Lu, 2014). The Yellow River (Huanghe) once carried the second
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largest sediment load in the world (Milliman et al., 1987), but now carries only 14% of the widely cited estimate of the peak load of 1.08 Gt/y, due to both natural processes and human activities over the past 56 years (Wang et al., 2007). This dramatic decrease in sediment load has caused the whole delta to switch from a state of accretion to erosion (Peng et al., 2010). As the second largest river China, and the 25th largest river in the world in terms of water discharge, the Pearl River is considered one of the world's most complicated fluvial networks. It is a compound river system, comprising three major tributaries (the West, North, and East rivers), and some other small rivers draining into the Pearl River delta (Fig. 1), which occupies an area of ~17,200 km2. The Pearl River delta is one of the most developed areas in China owing to implementation of China's open-door and reform policies since the 1980s, and at least 8636 reservoirs had been constructed in the watershed by the late 1990s. As a consequence, the hydrological regime of the Pearl River basin has been substantially altered by this intensive human activity, and the Pearl River has become one of the most highly impacted rivers in the world (Nilsson et al., 2005). Over recent years, previous studies have focused on various aspects of the changing water and sediment load of the Pearl River
(e.g., Dai et al., 2008; Zhang et al., 2008, 2009; Wu et al., 2012; Wu et al., 2012; Zhang et al., 2012; Liu et al., 2014) and channel changes (Lu et al., 2007). These papers indicate that human activities, in particular dam construction and soil conservation, have altered the natural sediment transport regime. However, to the best of our knowledge, no previous work has demonstrated the collective impact of changes to the subaqueous delta of the Pearl River caused by the changing sediment supply. Therefore, in this paper, we investigate the evolution of the subaqueous delta in response to human activities, based on hydrological data and bathymetric maps covering the period from 1950s to 2008. 2. Physical setting The Pearl River originates on the Yunnan Plateau and crosses low-terrain hills and even mountains as it runs 2400 km eastwards to South China Sea (Wu and Zhou, 2001). Due to its long-term average annual water discharge of 3260 108 m3/y, it plays an important role in fresh water supply to the large cities in the Pearl River delta region, such as Macau, Hong Kong, Zhuhai, and Guangzhou. The annual suspended sediment load of the river is
Fig. 1. (a) The Pearl River basin in South China. (b) The Pearl River chorographic map, including the Longtan Dam (LTD) and the Gaoyao, Shijiao, and Boluo gauging stations. (c) the Pearl River subaqueous delta, showing the Lingding Bay and Modaomen areas (dashed boxes). Numbers on the map represent eight “gates”: 1, Humen; 2, Jiaomen; 3, Hongqili; 4, Hengmen; 5, Modaomen; 6, Jitimen; 7, Hutiaomen; 8, Yamen.
Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
C.S. Wu et al. / Quaternary International xxx (2015) 1e8
88.7 Mt/y. The Pearl River is comprised of three major tributaries (the West, North, and East rivers), and some other small rivers draining into the Pearl River delta (Fig. 1). The West River, as the largest branch, (Xijiang), has a total length of 2214 km and a drainage area of 35.5 104 km2; the North River (Beijiang) has a total length of 468 km and a drainage area of 4.68 104 km2; while the East River (Dongjiang) has a total length of 562 km and a drainage area of 3.24 104 km2. The annual average water and suspended sediment discharges via the West River are 2281 108 m3/y and 6567 104 t/y, respectively, while for the North River they are 449 108 m3/y and 864 104 t/y, respectively, and for the East River are 234 108 m3/y and 236 104 t/y, respectively. The total water and sediment discharges of these three major rivers account for more than 80% and 95%, respectively, of the load entering the sea, which causes the seaward extension of the mouth region of the Pearl River at a rate of 40 m per year (Gong and Chen, 1964). The basin is located on the Tropic of Cancer, and has a subtropical and tropical monsoon climate with a dry season from October to March and a rainy season from April to September. The mean annual temperature is 14 e22 C and annual precipitation is 1470 mm (Dai et al., 2008). After entering the delta plain, the river bifurcates continuously and forms a complex network of distributaries that eventually discharge into estuarine bays. The Pearl River delta, the third largest in China (after the Yangtze and Yellow river deltas), is located between 21400 and 23 N, and 112 and 113 E (Fig. 1b). Sediment load from the rivers plays an important role during the delta forming process (Li et al., 2001; Wu et al., 2010). Eventually water discharge and sediment load enter a complex network of distributaries, with a channel density of 0.68 km/km2e1.07 km/km2, and flow out through the eight “gates”, Humen, Jiaomen, Hongqili, Hengmen, Modaomen, Jitimen, and Hutiaomen, into estuarine bays (Chen and Chen, 2002) (Fig. 1c). The delta was created by the deposition of river sediments into the receiving basin after the end of the Holocene transgression over the past 6000 years (Li et al., 2001), and consists of the West, North, and East river sub-deltas. 3. Materials and methods An annual record of water discharge and sediment load from the Pearl River basin for the period 1954e2011 was obtained from the Ministry of Water Resource of China (MWRC). These measurements were taken at the Gaoyao, Shijiao, and Boluo stations, the lowermost hydrological stations in the drainage basins (Fig. 1c), which serve as the controlling stations for monitoring water discharge and sediment load from the Pearl River to the delta. In order to calculate the temporal and spatial changes in the Pearl River subaqueous delta, bathymetric maps at scales of 1:25,000, 1:50,000, and 1:75,000, surveyed in 1955, 1964, 1977, 1989, 1998, and 2008, were provided by the Guangzhou Maritime Safety Administration and China People's Liberation Army Navy Command Assurance Department of Navigation. Due to the size and complexity of the delta, no single map covered the entire area. Therefore, the Lingdingyang and Modaomen deltas are the focus of this paper. The largest possible area of these two deltas was selected to ensure that the data are representative, and the accretion/erosion rates were calculated over an interval of two decades. Each set of depths and isobaths from the maps was processed and a digital elevation model (DEM) constructed using ArcGIS software, then each set of depth soundings was interpolated onto a grid of 50 50 m cells using Kriging interpolation. Digitized maps were used to calculate the vertical accretion/erosion rates and to delineate accretion/erosion areas. For each grid, deduction of the later depth from the earlier depth gave the thickness of accretion (positive) or erosion (negative). The total volumes of accretion and
3
erosion, and the difference between them, were then calculated. The rate of annual accretion or erosion could then be calculated from the net volume divided by the area and time period (years). 4. Results and discussion 4.1. Changes in water discharge and sediment load Annual water discharge and sediment load have been analyzed for the Pearl River in the different periods, taking account of peak values and human effect (Fig. 2). Fig. 2 shows that water discharge has remained relatively constant, except for occasional unusually dry or wet years, since the 1950s, averaging ~282 km3/y as calculated from the stations at Gaoyao, Shijiao, and Boluo. The variations in annual water discharge are closely related to precipitation changes in the region (Zhang et al., 2009; Wu et al., 2012). However, the Pearl River to the estuary began to show a dramatically decreasing trend after 1994 (Fig. 2). Before the 1990s, the sediment load in the river basin was mainly determined by hydro-climatic factors, i.e., fluctuations in sediment load were synchronous with fluctuations in water discharge. This is confirmed by the statistically significant correlation coefficients between water discharge and sediment load of 0.56e0.88 (Fig. 3). Fig. 3 also shows that sediment load transport reached its peak during the period 1983e1994. This upward trend in sediment load was probably related to the increasing rocky desertification of the surrounding karst caused by the flow regime in the upper reaches of the Pearl River basin (Zhang et al., 2012). After 1994, much of the river's sediment load was trapped by the reservoirs (Wu et al., 2012). For example, during 1994e2006, sediment load fell from 88.0 to 57.9 Mt/y due to the completion of the Yantan Dam (YTD) in 1992. Longtan Dam (LTD), as the second largest dam in China, was completed in the Pearl River. After operating in 2006, the sediment load to fall from about 85.9 Mt/y in the 1980s and 1990s, to 29.2 Mt/ y from 2006 to 2011, and approximately 66% of the sediment load is now deposited in the reservoirs. Many reservoirs have been constructed along the Pearl River for flood control and power generation since the 1990s, and the total storage capacity had reached 621 km3 by 2006 (Fig. 4). These data indicate that human activities in the Pearl River basin, especially construction of the LTD, have become the dominant control on sediment delivery from the Pearl River to the South China Sea. 4.2. Morphological changes on the Pearl River delta Unlike the Yangtze and Yellow river deltas, the Pearl River delta is filled by a large amount of silt in the mouths of the West, North and East rivers. At present, there are numerous islands along the Pearl River delta coast which act as nuclei for the deposition of sediment carried by the rivers. It remains unclear how the subaqueous delta of the Pearl River estuary has responded to changes in river flow. In an attempt to identify temporal trends in the Pearl River delta bathymetry and morphology, we compared bathymetric data to analyze the delta evolution from 1950s to 2000s. 4.2.1. Lingding Bay delta The delta of Lingding Bay, located in the northeast of the Pearl River estuary, was initially part of a coastal embayment during the mid-Holocene (Wang et al., 2007), but was filled by the large amount of silt provided by the Humen, Jiaomen, Hongqimen, and Hengmen rivers, known as the ‘four east men’, which accounted for ~50% of the total sediment load from the Pearl River (Table 1) (Luo et al., 2007). To determine the temporal and spatial trends associated with the development of the Lingding Bay subaqueous delta, we compared data from 1955, 1977, 1989, 1998, and 2008 over an
Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
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Fig. 2. Annual water discharge and sediment load for the Pearl River basin from 1954 to 2010 as measured at the Gaoyao, Shijiao, and Boluo stations.
area of 862 km2, which accounts for ~84% of the whole Lingdingyang area. For the period between 1955 and 1977, the bathymetric data indicate that most of the Lingding subaqueous delta was accumulating sediments over an area of 600 km2 (Table 2), with sediment being supplied from the Pearl River into Lingding bay at an annual average of 39 Mt/y. The accumulation rate was between 0 and 36 mm/y. The net deposition rate was approximately 1.66 cm/y (Table 2). The total amount of sediment deposited was 428 Mt, which is equivalent to ~80% of the total sediment load (546 Mt) over this period (1.23 g/cm3). The results indicated that most sediment load from ‘four east men’ is deposited in Lingdingyang bay. The other 20% of sediment load may be deposited in the channel or enters the South China Sea. Between 1977 and 1989, the area of the Lingding subaqueous delta continued in a depositional state. The depositional area decreased to 534 km2, and the erosional area increasing during this period, especially in the northern section where the scour depth exceeded 1 m due to commercial sand extraction. The urbanization process accelerated at this time to meet the needs of economic development, and there was considerable development activity in the Lingdingyang estuary. According to incomplete statistics, the average dredging amount was ~10 106 m3/y (Yao, 2007). In addition, the Lingdingyang channel has been regularly dredged to 8.6e9.0 m water depth to facilitate navigation since 1979. Therefore, the total dredged amount reached 205 106 m3 from channel
maintenance and sand extraction. If we ignore sediment removal by sand mining, the accumulation rate is 11.1 mm/y, which represents a decrease of 33% over this period. However, the amount removed by dredging had a significant impact on delta evolution. Therefore, the actual accumulation rate was up to 29.4 mm/y (Fig. 5), while the sediment load entering the Lingdingyang was 44.2 Mt/y. Between 1989 and 1998, the area under erosion continued to expand, to about 374 km2, more than 50% of the total study area. The east shoal of the study area was eroding as in 1977e1989 (Fig. 5). According to the calculation results of the bathymetric maps between 1989 and 1998, the Lingdingyang subaqueous delta was transitioning from deposition to erosion, with an erosion rate of 3.1 mm/y (Fig. 5; Table 2). However, during this period, there were many human activities in the Lingdingyang estuary, such as sand mining and channel dredging. If we want to know the evolution of the subaqueous delta responding to sediment supply, we cannot ignore this volume, which can give rise to delta erosion. According to incomplete statistics, since the 1990s, the average sand removal for dredging and navigation maintenance accounts for ~19.53 106 m3 (Luo et al., 2007; Yao, 2007). If the account of sediment is added into the former volume (0.027 km3) (Table 2), as a consequence, the delta would actually be accumulating sediment, and the accumulation rate was 16.5 mm/y if there were no sand mining and channel dredging activities. Between 1998 and 2008, the erosional area became increasingly large. The calculated erosion rate is 2.5 mm/y, but the deepening of the Lingding channel which was from 9 to 11.5 m in 2000 played
Fig. 3. Relationship between water discharge and sediment load in Pearl River during 1954e1963, 1963e1983, 1983e1994, 1994e2006 and 2006e2010.
Fig. 4. Total storage capacity of the reservoirs on the Pearl River during the period 1954e2010.
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Table 1 The ratios of water discharge and sediment load in the eight “gates” of Pearl River estuaries (unit: %).
Water discharge Sediment load
Humen
Jiaomen
Hongqimen
Hengmen
Modaomen
Jidimen
Hutiaomen
Yamen
18.5 9.3
17.3 18.2
6.4 7.3
11.2 13.0
28.3 33.0
6.0 7.0
6.2 7.1
6.0 5.1
Table 2 Decrease in accretion rate response to decline in riverine sediment supply in Lingdingyang bay during different periods(The bold represent sediment rate without sand mining and channel dredging activities). Time interval
Accretion 2
1955e1977 1977e1989 1989e1998 1998e2008
Erosion 3
Net accretion 2
3
3
Sediment supply (Mt/yr)
Area (km )
Vol (km )
Rate (cm/yr)
Area (km )
Vol (km )
Rate (cm/yr)
Vol (km )
Rate (mm/yr)
600 534 488 468
0.439 0.339 0.321 0.520
3.05 4.90 6.58 10.01
262 328 374 394
0.096 0.215 0.348 0.545
1.53 5.04 9.28 12.53
0.342 0.125 0.027 0.025
16.6 11.1/29.4 3.1/16.5 2.6/1.6
an important role in the evolution of the Lingdingyang. The total dredged amount was about 40.0 106 m3 (Yao, 2007). If we add this amount to the results between 1998 and 2008, the subaqueous delta in Lingdingyang would also be undergoing accumulation, but the accumulation rate is only 1.6 mm/y. Compared to the period from 1989 to 1998, the sedimentation rate dropped dramatically mainly due to the decreased sediment supply from the Pearl River. Sediment supply to Lingding Bay was 20.9 Mt/y, which is a decrease of 44% compared with the period 1989e1998. This decline was caused by the construction of many large reservoirs in the Pearl River basin. The total storage capacity of these reservoirs was 455 km3, which equates to 167% of the storage capacity during the period 1958e1998 (Fig. 4).
39.0 44.2 37.1 20.9
Based on the bathymetric changing analysis in the Lingdingyang over the past 50 years, the results indicate that the evolution of the subaqueous delta in the study area has undergone two stages of rapid growth and rapid decline due to sediment supply from the rivers. There is a good correlation between the accumulation rate and sediment load as measured in the Pearl River (R2 ¼ 0.92) (Fig. 6). The linear relationship suggests that erosion of the subaqueous delta will occur if the sediment load from the river is less than ~21.7 Mt/y. 4.2.2. Evidence from the Modaomen delta The Modaomen delta is located in the southern part of the Pearl River delta. As the main waterway of the West River, it is much
Fig. 5. A) Accumulation/erosion in the subaqueous delta of the internal LingDing Bay (negative value represents erosion; positive value represents accretion) during the periods 1955e77, 1977e89, 1989e1998 and 1998e2008. B) Comparison of sediment load and sediment accumulation/erosion rate into Lingdingyang estuary. Circle grid represents natural accumulation/erosion condition based on no sand mining and channel dredging hypothesis.
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Fig. 6. Comparison of subaqueous delta accumulation/erosion between various bathymetric surveys and mean sediment load in Lingdingyang estuary during the periods 1954e77, 1977e89, 1989e1998 and 1998e2008 (R: correlative coefficient; P: level of significance).
more strongly influenced by river runoff than by tides, with the ratio of mean annual runoff to tidal discharge being 5.77 (Jia et al., 2009). The annual average river sediment load is ~23.4 Mt/y, which
is ~30.4% of the total for the whole area (Luo et al., 2007; Wu et al., 2010). Over the past 50 years, the delta has been widely affected by human activity and highly utilized. Previous studies of the delta have concentrated on hydraulics, and sand bar and morphological evolution (Wu et al., 2010; Mo et al., 2011; Jia et al., 2009). To assess the response of the subaqueous delta to changing sediment load, bathymetric data for 1955, 1964, 1977, and 2005 were obtained from the Guangzhou Maritime Bureau. The study area, which covers ~682 km2, is shown in Fig. 1c. Between 1955 and 1964, the common study area covered 394 km2, only about 58% of that analyzed in the two later periods, due to the bathymetric data being unavailable for 1955. The accumulation rate over this restricted area was 9 mm/y. We used the least squares method to calculate that the accumulation rate for the whole area was ~1.6 mm/y, based on the Pearl River discharging an average sediment load of 23.5 Mt/y into the Modaomen Estuary (Table 3). From 1964 to 1977, the outer subaqueous delta of the Pearl River continued to prograde due to the increased sediment discharge (Fig. 7). During this 14-year period, 44% of the area shoaled between 0 and 0.5 m, and 36% eroded generally between 0 and e0.5 m in the north of the area. The calculated net shoaling for the entire study area increased to 2.4 mm/y as the sediment load increased to 28.2 Mt/y. From 1977 to 2005, the erosion area in the north of the study area increased in size. Although the river sediment load decreased
Table 3 Decrease in accretion rate response to decline in riverine sediment supply in Modaomen estuary during different periods(Rates basically represent natural evolution without human intervention). Time interval
1955e1964 1964e1977 1977e2005
Accretion
Erosion
Net accretion
Sediment supply (Mt/yr)
Area (km2)
Vol (km3)
Rate (mm/yr)
Area (km2)
Vol (km3)
Rate (mm/yr)
Vol (km3)
Rate (mm/yr)
210 378 270
0.155 0.114 0.121
66.84 21.46 15.49
183 328 412
0.151 0.091 0.051
74.99 21.37 4.26
0.003 0.023 0.071
0.75 2.56 3.56
23.9 28.2 25.9
Fig. 7. A) Accumulation/erosion in the outer subaqueous delta in the Modaomen estuary (negative value represents erosion; positive value represents accretion) during the periods 1955e77, 1977e89, 1989e1998 and 1998e2008. B) Comparison of sediment load and sediment accumulation/erosion rate into Lingdingyang estuary.
Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009
C.S. Wu et al. / Quaternary International xxx (2015) 1e8
from 28.2 to 25.9 Mt/y during this period, the whole subaqueous delta continued to prograde, but at only 1.5 mm/y. Compared with the former period, the accumulation rate decreased by 38%, whereas the sediment load decreased by only 8%. After the closure of the LTD in 2006, the sediment load flowing into the Modaomen Estuary decreased sharply to 9.36 Mt/y. As a consequence, the outer delta is now experiencing sediment starvation, so we must focus on the evolution of the Modaomen delta in the next decades. 5. Conclusions The sediment load from the Pearl River basin, as measured at Gaoyao, Shijiao, and Boluo, carried into the South China Sea, increased from ~69.8 Mt/y (1954e1963) to 88.0 Mt/y (1984e1993), and then decreased to 29.2 Mt/y after the closure of the LTD in 2006. In contrast, water discharge from 1954 to 2008 remained relatively stable. These results indicate that the sediment load carried into the sea has been significantly disturbed by human activities, and primarily trapped behind dams. In response to the dramatic decrease in sediment supply, the subaqueous delta in Lingdingyang has experienced sediment starvation, and the accretion rate has decreased from 17 to 1.6 mm/y. This decrease has been more rapid than the decline in river sediment load over the same period. Thus, net erosion is likely to occur first in Lingdingyang as the decline in sediment load continues. The conversion of the delta from accretion to erosion appears to have occurred when the fluvial sediment input to Lingdingyang fell below 22 Mt/y. However, the accumulation rate of the Modaomen outer subaqueous delta has increased from 1.3 mm/y to 3.5 mm/y in the past 50 years due to sediment supply. The account of sediment supply during 1980e2005 is balanced with the period of 1954e1964. In the coming decades, the LTD's operation and other human activities were initiated in the watershed from 2006 to 2008, the sediment load carried into the Modaomen Estuary decreased to 9.6 Mt/y, which is ~40% of the sediment load during the period 1980e2005. Therefore, the outer subaqueous delta in Modaomen may undergo significant changes in the future. Based on the Comprehensive Treatment Plan in Pearl River Estuary approved by the China State Council in 2010, the river sediment load carried into the sea has been increasingly disturbed by human activities and is likely to continue to decline. Consequently, erosion of the subaqueous delta is likely to continue over coming decades. Acknowledgements This work was supported by the Natural Science Foundation of China (41021064) and the Ministry of Science and Technology of China (2010CB951202). The Natural Science Foundation of China (41021064) and the Ministry of Science and Technology of China (2010CB951202), China Postdoctoral Fund (2014M551773), the Science and Technology Plan for Zhejiang Province (2012F20049), and the Natural Science Foundation of China (LY13E090001). References nchez, A., 1999. Sedimentation in the Colorado River delta and Carriquiry, J.D., Sa Upper Gulf of California after nearly a century of discharge loss. Marine Geology 158, 125e145. Chen, X.H., Chen, Y.D., 2002. Hydrologicla change and its causes in the river network of the Pearl River Delta. Acta Geographica Sinica 57 (4), 430e436 (in Chinese). Day, J.W., Giosan, L., 2008. Survive or subside? Nature Geoscience 1, 156e157.
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Please cite this article in press as: Wu, C.S., et al., Delta changes in the Pearl River estuary and its response to human activities (1954e2008), Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.04.009