Recent changes in hydrodynamic characteristics of the Pearl River Delta during the flood period and associated underlying causes

Ocean and Coastal Management 179 (2019) 104814

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Recent changes in hydrodynamic characteristics of the Pearl River Delta during the flood period and associated underlying causes

T

Changjie Liua, Minghui Yua,∗, Huayang Caib, Xiaoqi Chena a b

State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, 430072, China Institute of Estuarine and Coastal Research, School of Marine Sciences, Sun Yat-sen University, Guangzhou, 510275, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Residual water level Human activities River-tide dynamics Pearl river delta

It is well-known that both human activities and river-tide dynamics are significant factors affecting the hydrodynamic process in estuarine and coastal regions. In this respect, it is considered that riverbed down-cutting exacerbated by human interventions (such as dam construction, sand excavation, and dredging for navigation) may exert dramatic changes in the hydrodynamic characteristics of the Pearl River Delta (PRD). In this study, changes in the rating curves of residual water level (RWL) versus residual water discharge (RWD) at upstream stations of the PRD are examined between three extreme floods (here termed as “986,” “056,” and “086,” respectively), in addition to variations in RWL along the West, North, and East Rivers. The results show a dramatic downshift of the rating curves (0.65–4.9 m) in “056” and “086”, compared with the case in “986”. Besides, the RWL along the three tributaries fell significantly at the same RWD and the maximum reduction in the RWL occurred in the East River. It is considered that the main reason for this occurrence could be contributed to uneven deepening of river channels induced by uncontrolled and disorderly human activities. In addition, the decrease in RWL was more significant in the upper reaches than that in the lower reaches, which reduced the RWL slope and enhanced the PRD tidal dynamics. However, compared with the case in “056,” the rating curves in “086” are seen to shift up slightly (0.11–0.29 m) and the RWL increased in upstream parts but decreased in downstream parts in the West and North Rivers. This phenomenon can be attributed to the impacts of river-tide dynamics: river flow plays an important role upstream, and the flood wave in “086” delivered more water volume into the PRD for the same range of RWD and raised the RWL, whereas tidal dynamics contributed considerably to the effects downstream and the small tidal range in “086” lowered the RWL. It is anticipated that these results can be used as scientific guidelines for general water management and particularly for flood control in the PRD.

1. Introduction

may far exceed those of natural changes in deltas situated in highly populated areas that are undergoing rapid economic development (Wang et al., 2015). Cumulative human activities have caused dramatic changes in riverbed topography and estuarine morphology, and have resulted in the modification of hydrodynamic characteristics further. For instance, due to the construction of the Three Gorges Dam, which has trapped large-scale sediment, the riverbed of the lower reaches in the Yangtze River has changed from one that is depositional to erosional (Chen et al., 2001; Yang et al., 2007; Dai et al., 2016), riverbed down-cutting is occurring in channels and the yearly mean water level has considerably decreased (Dai and Liu, 2013). It is evident that human activities are an important factor accelerating the evolution of hydrodynamic characteristics; therefore, considerable efforts are required to study the changes in hydrodynamic characteristics induced by

Most large-scale deltas throughout the world, such as the Mississippi Delta, the Mekong Delta, the Yangtze Delta, and the Pearl River delta (PRD), occur where a river enters an ocean. As the primary link between rivers and oceans, the river channels in deltas have extremely complex hydrodynamic characteristics that are influenced by interactions between river and tide dynamics (Ludwig et al., 1996; Milliman and Farnsworth, 2011; Cai et al., 2018a). The evolution of hydrodynamic characteristics in deltas is affected by both natural changes (such as sea level rise and global climate change) and human activities (such as dam construction, sand excavation, and dredging for navigation) (Day and Giosan, 2008; Liu et al., 2012, 2017; Cai et al., 2018b). However, the impacts of long-term and high-intensity human activities

∗

Corresponding author. E-mail address: [email protected] (M. Yu).

https://doi.org/10.1016/j.ocecoaman.2019.104814 Received 6 December 2018; Received in revised form 10 May 2019; Accepted 15 May 2019 Available online 12 June 2019 0964-5691/ © 2019 Elsevier Ltd. All rights reserved.

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to the influence of the summer monsoon, approximately 80% and 20% of annual precipitation occurs in the wet season (from April to September) and the dry season (from October to March), respectively. The seasonal water discharge distributions are strongly correlated with precipitation patterns; therefore, major flooding occurs in June and July within the PRD. The channel network in the PRD is considered to be one of the most complicated river networks in the world, and it has more than 300 longitudinal and transverse tributaries with a channel length density per unit of 0.68–1.07 km/km2 (Zhang et al., 2011). The PRD has three main tributaries: the West River, North River, and East River, which contribute 72%, 14%, and 7.6% of the water discharge debouching into the sea, respectively (Zhang et al., 2012b). After the West and North Rivers meet at Sixianjiao, the water discharge is redistributed, and it flows through Makou station into the West River networks and through Sanshui station into North River networks, respectively. In contrast, the East River networks are relatively independent downstream of Shilong station. Ultimately, the flows pass through eight outlets into estuarine bays, where Yamen and Hutiaomen are situated adjacent to Huangmaohai Bay; Jitimen and Modaomen are adjacent to the outer Modaomen Bay; and Hengmen, Hongqimen, Jiaomen, and Humen are adjacent to Lingdingyang Bay (Xia et al., 2004). The PRD mainly has a mixed semi-diurnal tidal regime with microtidal characteristics (an average tidal range of 1.0–1.7 m). Huangmaohai Bay and Lingdingyang Bay are funnel-shaped, tidedominated regions with high tidal range values and low RWL values, and they lie on the eastern and western sides of the PRD, respectively. In contrast, the central part of the channel networks in the PRD is a river-dominated region, where the tidal range and RWL have a low and high value, respectively (Cai et al., 2018a).

both natural changes and human activities in delta regions, as this is imperative for efficient and effective water management relating to water resource utilization, flood control, and prevention of saltwater intrusion. The PRD has suffered from intensifying human interventions since the implementation of China's open-door and reform policies in the late 1970s (Zhang et al., 2009, 2011), and the Pearl River has become one of the world's most regulated rivers (Nilsson et al., 2005). Due to dam construction and afforestation, the sediment load delivered into the PRD in the 2010s decreased to 32.6% of the level in the 1950s (Zhang et al., 2008; Liu et al., 2014; Tan et al., 2017). In addition, uncontrolled and large-scale sand excavation has occurred in all 324 tributaries of the PRD since the 1980s, to satisfy the demands of urban construction (Luo et al., 2007). Both the reduction in sediment load and the practice of sand excavation have resulted in considerable geomorphological changes occurring in the river channels of the PRD (Zhang et al., 2015a). Numerous studies have investigated the hydrodynamic responses to the combined influence of diverse human activities, and it has been determined that the water level has decreased in the upstream of the PRD, there have been adjustments to the divided flow ratio between various watercourses, and both the tidal range and flood tide duration have increased at most stations within the PRD (Lu et al., 2007; Zheng et al., 2014; Zhang et al., 2010, 2017). Previous studies have focused extensively on long-term variations and used annual or monthly time series data (e.g., Zhang et al., 2009, 2010; Liu et al., 2017). However, considering that the water level and discharge vary dramatically within a few days or hours during flood periods, analyses conducted on annual or monthly hydrological series may mask details of the shift in hydrodynamic characteristics; in particular, changes in the residual water level (RWL) averaged over a lunar day. Recently, Cai et al. (2018a) detected that the RWL distribution in the PRD has a complex pattern due to the highly nonlinear interaction between river and tide dynamics and that RWL is mainly determined by variations in water discharge during the flood period. It is considered that geomorphologic changes in the PRD induced by human activities may exert dramatic changes on the relationship between the RWL and residual water discharge (RWD). In addition, although there may be alternations in the distribution of the RWL, variations in the RWL along the river channels have not yet been fully examined. Therefore, this study detects changes in hydrodynamic characteristics during the flood period in terms of RWL using hourly hydrological series data from 30 stations over the whole PRD for three different flood periods. The main objectives of this study are as follows: (a) to examine and compare changes in the rating curves of RWL versus RWD at upstream stations of the PRD; (b) to investigate variations in the RWL along different river channels of the three main tributaries; (c) to detect changes in the RWL slope between the three flood periods; and (d) to explore the underlying causes of recent changes in hydrodynamic characteristics. A detailed analysis of the response of RWL to the geomorphologic changes induced by human activities is conducted, and the impacts of river-tide dynamics on the RWL are discussed. Recent changes in hydrodynamic characteristics during the flood period are presented in this study, and the results can be used as a scientific basis for water management in the PRD.

3. Data and methods 3.1. Datasets Over two recent decades, three extreme floods occurred in the PRD (June 1998, June 2005, and June 2008, respectively, which are referred to here as “986,” “056,” and “086”). The flood volumes in “986” and “056” were the largest in a century, with a total water discharge of more than 62300 m3/s at MK and SS stations, and the flood volume in “086” represented a more than fifty-year return period, with a peak water discharge of 60400 m3/s. In this study, three large-scale measurements conducted during the corresponding periods were utilized to detect recent changes in hydrodynamic characteristics within the PRD during the flood periods. The datasets were provided on an hourly scale by the Guangdong Hydrology Bureau of the People's Republic of China, and they include measurements of the water discharge at seven hydrological stations and water levels at 30 gauging stations distributed over the PRD. A list of the 30 gauging stations with corresponding periods relating to the three floods are provided in Table 1, and their locations are presented in Fig. 1. It is of note that all water levels measured at the different stations were corrected to mean sea level of Pearl River datum. To examine morphological changes in the river channels, the following hypsography data relating to BJ-54 coordinates and Pearl River datum were collected: cross-sectional profiles at upstream stations (such as GY, SJ, and BL stations) in 1998, 2005, and 2008; cross-sectional profiles across the Pearl River networks in 1999; and a waterway topographic map of the PRD in 2008. The cross-sectional profiles and waterway topographic map were supplied by the Guangdong Research Institute of Water Resources and Hydropower and the Guangdong Hydropower Planning and Design Institute, respectively, and these were strictly inspected by the relevant authorities prior to release.

2. Overview of the PRD The Pearl River is China's second-ranked river in terms of water discharge. The PRD is located in Guangdong Province in southern China, and it occupies a drainage area covering more than 9750 km2 (Fig. 1). Data recorded at Makou, Sanshui, and Boluo stations show that the PRD delivers 286 × 109 m3/yr of freshwater discharge into the South China Sea (Zhang et al., 2012a). The PRD is dominantly influenced by a south sub-tropical monsoon climate, the annual mean temperature across the delta is 22 °C–22.5 °C, and annual mean precipitation ranges from 1500 mm to 2200 mm (Zhang et al., 2015b). Due 2

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Fig. 1. Sketch of the PRD displaying the locations of tidal gauging and hydrological stations.

Table 1 List of gauging stations with three flood periods. Tributaries

Stations

Abbr.

Longitude

Latitude

Data types

Period of “986″

West River

Gaoyao Makou Ganzhu Tianhe Jiangmen Baiqing Hengshan Xipaotai Da'ao Zhuyin Denglongshan Nanhua Xiaolan Hengmen Shijiao Sanshui Sanduo Sanshanjiao Banshawei Fengmamiao Wanqingsha Zidong Lanshi Sanshakou Boluo Shilong Dasheng Laoyagang Shizui Dawanshan

GY MK GZ TH JM BQ HS XPT DA ZY DLS NH XL HM SJ SS SD SSJ BSW FMM WQS ZD LS SSK BL SL DSH LYG SZ DWS

112°27′E 112°47′E 113°04′E 113°05′E 113°07′E 113°10′E 113°11′E 113°06′E 113°13′E 113°17′E 113°23′E 113°06′E 113°14′E 113°31′E 112°57′E 112°49′E 112°58′E 113°17′E 113°21′E 113°29′E 113°33′E 112°59′E 113°05′E 113°30′E 114°07′E 113°50′E 113°31′E 113°10′E 112°53′E 113°43′E

23°02′N 23°06′N 22°48′N 22°44′N 22°36′N 22°30′N 22°20′N 22°14′N 22°29′N 22°21′N 22°13′N 22°45′N 22°41′N 22°34′N 23°33′N 23°9′N 22°58′N 22°53′N 22°48′N 22°42′N 22°39′N 23°01′N 22°58′N 22°53′N 23°09′N 23°07′N 22°02′N 23°13′N 22°28′N 21°55′N

WD,WL WD,WL WL WL WL WL WL WL WL WL WL WL WL WL WD,WL WD,WL WL WL WL WL WL WL WL WL WD,WL WL WL WD,WL WD,WL WL

Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 Jun.21 – Jun.21 Jun.21 Jun.21 Jun.25 Jun.25 Jun.25 – – –

North River

East River

Others

Note: WD: water discharge; WL: water level; “—” indicates no data. 3

Period of “056″

4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 4 h-Jun.30

8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h 8h

4 h-Jun.30 4 h-Jun.30 4 h-Jun.30 8 h-Jun.29 8 h-Jun.29 8 h-Jun.29

8h 8h 8h 8h 8h 8h

Jun.17 Jun.17 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.20 Jun.17 Jun.17 Jun.20 Jun.20 Jun.20 – Jun.20 Jun.20 Jun.20 Jun.20 Jun.17 Jun.20 Jun.20 – – Jun.17

0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1 0 h-Jul.1

Period of “086″ 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h 10 h

0 h-Jul.1 10 h 0 h-Jul.1 10 h 0 h-Jul.1 10 h 0 h-Jul.1 10 h 0 h-Jun.30 20 h 0 h-Jul.1 10 h 0 h-Jul.1 10 h

0 h-Jul.1 10 h

Jun.11 Jun.11 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.11 Jun.11 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.14 Jun.11 Jun.14 Jun.14 Jun.11 Jun.11 Jun.11

21 h-Jun.24 21 h-Jun.24 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 21 h-Jun.24 21 h-Jun.24 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 12 h-Jun.23 21 h-Jun.24 12 h-Jun.23 12 h-Jun.23 21 h-Jun.24 21 h-Jun.24 21 h-Jun.24

0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h 0h

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boundaries at the open sea extend to DWS station, which is approximately situated at the 30 m isobath off-shore. The spatial resolution of the grid ranges from 2.5 km in the offshore region to 50 m in the river channel region. A sufficient grid resolution is provided to adequately schematize the bottom elevation of the river channels when using the hypsography data from 2008. As shown in Fig. 2, hourly water discharges in “086” are specified as the inflowing boundary conditions at upstream stations of the West, North, and East Rivers (GY, SJ, and BL stations, respectively), while constant flows with averaged discharges during the same periods are specified at the other upstream boundaries (LYG and SZ stations). In addition, the water levels at the offshore open boundary are forced by the tidal elevation provided by TPXO models (Egbert and Erofeeva, 2002), to allow tidal flow to freely propagate in and out of the model domain. The water level was calibrated by adjusting the bottom roughness height (z0) so that modeled water level agreed with observations. After several model tests, a uniform roughness height of 3 mm was found to perform well enough when compared with observations. Comparisons between modeled water level and RWL against observations at five stations along the West River are shown in Fig. 3a and Fig. 3b, respectively, and the root mean square error (RMSE) between modeled and observed data is 0.12 m and 0.09 m for water level and RWL, respectively. The comparisons of modeled water discharge and RWD against observations are shown in Fig. 3c and d, respectively, and are found to be well-matched with observations with RMSEs of 778.9 m3/s and 761.2 m3/s, respectively. The modeled results exhibit good agreements with observations, which indicates that the model can accurately reflect the hydrodynamic process of the West River during the flood period of “086.”

3.2. Fourier transform To obtain the RWL and RWD at different stations within the PRD, the Fourier transform was applied to original datasets. Assuming that the time series considered is Y(n) (n = 0,1,2, …, N - 1, where N is the length of the time series), the discrete Fourier transform of the time series Y(n) is defined as N −1

∑ Y (n)⋅e−i2πnf ,

H (k ) =

(k = 0, 1, 2, …, N − 1), (1)

n=0

where H(k) is the result of the Fourier transform, and f is the frequency component of H(k). H(k) is typically a complex number, and its mode represents the amplitude of the corresponding period. The amplitude of periodicity less than T was set to zero, and the influences of periodic oscillation less than T on the time series were eliminated by the following formula of the inverse discrete Fourier transform,

YL (n) =

1 N

N −1

∑ H (k )⋅ei2πnf ,

(n = 0, 1, 2, …, N − 1),

i=0

(2)

where YL(n) is the time series of low-pass filtering (Yang et al., 1996; Liu et al., 2014). In this study, the Fourier transform was applied to the hourly hydrological series of water level and water discharge, T was set as 25 h, and the corresponding components of low pass filtering were thus RWL and RWD, respectively. 3.3. Numerical model and model setup To verify the impacts of river-tide dynamics on the RWL, which are discussed in Section 5.2, the EFDC (Environmental Fluid Dynamics Code) model was utilized to simulate the hydrodynamic processes of the PRD. This model has been successfully applied in a wide range of coastal and estuarine studies (Xia et al., 2007; Gong and Shen, 2009; Gong et al., 2012). In the current study, the model uses a curvilinear, orthogonal grid for the horizontal coordinates and one sigma layer for the vertical coordinates, thereby solving the two-dimensional (2-D) continuity and free surface equations of motion (Hamrick and Wu, 1997). Fig. 2 shows the model grid and bathymetry of the PRD. The model covers the entire Pearl River network and coastal regions, and its

4. Results 4.1. Changes in rating curves The rating curve of water level versus water discharge can reflect the complexity of the interaction between river flow and channel morphology (Lu et al., 2007; Luo et al., 2007). Both the impacts of river-tide interaction and diverse human activities (such as dam construction and sand excavation) can cause upward and downward shifts in rating curve trends (Zhang et al., 2015b). Fig. 4 shows the rating curves of RWL versus RWD during the three different flood periods (“986,” “056,” and “086”) in the West, North, and East Rivers, where it is found that variability of RWL is mainly controlled by RWD during the flood period, and RWL increases with an increase in RWD at all hydrological stations in the upstream parts of the three tributaries. For the West River, the rating curves at both GY and MK stations (Fig. 4a and b) show the same trends with respect to temporal changes. Compared to the case in “986,” the rating curves in “056” move downward dramatically with an average shift distance of 1.45 m for the two stations. In addition, compared with the case in “056,” the rating curves at the two stations move upward slightly in “086,” with an averaged shift distance of 0.29 m, which is still lower than the case in “986.” However, with respect to the North River, the rating curves at SJ and SS stations (Fig. 4c and d) exhibit different temporal change trends. It is obvious that the rating curve at SJ station moves downward continuously during the three flood periods: compared with the case in “986,” the rating curve at SJ station drops by 1.15 m in “056” and by 1.75 m in “086,” respectively. Although the rating curve at SS station drops by 0.76 m on average in “056” compared with the case in “986,” it rises by approximately 0.11 m in “086” compared with “056.” The temporal changes in rating curves at SS station have similar trends to those at stations in the West River, which suggests that the hydrodynamic characteristics of the North River may be affected by the West River during flood periods. With respect to the East River, the rating curve at BL station

Fig. 2. Grid and bottom elevation of EFDC model of the PRD. 4

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Fig. 3. Comparison between modeled results against observations along the West River in “086” (a: water level, b: RWL, c: water discharge, d: RWD).

the RWL along different river channels in “056” and “086,” but there is a greater reduction in RWL at upstream stations than at downstream stations. Compared with the case in “986,” for given identical RWD at SS station, the RWL in “056” and “086” decreases by an average of 0.39–0.41 m and 0.42–0.46 m, respectively, which is less than the reduction in RWL within the West River. Additionally, the variations in RWL between “086” and “056” in the North River are similar to those of the West River, where rising RWL occurs in the upstream reaches but falling RWL occurs in the downstream reaches. For the East River (Fig. 5f), there are similar RWL variation patterns between “986” and “056” as those occurring in the West and North Rivers. At the same RWD at BL station, there is an average decrease of 1.24 m in the RWL along the river channel in “056” compared to “986.” The RWL continues to fall during the flood period of “086,” and it decreases by an average of 0.61 m and 1.84 compared to “056” and “986,” respectively, thereby representing the largest RWL reductions of the three tributaries and indicating that the East River is the most affected by excessive anthropogenic changes. It is also observed that the decrease in RWL is greater at upstream stations than at downstream stations.

(Fig. 4e) shows a sustained downward trend during the three flood periods, and it drops by an average of 2.94 m in “056” and 4.90 m in “086” compared with “986.” In addition, the shift distance of the rating curve at BL Station is the largest at all hydrological stations located in the upstream parts of the PRD, which indicates that the East River is probably the most seriously influenced by human activities of all three tributaries.

4.2. Variations in RWL along river channels From water level observations throughout the PRD, it is possible to investigate variations in RWL along river channels during flood periods. For identical RWDs of stations at the apex of Pearl River networks, RWL differences between the three different flood periods (“986,” “056,” and “086”) can be calculated and used as indicators to evaluate variations in RWL along different river channels: Fig. 5 shows mean RWL differences with standard deviations at different stations along the West, North, and East Rivers during the three flood periods, where positive or negative values indicate rises or falls in RWL at the same RWD. For the West River (Fig. 5a, b, c), there is a fall in the RWL during “056” and “086” at all stations along the river channels compared to “986.” For given identical RWDs at MK station, the average RWL reduction along the different West River channels ranges from 0.45 m to 0.62 m in “056,” but 0.47 m–0.58 m in “086.” It can also be observed that when the locations of stations are situated further upstream, there is more variability in the RWL in all of the river channels of the West River. This suggests that RWL in the upper reaches is even more sensitive to the overall effects of human activities than in the lower reaches of the PRD. However, the variations in RWL between “086” and “056” upstream are inconsistent with those downstream in the West River. In other words, compared with the case in “056,” the RWL in “086” rises at upstream stations but falls at downstream stations. With respect to the North River (Fig. 5d and e), there is also a fall in

4.3. Changes in RWL slope The RWL slope is mainly balanced by the tidally averaged friction term in the subtidal momentum equation (Buschman et al., 2009; Cai et al., 2014, 2018a), which suggests that the decreasing RWL slope (and hence, the bottom friction) caused by the impact of human activities results in an increasing tidal range. Therefore, changes in the RWL slope can denote the extent of tidal impacts on the hydrodynamic characteristics of river channels. The RWL slope during the three flood periods in different river channels of the PRD is shown in Fig. 6, where it is evident that the RWL slope is mainly controlled by the RWD at upstream hydrological stations: specifically, the RWL slope in the West, 5

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Fig. 4. Rating curves of RWL versus RWD during three different flood periods (“986,” “056,” “086”) in the West (a, b), North (c, d), and East (e) Rivers.

these are the maximum RWL slope reductions among the three tributaries, and they are consistent with the reductions in RWL previously mentioned (Fig. 4). The apparent reduction in the RWL slope in different river channels illustrates that the tidal dynamics of the entire PRD may have strengthened over time. Furthermore, previous studies have indicated that there is an increasing trend in the tidal range at most stations in the PRD and that the tidal limit has moved upstream in recent years compared with the 1980s (Zhang et al., 2010; Yuan and Zhu, 2015).

North, and East Rivers is determined by the RWD at MK, SS, and BL stations respectively. In addition, the RWL slope tends to increase with increasing RWD. As shown in Fig. 6, the West and North Rivers exhibit the same RWL slope change trends during the three flood periods. With respect to the greater decrease in RWL at upstream stations than downstream stations (Fig. 5), a significant decrease in the RWL slope can be observed in “056” and “086” compared to “986”. For the same RWD, the RWL slope in “056” decreases by 14.5% on average in the West River (Fig. 6a, b, c), and by 12.2% in the North River (Fig. 6d and e), and it exhibits a decrease of 11.5% and 9.2% in “086” in the West and North Rivers, respectively. It is also evident that the RWL slope reduction in “086” is less than that in “056” in the West and North Rivers; this is mainly caused by the upward movement of rating curves at MK and SS stations (Fig. 4b, d) between the two flood periods. For the East River, the RWL slope displays a sustained and significant decrease during the three flood periods. Compared with the case in “986,” the RWL slope decreases by 29.7% and 49.7% in “056” and “086,” respectively (Fig. 6g);

5. Discussion 5.1. Impact of human activities In general, the hydrodynamic characteristics in natural channels vary slowly, if riverbed changes are associated only with sediment deposition and erosion (Braca and Futura, 2008; Zhang et al., 2015b). However, human activities have had an important impact over the past 6

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Fig. 5. Variations in RWL along different river channels of West (a, b, c), North (d, e), and East (f) Rivers.

than 8.7 × 108 m3 of sand was excavated between 1986 and 2003 (Luo et al., 2007). Therefore, in this study, a detailed discussion of the responses of rating curves to changes in cross-sectional profiles is conducted, and comparisons are made between RWL variations and bottom elevation changes along the river channels.

decades and have accelerated the evolution of hydrodynamic characteristics in estuarine and coastal regions. In particular, down-cutting of the river bed induced by the overall impact of human interventions (such as dam construction, sand excavation, and navigation channel dredging) has caused dramatic changes in the hydrodynamic characteristics (Luo et al., 2007; Zhang et al., 2009; Liu et al., 2014). Previous studies have shown that river channels in the PRD have turned from being depositional to erosional since the 1980s, due to the decreased sediment load that has been mainly induced by dam construction and afforestation (Zhang et al., 2012a; Liu et al., 2017). In this respect, during the 1990s–2000s, several large reservoirs and dams were constructed in the Pearl River basin, including the Tianshengqiao Reservoir with a storage capacity of 102.6 × 108 m3 and the Longtan Reservoir with a storage capacity of 273.0 × 108 m3. The cumulative storage capacity and annual deposition rate in the 2000s were three and five times those of the 1980s, respectively (Fig. 7a). As a result, even though the amount of water discharge remained almost unchanged, there was significant decrease in the sediment load. Furthermore, the annual sediment load decreased to 0.52 × 103 kg/s in 2009, nearly half that of the 1980s (Fig. 7b) (Liu et al., 2014, 2017). Large-scale sand excavation in response to the demand of urban construction occurred in all river channels from the 1980s–2000s, and it is estimated that more

5.1.1. Changes in cross-sectional profiles at upstream stations Fig. 8 shows the temporal changes in cross-sectional profiles at upstream stations of the PRD in 1998, 2005, and 2008, corresponding to the three flood periods. As shown in the figure, there is significant down-cutting of the riverbed in all of the cross-sectional profiles at upstream stations between 2005 and 1998, with respect to the increase in human activities. As a result, the bankfull cross-sectional area in 2005 is increased by an average of 8.2%, 8.1%, and 11.8% in the West, North and East Rivers, respectively, compared with the case in 1998 (Table 2). This change is mainly responsible for the dramatic downward movement of the rating curves between “056” and “986” (Fig. 4). Furthermore, continuous and apparent down-cutting of the riverbed is evident in the cross-sectional profiles of SJ and BL stations in 2008 compared to 2005 (Fig. 8c, e), and the bankfull cross-sectional area is increased by 6.5% and 6.9% at SJ and BL stations, respectively (Table 2). The rating curves at the two stations thus also display a 7

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Fig. 6. RWL slope during three different flood periods in the West (a, b, c), North (d, e), and East (f) Rivers.

Fig. 7. (a) Temporal changes in cumulative storage capacity and sediment deposition of major dams in the Pearl River basins; and (b) water discharge and sediment load in the PRD since 1980s.

continuous downward movement between “086” and “056” (Fig. 4c, e). In addition, with respect to GY, MK, and SS stations, the cross-sectional profiles are slightly cut down in 2008 compared to 2005, and such increments in the bankfull cross-sectional area range from 0.1% to 2.2%, which is in favor of a downshift in the rating curves. Conversely,

a slight upward movement of the rating curves is observed at the three stations between the corresponding flood periods (Fig. 4a, b, d). As shown in Table 2, the most dramatic cross-sectional profile change occurs at BL station: the bank-full cross-sectional area in 2008 is 19.5% larger than that in 1998. This results in the largest shift distance

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Fig. 8. Temporal changes in the bottom elevations of cross-sectional profiles in the West (a, b), North (c, d), and East (e) Rivers.

different interactions between river-tide dynamics, may have affected the RWL more than human activities during the two flood periods. The impacts of river-tide dynamics on the RWL are further discussed using a 2-D numerical model in the following section.

Table 2 Percentage changes in bankfull cross-sectional areas at upstream stations. Tributaries

Stations

1998–2005

2005–2008

1998–2008

West River

GY MK

7.2% 9.1%

1.8% 2.2%

9.2% 11.6%

North River

SJ SS

6.3% 9.9%

6.6% 0.1%

13.3% 9.9%

East River

BL

11.8%

6.9%

19.5%

5.1.2. Changes in averaged bottom elevation along river channels Fig. 9 shows the averaged bottom elevation (ABE) of the riverbed along different river channels of the PRD in 1999 and 2008. The river channels from the apex to the estuary mouth of the PRD were divided up into sections every 5 km. The ABE was calculated from the bottom elevation of the riverbed below 0 m in the West (Fig. 9a, b, c) and North (Fig. 9d and e) Rivers, and below 5 m in the East River (Fig. 9f). As shown in Fig. 9, there is obvious down-cutting of the riverbed in the different river channels across the PRD, which is mainly caused by extensive and intensive human activities. For the channels of West River (Fig. 9a, b, c), the ABE in 2008 is 0.62–1.46 m lower on average compared with 1999, and it decreases by an average of 0.85–1.10 m in the channels of the North River (Fig. 9d and e). For the East River (Fig. 9f), the reduction in ABE along the river channel ranges from 1.35 m to 10.22 m. As a result, the RWL along the river channels across the PRD is seen to decrease substantially in “056” and “086” compared

of the rating curves (Fig. 4e) and reveals that the East River is the most seriously influenced by human activities of all three tributaries. The dramatic downward movement of rating curves between the three flood periods, which is consistent with the down-cutting of cross-sectional profiles, suggests that human activities play an important role in the recent (1998–2008) changes in the hydrodynamic characteristics of the PRD. However, the slight upward movement of the rating curves between “086” and “056,” which shows the opposite trend to down-cutting of the riverbed, reveals that other underlying factors, such as

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Fig. 9. Averaged bottom elevation (ABE) and cumulative ABE differences within various river channels in the West (a, b, c), North (d, e), and East (f) Rivers.

to “986” at the same RWD of upstream stations (Fig. 5). Human activities have been conducted in an uncontrolled and disorderly manner within these three tributaries of the PRD, particularly with respect to sand excavation of river channels, which has been conducted in response to market demands. During the period of 1999–2008, the upper reaches of the East River, and particularly at BL station (Fig. 7e), suffered the most dramatic impact from human activities compared to the other two tributaries, and the maximum reduction in ABE occurred here (Fig. 9f). Therefore, the largest decrease in the RWL and RWL slopes occur in the East River (Fig. 5f). Because the decreases in ABE in the West and North Rivers are not as severe as those in the East River, the reduction in RWL was less in the West and North Rivers (Fig. 5) and the RWL slope is more stable than that of the East River (Fig. 6). Riverbed down-cutting is also uneven along the river channels. The slope of the cumulative ABE difference was used to detect the unevenness of riverbed down-cutting along the channels (Fig. 9). For the West and East Rivers, the slope of the cumulative ABE difference in the upper parts is approximately two to four times larger than that in the lower parts (Fig. 9a, b, f), which indicates that down-cutting of the upstream reaches is more severe than in the lower reaches. For given identical RWD, the RWL corresponds to the bottom elevation to a great

extent. Riverbed down-cutting that is more significant upstream is advantageous for decreasing RWL. Significant deposition can also be observed in a certain downstream channel of the West River (Fig. 9c), which is disadvantageous for lowering the RWL. With respect to the North River, the steep slope of the cumulative ABE difference occurs both in upstream and downstream channels (Fig. 9d and e), which suggests the existence of more obvious riverbed down-cutting. The downstream channels are affected by strong tidal dynamics, where mean sea level assists in maintaining a low and stable RWL. Riverbed down-cutting in the lower reaches enhances the tidal dynamics of the PRD, which is beneficial for offsetting the impact of riverbed downcutting. Hence, the combined effects of significant down-cutting upstream and strong tidal dynamics downstream cause the RWL at upstream stations to decrease more than at downstream stations in the PRD. Consequently, there is an apparent decrease in the RWL slope in the PRD during the three flood periods (Fig. 6). 5.2. Impact of river-tide dynamics Even without the influence of human activities, RWL behavior is still extremely complex due to the highly nonlinear interaction between river flow and tide. Previous studies have indicated that the RWL is 10

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Fig. 10. Variations in (a) RWD at MK and SS stations and (b) water level at DWS station in “056” and “086.”

river-tide dynamics in “056.” The boundary condition for the West River was forced by WD2 (distorted flood wave based on WD1), which had only one flood peak, and the duration of the water-rising stage was 20 h shorter than that in WD1. The other boundary conditions in Case 2 were given the same time series as Case 1. Case 3 was set in relation to the river-tide dynamics in “086,” where WL2 with a tidal range of 0.62 m representing the neap tide was used to replace WL1 as the downstream boundary condition, and other boundary conditions were the same as with Case 1. As shown in the modeled results, compared with Case 2 for the same RWD imposed into the channel, the flood wave in Case 1 causes a rise in the RWL along the channel of MK-DLS (Fig. 11b), and causes the rating curve (Fig. 11c) and RWL slope (Fig. 11d) to shift upwards at MK station. Furthermore, the effect of flood wave distortion on RWL tends to decrease in a seaward direction, and it can be ignored at DLS station, which is located at the mouth of Modaomen Estuary. A comparison between Cases 1 and 3 shows that the increasing tidal range also causes a RWL rise (Fig. 11b), but the effect of the tidal range decreases in a landward direction and is almost negligible at MK station, which is situated at the apex of the West River network. For this reason, there is no significant difference in the rating curve at MK station between Cases 1 and 3 (Fig. 11c); whereas the RWL slope decreases appreciably in Case 1 for the same RWD as with Case 3 (Fig. 11d). Moreover, compared with Case 2, the RWL in Case 3 rises at the upper reaches where the river flow plays a more important role than the tide, but it falls at the lower reaches where the effect of tidal range is much stronger than that of flood wave distortion (Fig. 11b). The modeled results based on the same bathymetry between Cases 2 and 3 have good agreement with the observed results between “056” and “086” described in Section 4, which illustrates that the impact of river-tide dynamics on RWL may have exceeded that of the influences of human activities between the two flood periods. Although the boundary conditions in “086” may be different to those of “986,” the changes in RWL between the two flood periods are one order of magnitude larger than that between “086” and “056,” which indicates that human activities contributed significantly to the recent dramatic changes between 1998 and 2008, with respect to the hydrodynamic characteristics of the PRD.

mainly controlled by variations in freshwater discharge in river-dominated regions. However, conversely, variations in RWL are modulated fortnightly in tide dominated regions, which results in a lower RWL during the neap tide than during the spring tide (Jay et al., 2015; Guo et al., 2015; Cai et al., 2018a,b). Comparisons between RWD observed at MK and SS stations in “056” and “086” were conducted, and variations in water level at DWS station were compared during the same periods, as this station is situated far enough from the estuary to be affected by river flow (Fig. 10). The variations in RWD at both MK and SS stations provided evidence of a typical flood wave in “056” and “086.” However, for given identical range of RWD during the water-rising stage, the flood duration in “086” was 15–20 h longer than that in “056” (Fig. 10a), which enabled a greater volume of water to flow into the PRD. For example, although the RWD of the two floods ranged from 25000 m3/s to 45000 m3/s at MK station, the water volume delivered into the PRD in “086” was 30.0 × 108 m3 greater than that in “056” (represented by the area difference of S2–S1 in Fig. 10a). Furthermore, the flood wave in “086” caused another flood peak at both MK and SS stations during the waterfalling stage, which precipitated the greater volume of water being debouched into the PRD as well: the flood wave in “086” delivered 22.4 × 108 m3 more water than that in “056” during the water-falling stage for the same given range of RWD from 40000 to 35000 m3/s at MK station (represented by the area of S3 in Fig. 10a). Therefore, the flood wave in “086” delivered considerably more water into the PRD than in “056,” both at the water-rising stage and the water-falling stage for the same range of RWD, and this was responsible for the rising RWL in “086” at the upstream reaches of the PRD (Fig. 5). With respect to the water level at DWS station, the water level in “086” was dominated by a neap tide with an average tidal range of 0.62 m, while that in “056” was mainly controlled by a spring tide with an average tidal range of 1.15 m (Fig. 10b), which was more advantageous than the case of “086” for increasing the RWL at the same RWD in the downstream parts of the PRD (Fig. 5). To further verify the interpretation mentioned above, a 2-D (depthaveraged) EFDC model based on topography data from 2008 was applied to the PRD to explore the response of RWL to the imposed water discharge of rivers and the water level of open sea. The model is calibrated in Section 3.3, and it provides reliable modeled values of the RWL along the West River during the flood period. As shown in Fig. 11a, three numerical experiments were conducted. Case 1 was the control experiment, where WD1 (observed flood wave at GY station in “086”) was specified as the inflowing boundary condition for the West River. The other upstream boundary conditions (e.g., the North and East Rivers) in Case 1 were given constant flows (average water discharges at corresponding stations in “086”), and the downstream boundary condition was forced by WL1 with a tidal range of 1.15 m, thereby representing the spring tide. Case 2 was set to represent the

6. Conclusions In this study, recent changes (1998–2008) in the hydrodynamic characteristics of the PRD were examined in terms of RWL during the flood period, using simultaneous hydrological series data obtained from approximately 30 stations distributed over the delta, which were measured during three extreme floods (here termed as “986,” “056,” and “086,” respectively). The under-lying causes of RWL changes were also analyzed, and were found to be mainly attributed to topographic changes induced by diverse human activities, such as dam construction 11

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Fig. 11. (a) Boundary conditions of different cases and modeled results: (b) variations in RWL along West River; (c) rating curves at MK station; (d) RWL slope of West River.

contributed greatly, and the small tidal range in “086” lowered the RWL. The results of this study are very useful for understanding recent changes in the hydrodynamic characteristics of the PRD and their underlying causes, and this is particularly important for advising the protection and management of the complex Pearl River networks with respect to flood control and the prevention of saltwater intrusion.

and sand excavation. In addition, the impacts of river-tide dynamics on hydrodynamic characteristics were investigated by comparing flood waves observed at MK and SS station and tidal levels measured at DWS station between different flood periods, respectively. To further understand the response of RWL to different boundary conditions, a 2-D (depth-averaged) EFDC model constrained into the PRD was used to detect the impacts of river-tide dynamics on RWL, where the West River was used as an example. The results indicate that the dramatic riverbed down-cutting caused by human activities played an important role in the changes of RWL between the three floods. Compared with the case in “986,” the rating curves at upstream stations show an apparent downshift in “056” and “086.” For identical given RWD, the RWL decreased more significantly at upstream stations than at downstream stations because of the strong tide dynamics at the lower reaches, which assisted in preventing the impacts of riverbed down-cutting. Besides, the lowered RWL slope suggests that tidal dynamics in the PRD have been greatly enhanced since the 1990s (Zhang et al., 2010; Yuan and Zhu, 2015). Results also show that human activities have been more intense in the East River than in the West and North Rivers, where there are maximum reductions in the RWL and RWL slopes. However, the impacts of river-tide dynamics on RWL exceed the influences of human activities between the floods of “056” and “086” in the West and North Rivers. When compared with the case in “056,” the rating curves in “086” shift up slightly, which is opposite to the effect of riverbed down-cutting. Meanwhile, for given identical RWD, the RWL in “086” increased at the upper reaches but decreased at the lower reaches compared to “056.” Both the observed and modeled results reveal that river flow played an important role in the upstream parts, where the flood wave in “086” was beneficial for providing a greater water volume to debouch into the channels and assisted in the rise of RWL. In addition, the tidal dynamics in the downstream areas

Declarations of interest None. Acknowledgments This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFC0402604). The authors also wish to thank the editor and anonymous reviewers, whose invaluable and constructive suggestions have greatly improved the scientific quality of the original manuscript. In addition, we are grateful to Liu Huamei, Wang Xin, and He Jun for providing the original datasets used in this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ocecoaman.2019.104814. References Braca, G., Futura, G., 2008. Stage-discharge relationships in open channels: practices and problems. Università di Trento. Dipartimento di Ingegneria Civile e Ambientale 978–88–8443–230–8. Buschman, F.A., Hoitink, A.J.F., van der Vegt, M., Hoekstra, P., 2009. Subtidal water level variation controlled by river flow and tides. Water Resour. Res. 45 (10), W10420.

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