Impacts of physical alterations on salt transport during the dry season in the Modaomen Estuary, Pearl River Delta, China

Estuarine, Coastal and Shelf Science 227 (2019) 106345

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Impacts of physical alterations on salt transport during the dry season in the Modaomen Estuary, Pearl River Delta, China

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Changjie Liua, Minghui Yua, , Liangwen Jiab, Huayang Caic, Xiaoqi Chena a

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

A R T I C LE I N FO

A B S T R A C T

Keywords: Salt transport Land reclamation Riverbed down-cutting Modaomen estuary

Physical alterations (e.g., land reclamation and riverbed down-cutting) caused by human activity may significantly affect the process of salt transport in estuarine systems. Between the 1970s and 2010s, the Modaomen Estuary, an outlet of the Pearl River Delta (PRD), was subject to large-scale land reclamation and riverbed downcutting, which exerted dramatic changes in the transport of salt. In this study, a three-dimensional hydrodynamic model (EFDC) is used to quantitatively evaluate the impacts of different physical alterations in the estuary during the dry season. The modeled results indicate that large-scale land reclamation has decreased landward salt transport, which has weakened the tidal mixing and diffusion in the estuary and has resulted in decreased salinity and a shortened salt intrusion length. In addition, the significant degree of riverbed downcutting has caused the import of more salt into the estuary, which has enhanced the estuarine circulation to cause further increases in salinity and an extended salt intrusion length. Further, the enhancement of landward salt transport owing to the riverbed down-cutting is much stronger than the weakening effect of land reclamation. All of these local physical alterations have significantly augmented the salt intrusion extent and has intensified the salinity stratification. Consequently, the Modaomen Estuary has changed from partial stratification to high stratification during the past four decades. Although both upstream riverbed down-cutting and sea-level rise facilitate salt intrusion, the impacts from these processes are one order of magnitude smaller than those from the physical alterations made in the estuary. The results obtained in this study have significant implications for the sustainable development of estuarine systems and provide scientific guidelines for general water management, particularly for the prevention of brine tides in the PRD.

1. Introduction Estuaries, the primary transition zones between river and maritime environments, present a continuum along a fresh–brackish–saltwater gradient and serve as one of the most productive natural habitats in the world (Costanza et al., 1993; Walling and Fang, 2003; McLusky and Elliott, 2004). The water quality of estuaries is largely affected by the process of salt transport, which controls the intrusion of salt water, the longitudinal and transverse density gradient, and the baroclinic circulation, and also influences the transport of sediment and nutrients (Gong et al., 2014). The salt transport process in estuaries is particularly sensitive to natural changes (e.g., sea-level rise and global climate change) and physical alterations (e.g., land reclamation, dredging for navigation, and riverbed deepening; Alber, 2002; Zhang et al., 2009; Wan et al., 2015). Such changes can alter both the hydrodynamics and

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the rate of mixing in the coastal ocean, thereby having a profound effect on salt transport in estuaries. For example, a study of Chesapeake Bay, eastern United States (U.S.), showed an increase of the residence time of freshwater and the penetration of upstream salt water due to the sealevel rise (Hong and Shen, 2012). The historic dredging and deepening of the navigation channel in the Caloosahatchee Estuary, Florida, U.S., has caused a significant system-wide increase in salt transport (Sun et al., 2016). In China, since the implementation of economic reform and opening-up policies (EROP) in the late 1970s, physical alterations have become increasingly evident in the estuarine systems, particularly in the Pearl River Delta (PRD; Fig. 1a). Owing to the construction of numerous dams and large-scale excavation of sand, considerable geomorphologic changes have occurred in the Pearl River network during the past four decades (Luo et al., 2007; Zhang et al., 2015; Liu et al.,

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

https://doi.org/10.1016/j.ecss.2019.106345 Received 5 June 2019; Received in revised form 10 August 2019; Accepted 12 August 2019 Available online 13 August 2019 0272-7714/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Sketch of the PRD displaying the location of the Modaomen Estuary including hydrological and mooring stations. (b) Coastlines of the Modaomen Estuary in the 1970s and (c) the 2010s. (d) Changes in the bottom elevation of the Modaomen Estuary between 1977 and 2010. Positive (negative) values indicate riverbed deposition (erosion).

The results obtained provide significant implications for the sustainable development of estuarine systems with human activity and provide scientific guidelines for general water management in the PRD, particularly for preventing brine tides.

2014, 2017). Consequently, changes in the tidal dynamics in the PRD have occurred, including an increased tidal range and an extended tidal limit (Zhang et al., 2010). This has led to increased penetration of salt water into the estuary (Zhang et al., 2013; Yuan and Zhu, 2015), which could contaminate the upstream freshwater resources, thereby threatening the freshwater supply available for local residential, agricultural, and industrial uses. The Modaomen Estuary, an outlet of the PRD, has also experienced dramatic physical alteration through large-scale land reclamation and significant riverbed down-cutting. In recent years, the increased saltwater intrusion has severely limited the water supply for surrounding cities during the dry season from October to March (Wen et al., 2007; Gong et al., 2012). A significant amount of research has been conducted to reveal the spatial and temporal variations in salinity and the roles of river flow and tidal dynamics in regulating salt transport in the estuary (Lv and Du, 2006; Bao and Liu, 2008; Gong and Shen, 2011; Gong et al., 2012, 2014). However, the differences in salinity distribution induced by the long-term human activities since the implementation of EROP until present are not fully understood. In addition, the effects of largescale land reclamation and significant riverbed down-cutting occurred in the Modaomen Estuary on the changes in salt transport have not been quantitatively evaluated so far. In this study, a three-dimensional numerical model (Environmental Fluid Dynamics Code, EFDC) is used to investigate the impacts of these physical alterations on salt transport during the dry season in the Modaomen Estuary. The purpose of this study is to quantitatively evaluate the changes in salinity distribution induced by land reclamation and riverbed down-cutting conducted during the past four decades and to identify the factor most responsible for the changes in salinity.

2. Method 2.1. Study area The PRD is located in Guangdong Province in southern China, which incorporates three main tributaries (i.e., the West, North, and East rivers) and eight outlets (i.e., Humen, Jiaomen, Hongqimen, Hengmen, Modaomen, Jitimen, Hutiaomen, and Yamen), where fresh water flows into estuarine bays (Fig. 1a; Cai et al., 2018). The Modaomen Estuary, as one of the eight outlets, is roughly defined for the purpose of this study as the downstream part of the Modaomen Waterway, a river channel of the West River, from stations ZY to DHQ. Many water treatment plants have been built along this estuary to provide fresh water for the surrounding cities. The Modaomen Estuary is approximately 40 km in length, less than 3 km in width, and approximately 5 m in mean water depth. Of the eight outlets in the PRD, it contributes the largest portion of the river flow debouching into the South China Sea. It is estimated that the annual freshwater discharge from the estuary reaches 88.4 billion m3, which is about 37.86% of the flow from the West River (Gong and Shen, 2011). The Modaomen Estuary has an irregular semi-diurnal tidal regime and features a microtidal character with an average tidal range of only 0.86–1.11 m. The tidal range decreases even further when the tide propagates from the estuary mouth to the upstream channels. 2

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open boundary conditions, allowing the tidal flow to freely propagate across the model domain. The water levels at the offshore open boundary were forced by the tidal elevations provided by the TPXO models (Egbert and Erofeeva, 2002). Eight principal tidal constituents, namely Q1, O1, P1, K1, N2, M2, S2, and K2, account for most of the tidal energy in the South China Sea (Fang et al., 1999). The incoming salinities at the offshore open boundary were specified as 34 ppt as given by WL-Delft hydraulics (2008) and Gong et al. (2012). The simulation period for the model calibration and verification was from November 1 to December 31 in 2009 and 2016, respectively. The available boundary conditions during the period were implemented into the model. The simulation period for the evaluation of the impacts from physical alteration was the same as model verification, whereas the constant flow was used as the inflowing boundary condition. Based on the data collected during the dry season between 1959 and 2016, the mean water discharges of 2900 m3/s, 600 m3/s, and 400 m3/s were specified at Wuzhou, Shijiao, and Boluo, respectively. With regard to the initial hydrodynamic conditions, the water elevation and current velocity were both set as zero over the domain. To obtain the initial conditions for salinity, the model was run iteratively for approximately 60 days using the forced boundary conditions. The resulting salinity distribution at the end of the simulation was used as the initial salinity condition in all cases. To exclude the model spin-up effect, only the results from the last 30 days were used in the following analysis.

With the development of the local economy and the enhancement of urbanization during the past few decades, the Modaomen Estuary has been significantly altered from its natural state by human intervention. Since the implementation of EROP, the estuary has experienced physical alterations with increasing intensity. In the 1970s, the open sea occupied the vast majority of the Modaomen Estuary. The estuary occupied a nearly funnel-shaped geometry and its mouth was located at GDJ station, where the river flow from the Modaomen Waterway debouched into Sanzao Bay and Hongwan Waterway (Fig. 1b). However, dramatic coastline changes have occurred in the estuary owing to land reclamation practices at Hezhou, Sanzao Island, and Hengqin Island conducted during the 1980s to satisfy the demands of urbanization (Fig. 1c). Consequently, Sanzao Bay has almost disappeared, and its water area has decreased by 117.32 km2, or approximately 77% of the 1970s level. Hongwan Waterway has become extremely narrow and has a current width of only 500 m, or approximately one-third of its original width. Compared with 1970s data, the water area in the estuary has decreased by an estimated 172.81 km2, and the Modaomen Waterway has extended seaward by 16 km during the 2010s, resulting in the movement of the estuary mouth from station GDJ to station DHQ. Besides, several large reservoirs and dams were constructed in the Pearl River basin since 1980s, which cut off the source of the riverine sediment load, sand excavation performed booming during the early 1990s, and dredging for navigation was implement during the 2000s (Luo et al., 2007). As a result, the PRD has been dominated by riverbed erosion, and significant riverbed down-cutting has occurred in the Modaomen Estuary. A comparison of 1977 and 2010 topographic maps of the waterway revealed that the bottom elevation of the estuary has cut down 2.75 m on average, and its maximum deepening depth has reached 12 m (Fig. 1d).

2.3. Design of the numerical simulation As shown in Table 1, three scenarios were simulated to quantify the potential influence of local physical alterations on the salt transport of the Modaomen Estuary. The Pre-EROP Case is based on the coastlines during the 1970s (Fig. 1b) and the bathymetry map of the Modaomen Estuary from 1977. The Transition Case is defined as the scenario to evaluate the impact of land reclamation that occurred predominantly during the 1980s, using the coastlines from the 2010s (Fig. 1c) but with the same bathymetry as that of the Pre-EROP Case. The Present Case is defined as the simulation detecting the impact from the riverbed downcutting that was primarily induced by the sand excavation boom of the 1990s. It uses the same coastline as that for the Transition Case, although the bathymetry of the Modaomen Estuary is specified as data collected in 2010. The bottom elevation of PRD regions other than the Modaomen Estuary was generated in all simulations using the bathymetry data from 2008. Changes to the coastline induced by land reclamation between the 1970s and the 2010s were represented by modifying the grid at the corresponding locations in the model.

2.2. Numerical model and model setup To investigate the impact of the physical alteration on the salt transport, the EFDC model was utilized to simulate the water level, velocity, and salinity of the Modaomen Estuary for this study. The model uses curvilinear and orthogonal horizontal coordinates and stretched (or sigma) vertical coordinates to represent the physical characteristics of a water body (Hong and Shen, 2012). The model also solves the three-dimensional continuity and free surface equations of motion (Hamrick and Wu, 1997). The Mellor and Yamada level 2.5 turbulence closure scheme was implemented in this model (Mellor and Yamada, 1982; Galperin et al., 1988), which has been successfully applied in a wide range of environmental studies concerning the PRD and other estuarine systems (Xia et al., 2007; Gong and Shen, 2009; Gong et al., 2012). Fig. 2 shows the model grid and bottom elevation of the PRD. The model covers the entire Pearl River network as well as the coastal regions. To ensure that the Modaomen Estuary and its river plume were fully covered by the model, the boundary with the open sea was extended approximately to the 70 m isobath offshore. The eastern boundary was close to 115.00° E, and the western boundary was near 112.20° E. A curvilinear and orthogonal grid was used over the domain, and this refined grid was utilized for the Modaomen Estuary. The spatial resolution ranged from 10 km in the offshore regions to 50 m in the area near the river channels. After several model sensitivity tests, 13 sigma layers were used in the vertical direction. Sufficient grid resolution was provided to adequately schematize the bottom elevation of the Modaomen Estuary. Daily water discharges were specified as the inflowing boundaries at the upstream stations of the West, North, and East rivers (Wuzhou, Shijiao, and Boluo, respectively). The water levels observed during the same periods were specified at other upstream boundaries (Laoyagang and Shizui). The upstream boundaries were set with sufficient distance from the Modaomen Estuary to ensure that any effects from physical alterations were negligible. The water levels were specified for offshore

3. Model calibration and verification To evaluate the performance of the model in the simulation of tidal dynamics and salt transport during the dry season, the results in the Present Case were compared with the available observational data. Hourly observations from December 9 to 16, 2009, which spanned a neap–spring tidal cycle, were utilized for model calibration; these include the water levels observed at the four hydrological stations shown in Fig. 1a (DHQ, GDJ, ZPS, and ZY) and the velocities and salinities measured at three mooring stations (M1, M2, and M3). The mooring stations are situated at the oligohaline, mesohaline, and hyperhaline regions of the estuary, respectively, and include acoustic Doppler current profiles (ADCPs) and conductivity–temperature–depth (CTD) sensors that obtain information concerning water currents and the salinities of the surface, middle and bottom layers. The water level and depth averaged velocity collected hourly at stations along the West River from December 1 to 7, 2016, were used for model verification of the tidal dynamics. The surface salinity data that were measured daily at the four hydrological stations (DHQ, GDJ, ZPS, and ZY) from December 6 to 19, 2009, were used for the model verification of salt transport. 3

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Fig. 2. Model grid showing bottom elevation and the locations of the upstream boundaries.

The root–mean–square error (RMSE) and Nash–Sutcliffe efficiency coefficient (NSE) were used to assess the model accuracy of the model. The RMSE of n model data comparisons is defined as

Table 1 Coastlines and bathymetries of the Modaomen Estuary in the three simulated scenarios. Scenario

Coastline

Bathymetry

Pre-EROP Case Transition Case Present Case

1970s 1970s 2010s

Measured data for 1977 Measured data for 1977 Measured data for 2010

RMSE =

Σ (M − D)2 . n

(1)

The NSE of the modeled result is calculated as

Fig. 3. Comparisons between the modeled and observed water levels at the stations in the Modaomen Estuary. The solid and dotted lines indicate model results and observations, respectively. 4

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NSE = 1 −

Σ (M − D)2 , Σ (D − D ‾)2

detected that the impacts of varying wind magnitude and direction on salt transport are significant and extremely complex in the PRD during the dry season. However, this study is limited to evaluate the impacts of physical alterations on salt transport, hence the effect of wind is not considered herein. Compared with the observations, the modeled results at station M3 showed excellent performance, with NSE values greater than 0.75. The modeled velocity results matched the observations well at stations M1 and M2, with NSE values ranging from 0.50 to 0.72.

(2)

where D is the observational data, D ‾ is the mean of the observational data, and M is the corresponding modeled data. Perfect agreement between the modeled results and observation will yield an NSE value of 1; an NSE value of 0 indicates that the predictions made by the model are equivalent to the mean of the observations, and a negative NSE value suggests that the model is less predictive than the mean of observations. An NSE value close 1 indicates high model accuracy. Generally, levels for the model performance are categorized as > 0.65: excellent; 0.65–0.5: very good; 0.5–0.2: good; and < 0.2: poor (Allen et al., 2007; Ritter and Muñoz-Carpena, 2013).

3.3. Calibration of salinity Fig. 5 shows comparisons between the modeled and observed salinities in the estuary. The model results were particularly accurate when reproducing the hourly salinity patterns of the surface, middle, and bottom layers. The averaged RMSE was 1.4, 2.1, and 1.7 ppt at stations M1, M2, and M3, respectively, and those of the middle and bottom layers were greater than those of the surface layer. The NSE values at the three mooring stations ranged from 0.42 to 0.76, suggesting that the model is capable of accurately simulating the process of salt transport. Although the discrepancies between the modeled and observed salinities were significantly greater than those for simulations of the velocity and water level, the salinities modeled in this study are generally considered to be acceptable.

3.1. Calibration of water level The modeled water elevations were compared with the observations. When the bottom roughness height was set as 2 mm, the modeled water levels yielded the best results. As shown in Fig. 3, the modeled water levels agreed with the observations at the four stations in the Modaomen Estuary. The averaged RMSE between the modeled and observed data was 0.12 m. The NSE values for the results at different stations varied from 0.92 to 0.94, indicating that the modeled water levels achieved almost perfect performance. 3.2. Calibration of velocity

3.4. Model verification Fig. 4 shows the calibration of velocity by comparing the modeled and observed water currents at three mooring stations in the estuary. The across-channel velocities were significantly weaker than the alongchannel values in natural channels. Therefore, both the modeled and observational velocities were rotated locally in the along-channel direction, and only the along-channel velocities were assessed. The RMSEs at the mooring stations ranged from 0.10 to 0.24 m/s, and those of the surface layer were significantly greater than those of the middle and bottom layers. A likely reason for such deviation is that the effects from wind were neglected in the model. Gong et al. (2018a, 2018b)

Fig. 6 presents the model verification for the tidal dynamics of the West River, including the Modaomen Estuary. Both the modeled water level and the depth averaged velocity compared well with the observation. The mean RMSE of the water level at the stations along the West River was 0.13 m, and the averaged RMSE of the velocity was 0.08 m/s. Further, the NSE values of both the water level and velocity were 0.90, indicating that the model is particularly accurate in reproducing the hydrodynamic processes of the PRD in the 2010s. As shown in Fig. 7, the subtidal salinity, which hereafter refers to

Fig. 4. Comparisons between modeled and observed water currents along the Modaomen Estuary. The solid and dashed lines indicate model results and observations, respectively. 5

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Fig. 5. Comparisons between modeled and observed salinities along the Modaomen Estuary. The solid line and dashed lines indicate model results and observations, respectively.

along the West River. The tidal range of 0.75 m in the estuary decreased to 0.35 m in the upstream channels in all three cases (Fig. 8a). When compared with the Pre-EROP Case, the decreases in tidal range in the Transition Case with the maximum reduction of 0.14 m at station ZPS suggests that large-scale land reclamation occupies much of the previous water area of the estuary and increases the resistance to tidal wave propagation, thereby weakening the tidal dynamics and the effects of mixing and diffusion. Because the significant extent of riverbed down-cutting reduces the bottom friction in river channels, the tidal range has apparently increased in the Present Case. Compared with that in the Transition Case, the maximum increase of 0.27 m in tidal range occurred at station ZY. This increase is significantly greater than the decrease caused by land reclamation, illustrating that the effect of riverbed down-cutting is significantly greater than that of land reclamation. In addition, the spatial scope of the area in which the tidal dynamics are affected by land reclamation extends to station GZ, which is located approximately 100 km from the mouth of the estuary. The scope of the effect from the riverbed down-cutting is substantial, reaching station GY nearly 200 km from the estuary mouth (Fig. 8a). Fig. 8b shows the temporal variation in water level at station ZY,

the 25-h averaged value of hourly output from the simulation, was compared with the daily observations taken at stations in the estuary for verification of the model with regard to salinity. Stations closer to the lower estuary had better model performance. According to the performance statistics, the RMSEs at the four stations were all less than 1 ppt, and the NSEs were all larger than 0.74, suggesting accurate model performance in simulating the daily salinity.

4. Results 4.1. Changes in tidal range Tidal range is an important index for the measurement of tidal dynamics in estuaries and is closely related to tidal mixing and the salinity stratification (Zhong et al., 2008; Hong and Shen, 2012). This index is sensitive to changes in coastline configuration and bathymetry; therefore, physical alterations can cause variations in the tidal range (Zhang et al., 2010; Yuan and Zhu, 2015). To illustrate the changes in tidal range induced by physical alterations made to the Modaomen Estuary, Fig. 8a presents the monthly mean tidal ranges at the stations

Fig. 6. Model verification of (a) water level and (b) depth averaged velocity at stations along the West River. 6

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Fig. 7. Model verification of salinity at stations in the Modaomen Estuary. The solid line, dashed line, and dots indicate hourly model results, daily model results, and daily observations, respectively.

induced by the effects of the spring–neap tidal cycles (Fig. 9b). The maximum (minimum) subtidal salinity occurred one to two days after neap (spring) tide in all three cases, suggesting that physical alterations had little effect on the arrival time of the subtidal salinity peaks. However, such alterations led to an apparent change in the occurrence time of the tidal salinity peaks. Banas et al. (2004) demonstrated that stronger landward salt transport occurs during neap tides in an exchange flow dominated estuary while stronger landward salt transport happens during spring tides in a tidal diffusion dominated estuary. Tidal salinity peaks generally occurred at spring tide in the Pre-EROP Case, indicating that a larger tidal range led to higher tidal salinity and the estuary was typical tidal diffusion dominated in the 1970s. However, the land reclamation weakened the effects of tidal mixing and diffusion, and the riverbed down-cutting enhanced the salinity stratification and estuarine circulation (detected in section 4.4), suggesting that the estuary turned into an exchange flow dominated one, hence the occurrence time of tidal salinity peaks changed to the neap tide in the Transition and Present cases. The difference in salinity between the three cases was used as an indicator to evaluate the changes in the salinity time series (Fig. 9c). The large-scale land reclamation in the Transition Case narrowed the waterway and decreased the landward salt transport. The subtidal estuary averaged salinity therefore decreased slightly, by 0.2–1.3 ppt.

where the tidal dynamics are mostly changed by local physical alterations. The land reclamation produced little effect on the tidal phase, whereas the riverbed down-cutting advanced the tidal phase by 1–2 h. The deepening channel in the Present Case has increased the phase velocity of the tidal wave propagation, which has led to high and low tides that arrive earlier than those in the Pre-EROP and Transition cases (Fig. 8b). These results indicate that increase in tidal dynamics owing to riverbed down-cutting far exceeds the decreasing effect from land reclamation from both spatial and temporal perspectives. Hence, the overall effects of the local physical alterations from the 1970s to the 2010s have caused system-wide enhancement in the tidal dynamics of the Modaomen Estuary, which has facilitated increases in salinity and saltwater intrusion.

4.2. Changes in salinity Fig. 9a shows the temporal variation in the water level at station DWS, where the impact from physical alterations on the tidal dynamics can be neglected. During the model days, the tidal elevation at this station spanned two spring–neap–spring tidal cycles, with tidal ranges less than 1 m and more than 2 m in neap and spring tides, respectively. The average salinity in the Modaomen Estuary shows a clear semimonthly variation with two subtidal salinity peaks within 30 days

Fig. 8. (a) Mean tidal ranges at stations along the West River. (b) Temporal variation in water level at station ZY. 7

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Fig. 9. (a) Temporal variation in the water level at station DWS. (b) Temporal variation in the averaged salinity of the Modaomen Estuary. (c) Temporal variation in the averaged salinity difference in the Modaomen Estuary. Positive (negative) values indicate salinity increases (decreases), and solid and dashed lines indicate subtidal and tidal results, respectively. (d) Spatial variation in subtidal depth averaged salinity along the Modaomen Estuary. Solid (dashed) lines indicate neap (spring) tide.

steep longitudinal salinity gradient of 1.0 ppt/km. However owing to the large-scale land reclamation, the depth averaged salinity obviously decreased in the Transition Case, with a maximum reduction of 6.5 ppt. Although the longitudinal salinity gradient was almost unchanged, the distance at which salt was detected is 2.3 km closer, indicating a weakened extent saltwater intrusion. Because of the significant riverbed down-cutting, an increase in the system-wide salinity occurred in the Present Case. The maximum increase in salinity along the estuary in the Present Case was 13.0 ppt greater than that in the Transition Case and thus, significantly greater than the decrease caused by land reclamation. In addition, the longitudinal salinity gradient decreased to 0.5 ppt/km and salt filled the entire estuary over a range of 40 km, suggesting a reduced rate in the decay of salinity and enhanced salt intrusion. Less salt is transported into the estuary during spring tide than that during neap tide. Therefore, the decreased salinity occurred along the entire estuary, and the distance at which salt was detected is approximately 75% of that during neap tide (Fig. 9d). Moreover, the longitudinal salinity gradient in the Present Case was smaller than that in the Pre-EROP and Transition cases during spring tide and similar to that during neap tide.

However, the significant riverbed down-cutting in the Present Case led to more salt being imported into the lowered estuary, which dramatically increased the subtidal estuary averaged salinity, by 3.7–8.1 ppt. The smaller tidal range in the neap tide contributed to stronger salinity stratification and suppressed the weakening effect of land reclamation on tidal diffusion, thereby the land reclamation decreased the salinity more during spring tide in the Transition Case. However, the larger tidal range in the spring tide led to stronger mixing and suppressed the enhancing effect of riverbed down-cutting on estuarine circulation, thus the riverbed down-cutting increased the salinity more during neap tide in the Present Case. Consequently, the salinity difference displays an apparent spring–neap variation, as shown in Fig. 9c. Because the increase in salinity caused by the riverbed down-cutting is significantly greater than the decrease induced by land reclamation, the overall effects from the physical alterations were evident as an overall increase in salinity of 2.9–7.6 ppt in the estuary. This effect led to intensification of the saltwater intrusion during the 1970s–2010s. The spatial variation in depth averaged salinity along the longitudinal section at the thalweg of the estuary (Fig. 1a) is shown in Fig. 9d. During neap tide, salt was present at lower reaches of the estuary in the Pre-EROP Case, occupying a length of nearly 20 km with a

Fig. 10. (a) Temporal variations in the salt intrusion length in the Modaomen Estuary. (b) Temporal variations in the difference in salt intrusion length. Positive (negative) values indicate length increases (decreases), and solid (dashed) lines indicate subtidal (tidal) results. 8

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highly stratified by time of the Transition Case. The Sp increased to 1.1–1.3, the salinity isohaline became nearly horizontal, and a characteristic salt wedge type displayed at the lower estuary with a range of 18.5 km from station DHQ (Fig. 11, middle panel). The bottom salinity in the Transition Case, particularly at ebb peak (Fig. 11g) and ebb slack (Fig. 11h), increased by approximately 5 ppt. Further, the bottom salinity front (e.g., 20-ppt isohaline) extended landward by approximately 4.1 km at flood peak (Fig. 11e) and flood slack (Fig. 11f), whereas the surface salinity front (0.5-ppt isohaline) retreated by approximately 6.0 km. These results indicate that the length of the salt intrusion decreased (Fig. 10). In the Present Case, the estuary is still highly stratified with Sp greater than 1 and a horizontal isohaline (Fig. 11, right panel). Owing to the significant extent of riverbed downcutting, both the tidal dynamics and estuarine circulation in the estuary were largely enhanced in the Present Case. As a result, the salt wedge extended landward significantly over a distance of 38.5 km from station DHQ and filled almost the entire estuary (Fig. 11, right panel). The bottom salinity in the Present Case was 5 ppt larger than that of the Transition Case. Moreover, the bottom and surface salinity fronts were extended landward by 10.2 km and 19.9 km, respectively, which caused a dramatic increase in the length of the salt intrusion, thus threatening the freshwater supply (Fig. 10). With the increased tidal range in the spring tide, the stronger tidal mixing effect generally weakened the stratification of the water column and destroyed the salt wedge in the corresponding cases (Fig. 12). In the Pre-EROP Case, the Sp was between 0.02 and 0.4 during spring tide, which is significantly lower than that during neap tide, indicating that the estuary has undergone a transition between well-mixed and partially stratified status (Fig. 12, left panel). For example, the estuary was well mixed at flood slack (Fig. 12b) and was partially stratified at ebb slack (Fig. 12d). The tidal mixing enhanced by spring tide was suppressed by the land reclamation and the estuary became partially stratified in the Transition Case. Sp ranged between 0.1 and 0.7, and the salinity isohaline formed an obvious angle with regard to the horizontal (Fig. 12, middle panel). Although the bottom salinity front (20-ppt isohaline) progressed landward slightly by about 1.2 km, the front of the surface salinity moved seaward by approximately 4.1 km. The salt was expelled from the estuary by the fresh water at ebb slack (Fig. 12h), indicting a reduced risk of salt intrusion. With regard to the Present Case, enhanced tidal mixing during spring tides did not shut down the development of riverbed down-cutting induced stratification (Fig. 12, right panel). Sp was between 0.5 and 0.8, which is less than that during neap tide, indicating partial stratification. The angle formed between the salinity isohaline and the horizontal decreased, particularly at flood slack (Fig. 12j), indicating that the salinity stratification was more obvious than that in the Transition Case. Additionally, compared with the Transition Case, the bottom salinity increased by 5–10 ppt and the surface salinity front made an apparent landward movement of about 12.9 km in the Present Case. These factors also increased the risk to the freshwater supply. The modeled results mentioned above indicate that both land reclamation and riverbed down-cutting are capable of enhancing the salinity stratification but through different mechanisms. The geometry plays an important role in modulating the mixing process of salinity in estuaries, which largely controls the variation of tidal range, current during tidal cycles, and the water exchange among the upstream river channels (Gong and Shen, 2011). The Modaomen Estuary had a nearly funnel-shaped geometry and was typical tidal diffusion dominated in 1970s. Land reclamation changed the estuary into a restricted and narrow channel, which decreased the tidal dynamics. The weakened tidal diffusion decreases the estuary averaged salinity (Fig. 9c) and facilitates the seaward movement of the surface salinity front; whereas, the reduced vertical mixing caused by decreased tidal range increases the bottom salinity and facilitates the landward movement of the bottom salinity (Figs. 11 and 12). Hence, the salinity stratification is enhanced, but the length of the salt intrusion is decreased. On the

4.3. Changes in salt intrusion length Fig. 10a presents a time series for the length of salt intrusion in the Modaomen Estuary under the three simulation cases, which is defined as the distance from the estuary mouth (station DHQ; Fig. 1a) to the salt intrusion limit in the upper estuary where the bottom 0.5 ppt isohaline occurs (Bao and Liu, 2008; Gong and Shen, 2011). Although many factors such as river discharge, tidal and wind mixing, and estuarine circulation play important roles in salt intrusion (Hong and Shen, 2012; Gong et al., 2018a, 2018b), this study is limited to evaluate the impacts of physical alterations during the dry season and the effects of varied river discharge and wind are not considered. Hence, the length of salt intrusion is determined directly by the amount of salt imported into and expelled from the estuary during tidal cycles. Previous studies have shown that the Modaomen Estuary gains salt during neap tides and loses salt during spring tides (Gong and Shen, 2011, 2012; Wang et al., 2012). As shown in Fig. 10a, the maximum (minimum) length of salt intrusion occurs during the transition between neap and spring (spring and neap) tides, generally one to two days after neap (spring) tide. Although the physical alterations caused by diverse human activity did not change the time of salt intrusion, these changes apparently altered the length of the salt intrusion. In the Pre-EROP Case, the length of the subtidal salt intrusion featured a slight fluctuation between 16.2 km and 22.2 km during the model days, a slight decrease to 10.4–19.5 km afterward in the Transition Case, and then a dramatic increase to 20.4–49.0 km in the Present Case (Fig. 10a). The difference in salt intrusion length is utilized to quantitatively assess the different impacts caused by various physical alterations in the three cases (Fig. 10b). The land reclamation makes it more difficult for salt to be imported into the estuary. Therefore, the length of the subtidal salt intrusion decreases by 2.4–5.7 km without significant springneap variations. However, the significant riverbed down-cutting enhances estuarine circulation and allows more salt to flow into the estuary (Fig. 11). This greatly increases the length of the salt intrusion by 9.8–31.8 km with an obvious spring–neap variation: The maximum (minimum) increment occurs during the transition between neap and spring (spring and neap) tides. Furthermore, the maximum increment was two to three times that of the minimum increment during the corresponding periods. The difference in salt intrusion length induced by riverbed down-cutting was much greater than the decrease owing to land reclamation (Fig. 10b), which means that overall, the physical alterations have significantly increased the salt intrusion. The evaluation suggests that the length of the salt intrusion has increased by 4.2–26.8 km since the 1970s as a result of local physical alterations, which obviously heightens the risk to freshwater supplies for the surrounding cities. 4.4. Changes in vertical stratification The distributions in salinity along the longitudinal section (Fig. 1a) in the three cases were compared to detect changes in the vertical stratification of the Modaomen Estuary, as shown in Figs. 11 and 12 for neap and spring tides, respectively. The stratification number, Sp, was utilized to represent the overall stratification of the water column: Sp = dS/S0, where dS is the salinity difference between the surface and the bottom, and S0 is the depth averaged salinity (Hansen and Rattray, 1966; Gong et al., 2014). As shown in the figure, salt is imported into the estuary during the flood-tide periods; the salinity thus increases, and the maximum salt intrusion occurs at flood slack. However, salt is expelled from the estuary during ebb-tide periods; the salinity decreases, and the minimum salt intrusion occurs at ebb slack. During neap tide, the estuary was mainly in a partially stratified state in the Pre-EROP Case, where Sp was between 0.2 and 0.8, and an obvious angle was formed between the salinity isohaline and horizontal direction (Fig. 11, left panel). However, because of the reduction in tidal mixing effect induced by land reclamation, the estuary became 9

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Fig. 11. Longitudinal distribution of salinity in the Modaomen Estuary during neap tide. The left, middle, and right panels show the Pre-EROP, Transition, and Present cases, respectively.

the temporal changes in cross-sectional profiles of the West (i.e., station MK) and North (i.e., station SS) rivers that have occurred since the 1970s. The MK cross-sectional profile shows no evident change (Fig. 13a) during the period 1974–1995 because of the dynamic balance between deposition and erosion. However, during 1995–2008, obvious riverbed down-cutting occurred with a 6.9 m deepening of the thalweg caused mainly by the booming sand excavation (Fig. 13b). During 2008–2015, sand excavation was prohibited by the government; therefore, a change in the cross-sectional profile was not obvious at station MK. With regard to the cross-sectional profile at station SS, the riverbed down-cutting occurred after 1985, which is earlier than that at station MK, and the thalweg elevation was lowered by 7.1 m in 2005. After 2005, no obvious change was observed in the cross-sectional profile at station SS. Owing to the significant extent of upstream riverbed down-cutting, the tidal dynamics at stations MK and SS were dramatically enhanced (Fig. 13d). The tidal range has been significantly increased since the mid-1980s, with values doubled by the 2010s. This indicates that the tidal dynamics have become much stronger, which facilitates salt intrusion. Moreover, the riverbed down-cutting is uneven and has caused changes in the divided flow ratio (Fig. 13e). The divided flow ratio at station MK dropped to 78.3% in the 2010s from 85.4% in the 1970s, although that at station SS increased to 21.7% from 14.6% during the same period. These results reveal that about 7.1% of the fresh water flows into the North River from the West River; therefore, less fresh water debouches into the Modaomen Estuary. The numerical results based on the Present Case with 10% less river discharge shows that the

contrary, although the riverbed down-cutting increases the tidal range in the estuary and strengthens the vertical mixing to some extent, the dramatically deepening riverbed with an inverse slope greatly reduces the topographic resistance to salt transport, which facilitates the formation of large-scale estuarine circulation and a salt wedge. Consequently, both the bottom and the surface salinity fronts make significant landward movement, and the scale of salinity structure becomes twice as large as before, resulting in severe salt intrusion. Considering the fact that the surface salinity front with faster current moves further upstream than the bottom salinity front, the salinity stratification is significantly increased by the riverbed down-cutting. In summary, the Modaomen estuary has been substantially changed from partially stratified to highly stratified by the local physical alterations during the past four decades. 5. Discussion 5.1. Impacts of upstream physical alterations Upstream physical alterations may strongly affect the fresh water flowing into the Modaomen Estuary, which further alters the salt transport. Numerous studies (Luo et al., 2007; Zhang et al., 2010; Liu et al., 2014, 2017) have shown that sand excavation caused significant riverbed down-cutting in the upstream of the PRD. It is estimated that more than 8.7 × 108 m3 of sand was excavated from the 324 tributaries of the PRD between 1986 and 2003 (Luo et al., 2007). This has resulted in dramatic and uneven riverbed down-cutting. Fig. 13a, b, and c show 10

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Fig. 12. Longitudinal distribution of salinity in the Modaomen Estuary during spring tide. The Pre-EROP, Transition, and Present cases are indicted in the left, middle, and right panels, respectively.

water levels at the offshore open boundary were lowered by 0.07 m in the Present Case to eliminate the effects of sea-level rise during the past forty years. The differences between the results of the two simulations were used to evaluate the impacts of sea-level rise on salinity in the Modaomen Estuary. Fig. 14 presents the impact of sea-level rise on the tidal range, estuary averaged salinity, and the length of salt intrusion in the Modaomen Estuary. As the sea-level rise, the tidal dynamics gets enhanced and the baroclinic gradient force increases, which is beneficial for the transport of more salt into the estuary (Yuan et al., 2015). Owing to the 0.07 m rise in sea-level, the monthly averaged tidal range was almost unchanged at the stations in the estuary but increased by 0.01 m at upstream stations, such as stations ZY and DA (Fig. 14a). The estuary averaged salinity increased by 0.4–0.9 ppt, with no obvious variation associated with the spring–neap cycle (Fig. 14b). Because of the increase of baroclinic gradient force caused by sea-level rise, the landward residual current increased to 0.039 m/s from 0.036 m/s in the estuary. Even though the changes in tidal range and estuary averaged salinity were extremely small, the salt intrusion extended landward with an apparent subtidal increase of 0.8–4.8 km (Fig. 14c). Contrary to the impact caused by the physical alterations, it is clearly indicated that the sea-level rise increased the length of the salt intrusion more during spring tide than that in neap tide. In addition, the impacts from sealevel rise are one order of magnitude smaller than those from the physical alterations, suggesting that human activities played a more important role in changing the salt transport processes in the estuary and that the impacts from sea-level rise are almost negligible relative to

estuary averaged salinity increased 0.5–1.5 ppt and the subtidal salt intrusion length increased 0.7–4.5 km. The simulated results of Gong and Shen, (2011) indicate that the steady-state salt intrusion length varied with the river discharge during the dry season in the 2010s, following the power law L = 1886.1*Q−0.4928, where Q is the total river discharge from Wuzhou and Shijiao. If the divided flow ratio at station MK remains at the level of 1970s and the same amount of water is released from the upstream boundaries, the relationship mentioned above will be L = 1807.0*Q−0.4928, matching well with our simulated results (Fig. 13f), which demonstrates that the length of salt intrusion will decrease by 4.2%, or 1.1–2.1 km, compared with the Present Case. As previously discussed, upstream physical alterations can also affect the salt transport in the Modaomen Estuary. Both the increased tidal range and decreased divided flow ratio at station MK can lead to enhanced salt intrusion in the estuary. However, the impact of the upstream riverbed down-cutting is significantly less than that of the physical alteration that occurred in the estuary. 5.2. Impacts of sea-level rise Previous studies have indicated that sea-level rise is another important factor in estuarine and coastal regions, affecting not only tidal dynamics but also resulting in an increased salinity (Grabemann et al., 2001; Hilton et al., 2008; Hong and Shen, 2012). It is estimated that the mean rate of globally averaged sea-level rise was 1.7 mm/y between 1901 and 2010, as stated in the recent Intergovernmental Panel on Climate Change (IPCC) Report (Pachauri and Meyer, 2014). Hence, 11

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Fig. 13. Temporal changes in the bottom elevation of the cross-sectional profiles at (a) station MK Ⅰ, (b) MK Ⅱ, and (c) station SS. (d) Temporal variations in tidal range at stations MK and SS. (e) Temporal changes in the divided flow ratio at stations MK and SS. (f) Relationship between river flow and salt intrusion length.

Fig. 14. (a) Monthly averaged tidal range differences along the West River. (b) Estuary averaged salinity differences and (c) salt intrusion length differences in the Modaomen Estuary. Positive (negative) values indicate physical parameter increases (decrease), and the solid (dashed) lines indicate subtidal (tidal) results. 12

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Appendix A. Supplementary data

the significant impacts from physical alterations that occurred during the past few decades.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecss.2019.106345.

6. Conclusions

References

In this study, the EFDC model was used to quantitatively evaluate the impacts from different physical alterations, specifically land reclamation and riverbed down-cutting, on salt transport in the Modaomen Estuary during the dry period between the 1970s and the 2010s. The model calibration and validation using observed data collected in December 2009 and in December 2016 indicate that the model successfully simulated the dynamic processes and salinity distribution in the estuary for the dry season. The impacts of upstream riverbed down-cutting and downstream sea-level rise were compared with those from physical alterations that have occurred within the estuary. The modeled results indicate that the large-scale land reclamation in the Modaomen Estuary decreased the tidal range and weakened the tidal mixing and diffusion, causing a slight increase in bottom salinity and an obvious decrease in surface salinity, which led to less salt being imported into the estuary. Consequently, both the estuary averaged salinity and the length of salt intrusion decreased, whereas the vertical stratification was enhanced. However, the significant extent of the riverbed down-cutting increased the tidal range and enhanced the estuarine circulation, facilitating the transportation of salt into the estuary. As a result, both the estuary averaged salinity and the salt intrusion length increased. Because the surface salinity front moves landward more than the bottom salinity front, the vertical stratification was further intensified. The increase in salt transport caused by riverbed down-cutting is significantly stronger than the weakening effect from land reclamation, hence the overall effects of the local physical alterations are the augmentation of the salt intrusion and the high stratification of the estuary from a partially stratified status. In addition, both the uneven riverbed down-cutting at upstream channels and the sea-level rise at the downstream boundary are favorable for salt intrusion. But their impacts are one order of magnitude smaller than those from local physical alterations, suggesting that human activities have contributed more to the changes of salt transport in the Modaomen Estuary during the past decades. With the intensification of human activity, the impacts from physical alterations on estuarine systems have become increasingly obvious worldwide and may far exceed the impacts caused by natural changes. In south Florida, U.S., many water control structures have been constructed near the head of the estuary for navigational purposes (Antonini et al., 2002; Ogden et al., 2005). In the Mississippi Delta, dam and dike constructions cut the access to sources of riverine sediment (Barras et al., 2004; Day et al., 2005). Such physical alterations have caused dramatic changes in the geomorphology and hydrology of estuaries. Since the dramatic physical alterations induced by human activities are unlikely to be reversed, the results obtained will provide significant implications for the development and protection of estuarine systems. It is anticipated that these results can be used as scientific guidelines for general water management, particularly for the prevention of brine tide in the PRD.

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Funding This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFC0402604).

Acknowledgments 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. 13

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