Numerical simulation of hydrodynamic environment effects of the reclamation project of Nanhui tidal flat in Yangtze Estuary *

Di-fan Cao , Yong-ming Shen , Mei-rong Su, Chun-xue Yu

1. Institute of Environmental and Ecological Engineering, Guangdong University of Technology, Guangzhou 510006, China

2. Ocean & Hydraulic Engineering Institute, Power China Huadong Engineering Corporation Limited, Hangzhou 311122, China

3. State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024,China

4. Research Center for Eco-Environmental Engineering, Dongguan University of Technology, Dongguan 523808,China

Abstract: The reclamation is the main method in the coast exploitation, and the assessment of the hydrodynamic environment effect of the reclamation project is important for project's site selection and environmental protection. With consideration of the baroclinic water, a 3-D numerical model MIKE3 is applied to simulate Yangtze Estuary's hydrodynamic environment to predict the impacts of the reclamation project of the Nanhui tidal flat. The simulated results of the model agree well with the field data of the tide level, the current speed, the current direction, the temperature, the salinity and the water quality, and it is indicated that after the reclamation project, the high tide level will be lower, while the low tide level will be higher in the South Branch in general. During the spring tide in the dry season, the peak velocity during the ebb tide in the North Channel will be reduced by 13%, while it will be increased by 21%in the South Channel in average. The salinity will be increased in the North Channel, while reduced in the South Passage, besides, the reclamation project will aggravate the saltwater intrusion of the North Branch. The value of N/P will be increased by about 4% in the whole South Branch except for the North Channel, leading to a slight aggravation of the phosphorus restriction effect in the Yangtze Estuary.

Key words: Yangtze Estuary, 3-D numerical model, hydrodynamics, nutrients, reclamation project

Introduction

The land is crucial to human survival, and the land shortage is one of the most significant constraints for the economic sustainable development. The reclamation is the human activity which changes some part of the natural ocean into a land by building dykes and dams and by dumping the earth-rock. In order to acquire more land resources, coastal cities carried out large scale reclamation projects, which impact the coastal hydrodynamic conditions, the salinity, the sediment transport and the water quality.

The estuary is the end segment of a river. The water in the estuary is affected by runoff and tide simultaneously, resulting in the complexity of the hydrodynamic condition in this area. There were many successful studies of the estuary. For example,Shen et al.[1]simulated and predicted the influence of reclamation on saltwater intrusion and storm surge in the Pearl River Estuary, China by using the FVCOM Surface Wave Module, and the study showed that the reclamation had a great influence on tidal velocity and direction, especially in some areas where narrow waterways were formed. Guo et al.[2-3]established a three-dimensional model based on the FVCOM to reproduce the storm surge generated by the Typhoon Agnes, studied the effects of the land reclamation project and the cyclonic parameters on the hydrodynamics in the Hangzhou Bay and presented a mathematical model using the finite volume method with unstructured mesh to simulate the tide-induced water elevation, the current velocity, the bed deformation, and the suspended sediment transport in the Qiantang Estuary. Wan et al.[4]used a 3-D hydrodynamic model CH3D to calculate the residence time in the Caloosahatchee Estuary in Florida, USA. Liu et al.[5]developed a hydrodynamic model by integrating the hydrological simulation program fortran (HSPF)and the environmental fluid dynamics code (EFDC) to simulate the time-varying surface water elevation, the velocity, the salinity, the water temperature and the fresh water discharge from the upstream water- sheds in St. Louis Bay Estuary. Passeri et al.[6]evaluated the geophysical influence of the combined effects of the historic sea level rise (SLR) and the morphology on the tidal hydrodynamics in the Grand Bay estuary in the Mississippi Sound. Xu et al.[7]used the radium isotopes as the tracers to characterize the coastal hydrodynamics and the submarine groudwater discharge (SGD) in the Yellow River Estuary in order to assess the ecological effects in one of the most turbid estuaries in the world. Zhang et al.[8]established a model performed with a coupled wavecurrent hydrodynamic model using TELEMAC and TOMAWAC to examine the influence of wind-waves on the mouth-bar, and the results suggested that the dominance of river discharge was limited to an area inside the mouth, while outside, the mouth-bar was tide-wave dominant.

The Yangtze Estuary (see Fig. 1) is the biggest estuary in China, also an important urban water supply area in Shanghai. Except for some areas in the South Channel, the water depth is generally more than 10 m in the estuary area, and 10 000 t ships can navigate straight to the Nanjing Harbor, therefore the channels in the Yangtze Estuary are called golden channels, and the Shanghai Harbor becomes the largest harbor in China. The runoff of the Yangtze River brings abundant nutrients to the fish and the sea creatures in the Yangtze Estuary, making the Zhoushan Fishery and the Lvsi Fishery at the outside of the Yangtze Estuary the largest fisheries in China. In recent years,due to increasingly serious environmental pollution,the coastal eutrophication has become more salient,especially for the nutrients in the Yangtze River.According to several decades' monitoring results, the concentrations of N and P in the Yangtze Estuary are the highest in all Chinese estuaries, therefore leading to the most frequent red tide in this area. There are also rich tidal flat resources in the Yangtze Estuary,and the scientific exploitation of the tidal flat resources is the key to maintain the coordinated development of the ecological environment and the economy in the Yangtze Estuary region.

The present study makes an assessment of the impacts of the reclamation project of the Nanhui tidal flat on the hydrodynamic environment of the Yangtze Estuary, to provide some insight for the subsequent reclamation and environmental protection. A 3-D hydrodynamic numerical model MIKE3 is implemented to simulate the hydrodynamic environment in the Yangtze Estuary.

Fig. 1 Yangtze Estuary and its river regime

1. Study area

The Yangtze Estuary is a triple-bifurcation,four-outlet, multi-shoal system, with a length of about 120 km and a width of about 90 km at the mouth[9].Specifically, the Yangtze River is divided into the North Branch and the South Branch by the Chongming Island. The South Branch is divided into the North Channel and the South Channel by the Changxing Island and the Hengsha Island. The South Channel is divided into the North Passage and the South Passage by the Jiuduansha Shoal. The river regime of the Yangtze Estuary is shown in Fig. 1.

The Nanhui tidal flat locates at the confluence of the Yangtze Estuary and the Hangzhou Bay, the south of the South Passage and the east of the Lingang. The sediment from the Yangtze River silts at the Nanhui tidal flat, and the rapid development of the tidal flat leads to the richness of the tidal flat resources in this place. The Nanhui tidal flat is therefore a crucial resource for the development of Shanghai. The total area of the reclamation project of the Nanhui tidal flat is about 149 km2, the largest area of the reclamation projects in the Yangtze Estuary in recent years. The hydrodynamic environment of the Yangtze Estuary was extensively studied. Yang et al.[10]analyzed the influence of the human activity on the sea water content, the sediment content and the regional transport situation in the Yangtze Estuary. Wang et al.[11]established a hydrodynamic and water quality model to simulate the characteristics of the high phosphorus concentration area in the Yangtze River Estuary based on the DHI's open platform Ecolab.Gong et al.[12]carried out a stochastic dynamic analysis by establishing a forecast model to predict the sea-level rise of the East China Sea based on the Topex/Poseidon altimeter data obtained during the period of 1993-2005. Jiang et al.[13]assessed the morphological changes by analyzing the digitized bathymetric data of the Yangtze Estuary prior to and after the navigational engineering works. Dai et al.[14]analyzed the suspended sediment concentration (SSC)in the surfacial water measured at different stations in the Yangtze Estuary and concluded that the spatial characteristics of the annual mean SSC around the mouth bar area show no apparent change yet, even though the Three Gorges Dam was constructed with an ascending trend at the upper part of the estuary.Yin et al.[15]selected 5 classes of antibiotics to conduct orthogonal experiments in order to explore their combined effects on the denitrification. Wan and Zhao[16]pointed out that the salinity-induced baroclinic pressure gradient is a major factor controlling the vertical velocity structure and the salinity and sediment transport of residuals generated by the internal tidal asymmetry plays a dominant role in maintaining a stable density stratification interface near the estuarine front. Kuang et al.[17]studied the seasonal and long-term changes of sea level based on the four characteristic discharges and the mouthaveraged river discharge from 1950 to 2011.

Fig. 2 The larger grid of the model

2. Model configuration

The model is implemented by two nested grids.In both of them the wet/dry treatment is adopted to characterize the extensive tidal flat. The larger grid covers the entire Yangtze Estuary and Hangzhou Bay,as shown in Fig. 2. The topography and the observed stations are shown in Fig. 3. Changxing, Hengsha,Zhongjun, Jinshanzui, Luchaogang, Tanhu,Lvhuashan, Shenjiamen, Meishan and Qitoujiao are the tide level stations. JS1, SH4, CJ3, CJ4, HB1 and HB2 are the current stations. C1-C5 are the temperature and salinity stations. There are 8 857 elements,4 883 nodes and 10 vertical sigma layers in this grid,and the gradual unstructured triangle elements are compacted near the coast, resulting in a horizontal resolution of between 2 km to 18 km and a vertical resolution of between 0.2 m to 8 m in most areas.

The three open boundaries are driven by a tide level generated from the global tide model in the MIKE21 Toolbox with the consideration of 4 tidal constituents, i.e., M2, S2, K1, O1. The values of the temperature and the salinity on the open boundaries are interpolated from the values in the marine atlas,and vary in time and along boundaries. The discharge of the Yangtze River, the Qiantang River and the Huangpu River are considered in this study, and the daily average discharge of the Datong station in the Yangtze River, the monthly average discharge of the Qiantang River and the yearly average discharge of the Huangpu River are adopted. The meteorological parameters (the air temperature, the relative humidity,the total cloud cover, the wind components, the precipitation and the evaporation) are obtained from the database of the european centre for medium-range weather forecasts (ECMWF) with a resolution of 0.75°×0.75°. In the flow model simulation, the initial water level and velocity take the value 0, and the last step of the last year's simulated temperature and salinity field is adopted as the initial temperature and salinity condition.

Fig. 3 The topography and the observed stations

Fig. 4 The smaller grid of the model

The smaller grid covers the estuary and the offshore area of the Yangtze Estuary, as shown in Fig.4, and the model is coupled with the hydrodynamics,the temperature, the salinity and the water quality in terms of 7 water quality variables, i.e., BOD, DO,Chlorophyll-a, NH4+, NO2-, NO3-and PO43-. The hydrodynamic and environmental conditions from Jan,2004 to Mar, 2005 are successfully simulated with the smaller grid. The values of the tide level, the temperature and the salinity on the three open boundaries of the smaller grid are derived from the larger grids' simulated results. The small grid's boundary condition for BOD is the one with zero gradient. The boundary condition for Chlorophyll-a is extracted and interpolated from the marine atlas. The boundary condition for DO is extracted andinterpolatedfromtheworkofLietal.[18],andtheboundary conditions for NH4+, NO2-, NO3-and PO4-are extracted and interpolated from the work of Wang et al.[19]. The pollutants mainly include the input of the Yangtze River and the Huangpu River, as well as the input of three main sewage draining exits of Shanghai,thus there are 5 point sources in the model. The initial conditions of the model are set to the long-time average annual values.

Fig. 5 The verifications of tide level and current at some observed stations

3. Model verification

3.1 Verifications of tide level and current

The simulated tide level η of the larger grid from 2002-3-15 00:00 to 2002-3-17 00:00 is selected to compare with the field data of the Hengsha,Changxing and Zhongjun stations, and that from 2005-7-6 00:00 to 2005-7-8 23:00 is selected to compare with the field data of the Luchaogang station.Meanwhile, the simulated current speed U and current direction D from 2005-11-3 17:00 to 2005-11-4 19:00 are selected to compare with the field data of the CJ4 station, and those from 2005-11-2 9:00 to 2005-11-3 11:00 are selected to compare with the field data of the HB2 station. Due to the limited space, only 4 tide level verifications and 2 current verifications are given here. The comparison(see Fig. 5) indicates that the simulated results agree well with the field data in general. So the simulated tide field can provide the background field of the tide for the smaller grid.

Fig. 6 The verifications of sea surface temperature at C1, C2 stations

3.2 Verfications of temperature and salinity

The volume of the runoff, the water diversion ratio of the outlets, the tidal strength and the topographic condition are the main elements for controlling the temporal and spatial variations of the salinity. The spatial characteristics of the salinity distribution in Yangtze Estuary can be described as follows. The salinity in the North Branch is much higher than that in the South Branch, the North Channel and the South Channel. The salinity in the entrance of the North Channel is less than that in the North Passage, while the salinity in the entrance of the North Passage is less than that in the South Passage.

The change of the Yangtze River's volume of the runoff has a significant impact on the salinity in the North Branch and the entrances of the North Channel and the South Channel. During the spring tide in the dry season, the saltwater and the fresh water in the Yangtze Estuary are mixed intensively, with a small vertical salinity difference, as a feature of a strongly mixed type.

Fig.7 The verifications of sea surface salinity at C1, C2 stations

The simulated results of the temperature T and the salinity S at the C1 through C5 stations also agree well with the statistical data, as shown in Figs. 6, 7,and due to the limited space, only the varifications at the C1, C2 stations are given here. The simulated distributions of the temperature and the salinity of the surface layer in winter and summer during the ebb tide are shown in Fig. 8. The temperature is low near the coast and in the estuary, high in the outer sea in winter,while the temperature is high in the estuary and the outer sea, low around the Zhoushan Islands in summer.The salinity is low in the estuary and at the top of the Hangzhou Bay, high in the outer sea in both winter and summer. With the comparison between the distributions of the salinity in winter and summer, it can be seen that the salinity in winter is higher than that in summer generally, and the contours of the 20 PSU, 32 PSU expand distinctly in summer, due to the larger discharge of the Yangtze River and the Qiantang River as well as the more precipitation. In addition, the area of the saltwater intrusion can beseeninthesimulateddistributionsofthesalinity,which is in accord with the conclusion that the saltwater can generally reach the upper side of the North Branch and the middle of the South Branch in the dry season, as well as the middle of the North Branch and around the sandbar of the South Branch in the flood season. In conclusion, the simulated distributions of the temperature and the salinity are close to the statistical results of the marine atlas, and the variations of the temperature and the salinity in the Yangtze Estuary are well simulated with the larger grid, which can therefore provide the background field of the temperature and the salinity for the smaller grid.

Fig. 8 The simulated distributions of temperature and salinity

Fig.9 Simulated distributi of DIN, PO4 3- and Chl-a of the surface layer in Yangtze Estuary

3.3 Verifications of water quality

The simulation time for the water quality is from 2004-1-1 00:00 to 2005-3-1 00:00. The measured distribution of NO3-is like that of the salinity,decreasing from the estuary to the outer sea. The highly nutrient flow spreads from the estuary to thenortheastinsummer,andtothesoutheastinwinterdue to the decrease of the discharge, with a highly nutrient area's retraction to the estuary. The measured distribution of PO43-is tougue-shaped, high in north and south, while low in the middle and the outer sea.The concentration of Chlorophyll-a is significantly changed by season, high in spring and summer, low in autumn and winter. The measured high chlorophyll-a concentration area is mainly between 122.0°E and 122.5°E in summer and autumn, the west of 122.0°E in winter, between 122.5°E and 123.0°E in spring.Although the simulated distributions of DIN, PO43-and Chlorophyll-a (see Fig. 9) are a little different from the measured distributions (see Fig. 10), from Chai[20], they are similar numerically and spatially in general, and in accord with the conclusion that the high value areas of phytoplankton and Chlorophyll-a on the surface are generally in the front of the diluted water, instead of within the estuary with lots of nutrients inside. So the coupled model of this part can be a reliable foundation for simulating the water quality after the reclamation project of the Nanhui tidal flat in the Yangtze Estuary.

Fig.10 Measured distributiof DIN, PO4 3- and Chl-a of the surface layer in Yangtze Estuary

Fig.11 The sketch map of the reclamation project and the locations of the feature points

4. Results and discussions

The sketch map of the reclamation project of the Nanhui tidal flat is shown in Fig. 11. For the convenience of analyzing, other projects around are neglected, and only the impact of the reclamation project of the Nanhui tidal flat is considered in this study. The area of the project is 149 km2, and the whole project is 23.9-27.3 km long in the N-S direction, and 2.6 km-9.8 km wide in the E-W direction. According to the feasibility study of the reclamation project of the Nanhui tidal flat, the whole project is planned to be finished in 2022. The reclaimed land is mainly for farming. The well verified hydrodynamic model mentioned above coupled with the temperature, the salinity and the water quality in the Yangtze Estuary is modified to fit the new coastline after the reclamation project. The model and the modified model are used to simulate the hydrodynamic envi- ronment before and after the reclamation project from 2004-1-1 00:00 to 2005-3-1 00:00 in the Yangtze Estuary, and the simulated results can provide some scientific guidance for the subsequent reclamation projects and environmental protection.

Three feature points are selected in each part of the South Branch of the Yangtze Estuary, as shown in Fig. 11. The Points 1-3 are in the North Channel, the Points 4-6 are in the South Channel; the Points 7-9 are in the North Passage; the Points 10-12 are in the South Passage and the Points 13-15 are around Nanhui.

4.1 Impact on velocity

Thesimulateddepth-averagedvelocityfrom2005-2-14 00:00 to 2005-2-16 00:00 is selected for analyzing. The rate of the velocity change at each feature point is calculated as(1V ,2V represent the simulated velocities before and after the reclamation project at each feature point), and the results are shown in Table 1. The results indicate that,the peak velocity during the ebb tide in the North Channel is reduced the most at the Point 2 by 17.12%,and by 13% in average, while it is increased the most at the Point 4 by 33.84%, and by 21% in average in the South Channel after the project. The peak velocity during the ebb tide is also increased in the North Passage and the South Passage in general, while it is decreased around Nanhui after the project. The peak velocity during the flood tide changes but slightly, and is generally increased in the South Channel and the South Passage, while it is decreased around Nanhui after the project. The velocity declines in the North Channel and the velocity increases in the South Channel and the South Passage during the ebb tide after the project, which is mainly due to the changesof the salinity front, as shown in Fig. 12.

Table 1 Simulated depth-averaged velocity before and after reclamation during spring tide in dry season

Table 2 Simulated tide level before and after reclamation during spring tide in dry season

Fig.12 Simulated distributions of sea surface salinity in Yangtze Estuary before and after reclamation

The increase of the salinity in the North Channel prevents the runoff from flowing into the North Channel, therefore leading to a velocity decline during the ebb tide in the North Channel after the project.Meanwhile, the project weakens the salinity front in the South Passage, and the weakened salinity front allows more runoff to flow into the South Passage,leading to a velocity increase during the ebb tide in the South Passage after the project. Furthermore, more saltwater from the North Branch intrudes into the South Branch, and with the blocking effect of the North Channel and the discharging effect of the South Passage, the velocity during the ebb tide in the South Channel increases significantly after the project. It also can be inferred that, the water diversion ratio of the outlets above are changed, and the water diversion ratio of the South Channel and the South Passage are increased, and the specific extent of the ratio increase deserves a further study.

4.2 Impact on tidal level

The simulated tide level from 2005-2-14 00:00 to 2005-2-16 00:00 is selected for analyzing. The analysis of the tide level is similar to the analysis of the velocity, and the results are shown in Table 2. The results indicate that, in most of the area covered by the feature points, the mean high tide level is declined and the mean low tide level is increased except for the North Channel area, generally leading to a smaller tidal range after the project. A significant increase of the mean low tide level and a decline of the mean high tide level, by 19%, 5.17%, respectively are observed after the project in the South Channel. The above phenomena can mainly explained by the changes of the water diversion ratio and the tidal prism. During the ebb tide, more water in the South Branch flows into the South Channel, resulting in an increase and a decline of the low tide level, respectively, in the South Channel and the North Channel after the project. The funnel shape in the South Passage is weakened after the project, therefore the tidal prism of the Yangtze Estuary is reduced, leading to a common decline of the high tide level in all outlets. Furthermore, the decline of the high tide level is bad for the navigation,especially, in the North Channel and the South Channel.

4.3 Impact on salinity

The simulated distributions of the sea surface salinity at 2005-2-13 13:00 before and after the project are selected for analyzing, as shown in Fig.12.

The results indicate that, the saltwater in the North Channel intrudes more deeply, and the contour of 2 PSU reaches the middle of the Changxing Island.The contours of the salinity become sparser in the South Passage, leading to a salinity decline around the project area. The saltwater in the North Branch intrudes more widely into the South Branch. The salinity change can mainly explained by the blocking effect of the project during the flood tide. On the one hand, the reclamation project prevents the outer flood current in the east and the south of the project from flowing into the South Passage, leading to a velocity decline at the Points 12-14 during the flood tide.Therefore, the salinity is reduced and the salinity front is weakened around the reclamation project. On the other hand, the tidal strength out of the North Channel is enhanced relatively because that of the South Passage is weakened during the flood tide, leading to more flood current out of the North Channel flowing into the North Passage, and the contours of the salinity out of the North Channel expanding more widely into the North Channel and the North Passage. The velocity decline at the Points 2, 3 and the velocity increase at the Points 1, 7 well demonstrate the above inference that, the velocity change at the Points 2, 3 and 7 is due to the change of the water diversion ratio of the flood current out of the North Channel, while the velocity increase at the Point 1 is due to the significant velocity increase in the Hengsha Passage,leading to more flood current from the North Passage flowing into the North Channel. The aggravation of the saltwater intrusion of the North Branch mainly results from the velocity increase in the South Channel during the ebb tide. The velocity increase draws more saltwater from the North Branch into the South Channel, as shown in Fig. 12(b).

4.4 Impact on water quality

Three feature points are selected in each part of the South Branch of the Yangtze Estuary, as shown in Fig. 11. The Points 1-3 are in the North Channel, the Points 4-6 are in the South Channel; the Points 7-9 are in the North Passage, the Points 10-12 are in the South Passage and the Points 13-15 are around Nanhui.

The simulated monthly-averaged concentration C in the surface layer of DIN, PO43-, Chlorophyll-a and the value of N/P before and after the reclamation project at each feature point in February, 2005, which are, respectively, simulated by the coupled and the modified coupled model, are shown in Fig. 13. The results indicate that, the concentrations of DIN and PO43-are both reduced in the North Channel and the South Channel, especially at the Point 2, while they are increased around the reclamation project,especially at the Point 12. The former is probably due to more flood current with low concentration nutrients out of the North Channel flowing into the North Channel and the South Channel, and the latter is probably due to the reclamation project's effect of preventing the water with high concentration nutrients from spreading around. Although the concentrations of DIN and PO43-change similarly in general, the specific amplitudes of their variations are different,which deserves to be further studied. The value of N/P is reduced in the North Channel, while increased by about 4% in the other outlets, which slightly aggravates the phosphorus restriction effect in the Yangtze Estuary. The concentration of Chlorophyll-a changes in the opposite way as that of N/P, it is increased in the North Channel, while reduced in the other outlets in general. So the reclamation project restricts the growth of the phytoplankton in the Yangtze Estuary. Fang[21]pointed out that, under a high irradiance condition, adding Phosphorus promotes phytoplankton to absorb PO43-, NO3-, NO2-and SiO32-, while adding NO3-suppresses phytoplankton's absorption of nutrients and increases Chlorophyll-a and PH values, as in accord with the simulated variations of N/P and Chlorophyll-a.

Fig. 13 Simulated values of DIN, PO43-, N/P and Chlorophyll-a

5. Conclusions

In the present paper, with the verification of the larger grid, a 3-D numerical hydrodynamic model MIKE3 coupled with the TS module and the Ecolab module is implemented to simulate the hydrodynamic environment in the Yangtze Estuary. After analyzing the simulated results before and after the reclamation project, the main conclusions are drawn as follows:

(1) During the spring tide in the dry season, the peak velocity during the ebb tide in the North Channel is reduced by 13% in average, while it is increased by 21% in average in the South Channel after the project.The peak velocity during the ebb tide is also increased in the North Passage and the South Passage in general,while decreased around Nanhui after the project. The peak velocity during the flood tide changes but slightly, and is generally increased in the South Channel and the South Passage, while decreased around Nanhui after the project.

(2) During the spring tide in the dry season, a significant increase of the mean low tide level by 19%is observed, as well as a decline of the mean high tide level by 5.17% in average in the South Channel. In most of areas covered by the feature points, a decline of the mean high tide level and an increase of the mean low tide level are observed except for the North Channel area, generally leading to a smaller tidal range after the project, which is bad for the navigation,especially, in the South Channel.

(3) After the project, the saltwater in the North Channel intrudes more deeply, and the contour of 2 PSU reaches the middle of the Changxing Island. The contours of the salinity become sparser in the South Passage, leading to a salinity decline around the project area. The saltwater in the North Branch intrudes more widely into the South Branch.

(4) The concentrations of DIN, PO43-are both reduced in the North Channel, the South Channel,while increased around the reclamation project. The value of N/P is reduced in the North Channel, while increased by about 4% in the other outlets of the South Branch, leading to a slight aggravation of the phosphorus restriction effect in the Yangtze Estuary.The concentration of Chlorophyll-a changes in the opposite way as that of N/P.