Based on the geometry and surface in Daya Bay, we artificially divided the reclamation projects into three periods to analyze the influences and changes on hydrodynamic conditions as a result of the reclamation projects. Three periods of tidal current fields, tidal prisms, and water exchange capacity are simulated by the Finite-Volume, primitive equation Community Ocean Model and the characteristics and trends of hydrodynamics in Daya Bay are discussed. The combination of observation and simulation in this paper gives a good description on the tidal dynamic system in Daya Bay. As indicated by model results, the tidal current velocity in the Bay totally decreases after numerous activities associated with reclamation construction. The decreasing current velocity region is mainly distributed near the Xiachong and Gangkou chain islands. The current velocity in 2015 decreases by approximately 5 cm s−1 compared with velocities before 2000. Future reclamation activities will exacerbate these decreasing current velocity trends in some regions. Compared with 2015, the tidal prism has significantly decreased by 1.3622 × 107 m3 due to planned reclamation. The half-water exchange times for Daya Bay in 2015 and after planned reclamation are 178.9 and 177.4 days, respectively. The water exchange capacity in Fanhe Harbor is weaker than other water fields throughout Daya Bay.

Introduction

Daya Bay (DYB) is a semi-enclosed shallow bay in the northern South China Sea. It is a shallow, mixed, mainly semidiurnal-tide dominated bay with a micro to mesotidal range (Song et al., 2016), Some researchers use the model to study the astronomical tidal component and circulation structure of DYB (Wu et al., 1998; Wu et al., 2007; Wang et al., 2008). The DYB has a significant amount of investment and business environment potential, as well as city support, and is the only petrochemical base along the eastern coast of the Pearl River Delta, which is characterized by the comprehensive advantages of China's coastal areas. Throughout the last two decades, the DYB has experienced a large number of sea activities. Based on data collected on the development and use of the DYB, oceanutilization in the DYB can be divided into three stages (Figure 1). Several reclamation projects occurred before 2000 in the first, i.e. original stage. The second stage was a series of reclamation projects that were approved and carried out between 2000 and 2015, which accounted for a total of 1,163 hectares and was mainly distributed near the cities of XiaChong and Aotou Harbor. The third stage is planned future reclamation, which includes reclamation projects under application and reclamation planning. The former has an area of 1,194.9 hectares and is mainly distributed in coastal areas around Fanhe Harbor. The latter has an area of 824.7 hectares near the cities of XiaChong and Aotou Harbor. The construction of offshore engineering makes the local topography change dramatically. The excavation of channel harbor makes the 10 m and 20 m contour areas close to shore, while the construction of reclamation projects leads to the deposition or disappearance of 0 m and 2 m contour lines to the sea side. For these three stages, there is a necessary and urgent call to investigate the effects that reclamation has on marine environments, especially with respect to the hydrodynamic environment in the DYB. However, at present, there are very few studies that have analyzed the effects that reclamation has on the marine environment in the China (Zhang et al., 1997; Wang et al., 2000; Chen et al., 2008; Liu et al., 2012; Yang et al., 2017), especially the influence of reclamation on tidal current in DYB. In this paper, the tidal characteristics of DYB are studied by combining the measured data and numerical simulation, and the tidal current, tidal prism and water exchange processes at different stages are revealed.

Model descriptions and configuration

Model description and setup

The Finite-Volume, primitive equation Community Ocean Model (FVCOM) implemented in this study is an unstructured-grid, finite-volume, 3-D primitive equation free-surface coastal ocean model originally developed by Chen et al. (2003). The Surface Water Model System (SMS) constructs the high-quality unstructured triangular mesh (Figure 2), which is more suitable for the DYB’s complex shoreline and internal islands. The barotropic tidal model has a higher resolution (about 30 m) near coast (Figure 2) with 5 sigma layers in the vertical. A wet/dry treatment with a critical depth of 0.01 m is embedded in FVCOM, which can provide an accurate simulation of flooding and drainage process on tidal flats (Song et al., 2016). The model was driven by 18 tidal components (Q1, O1, M1, P1, K1, J1, OO1, N2, M2, S2, K2, L2, 2N2, MU2, T2, MTM, MF, and MSF) that derive from a global ocean tide model (NAO.99b model) (Matsumoto et al., 2000) and three shallow water tides (M4, MS4, and MN4) originate from the OTPS (Oregon State University Tidal Prediction Software). The model was initialized with zero conditions, no wind and river was given as we focused on tides. Based on previous studies (Lai et al., 2010; Shi et al., 2011), sensitivity tests of bottom friction drag coefficient and horizontal eddy viscosity are carried out. By comparing the test results with the measured, the bottom friction drag coefficient is set to 0.25 × 10−3 and the horizontal eddy viscosity is set to 0.2.

Data presentation

The original coastline data derive from the results of a revision survey that was approved by the Guangdong Provincial Government in 2008. The data for changes in the coastline due to reclamation projects derive from the Dynamic Surveillance and Monitoring Center of Sea Area Use (Guangdong Province). A portion of the topographic data comes from sea maps published by the Maritime Bureau of the People's Republic of China: Daya Bay (No. 83101; 2013 edition) and Honghai Bay to Dangguan Island (No. 83001). The other areas come from topographic survey data collected in-situ at the areas of reclamation projects. The fused digitized chart and survey data are used as topographic data for model calculations.

In this study, from 2002 to 2010, we collected data for tidal level and currents in the DYB. We use observations from May 3, 2009, to June 8, 2009, to calibrate and verify the FVCOM with three tidal level observation stations, i.e. XiaChong (XC), DongChong (DC), and GangKou (GK), as well as seven tidal current stations located throughout the DYB (Figure 3).

Model verification

Figure 3 shows all points used for comparison in this study. Figure 4 presents comparison between the simulated tidal levels and observation (W01, W02 and W03). The modeled tidal levels matched the measured data well with higher correlation correlations (CC) of 0.87, 0.97, and 0.95 in XC, DC and GK, respectively, with root mean square error (RMSE) of 0.21 m, 0.13 m and 0.16 m, respectively.
formula
(1)
formula
(2)

Where N is the number of prediction-observation pairs, Oi is the value of the th observed data, Pi is the value of the th model data, is the mean value of the model data, is the mean value of the observation.

As the Figure 4 shows, double high tides and low tides are observed in DYB every solar day. The altitudes of two high tides or two low tides are different, and the tidal diurnal tides are unequal. The tidal verification results for these two stations (DC and GK) are better than results for the XC station. On the one hand, changes in topography deform the tidal waves that propagate into the DYB, which may complicate tidal processes. On the other hand, the intensity of human activities along the DYB’s northern coast has greatly increased in recent years. In particular, reclamation work performed through large-scale projects, such as by the South China Sea Petrochemical Company, have modified the coastline and water depth at the entrance to the DYB, which has an influence on tidal processes. Compared with the entrance to the DYB, which is characterized by noticeable changes in the water depth and shoreline, the DC and GK stations, located in open sea areas of the DYB, have changed little in terms of geographical conditions due to scarce human activity.

Figure 5 shows comparisons between the averaged measured tidal current and simulations from May 3, 2009, to June 8, 2009. We find that the average simulated velocity and direction from station V01 to V07 are in good agreement with the measured data. As summarized in Table 1, the computed values match the measured values closely, with an average mean RMSE in velocity magnitude of 0.02 cm/s and an average RMSE in velocity direction of 43°. The correlation correlations (CC) of velocity magnitudes is 0.83, and the average CC in velocity directions is 0.88. The FVCOM reproduces the tidal current processes that occur in the DYB, although there are deviations between the calculated velocities and measured values at several stations (V01 and V02) for specific periods, which are due to objective characteristics, such as numerous islands, a complex coastline, and weak hydrodynamic conditions in the DYB.

Nevertheless, the model results are in reasonable agreement with the observations. It can reflect the characteristics of tides and tidal currents that occur in the DYB, which allows us to use these results for further statistical analyses.

Results and discussion

Based on the reclamation characteristics in the DYB, we artificially divide the reclamation projects into three periods to analyze the influences and changes on hydrodynamic conditions as a result of the reclamation projects. The first period is before 2000, which had relatively little reclamation activity. The second period is between 2000 and 2015, when has experienced a large number of sea activities. The third period coincides with current and future reclamation projects.

Tidal current field

Reclamation possibly modifies the sea area and depth of the DYB and influences the hydrodynamic field throughout the bay. Based on the shoreline and actual water depths at reclamation areas, we calculate the hydrodynamic field and variations in velocity for the three stages. We chose 12 points (S01–S12) in the DYB (Figure 3) to conduct a comparative analysis of the changes in the flow field. Table 2 lists the changes in the maximum vertical velocity at various points during the three periods. Figure 6 shows a comparison of the flood and ebb current fields for the mean vertical current between 2000 and 2015, as well as before 2000. Figure 7 displays a comparison between the flood and ebb current fields for the mean vertical current beginning in 2015 and for planned reclamation projects.

Figure 6 shows a comparison of the flooding and drainage fields for the mean vertical current during three stages. Reclamation projects led to a reduction in the tidal current velocity in the DYB between 2000 and 2015, which is due to a large reclamation project (petrochemical project) near the XC station. The current field in the mid-eastern region of the DYB has notably changed and the maximum current velocity has decreased by approximately 5 cm s−1. In addition, the current velocity increased to a range below 10 cm s−1 in some waters northwest of Chunzhou. Characteristic variations in neap tides are similar to variations for spring tides but the decreasing range actually increased. Based on maximum velocity changes, decreases in velocity in Fanhe Harbor are less than 3% while the amplitude of the decrease at XC is significant, i.e. up to 30.7%. The maximum decreasing range (9.9%) occurred at S07 in western water fields and the decreasing range was generally less than 5 cm s−1 in eastern fields at the Zhongyang chain islands. Differences in the current velocity were insignificant at the entrance of the DYB.

Planned reclamation and influences on the tidal flow mainly occur in the Chunzhou area at northern areas of the DYB and Fanhe Harbor in the northeastern DYB. As shown in Figure 7, narrow channels enhance the current when it enters the DYB, which is separated by islands into two branches: one turns to the west towards Dapeng Cove and Aotou Harbor and the other turns to the east towards Fanhe Harbor. The current at the entrance to Fanhe Harbor noticeably decreases by approximately 10 cm s−1. For spring tides, the maximum increase is more than 20 cm s−1. The range and amplitude of ebb tides are all less than that of spring tides but there are several areas that are characterized by decreases above 20 cm s−1. In addition, there are several other areas in northern Fanhe harbor with current velocity increases of approximately 5 cm s−1, which is due to the narrowing of the flow channel caused by planned reclamation projects. As indicated in Table 2, characteristic variations at the stations in Fanhe Harbor also show significant decreases in mean vertical velocity with a maximum current velocity decrease that is 14.7 to 22.2% at S01 and S02, respectively. However, due to reclamation in Chunzhou, the local area has narrowed and, on the contrary, the current velocity at S05 has increased by as much as 18.9%. The tidal current velocities at S07 and S10 have decreased by 7.5 and 6.6%, respectively. The velocity at S08 in Yaling cove showed no significant change, with a slight increase, and the velocity at S12, near the entrance of the DYB, decreases slightly.

Tidal prisms

The tidal prism is defined as the water volume that flows into the bay via ebb to flood during a tidal cycle, which is mainly related to changes between the tidal levels at high and low tides and changes in sea areas. It is an important parameter describing the hydrodynamics of a bay and its tidal inlets. Tidal prism directly controls the bay’s ability to exchange water, and therefore the movement of pollutants, and its self-purification capability. The change of tidal prism in three periods would be discussed in this section. The formula for calculating tidal prism is as follows:
formula
(3)
where P is the tidal prism, S1 and S2 represent the water area corresponding to high and low tide, respectively, and H1 and H2 are the high and low tide level, respectively. In the finite volume method, the tidal prism is applied to each grid for accurate calculation. The formula (1) is extended to as follow:
formula
(4)
where Si, H1i, H2i are the grid area for i, high and low tide level respectively, and N is the total number of grids in the study area.

Based on the characteristics of the DYB, we set up three sections (Figure 1) in the DYB to calculate the effects that reclamation has on the tidal prism. Section A is located at the entrance of the DYB, which is used for total tidal prism statistics in the DYB. Since the reclamation project is mainly located along the northern and eastern coastline of the DYB, section B is situated in the middle of the DYB, where we partially take into account changes in the tidal prism induced by reclamation projects. The hydrodynamic force in Fanhe Harbor is overall weaker than the entire DYB, and its ecological environment is, therefore, more fragile. Section C is set at the entrance of Fanhe Harbor to study changes of the tidal prism in Fanhe Harbor.

During May 3, 2009, to June 8, 2009, the highest tidal level near the entrance to DYB is 1.06 m, whereas the lowest tidal level is –1.22 m. Calculation results indicate that the tidal prism in section A during the three periods is 1.1899 × 109, 1.1503 × 109, and 1.1181 × 109 m3 respectively. The tidal prism in 2015 decreases by approximately 3.96 × 107 m3 compared with that before 2000. After the planned reclamation, the tidal prism will decrease to 3.22 × 107 m3 compared with 2015. The tidal prism at section B during the three periods is 6.6141 × 108, 6.239 × 108, and 5.901 × 108 m3, respectively. The tidal prism in 2015 decreased by approximately 3.75 × 107 m3. After the planned reclamation, the tidal prism decreased by 3.38 × 107 m3 compared with 2015. The tidal prism in section C during the three periods is 7.365 × 107, 7.3416 × 107, and 5.9794 × 107 m3, respectively. The tidal prism in 2015 declined by 0.32%, i.e. approximately 2.34 × 105 m3, which indicates that reclamation has little effect on the tidal prism in Fanhe Harbor. Compared with 2015, the tidal prism will decrease significantly by 1.3622 × 107 m3 due to planned reclamation.

Water exchange capacity

In fact, the study of water exchange capacity in the bay is to study the physical self-purification ability of the bay, which is an important index and method to evaluate and predict the environmental quality of the bay. In this study, we use a water exchange half-life time (Luff et al., 1995) of conservative materials to calculate and analyze the water exchange capacity in the DYB. The method is summarized as follows: For an entire bay, we assume that the given dimensionless initial concentration of conservative substances is 100%. The time required, when the conservative substance concentration in the sea area is diluted by convection and diffusion to 50% of the initial concentration, is defined as the water exchange half-time, which represents the rate of change in water quality and is, therefore, indicative of the overall water exchange capacity in the entire bay.

Figure 7 presents the time-varying process of water exchange in the entire DYB in 2015. Since the process of water exchange after reclamation is similar to that in 2015, we do not provide it here. Based on the entire water exchange process, there are several changes in concentration at the entrance to the bay during the first 5 days, as well as the fact that the water exchange at the eastern side of the entrance increases after 10 to 15 days. Thirty days later, the concentration in approximately half of the DYB's eastern area dropped to below 50%. From day 60 to 90, the concentration had the tendency to further decrease along the eastern side, which continued to expand into the interior of the DYB. After 180 days, the concentration in most of the bay drops to below 50% but the concentration in Fanhe Harbor, XiaChong, Yaling Cove, and Dapeng cove were still high, i.e. areas where diffusion and exchange are difficult. After 210 days, instances of higher concentration still existed in several of the water fields mentioned above, which indicates that these areas have a relatively poor water exchange capacity.

In general, water exchange in the DYB begins from the eastern side of the mouth and gradually spreads to the north of the bay. The outflow of conservative materials from the west of the mouth occurs in a band between Sanmen Island and the mainland, flowing into Dapeng cove. The water exchange half-life time for the DYB in 2015 and after planned reclamations are 178.9 and 177.4 days, respectively, where the latter is 1.5 days faster than the former because reclamation reduces the sea area within the bay and, thus, reduces the water’s half-time. In addition, the planned reclamation area is small compared with the entire sea area of the DYB and the decrease in the water exchange half-time is, therefore, not noticeable.

Since Fanhe Harbor is the focus of a large part of the future reclamation, its area, which is smaller than other harbors, poses a problem with respect to hydrodynamic conditions in the bay. Therefore, we calculated the water exchange time in Fanhe Harbor. Figures 8 and 9 show the time-varying process of water exchange in Fanhe Harbor in 2015 and during the planned reclamation, respectively. In 2015, water exchange occurred in the first 15 days only near the entrance of Fanhe Harbor. The water exchange process is noticeable in most areas of Fanhe Harbor after 30 days but its concentration is still relatively high. On day 60, the exchange area has expanded, whereas the concentration has decreased, with most areas falling below 70%. After 90 days, the concentration continued to decrease compared with day 60 but the water exchange capacity in the northeastern area of Fanhe Harbor was weak and had no change in concentration, with an increasing range of diffusion outside the entrance. The diffusion area of conservative substances outside Fanhe Harbor is mainly located along the northwest side of Fanhe Harbor. For the planned reclamation, water exchange processes were similar to those in 2015 but reclamation areas near the entrance prolonged the water exchange process. The water half-time in Fanhe Harbor increased from 84.5 days in 2015 to 113 days for the planned reclamation, such that there was a significant negative influence on water exchange.

Conclusions

Based on collected data for the development and utilization of the DYB and the various impacts associated with reclamation in the DYB, we analyzed three artificial periods, i.e. before 2000, between 2000 and 2015, and from 2015 onwards. We establish a three-dimensional, fine resolution, unstructured grid, coastal ocean model to reproduce the tides in DYB. Double high tides and low tides are observed in DYB every solar day, the model results are in reasonable agreement with the observations. The tidal current, tidal prism and water exchange processes at different stages are revealed in this study.

The total current velocity in the DYB decreased after the construction of numerous reclamation projects. The decreasing velocity region is mainly distributed near the Xiachong and Gangkou chain islands, where the current velocity in 2015 decreases by approximately 5 cm s−1 compared with before 2000. For the planned reclamation, the influences on tidal flow predominantly concentrate in the Chunzhou area of the northern DYB and the Fanhe Harbor in the northeastern DYB. The current at the entrance to Fanhe Harbor noticeably decreased in a range of approximately 10 cm s−1, with a maximum of more than 20 cm s−1 for spring tides.

In this study, from May 3, 2009, to June 8, 2009, the highest tidal level near the entrance of the DYB was 1.06 m, whereas the lowest tidal level was –1.22 m. Compared with that before 2000, the DYB tidal prism in 2015 decreased to 3.96 × 107 m3 with a range of 3.3%. After the planned reclamation, the tidal prism decreased to 3.22 × 107 m3 with a range of 2.8%. The tidal prism decreased by more than 18.6% in Fanhe harbor.

The half-life water exchanges for the DYB in 2015 and after the planned reclamation are 178.9 and 177.4 days, respectively. The water exchange capacity in Fanhe Harbor is weaker than other water fields in the DYB. For the planned reclamation, the water exchange process is similar to that in 2015 but these processes are prolonged due to reclamation areas near the entrance. The water half-time for Fanhe Harbor increased from 84.5 days in 2015 to 113 days for planned projects, such that there is a significant negative influence on water exchange.

Acknowledgements

We wish to thank the Guangdong Provincial Department of Ocean and Fisheries and the Administration of Ocean and Fisheries of Huizhou for providing the depth data. The authors gratefully acknowledge the use of the HPCC for all numeric simulations and data analysis at the South China Sea Institute of Oceanology, Chinese Academy of Sciences. We specially wish to thank Ms. Kong Xiaoli for drawing Figure 1 in this study. This work is supported by Open project fund from the State Key Laboratory of Tropical Oceanography (South China Sea Institute of Oceanology Chinese Academy of Sciences) (LTO1810).

Note

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uaem.

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