Response of tidal current to the construction of Yangshan Deepwater Harbor, Shanghai, China

A high-resolution ocean model for the Yangshan Deepwater Harbor (YDH), Shanghai, China was established based on FVCOM. Changes of tidal current and water fluxes due the construction of YDH was then investigated thoroughly. Results show that the construction of YDH reduced current velocities in the harbor basin, but led to significant increase of tidal current in the channels.


Introduction
The Yangshan Deepwater Harbor (YDH) was constructed on the Qiqu Archipelago, which is located at the conjunction of the Changjiang Estuary and Hangzhou Bay (Figure 1).The islands in the Qiqu Archipelago arrange into two island chains.The constructions were mainly on the north-island chain.The sea area embraced by the two island chains is characterized by deep water (more than 12 m) and weak waves.This particularly dynamical feature makes this sea area to be an ideal place for ship berthing.The names of the islands and passages are denoted used in this study.
The construction of the YDH started in June 2002, and finished the first three phases in 2008.The fourth phase of the project was completed in 2018.Three important passages are closed, and the construction of the YDH construction reclaims a large area of seas (Table 1).The dynamics in the phase IV: land reclamation and diversion dikes

Methodology
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 [13].
The YDH sea area is located in the water and sediment exchange areas of the Changjiang estuary and Hangzhou Bay.Thus, the model domain covers the Changjiang Estuary and Hangzhou Bay.
Considering the computational efficiency of our numerical model, a model nesting technique is used in the model.As shown in Figure 2, the YDH model domain with a high spatial resolution of 50 m is nested within a well-validated Changjiang Estuary FVCOM (CJE-FVCOM) model developed by Ding Pingxing's group at East Normal University of China [14,15].
In the CJE-FVCOM model (Figure 2a), the upstream boundary of the Changjiang Estuary reaches Datong, which is about 630 km upstream from the river mouth.There is a representative hydrologic station that measures water and sediment flux is regarded as the Changjiang river into the sea.The upper boundary of the Qiantang River reaches Laoyancang.The model domain covers an extensive sea area stretching to 124.5°E to the east, 34.5°N to the north and 28.5°N to the south, embracing the Changjiang Estuary, the Hangzhou Bay and the Zhoushan Archipelago.
The domain of YDH-FVCOM model (Figure 2b) covers all islands in the Qiqu Archipelago.The maximum resolution of the model mesh at the open boundary is 1000 m.The mesh resolution is refined to 50 m on the coastlines and in the passages.The high-resolution unstructured triangular mesh, which suits the irregular coastlines and complex topographies in the YDH sea area.Moreover, the time step should be short enough, limited by the CFL condition.It is set to a small value as 0.2 s.The CJE-YDH numerical model has been well validated in Guo et al. [16,17].
Figure 2 The Changjiang Estuary-Yangshan Deepwater Harbor nesting FVCOM model mesh used in this study.The mesh is refined on the boundaries of constructions.

Tidal Characteristics
According to the results of harmonic analysis of water level data from tidal stations in Qiqu Archipelago, the tidal characteristics show that the semi-diurnal tide is obvious.However, there is a phenomenon of unequal daily tides.The shallow water tide is more significant, and the duration of the ebb tide is longer than that of the flood tide, so the tidal type in this sea area belongs to the irregular semi-diurnal shallow water tide type.
According to the tide level data statistics of Xiaoyangshan station from 1997 to 2004 and the tide level data statistics from 2018 to 2021(Table 2).The average annual tidal range has increased significantly from 2.74 m to 3.07 m.But the extreme tidal range decreased from ~ 5 m to ~ 4.5 m.The mean high water has been elevated from 3.88 m to 4.01m, and the mean low water decreased by about 0.20 m.All these statistics show that the tidal dynamics is modulated strongly by the construction of YDH.The current direction during ebb slack is nearly same to that during ebb maximum, but magnitude decreases much.While during flood slack, though the current is weak, relatively large velocities can be found in the North of Dashantang Island.
After the construction, snapshots of surface current during ebb and flood maximum and ebb and flood slacks are showed in Figure 4.During ebb tide, current is maintained flowing into the harbor area though the west entrance and flowing out through the other passages.The flow velocity distribution in the harbor area is more average.During flood tide, current still flows out the harbor area through the west entrance and flows in through the other passages.The gap between the flow velocity of ebb slack and flood slack still exists.While during flood maximum, relatively large velocities can be found in the North of Xiaoyangshan Island because of blocking the near-shore flood current and in the East Entrance due to the narrow channel.Similar flow field distribution also appears at the flood slack.
On the whole, the construction of the project reduces the flow velocity inside the harbor area and increases the flow velocity at the East Entrance.The flow velocity on the north side of Dashantang island which was greater than 2 m/s during the flood maximum before the project decreased significantly after the project.It can be clearly seen that the flow velocity after the project at the East Entrance is significantly higher than that before the project due to the tidal choking.The construction of the Yangshan Deepwater Harbor also makes the maximum flow velocity point of flood slack in the harbor area transfer.The maximum flow velocity before the project appears in the southwest area of the harbor area, and after the project the flow velocity in this area is greatly reduced.This shows that when the passages are blocked, the shore wall of the North Island chain is basically connected as a whole, preventing the flood current on its north side flowing into the harbor.Before the construction, in terms of the distribution of the tidal flux between flood and ebb tides, the flood and ebb tides at the West Entrance of the harbor are more balanced, while the ebb tides dominate at the remaining three channels except Kezhushan Passage.From the point of view of the total amount of tidal flux, the West Entrance has a larger total amount due to its longer length; the East Entrance has a larger sectional tidal flux due to its narrower topography and stronger hydrodynamic force.
After the construction, the East Entrance and West Entrance are more balanced than the other three sections in terms of flood tide and ebb tide flux.The total tidal flux of the east gate and the west gate is much larger than that of the other three passages.
Comparing the flux before and after the project, the balance between the flood and ebb tidal flux at the West entrance remains constant.From the perspective of the tidal flux change, the Shuanglianshan-Dashantang, Dashantang-Dayangshan passage of the south-island chain and the Kezhushan Passage of the north-island chain have an increase.But the West Entrance shows a decrease in flood tide fluxes.This has a certain connection with the blocking project on the north side.In terms of the balance of tidal flux, the flood and ebb tides are more balanced in several passages after the works than before the works, especially at the East Entrance As a part of the Yangshan Harbor channel, the flux balance of the East Entrance is of great significance.Upon completion of the project construction, significant changes were observed in the flow field within the basin.The newly-built coastline altered the tidal flow direction near the project area, while the offshore region, farther away from the project, experienced less impact.Furthermore, the closure of the branch channel resulted in a weakening of hydrodynamic forces in most sea areas within the harbor, with an enhancement observed at the east entrance.
Through numerical simulations, we calculated the water flux of five representative sections before and after the project.Our findings indicate that four sections exhibited a net outward water flux, while the Kezhushan Passage displayed a net inward water flux.Additionally, a comparison between flood and ebb tide flow flux revealed a flood-dominant current in the western region of the YDH.Notably, two sections, namely the Kezhushan Passage in the north-island chain and the Shuanglianshan-Dashantang Passage in the south-island chain, experienced a significant increase in water flux.This increase can be attributed to the connection of the shore wall of the Xiaoyangshan island chain when the channel was blocked.

Figure 1
Figure 1 Map of the Qiqu Archipelago where the Yangshan Deepwater Harbor (YDH) constructed.The names of the islands and passages are denoted used in this study.

Figure 3
Figure3displays snapshots of surface current during ebb and flood maximum and ebb and flood slacks before the construction of YDH.During ebb tide, current flows into the harbor area though the west entrance and flows out through the other passages.The ebb current flows roughly easterly, and the direction is modulated significantly by the complex topography.The funnelled geometry converges the ebb current in the harbor sea area, forcing enhanced velocities.During flood tide, current flows out the harbor area though the west entrance and flows in through the other passages.Forced by tidal choking effect, tidal current in the East Entrance is extremely strong, the velocity can be over 3.0 m/s.The current direction during ebb slack is nearly same to that during ebb maximum, but magnitude decreases much.While during flood slack, though the current is weak, relatively large velocities can be found in the North of Dashantang Island.After the construction, snapshots of surface current during ebb and flood maximum and ebb and flood slacks are showed in Figure4.During ebb tide, current is maintained flowing into the harbor area though the west entrance and flowing out through the other passages.The flow velocity distribution in the harbor area is more average.During flood tide, current still flows out the harbor area through the west entrance and flows in through the other passages.The gap between the flow velocity of ebb slack and flood slack still exists.While during flood maximum, relatively large velocities can be found in the North of Xiaoyangshan Island because of blocking the near-shore flood current and in the East Entrance due to the narrow channel.Similar flow field distribution also appears at the flood slack.On the whole, the construction of the project reduces the flow velocity inside the harbor area and increases the flow velocity at the East Entrance.The flow velocity on the north side of Dashantang island which was greater than 2 m/s during the flood maximum before the project decreased significantly after the project.It can be clearly seen that the flow velocity after the project at the East Entrance is significantly higher than that before the project due to the tidal choking.The construction of the Yangshan Deepwater Harbor also makes the maximum flow velocity point of flood slack in the harbor area transfer.The maximum flow velocity before the project appears in the southwest area of the harbor area, and after the project the flow velocity in this area is greatly reduced.This shows that when the passages are blocked, the shore wall of the North Island chain is basically connected as a whole, preventing the flood current on its north side flowing into the harbor.

Figure 3
Figure 3 Snapshots of surface current distribution at ebb maximum, ebb slack, flood maximum and flood slack before the construction of YDH

Figure 4
Figure 4 Snapshots of surface current distribution at ebb maximum, ebb slack, flood maximum and flood slack after the construction of YDH

Figure 5
Figure 5 Cross-section tidal flux during one month, unit ten thousand cubic meters.(left)before the construction, (right)after the construction

Table 1 :
Timeline of the Yangshan Deepwater Harbor construction Projects

Table 2 :
Tidal characteristic of Xiaoyangshan station