Numerical Study on the Influence of Tidal Inlet Restoration on Water Exchange in Wanning Lagoon

. Wanning Xiaohai is the largest lagoon in Hainan Province. In recent years, the body of water in Xiaohai has been polluted due to the untreated discharge of domestic sewage and aquaculture tailwater directly into Xiaohai. In order to improve the water quality of Xiaohai and relieve the pressure on the water environment, it was decided to restore the tidal inlet of Xiaohai. In this study, the water quality module of the finite volume ocean model was applied to simulate the change of pollution concentration using the Eulerian advection diffusion equation to analyze the half life time and tidal prism before and after the restoration project of the Wanning Xiaohai tidal inlet. The research results showed that after the restoration of the tidal inlet, the half life time of Xiaohai water body changed from 64 d to 58 d, shortened by 6 d, and the tidal prism increased from 8.8×10 6 m 3 to 9.6×10 6 m 3 , which can well enhance the exchange between Xiaohai and external water bodies, improve the water quality of Xiaohai and relieve the pressure of water environment.


Introduction
The lagoon is usually defined as a local seawater area separated from the outer sea by a sand spit, sand dam or coral. It is closely related to human production and life, and has important ecological, economic and tourism values, thus it has received wide attention from many scholars [1]. Some scholars have studied the lagoon through actual measurements or remote sensing data, such as El Zrelli et al assessed the ecological risk and environmental impact of the presence of seven metals in the surface sediments of the Bizerte lagoon [2]. Miyittah et al evaluated the water quality condition of Aby Lagoon using cluster analysis based on measured water temperature, dissolved oxygen, turbidity, conductivity, nitrate and phosphate data [3]. Maimouni et al used sentinel-2 data to analyze the turbidity of Moroccan lagoon water [4]. Other scholars have used numerical modeling methods to study the lagoon water environment, such as Li et al combined remote sensing data and numerical models to simulate the spatial distribution and evolution of water quality in Donghu [5]. Fang et al investigated the effects of anthropogenic nutrient inputs on the water environment of Xinchun Lagoon [6]. Kuang et al simulated the water circulation and transport characteristics of the Qilihai lagoon and calculated the water residence time to assess the water exchange capacity of the lagoon [7]. Hong Thi Pham et al calculated the environmental carrying capacity of the lagoon in the dry and rainy seasons based on Mike 3 [8].
Xiaohai is located in the eastern part of Wanning City, Hainan Province, and is the largest lagoon in Hainan Province, with an area of about 43 km 2 and waters ranging from 1 to 4 m. The lagoon is rich in biological resources, such as grouper and mud crab, and has high economic and environmental value. There are relatively few studies on the Xiaohai lagoon. Long et al used the finite element method to simulate the hydrodynamic field of the Xiaohai lagoon [9]. Luo investigated the spatial and temporal distribution of surface sediments in the Xiaohai lagoon [10]. In recent years, with the discharge of aquaculture tailwater and domestic sewage, the water quality of Xiaohai has deteriorated to poor category 4 water bodies, and the water quality is extremely poor. To improve the water quality of Xiaohai, it was decided to implement the tidal inlet restoration project for Xiaohai. In this study, a numerical model was used to assess the exchange capacity of the water bodies before and after the project of Xiaohai by introducing the half life time and the tidal prism, in order to provide a scientific reference for alleviating the environmental pressure of Xiaohai.

The hydrodynamic model
In this study, the finite volume ocean model is used to simulate the hydrodynamic field in the study area. The controlling equations of the model include momentum, continuity, temperature, salinity and density equations [11]. Figure 1 shows the grid diagram of the model calculation area. The model uses an unstructured triangular grid, which can be well adapted to complex coastlines. Figure  2 and Figure 3 are the water depth and grid diagram before and after Xiaohai project respectively. The bathymetric data are obtained from nautical charts, the shoreline data are obtained from 2020 Google-earth satellite maps, the open boundary uses tidal forcing, the open boundary tidal data are extracted from TPXO 9, and the tidal processes are obtained using eight major sub-tidal forecasts, M 2 , S 2 , N 2 , K 1 , K 2 , O 1 , P 1 , Q 1 . The wind field data are obtained from the European Centre for Medium-Range Weather Forecasts, the Fifth-generation Reanalysis data.

The water quality model
Where C is the concentration of the tracer, D is the water depth, u, v and ω are the x, y and σ components of the water velocity, K h is the vertical thermal diffusion coefficient, F c is the horizontal diffusion term. C 0 is the tracer concentration injected from a point source given as: C 0 x , y , σ , t 1 t s t t e ; σ k σ σ k n ; x x i ;y y i ; i 1, N 0 otherwise Where t s and t e are the start and end time of the tracer injection, σ k and σ k n refer to the upper and lower-bound σ levels in which the tracer is injected. i is the node ID, and N is the total number of nodes where the tracer is injected.

Model validation
The actual measured data of stations C1 and C2 near the project area from December 28, 2020 to December 29, 2020 of Hainan Academy of Marine and Fishery Sciences were used to compare with the model simulation data. Figure 4 shows the distribution of model validation station, Figure 5 shows the verification of C1 tidal level, Figures  6 and 7 show the C1 current velocity and direction verification, and Figures 8 and 9 show the C2 current velocity and direction verification, From the figures, we can see that the simulated tide level is consistent with the actual tide level in general, and there are only slight differences in the values, and the flow velocity and direction are very close to the measured values, which can simulate the hydrodynamic characteristics of the Xiaohai well.

Tidal Prism
Tidal prism is the maximum volume of tidal water that a bay or lagoon can hold. The tidal prism plays an important role in maintaining the ecological balance of the lagoon, and the larger the tidal prism, the stronger its self-cleaning capacity, calculated by the formula.
Where S i is the area of the i-th grid, H 1i and H 2i is the tide level at the moment of high and low tide of the i-th grid. Through the formula, it was found that the tidal prism of the Xiaohai was 8.8×106 m3, and after the restoration of the tidal inlet, its tidal prism became 9.6×106 m3, which increased by 8.0×105 m3 and increased its selfpurification capacity.

Water exchange
The strength of the water exchange capacity reflects the self-purification ability of the water body. There are many indicators to evaluate the exchange capacity of water bodies, such as half life time [12]. The half life time is the time required to reduce the concentration of conservative substances to 50% of the initial concentration. In this study, half life time was used to evaluate. Figure 10 and 11 show the half life time of the water bodies before and after the Xiaohai project. Figure 12 shows the difference of half life time of the water bodies before and after the Xiaohai project. Figure 13 shows the variation of total tracer concentration with time before and after the Xiaohai Project. It can be seen from the figures that before the project construction, the half life time of Xiaohai gradually increased from north to south, and the half life time was about 20 d in most areas of Xiaohai in the north, 60 d in the middle and 80 d in the south, and after the project construction, the overall half life time in most areas of Xiaohai was shortened by 0~10 d, and the half-life time in some areas was shortened by 30 d. As can be seen from Figure 13, the total half-exchange time of the Xiaohai before the project construction was 64 d, and the overall half life time became 58d after the tidal inlet was connected, which was shortened by 6 d. Therefore, it can be concluded from these indicators that the tidal channel restoration project can effectively improve the water exchange capacity of Xiaohai, enhance the exchange between Xiaohai and external water bodies, and improve the current poor water quality condition of Xiaohai.

Conclusion
In this study, a finite volume ocean model was used to simulate the hydrodynamic process of Wanning Xiaohai lagoon. By comparing the tidal prism and half life time of Xiaohai before and after the restoration of Xiaohai tidal inlet, it was concluded that the tidal prism of Xiaohai could be increased by 8×10 5 m 3 and the total half life time was reduced by 6 d after the restoration of Xiaohai tidal inlet, which could effectively enhance the self-purification capacity of Xiaohai and relieve the environmental pressure of Xiaohai water.