Study on Hydrodynamic Characteristics and Environmental Response in Shantou Offshore Area

Journal of Marine Science and Engineering

Article Study on Hydrodynamic Characteristics and Environmental Response in Shantou Offshore Area

Yuezhao Tang 1,2 , Yang Wang 1,*, Enjin Zhao 2,* , Jiaji Yi 1, Kecong Feng 1, Hongbin Wang 1 and Wanhu Wang 1

1 Haikou Marine Geological Survey Center, China Geological Survey, Haikou 570100, China; [email protected] (Y.T.); [email protected] (J.Y.); [email protected] (K.F.); [email protected] (H.W.); [email protected] (W.W.) 2 Marine Geological Resources Laboratory, China University of Geosciences, Wuhan 430074, China * Correspondence: [email protected] (Y.W.); [email protected] (E.Z.)

Abstract: As a coastal trading city in China, Shantou has complex terrain and changeable sea conditions in its coastal waters. In order to better protect the coastal engineering and social property along the coast, based on the numerical simulation method, this paper constructed a detailed hydrodynamic model of the Shantou sea area, and the measured tide elevation and tidal current were used to verify the accuracy of the model. Based on the simulation results, the tide elevation and current in the study area were analyzed, including the flood and ebb tides of astronomical spring tide, the flood and ebb tides of astronomical neap tide, the high tide, and the low tide. In order to find the main tidal constituent types in this sea, the influence of different tidal constituents on tide elevation and tidal current in the study area was analyzed. At the same time, the storm surge model of the study area was constructed, and the flow field under Typhoon “Mangkhut” in the study area was simulated by using the real recorded data. Typhoon wind fields with different recurrence periods and intensities were constructed to simulate the change in the flow field, the sea water level, and Citation: Tang, Y.; Wang, Y.; Zhao, E.; Yi, J.; Feng, K.; Wang, H.; Wang, W. the disaster situation along the coast. The results showed that under normal sea conditions, the sea Study on Hydrodynamic water flows from southwest to northeast at flood tide and the flow direction is opposite at ebb tide. Characteristics and Environmental The tidal range is large in the northwest and small in the southeast of the study area. The tides in the Response in Shantou Offshore Area. J. study area are mainly controlled by M2, S2, K1, and O1 tidal constituents, but N2, K2, P1, and Q1 Mar. Sci. Eng. 2021, 9, 912. https:// tidal constituents have significant effects on the high water level. The water level caused by typhoons doi.org/10.3390/jmse9080912 increases significantly along the coast of Shantou City. In the west area of the Rong River estuary, a typhoon with a lower central pressure than 910 hPa may induce a water increase of more than 2 m. Academic Editor: Yannis N. Krestenitis Keywords: tide; numerical simulation; tidal constituent; water elevation; typhoon

Received: 11 July 2021 Accepted: 16 August 2021 Published: 22 August 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Shantou City, located on the golden position of Hong Kong and Southeast Asia, is an with regard to jurisdictional claims in important node of the Maritime Silk Road. Shantou City faces the Taiwan Province Strait published maps and institutional afﬁl- to the east, 214 nautical miles away from Kaohsiung Port and 187 nautical miles away from iations. Hong Kong. At the same time, Shantou Port is located in the downtown area of Shantou. It is one of the 25 major coastal hub ports recognized by the Ministry of Transport of the People’s Republic of China and the ﬁve major ports in Guangdong Province. In order to ensure the production and life of Shantou coastal waters, the Shantou Marine Hydrological Weather Bureau has carried out relevant investigations. The survey Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. showed that the hydrological and climatic conditions of Shantou and its coastal areas are This article is an open access article complicated, with tortuous shorelines, rivers entering the sea, and a dense distribution of distributed under the terms and complex movement of seawater in some years. The changing tidal current affects the life of conditions of the Creative Commons the people in Shantou city and the operation of ships in Shantou Port. Coastal cities also Attribution (CC BY) license (https:// suffer from marine disasters such as typhoon storm surges and tsunamis [1]. Typhoons may creativecommons.org/licenses/by/ lead to serious water increases and storm surges. Since 1949, many typhoons have occurred 4.0/). in Shantou, causing serious disasters. For example, in 1969, Typhoon “Viola” caused the

J. Mar. Sci. Eng. 2021, 9, 912. https://doi.org/10.3390/jmse9080912 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, 912 2 of 28

water level to rise by more than one meter, killing 956 people and destroying 93,000 square kilometers of farmland. In 1979, Typhoon “Hope” killed 93 people, and destroyed more than 20,000 houses and 60,000 hectares of farmland. In 1986, Typhoon “Peggy” inundated 586,000 hectares of farmland, killed 206 people, sank 64 boats, and destroyed more than 81,000 houses. According to statistics, 201 tropical cyclones entered Shantou from 1950 to 2012. On average, there are 3.2 tropical cyclones every year. Among them, the tropical cyclones entering Shantou are mostly typhoons and severe tropical storms, accounting for 65% of the total. These typhoon paths are divided into five categories: landing from the west, landing from the front, landing from the east, passing by land, and passing by sea. About 60% of typhoons belong to the first two categories, and 35% of typhoons are landing from the east [2]. It can be seen that extreme weather does great harm to Shantou City. Generally speaking, the hydrodynamic conditions in the Shantou coastal area are complex and vulnerable to typhoons [3–5]. Therefore, in order to guarantee safety for the production and life of the port and coast, it is extremely necessary to establish a fine marine water environment dynamic simulation model in Shantou City. In this way, a hydrodynamic simulation of Shantou City can be carried out to better understand the tidal current in this sea area. On this basis, typhoon simulations can help us understand the disaster caused by typhoon storm surges, to provide a scientific basis for disaster prevention and mitigation. Compared with the traditional observation and acquisition, the use of numerical simulation is more efficient and more cost-effective [6,7]. By means of numerical simulation, we can better understand the distribution of tidal current in the Shantou sea area and the damage of storm surges. Numerical simulation technology has been applied well to the simulation of sea water movement and marine disasters. For example, Lai et al. [8] used FVCOM to simulate the circulation in the Pearl River Estuary and reproduce the observation characteristics of ocean currents. Liu et al. [9] studied the abnormal water increase in the Pearl River Estuary area caused by storm surges from Typhoon “Hato” through the numerical simulation of sea water movement. Xu et al. [10] analyzed the hydrodynamic characteristics and silt backfill in the channel in the western port area of Shenzhen Port through numerical simulation using a mathematical model. Jia et al. [11] established a two-dimensional hydrodynamic model of the Wanbao Lake based on MIKE21, and analyzed the dynamic characteristics of the lake and pollutant migration. In this paper, a detailed hydrodynamic model of the Shantou offshore area was established on the basis of existing research. The accuracy of the model was verified by comparing the measured ocean current data with the tide station water level. The simulated sea area was about 4 × 103 km2, mainly including the sea area in the southeast of Shantou City with a water depth of less than 30 m (Figure1). At the same time, the variations in sea water level and flow field in the study area were given. The variation in sea water flow field under different tidal currents and the distributions of water level and flow velocity under different wind conditions were discussed. In the Section2, the model is introduced and verified. The Section3 discusses the characteristics of the flow field in the study area at different times and the effects of different tidal constituents and wind fields on the results. The Section4 includes the conclusions. J.J. Mar. Mar. Sci. Sci. Eng. Eng. 20212021, ,99, ,x 912 FOR PEER REVIEW 33 of of 29 28

FigureFigure 1. 1. StudyStudy area area and and oceanographic oceanographic station station location. location. (The (The blue blue squares squares are are the the tide tide stations; stations; the the red red circles circles are are the the ADCPADCP points). points).

2.2. Model Model Introduction Introduction and and Verification Verification 2.1.2.1. The The Information Information of of Data Data Used Used TheThe data data used used in in the the model model included included shoreline shoreline data, data, topographic topographic water water depth depth data, data, open boundary forced water level data, wind field data, and river discharge data. open boundary forced water level data, wind field data, and river discharge data. The original shoreline data of the study area were obtained from the Global Self- The original shoreline data of the study area were obtained from the Global Self-con- consistent, Hierarchial, High-Resolution Geography Database. Geodas software was used sistent, Hierarchial, High-Resolution Geography Database. Geodas software was used for for extraction. Chart (NO. 81001) and remote sensing images were used to adjust details. extraction. Chart (NO. 81001) and remote sensing images were used to adjust details. The open sea area topographic data of the model were obtained from National Oceanic The open sea area topographic data of the model were obtained from National Oce- and Atmospheric Administration’s ETOPO1 data. ETOPO1 data represent a 1 arc-minute anic and Atmospheric Administration’s ETOPO1 data. ETOPO1 data represent a 1 arc- global relief model of Earth’s surface that integrates land topography and ocean bathymetry, minute global relief model of Earth’s surface that integrates land topography and ocean while the offshore topographic data were extracted from charts. bathymetry, while the offshore topographic data were extracted from charts. The open boundary forced water level data came from the data calculated by TPXO The open boundary forced water level data came from the data calculated by TPXO 2016. TPXO (Topex/Poseidon Crossover Solution) is a global tidal inversion model estab- 2016. TPXO (Topex/Poseidon Crossover Solution) is a global tidal inversion model established by Oregon State University with T/P intersection data. lishedThe by Oregon discharge State data University of Hanjiang with River T/P andintersection Rongjiang data. River came from the measured dataThe of Chaoan discharge hydrological data of Hanjiang observation River station and Rongjiang and Dongqiaoyuan River came hydrological from the measured observa- datation of station. Chaoan hydrological observation station and Dongqiaoyuan hydrological obser- vationWind station. field data were from the European Centre for Medium-Range Weather Fore- casts.Wind The field resolution data were of wind from field the wasEuropean 1/8◦ × Centre1/8◦ (forhttps://cds.climate.copernicus.eu Medium-Range Weather Fore- casts.(accessed The resolution on 15 July 2020)).of wind field was 1/8° × 1/8° (https://cds.climate.copernicus.eu (accessed on 15 July 2020)). 2.2. Introduction to the Model Used 2.2. IntroductionUsing numerical to the Model models Used is a good way to study the water. Numerical simulation has beenUsing widely numerical used in models the study is a of good the way flow to field study of the rivers, water. lakes, Numerical oceans, simulation around coastal has beenengineering, widely used and soin onthe [ 12study–15]. of the flow field of rivers, lakes, oceans, around coastal engineering, and so on [12–15]. The governing equation of the model is as follows:

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The governing equation of the model is as follows:

∂ζ ∂(Hu) ∂(Hv) + + = 0 (1) ∂t ∂x ∂y √ ∂u ∂u ∂u ∂ζ u2 + v2 ∂2u ∂2u 1 ∂P + + = + − − + + − a u v Fx c f v g gu 2 µ 2 2 (2) ∂t ∂x ∂y ∂x C H ∂x ∂y ρ0 ∂x √ ∂v ∂v ∂v ∂ζ u2 + v2 ∂2v ∂2v 1 ∂P + + = + − − + + − a u v Fy c f u g gv 2 µ 2 2 (3) ∂t ∂x ∂y ∂y C H ∂x ∂y ρ0 ∂y

τs = ρacd|uw|uw (4) Equation (1) represents the continuity equation, where ζ is the water level, t is time, H is water depth, x and y are coordinates in the Cartesian coordinate system, and u and v are the average velocity components along the water depth in the x and y directions, respectively. Equations (2) and (3) represent the conservation of momentum in the x and y directions, respectively, where Fx and Fy are source and sink items in the x and y directions, respectively, cf is the Coriolis force, g is gravitational acceleration, C is the Schetze coefficient, µ is the viscous force coefficient, ρ0 is the fluid density, and Pa is atmospheric pressure. Equation (4) is the calculation formula for the surface stress of the wind field, ρa is the air density, uw is the wind speed at 10 m high, and cd is the drag coefficient of wind stress [16]. According to Wu’s [17] empirical formula, when the wind −1 −1 speed is less than 25 ms , cd is 0.001255; when the wind speed is larger than 25 ms , cd is 0.002425. It is necessary to add fixed boundary conditions and initial conditions to make the model have unique solutions. The initial conditions and boundary conditions are shown in the following equation: ζ(x, y)|t=0 = ζ0(x, y) p(x, y)|t=0 = p0(x, y) (5) q(x, y)|t=0 = q0(x, y) C(x, y, t) = C∗(x, y, t) (6)

Vn = 0 (7) Equation (5) is the initial condition equation. Equation (6) is the open boundary condition, where C∗(x, y, t) are artiﬁcial set conditions (e.g., water level, ﬂow velocity, and discharge) at an open boundary. Equation (7) is the land boundary condition, where n is the normal direction of the land boundary and Vn is the water velocity in the normal direction at land borders.

2.3. Model Setup The study area is divided by a triangular unstructured grid (Figure2). The grid gradually becomes densified from the sea to the shoreline. The grid resolution of the nearshore part is 150 m, and that of the sea boundary part is 5000 m. There are a total of 58,444 grid nodes in the whole area. The study area is divided into 113,148 triangular grids. In the model, the normal velocity of sea flow at the land boundary is zero. The water level at the open boundary is calculated by the TPXO model. The water level is controlled by astronomical tides. To be more specific, the water level at the open boundary is calculated by taking into account 8 tidal constituents, which are M2, S2, K1, O1, N2, K2, P1, and Q1. The discharge data of Hanjiang River and Rongjiang River were used as input conditions in the model. At the same time, the meteorological data provided by ECMWF were input into the model as wind field conditions. The simulation period was from 1 June 2020 to 30 June 2020. The minimum time step of the model was 0.01 s and the maximum time step was 30 s. The eddy viscosity coefficient is given by the Smagorinsky formula and the value is 0.28 [18]. The bed resistance used the Manning number, which is 32 m1/3/s. J. Mar. Sci. Eng. 2021, 9, 912 5 of 28 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 5 of 29

FigureFigure 2.2. Grid division diagram ofof thethe studystudy area.area.

InIn fact,the model, some baroclinicthe normal conditions velocity of suchsea fl asow thermohaline at the land boundary can also is affect zero. theThe whole water hydrodynamiclevel at the open process boundary [19]. is Zhao calculated et al. [ 20by] the used TPXO the POM, model. which The water is a baroclinic level is controlled model, to calculateby astronomical the tidal tides. energy To of be Beibu more Gulf. specific, However, the water the focuslevel at of the this open study boundary was the seais calcu- level andlated storm by taking surges into caused account by typhoons.8 tidal constitu Comparedents, which to thermohaline, are M2, S2, K1, tides O1, and N2, wind K2, P1, play and a moreQ1. The important discharge role data in contributing of Hanjiang toRiver the researchand Rongjiang focus. AtRiver the were same used time, as the input barotropic condi- modeltions in has the already model. hadAt the a good same agreement time, the meteorological with measurements, data provided which can by beECMWF used in were the subsequentinput into the simulation model as (see wind Section field conditions.2.4). Thus, theThe density simulation of seawater period was was from set at 1 a June constant 2020 −3 1.05to 30 kgm June 2020.and doesThe minimum not vary withtime temperaturestep of the model or salinity was 0.01 in this s and model. the maximum Table1 shows time thestep parameter was 30 s. The setting eddy of viscosity each experiment coefficien int theis given following by the paper. Smagorinsky formula and the value is 0.28 [18]. The bed resistance used the Manning number, which is 32 m1/3/s. Table 1. Parameter setting of each experiment. In fact, some baroclinic conditions such as thermohaline can also affect the whole Astronomicalhydrodynamic Tidal Boundary process [19]. Zhao Wind et Field al. [20] used the POM, Time which is a baroclinic Other Conditions model, to calculate the tidal energy of Beibu Gulf. However, the focus of this study was the sea level Experiment for model remain the same as M2, S2, K1, O1, N2, K2, P1, Q1 normal wind 6-1 0:00–7-1 0:00 validation (Section 2.4) and storm surges caused by typhoons. Compared to thermohaline, tidesmodel and wind setup play a Experiment for more important role in contributing to the research focus. At the same time, the barotropic remain the same as astronomic tide M2, S2,model K1, O1, has N2, already K2, P1, had Q1 a good normal agreement wind with measurements, 6-1 0:00–7-1 0:00 which can be used in the model setup (Section 3.1) subsequent simulation (see Section 2.4). Thus, the density of seawater was set at a constant Experiment for tidal remain the same as 1.05M2, kgm S2,− K1,3 and O1 does not vary normal with temperature wind 6-1or 0:00–6-16salinity in 0:00 this model. Table 1 shows constituent (Section 3.2) model setup the parameter setting of each experiment in the following paper. Experiment for tidal remain the same as M2, S2, K1, O1, P1 normal wind 6-1 0:00–6-16 0:00 constituent (Section 3.2) model setup Experiment for Table 1. Parameter setting of each experiment. wind field of typhoon remain the same as Typhoon “Mangokhut” M2, S2, K1, O1, N2, K2, P1, Q1 6-11 0:00–6-15 0:00 Astronomical“Mangkhut” modelOther setup Condi- (Section 3.3) Wind Field Time Experiment of typhoon Tidal Boundarywind field calculated tions remain the same as in different recurrence M2, S2,Experiment K1, O1, N2, K2,for P1, Q1 according to the 6-6 0:00–6-9 0:00 M2, S2, K1, O1, remainmodel setup the same periods (Section 3.3) model validation formulanormal wind 6-1 0:00–7-1 0:00 N2, K2, P1, Q1 as model setup (Section 2.4) 2.4.Experiment Model Validation for M2, S2, K1, O1, remain the same astronomicIn order tide to verify the accuracy ofnormal the model, wind the 6-1 data 0:00–7-1 of three 0:00 tide stations in the N2, K2, P1, Q1 as model setup surrounding(Section 3.1) waters of Shantou City were selected, and the tidal current was measured in threeExperiment different for positions by ship. The water level data are the measured data of the tide remain the same station.tidal constituent The tidal flowM2, velocity S2, K1, andO1 direction normal are wind the measured 6-1 0:00–6-16 data 0:00 of ADCP. The ADCP −1 as model setup used(Section was Aquadopp 3.2) Profiler 600 kHz. The absolute error was ±0.5 cms , and the relative error was ±1%. The positions of tide stations and ADCP points are shown in Figure1. See Table2 for detailed information.

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Experiment for M2, S2, K1, O1, remain the same tidal constituent normal wind 6-1 0:00–6-16 0:00 P1 as model setup (Section 3.2) Experiment for wind field of ty- Typhoon M2, S2, K1, O1, 6-11 0:00–6-15 remain the same phoon “Mangokhut” N2, K2, P1, Q1 0:00 as model setup “Mangkhut” (Section 3.3) Experiment of typhoon in differ- wind field calcu- M2, S2, K1, O1, remain the same ent recurrence lated according 6-6 0:00–6-9 0:00 N2, K2, P1, Q1 as model setup periods (Section to the formula 3.3)

2.4. Model Validation In order to verify the accuracy of the model, the data of three tide stations in the surrounding waters of Shantou City were selected, and the tidal current was measured in three different positions by ship. The water level data are the measured data of the tide station. The tidal flow velocity and direction are the measured data of ADCP. The ADCP used was Aquadopp Profiler 600 kHz. The absolute error was ±0.5 cms−1, and the relative J. Mar. Sci. Eng. 2021, 9error, 912 was ±1%. The positions of tide stations and ADCP points are shown in Figure 1. See 6 of 28 Table 2 for detailed information.

Table 2. Information of tide stations and ADCP points. Table 2. Information of tide stations and ADCP points. Station Location Observation Time Station Location Observation Time Yunaowan tide station 117°06′ E, 23°24′ N ◦ 0 ◦ 0 Shantou tideYunaowan station tide station 116°44′ E, 23°20 117′ 06N E, 23 24 N6-6 00:00–6-30 23:00 Shantou tide station 116◦440 E, 23◦200 N 6-6 00:00–6-30 23:00 Haimen tide Haimenstation tide station 116°37′ E, 23°11 116′◦ 37N 0 E, 23◦110 N ADCP point ADCP1 point 1 116°52′ E, 23°08 116′◦ 52N 0 E, 23◦080 N 6-10 10:00–6-11 6-10 11:00 10:00–6-11 11:00 ADCP point ADCP2 point 2 116°56′ E, 23°22 116′◦ 56N 0 E, 23◦220 N 6-11 13:00–6-12 6-11 14:00 13:00–6-12 14:00 ◦ 0 ◦ 0 ADCP point ADCP3 point 3 116°41′ E, 23°06 116′ 41N E, 23 06 N 6-12 17:00–6-13 6-12 18:00 17:00–6-13 18:00

The model dataThe modelwere compared data were comparedwith the measured with the measuredtide levels tide of levelsthe three of the tide three sta- tide stations. tions. The temporalThe temporal resolution resolution of measured of measured tide level tide data level provided data provided by the three by the tide three sta- tide stations tions was oncewas an once hour. an hour.1 June 1 to June 5 June to 5 Junewas the was span the spantime timeof the of model. the model. The Theremaining remaining 25 days 25 days werewere used used for verification. for veriﬁcation. The Theresults results are areshown shown in Figure in Figure 3. 3.

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(a) Yunaowan tide station

(b) Shantou tide station

Figure 3. VerificationFigure 3. resultsVeriﬁcation of tide results level. of tide level.

Figure 3 showsFigure the3 showscomparison the comparison between the between measured the measured tide level tideand levelthe model and the data. model data. It It can be foundcan befrom found the fromtide changes the tide changesat the three at the tide three stations tide stations that there that are there two are unequal two unequal high high tides andtides low and tides low tidesin the in study the study area areawithin within a day. a day. Figure Figure 3a 3showsa shows the the verification veriﬁcation between between thethe measured measured tide tide level level and and the the model model results results of of the the Yunaowan Yunaowan tide tide station. The Yunaowan Yunaowan tide station is located in the southeast of Nan’ao Island, which is in the northwest of the study area. The average tide level in June here is about −0.03 m. The average tide level in June of the model is close to 0 m. According to the measured tide level in Yunaowan tide station, the maximum tidal range here is close to 2.5 m during the spring tide, while the tidal range is less than 1 m during the neap tide. It can be calculated that the average absolute error of the tide level in the model at Yunaowan tide station is 6.65 cm. Figure 3b is the verification of the measured tide level and the model results at Shan- tou tide station. Shantou tide station is located near Shantou Port and in the mouth of Rongjiang River. According to the measured water level of Shantou tide station, the average tide level here is −0.02 m, while the average tide level of the model results here is 0.03 m. According to the measured tide level of Shantou tide station, the tidal range of Shantou tide station is close to 1.8 m during the spring tide. The tidal range is about 0.8 m during the neap tide. The mean absolute error of the tide level in the model at Shantou station is 7.11 cm. Figure 3c shows the verification between the measured tide level and the model results of Haimen tide station. Haimen tide station is located near the southwest of the study area. According to the measured tide level, the average tide level here is about −0.02 m, while the average tide level of the model results is 0 m. According to the measured water level data of Haimen tide station, the tidal range of Haimen tide station is about 1.7 m during the neap tide. The tidal range is about 0.6 m during the neap tide. According to the calculation, the mean absolute error of tide level in the model at Haimen tide station is 4.05 cm. The comparison between the model and the measured tide level of the three tide stations shows that the model is in good agreement with the measured data. The mean absolute errors of the three locations are 6.65 cm, 7.11 cm, and 4.05 cm, respectively. The observation time of sea current at each place is 25 h. This covers a complete lunar day. The velocity verification is shown in Figure 4 and the direction verification is shown in Figure 5.

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tide station is located in the southeast of Nan’ao Island, which is in the northwest of the study area. The average tide level in June here is about −0.03 m. The average tide level in June of the model is close to 0 m. According to the measured tide level in Yunaowan tide station, the maximum tidal range here is close to 2.5 m during the spring tide, while the tidal range is less than 1 m during the neap tide. It can be calculated that the average absolute error of the tide level in the model at Yunaowan tide station is 6.65 cm. Figure3b is the verification of the measured tide level and the model results at Shantou tide station. Shantou tide station is located near Shantou Port and in the mouth of Rongjiang River. According to the measured water level of Shantou tide station, the average tide level here is −0.02 m, while the average tide level of the model results here is 0.03 m. According to the measured tide level of Shantou tide station, the tidal range of Shantou tide station is close to 1.8 m during the spring tide. The tidal range is about 0.8 m during the neap tide. The mean absolute error of the tide level in the model at Shantou station is 7.11 cm. Figure3c shows the verification between the measured tide level and the model results of Haimen tide station. Haimen tide station is located near the southwest of the study area. According to the measured tide level, the average tide level here is about −0.02 m, while the average tide level of the model results is 0 m. According to the measured water level data of Haimen tide station, the tidal range of Haimen tide station is about 1.7 m during the neap tide. The tidal range is about 0.6 m during the neap tide. According to the calculation, the mean absolute error of tide level in the model at Haimen tide station is 4.05 cm. The comparison between the model and the measured tide level of the three tide stations shows that the model is in good agreement with the measured data. The mean absolute errors of the three locations are 6.65 cm, 7.11 cm, and 4.05 cm, respectively. The observation time of sea current at each place is 25 h. This covers a complete lunar day. The velocity verification is shown in Figure4 and the direction verification is shown in Figure5. Figure4 shows the verification between the measured velocity of ADCP and the simulated velocity by the model. The velocity of seawater is affected by many factors, such as tide, wind, and even passing ships. Figure4a shows the verification result at ADCP point 1. This point is more than 20 km away from the land and nearly 30 m deep. The observation time is from 10:00 on 10 June 2020 to 11:00 on 11 June 2020. ADCP point 1 is located in the sea far away from the land. There are few islands around it, so the sea water flows smoothly here. During the observation period, the sea water flow velocity here is 0.30 ms−1 on average, and up to 0.6 ms−1 at flood tide and ebb tide. The sea water is slow at flat tide with the velocity being less than 0.01 ms−1. Figure4b is the verification result at ADCP point 2. This point is located at about 7 km on the south side of Nan’ao Island with a water depth of about 10 m. The observation time is from 13:00 on 11 June 2020 to 14:00 on 12 June 2020. ADCP point 2 is relatively close to the land. The water depth here is shallow, so the sea water here is more likely to be hindered by the terrain and the friction of the seabed. The sea water velocity here is slow. The average velocity is 0.13 ms−1. The maximum velocity here is only 0.2 ms−1. Figure4c shows the verification result at ADCP point 3. ADCP point 3 is similar to ADCP point 1. It is located on the open sea, but closer to the land than ADCP point 1 is. The water depth at this point is about 20 m. The average sea water velocity here is 0.26 ms−1. The velocity of sea water at ebb tide is smaller than that at flood tide. The maximum velocity of sea water at ebb tide is 0.32 ms−1, while it can reach 0.59 ms−1 at flood tide. Figure5a shows the flow direction at ADCP point 1. At the time of flood tide and ebb tide, the flow direction of the tidal current is significantly different. At flood tide, the flow direction is approximately between 40◦ to 50◦, pointing northeast. At ebb tide, the flow direction is approximately 220◦ to 240◦, pointing southwest. Figure5b shows the flow direction at ADCP point 2. The sea water flow direction has been rotating during a period of high tide to low tide. Figure5c shows the flow direction at ADCP point 3. The flow direction here is basically the same as that at ADCP point 1, pointing to the northeast at flood tide, and to the southwest at ebb tide. J. Mar. Sci. Eng. 2021, 9, 912 8 of 28 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 8 of 29

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(a) ADCP Point 1

(b) ADCP Point 2

Figure 4. VelocityFigure validation. 4. Velocity validation.

The velocity and direction of tide current simulated by the model have a good agreement with the data recorded by ADCP. The validation results of the tidal current show the reliability of the model, which indicates that the model is reliable in simulating the hydrodynamic conditions in this area. (c) ADCP Point 3 In order to better explain the error of the model, we calculated the deviation and root Figure 4. Velocitymean validation. square between the model data and the measured data. See Table3 for error values.

(a) ADCP Point 1

Figure 5. Cont.

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(b) ADCP Point 2

(c)ADCP Point 3

Figure 5. DirectionFigure verification. 5. Direction veriﬁcation.

Figure 4 showsBy comparingthe verification the mean between bias, the root-mean-square measured velocity error, of ADCP and determination and the sim- coefficient ulated velocitybetween by the the model. model The data velocity and the of measured seawater data,is affected it can by be many found factors, that the such error as of the model tide, wind, andis small even and passing the correlation ships. Figure between 4a shows the model the verification data and measured result at ADCP data is high.point This shows 1. This pointthat is more the model than 20 is reliablekm away and from the the model land data and can nearly be trusted. 30 m deep. The observation time is from 10:00 on 10 June 2020 to 11:00 on 11 June 2020. ADCP point 1 is located in the sea farTable away 3. Errorfrom analysisthe land. between There modelare few data islands and measured around data.it, so the sea water flows −1 smoothly here.Validation During the Item observation Mean period, Bias the Root sea water Mean flow Square velocity Error here Determination is 0.30 ms Coefficient on average, and up to 0.6 ms−1 at flood tide and ebb tide. The sea water is slow at flat tide tide elevation in with the velocity being less than 0.01 ms−1. Figure 4b is the verification result at ADCP Yunaowan tide 0.480 0.138 0.976 point 2. This point isstation located at about 7 km on the south side of Nan’ao Island with a water depth of abouttide 10 elevationm. The observation in time is from 13:00 on 11 June 2020 to 14:00 on 12 0.316 0.155 0.947 June 2020. ADCPShantou point tide 2 station is relatively close to the land. The water depth here is shallow, tide elevation in so the sea water here is more likely to0.318 be hindered by the 0.114 terrain and the friction of 0.964 the seabed. The seaHaimen water tide velocity station here is slow. The average velocity is 0.13 ms−1. The maxi- velocity in ADCP 1 0.144 0.084 0.851 mum velocity here is only 0.2 ms−1. Figure 4c shows the verification result at ADCP point velocity in ADCP 2 0.158 0.110 0.694 3. ADCP pointvelocity 3 is similar in ADCP to ADCP 3 point 0.011 1. It is located on 0.241 the open sea, but closer to 0.948 the land than ADCPflow point direction 1 is. inThe water depth at this point is about 20 m. The average sea 0.237 40.591 0.903 water velocity hereADCP is 0.26 1 ms−1. The velocity of sea water at ebb tide is smaller than that flow direction in −1 at flood tide. The maximum velocity of0.187 sea water at ebb tide 34.569 is 0.32 ms , while it can reach 0.709 0.59 ms−1 at flood tide.ADCP 2 flow direction in Figure 5a shows the flow direction0.536 at ADCP point 1. At 62.658 the time of flood tide and 0.779 ebb ADCP 3 tide, the flow direction of the tidal current is significantly different. At flood tide, the flow direction is approximately between 40° to 50°, pointing northeast. At ebb tide, the flow 3. Result and Discussion direction is approximately 220° to 240°, pointing southwest. Figure 5b shows the flow di- 3.1. Diagram of Water Level and Flow Field at Different Times rection at ADCP point 2. The sea water flow direction has been rotating during a period of high tide to lowFigure tide.6 showsFigure the 5c distributionshows the flow of tide direction levels andat ADCP velocity point under 3. The different flow conditions. direction hereThe is simulation basically the time same includes as that both at ADCP astronomical point 1, springpointing tide to andthe northeast neap tide. at Selected as- flood tide, andtronomical to the southwest spring tide at risingebb tide. time (23:00 on 6 June 2020), astronomical spring tide falling The velocitytime (17:00 and direction on 7 June of 2020),tide current astronomical simulated neap by the tide model rising have time a (17:00good agree- on 14 June 2020), ment with the data recorded by ADCP. The validation results of the tidal current show

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J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 11 of 29 astronomical neap tide falling time (23:00 on 14 June 2020), high tide ﬂat tide time (3:00 on 8 June 2020), and low tide ﬂat tide time (20:00 on 7 June 2020) are shown in Figure6.

(a) velocity vector and tide levels of spring flood tide (b) velocity distribution in spring flood tide

(e) velocity vector and tide levels of neap flood tide (f) speed distribution of neap flood tide

(g) velocity vector and tide levels of neap ebb tide (h) speed distribution of neap ebb tide

Figure 6. Cont .

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(i) velocity vector and tide levels of spring high tide (j) speed distribution of spring high tide

(k) velocity vector and tide levels of spring low tide (l) speed distribution of spring low tide

Figure 6. Distribution of tide levels and current speed in the study area.

When there is no strong wind field, field, the se seaa water current in the study area is mainly affected by by the the combined combined action action of of tide tide pote potentialntial and and topography. topography. Rivers Rivers also also play play a sig- a nificantsignificant role role in estuaries. in estuaries. The Thecurrent current in th ine thestudy study area area is approximately is approximately parallel parallel to the to coastline.the coastline. The current The current is cut-off is cut-off near the near island the island and split and into split two into currents two currents around around it. When it. theWhen tide the is tiderising, is rising,the sea the water sea flows water from flows so fromuthwest southwest to northeast. to northeast. The overall The overall flow direc- flow tiondirection is roughly is roughly parallel parallel to the to coastline the coastline (see (seeFigure Figure 6a,e).6a,e). When When the thetide tide is falling, is falling, the thesea watersea water flows flows from from northeast northeast to tosouthwest. southwest. It Italso also roughly roughly parallels parallels to to the the coastline coastline (see Figure 66c,g).c,g). TheThe velocityvelocity ofof seasea waterwater inin thethe southsouth ofof thethe studystudy areaarea isis fasterfaster thanthan thatthat inin thethe north. Especially inin somesome areaarea nearnear thethe southernsouthern border, border, the the speed speed during during astronomical astronomi- −1 caltides tides can can exceed exceed 1.2 1.2 ms ms.−1 However,. However, the the sea sea near near Nan’ao Nan’ao Island Island is is relativelyrelatively calm.calm. TheThe −1 velocity is generally lessless thanthan 0.50.5 msms−1 (see(see Figure Figure 66b,d,f,h,j,l).b,d,f,h,j,l). During highhigh tide,tide, itit tendstends toto be high inin thethe northeastnortheast and and low low in in the the southwest. southwest. However, However, at lowat low tide, tide, it is it opposite. is opposite. The Themaximum maximum tidal tidal range range in the in northeastthe northeast of the of studythe study area area can becan more be more than than 2 m, 2 where m, where this thisvalue value is less is thanless than 1.5 m 1.5 in them in southwest the southwest of the studyof the areastudy (compare area (compare Figures 6Figurei and6 k).6i and In Figurestudy area,6k). In there study are area, two there high are tides two and high two tides low and tides two in low one tides lunar in day. one Thelunar amplitude day. The amplitudedifference betweendifference adjacent between high adjacent tides and high low tides tides and is large. low Thetides tidal is large. type here The istidal irregular type heresemi-diurnal is irregular tides. semi-diurnal The flow velocity tides. The at flood flow tide velocity is slightly at flood greater tide thanis slightly that at greater ebb tide. than At high tide, the water level in the study area is high in the northeast but low in the southwest. that at ebb tide. At high tide, the water level in the study area is high in the northeast but At low tide, it is opposite. The water level difference at high tide is slightly higher than low in the southwest. At low tide, it is opposite. The water level difference at high tide is that at low tide in the study area. The tidal range in the northeast of the study area is large, slightly higher than that at low tide in the study area. The tidal range in the northeast of exceeding 2 m, while the tidal range in the southwest is low, less than 2 m. At flat tide, the the study area is large, exceeding 2 m, while the tidal range in the southwest is low, less whole study area is relatively calm. In addition, due to the topography and islands, near than 2 m. At flat tide, the whole study area is relatively calm. In addition, due to the to- the mouth of the river, and in some areas between the island and the land, the water also pography and islands, near the mouth of the river, and in some areas between the island flows fast. and the land, the water also flows fast. Figure6a,b shows the distribution of tide level and current velocity in the study area Figure 6a,b shows the distribution of tide level and current velocity in the study area at astronomical spring tide rising time. At this time, the sea water flows from southwest at astronomical spring tide rising time. At this time, the sea water flows from southwest to northeast, and the overall flow direction is roughly parallel to the coastline. Near the tomouth northeast, of the and Rongjiang the overall River, flow the bulgingdirection terrain is roughly of Dahao parallel Island to playsthe coastline. a role in Near blocking the mouththe current. of the Some Rongjiang seawater River, enters the bulging Haojiang terrain River, of while Dahao others Island bypass plays thea role island. in blocking At the theeast current. side of Some Dahao seawater Island, theenters velocity Haojiang of the River, sea waterwhile others can exceed bypass 1 msthe− island.1 at this At time. the east side of Dahao Island, the velocity of the sea water can exceed 1 ms−1 at this time. There

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There are also current diversions on both sides of Nan’ao Island, which are distributed from the east and west sides to bypass the island. This is also a place where the sea flow velocity is high, and the flow velocity can reach 1 ms−1. The tide level in the whole study area showed a trend of high in the southwest and low in the northeast, but the water level is not much different. The water level in the southwest was about 0.4 m higher than that in the northeast. There is a fast flowing area in the south of the study area, while the sea area east of Nan’ao Island is relatively calm. Figure6c,d shows the distribution of tide level and current velocity in the study area during astronomical spring tide falling time. The water now flows from northeast to southwest, also roughly parallel to the coastline. Compared with the flood tide, the flow velocity near Nan’ao Island is lower at ebb tide, but the flow velocity near Dahao Island is still faster, exceeding 1 ms−1. The sea water flows out of the estuary at ebb tide. At this time, the water level was high in the northeast and low in the southwest. The difference of water level was still small, about 0.4 m. The velocity distribution of sea water in the sea area is roughly the same as that in the spring tide. Figure6e–h show the distribution of tide level and sea velocity in the study area at astronomical neap flood tide and ebb tides, respectively. At this time, the distribution of tidal current in the study area is basically the same as that at astronomical spring tides. However, the flow velocity is low, generally lower than 0.5 ms−1. The difference in water level in the study area is small. The water level of the whole sea is relatively uniform. Figure6i,j shows the distribution of tide level and velocity during high tide in the study area. At this time, the current velocity of the whole sea area is slow, and the current velocity of the nearshore part of the whole study area is generally lower than 0.1 ms−1. The tide level of the whole Shantou sea area is high. The water level gradient is large, showing a trend of high in the northeast and low in the southwest, with a difference of more than 1 m. Figure6k,l shows the distribution of tide level and velocity at low tide in the study area. Consistent with the high tide, the current velocity is slow. On the contrary, the tide level is low in the northeast and high in the southwest, but the water level gradient is slightly smaller than that at the high tide. Several points are selected to compare the changes in tide level and velocity. Informa- tion of these points is shown in Table4.

Table 4. Comparison of tide level and tide current at different points.

Offshore Highest Lowest Mean Maximum Mean Flow Region Location Distance Water Level Water Level Range Flow Velocity Velocity 116◦390 E 0.902 50 m 0.839 m −1.054 m 0.434 ms−1 0.176 ms−1 23◦080 N m Chaoyang 116◦390 E 0.912 1500 m 0.841 m −1.071 m 0.540 ms−1 0.275 ms−1 District 23◦090 N m 116◦390 E 0.901 2500 m 0.842 m −1.071 m 0.515 ms−1 0.257 ms−1 23◦070 N m 116◦490 E 1.282 330 m 1.022 m −1.374 m 0.602 ms−1 0.333 ms−1 Haojiang 23◦140 N m District 116◦500 E 1.270 2000 m 1.028 m −1.351 m 0.594 ms−1 0.314 ms−1 23◦150 N m 116◦490 E 1.392 170 m 1.113 m −1.380 m 0.105 ms−1 0.037 ms−1 23◦220 N m Longhu 116◦490 E 1.389 700 m 1.109 m −1.379 m 0.106 ms−1 0.045 ms−1 District 23◦210 N m 116◦480 E 1.385 1200 m 1.104 m −1.379 m 0.147 ms−1 0.064 ms−1 23◦210 N m 117◦200 E 1.569 300 m 1.193 m −1.599 m 0.582 ms−1 0.261 ms−1 23◦300 N m Nan’ao 117◦080 E 1.564 800 m 1.192 m −1.599 m 0.508 ms−1 0.221 ms−1 island 23◦310 N m 117◦080 E 1.563 1300 m 1.192 m −1.599 m 0.490 ms−1 0.210 ms−1 23◦300 N m J. Mar.J. Mar. Sci. Sci.Eng. Eng. 20212021, 9,, x9, FOR 912 PEER REVIEW 13 of 28 14 of 29

TableAs 4, shownthe average in Table velocity4, Chaoyang in Haojiang District is District located in and the southwest,Nan’ao Island with theis greater smallest than 0.2 mstidal−1. range, which is less than 1 m. Longhu District is blocked by the island. The sea current is mostThe stable velocity with thein the slowest nearshore velocity. sea It can area be is seen generally from Table lower4 that than the average that at velocity the far shore, −1 whichof Longhu is the district result is of less the than shallow 0.01 ms water. Affected depth bynear the the topography, nearshore the and velocity sea bottom and tidal friction. Therange speed in Haojiang near the District coast is and greatly Nan’ao influenced Island are relativelyby the terrain. high. AsFor shown example, in Table Longhu4, the District average velocity in Haojiang District and Nan’ao Island is greater than 0.2 ms−1. is located between Dahao Island and Nan’ao Island. Because of its barrier, the flow veloc- The velocity in the nearshore sea area is generally lower than that at the far shore, itywhich is low. is the In result Haojiang of the District, shallow waterthe water depth flow near is the blocked nearshore by andthe seaisland bottom and friction. turns to converge,The speed so the near sea the water coast velocity is greatly is influenced relatively by high. the terrain. The topographic For example, effect Longhu in Districtthe vicinity of Nan’aois located Island between has Dahao a significant Island and influence Nan’ao on Island. the Becausetide, resulting of its barrier, in the the formation flow velocity of the cur- rentis low. around In Haojiang the island. District, the water flow is blocked by the island and turns to converge, so the sea water velocity is relatively high. The topographic effect in the vicinity of Nan’ao 3.2.Island Influence has a significantof Different influence Tidal Constituents on the tide, resulting in the formation of the current around the island. It is generally believed that the South China Sea is mainly affected by the four tidal constituents3.2. Influence ofM2, Different S2, K1, Tidal and Constituents O1, which enter from the Luzon Strait. The amplitude of these Ittidal is generally constituents believed increases that the as South the wate Chinar depth Sea is mainlybecoming affected shallow by the [21]. four For tidal example, Fangconstituents et al. [22] M2, numerically S2, K1, and simulated O1, which the enter tidal from current the Luzon distributions Strait. The of amplitudeM2, S2, K1, of and O1 inthese the tidalwhole constituents South China increases Sea, as including the water depththe Beibu becoming Gulf. shallow Wu et [21al.]. [23] For example,used the POM modelFang et to al. simulate [22] numerically the tides simulated and tidal the currents tidal current of M2, distributions S2, K1, and of O1 M2, in S2, the K1, South and China Sea.O1 in the whole South China Sea, including the Beibu Gulf. Wu et al. [23] used the POM modelHowever, to simulate in thethe tides calibration and tidal process currents ofof M2,the S2,hydrodynamic K1, and O1 in themodel, South we China found Sea. that if However, in the calibration process of the hydrodynamic model, we found that if only only these four tidal constituents are added to the open boundary, the validation effect of these four tidal constituents are added to the open boundary, the validation effect of the themodel model is not is ideal.not ideal. When When only M2, only S2, M2, K1, S2, and K1 O1, tidaland O1 constituents tidal constituents are considered are asconsidered tidal as tidalpotential, potential, taking taking Yunaowan Yunaowan tide station tide as anstation example, as an the example, verification the results verification are shown results in are shownFigure 7in. Figure 7.

FigureFigure 7. 7. TidalTidal constituentconstituent veriﬁcation verification for for M2, M2, S2, K1,S2, andK1, O1.and O1.

FigureFigure7 7 shows shows that that when when only only M2, M2, S2, K1,S2, K1, and and O1 tidal O1 tidal constituents constituents are considered, are considered, the tide levels in two high tides per day of the model data are basically the same. However, the tide levels in two high tides per day of the model data are basically the same. However, there is a signiﬁcant difference between the two water levels in the measured tidal data. thereOn this is a basis, significant the model difference added N2, between K2, P1, the and two Q1 tidalwater constituents levels in the successively. measured Thetidal data. Onsimulation this basis, results the show model that added the P1 N2, tidal K2, constituent P1, and plays Q1 antidal obvious constituents role in reducing successively. this The simulationerror. P1 is results the solar show declination that the dividual, P1 tidal consti mainlytuent caused plays by thean obvious tidal potential role in force reducing of this error.the sun, P1 withis the a periodsolar declination of 24.066 h. dividual, Figure8 takes main Yunaowanly caused tide by stationthe tidal as anpotential example; force it of the sun,shows with the simulationa period of results 24.066 after h. addingFigure the 8 P1takes tidal Yunaowan constituent totide the station M2, S2, as K1, an and example; O1 it showstidal components. the simulation results after adding the P1 tidal constituent to the M2, S2, K1, and O1 tidal components.

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FigureFigure 8. Tidal constituent constituent veriﬁcation verification for M2,for M2, S2, K1,S2, O1,K1, and O1, P1. and P1.

Figure8 8shows shows that that the the P1 P1 tidal tidal constituent constitu hasent ahas great a influencegreat influe onnce reducing on reducing the the error above. However, compared with the addition of M2, S2, K1, O1, N2, K2, P1, and error above. However, compared with the addition of M2, S2, K1, O1, N2, K2, P1, and Q1 Q1 (see Figure3a), there are still some errors. In the study of the tides in the South China (seeSea, Figure the four 3a), tidal there constituents are still M2,some S2, errors. K1, and In O1 the can study basically of the meet tides the in requirements the South China of Sea, theaccuracy. four tidal However, constituents the accuracy M2, ofS2, the K1, model and willO1 can be further basically improved meet the when requirements more tidal of accuracy.constituents However, are considered. the accuracy For example, of the Lumodel et al. will [24] consideredbe further eightimproved tidal constituents when more tidal constituentsM2, S2, K1, O1, are N2, considered. K2, P1, and For Q1 whenexample, studying Lu et tidal al. [24] currents considered in the northern eight tidal part ofconstituents the M2,South S2, China K1, O1, Sea. N2, Tao K2, et al. P1, [25 ]and considered Q1 when 13 commonstudying tidal tidal constituents currents in when the northern studying part of thethe South Nan’ao China Sea Area Sea. of Tao Shantou. et al. [25] considered 13 common tidal constituents when studying Shantouthe Nan’ao is close Sea to Area the South of Shantou. China Sea. M2, S2, K1, and O1 tidal constituents basically determine the level and the period of the tide in Shantou. However, there is an error in Shantou is close to the South China Sea. M2, S2, K1, and O1 tidal constituents basi- the lower high tide per day if we only consider these four tidal components. The P1 tidal callyconstituent determine can effectively the level reduceand the this period error. of the tide in Shantou. However, there is an error in the lower high tide per day if we only consider these four tidal components. The P1 tidal3.3. The constituent Influence of can the effectively Typhoon reduce this error. The typhoon is one of the most destructive weather systems. Storm surges and waves 3.3.caused The byInfluence typhoons of oftenthe Typhoon bring large damage to coastal areas. On 2 August 1922, a storm surgeThe occurred typhoon in Shantou is one of area, the with most more destructive than 70,000 weather deaths, whichsystems. was Storm one of surges the biggest and waves storm surge disasters of the 20th century [26]. Shantou City often encounters storm surges, caused by typhoons often bring large damage to coastal areas. On August 2, 1922, a storm mainly typhoons and severe tropical storms every summer [2]. surgeTyphoon occurred “Mangkhut” in Shantou is area, a super with typhoon more than that was70,000 generated deaths, at which 20:00 onwas 7 Septem-one of the big- gestber 2018storm over surge the Northwestdisasters of Pacific the 20th Ocean. centur On 10y September,[26]. Shantou the City typhoon often moved encounters to the storm surges,southwest. mainly At 8:00 typhoons on 11 September, and severe it tropical became astorms super typhoon.every summer On 15 [2]. September, the typhoonTyphoon passed “Mangkhut” through the northern is a super Philippines typhoon and that moved was generated to Guangdong at 20:00 Province on September of 7,China. 2018 Atover 17:00 the on Northwest 16 September, Pacific Typhoon Ocean. “Mangkhut” On September landed 10, in the Haiyan typhoon town moved with a to the southwest.maximum wind At 8:00 force on of September 14 and a central 11, minimumit became pressurea super of typhoon. 955 hPa. InOn the September afternoon 15, the typhoonof 17 September, passed thethrough typhoon the weakened northern toPhilippines a tropical depression,and moved and to thenGuangdong continued Province to of move to the northwest. The typhoon basically dissipated until 20:00 [27]. Figure9 shows China. At 17:00 on September 16, Typhoon “Mangkhut” landed in Haiyan town with a the wind speed and central pressure of Typhoon “Mangkhut”. maximumIn order wind to simulate force of the 14 impactand a central of typhoons minimum on Shantou pressure more of truly, 955 thehPa. parameters In the afternoon ofof September wind field (including17, the typhoon wind speed weakened and central to a tropical pressure) depression, in the model and are then the data continued of to moveTyphoon to the “Mangokhut”. northwest. ThisThe istyphoon equivalent basicall to movingy dissipated its route until east by20:00 1.2 longitude[27]. Figure and 9 shows thenorth wind by 1.7speed latitude, and central so that pres it transitssure of from Typhoon the northeast “Mangkhut.” side of the study area (see Figure 11). The simulation of the typhoon was conducted by hot start, and the preliminary results were taken as the initial conditions. The simulation lasted for 4 days. Figure 10a shows the distribution of the flow field and water level during the typhoon at the time of high tide. Figure 10b shows the flow field and water level distribution of the normal wind field.

J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 16 of 29 J. Mar. Sci. Eng. 2021, 9, 912 15 of 28

(a) wind speed variation of typhoon “Mangkhut”

(b) central pressure variation of Typhoon “Mangkhut”

Figure 9. WindFigure speed 9. Wind and speed air andpressure air pressure in the in the center center ofof TyphoonTyphoon “Mangkhut” “Mangkhut” (data of (data Typhoon of Typhoon “Mangkhut”“Mangkhut” according according to Liu [28]). to Liu [28]).

In the presence of the super typhoon, the whole flow field in the study area changed In ordersignificantly. to simulate The waterthe impact level distribution of typhoons shows on that Shantou the closer more to the typhoontruly, the center, parameters of wind fieldthe higher(including the water wind level. speed Compared and with cent normalral pressure) wind, the water in the level model increased. are The the data of Typhoon “Mangokhut.”water level in the wholeThis seais equivalent area increased to by moving more than 0.3its m.route Under east the influenceby 1.2 longitude of the and typhoon, the current direction in the study area was the far current pointing to the typhoon north by 1.7center, latitude, while so the that offshore it transits current was from similar the to northeast the counterclockwise side of rotationthe study of wind area field. (see Figure 11). The simulationThe flow velocity of the also typhoon increased was greatly, conduc and theted main by drivinghot start, force and of the the flow preliminary field in the results were takenstudy as the area initial changed conditions. from tidal potential The simulation to wind field. lasted for 4 days. Figure 10a shows The flood disaster caused by a typhoon storm surge is the main way typhoons damage the distributioncoastal of areas. the In flow this model, field the and path water of the typhoonlevel during center is locatedthe typhoon in the northeast at the of time the of high tide. Figurestudy 10b shows area, and the the flow landing field place and is in water the south level of Zhao’an distribution County. Takingof the thenormal typhoon wind field. before landing as an example, the calculation shows that when the typhoon passes through, the tide elevation in the south sea area of Zhao’an County increases by 0.32 m. The 0.24 m water level was increased in the sea area to the east of Nan’ao County, and −0.08 m near the Rongjiang River estuary of Dahao Island. The water increased about 0.08 m in the sea area around the Chaoyang area. It can be seen that the closer it is to the typhoon center, the more obvious the increase in water level. Five typhoon monitoring points were selected to compare the different responses of the water surface height relative to the general state when the typhoon transited, and the

(a) Typhoon “Mangkhut” is passing through

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J. Mar. Sci. Eng. 2021, 9, 912 16 of 28 11). The simulation of the typhoon was conducted by hot start, and the preliminary results were taken as the initial conditions. The simulation lasted for 4 days. Figure 10a shows thedifferent distribution response of the of speedsflow field when and there water was level no wind, during normal the typhoon wind, and at whenthe time the of typhoon high tide.transited. Figure See10b Tableshows5 andthe flow Figure field 11 andfor itswater distribution level distribution and information. of the normal wind field.

(a) Typhoon “Mangkhut” is passing through

(b) normal wind field

FigureFigure 10. 10. WaterWater level level line line and and flow flow field field map map of of typhoon typhoon transit transit compared compared with with normal normal wind wind field field conditions.conditions.

TableIn 5.theInformation presence of typhoonthe super monitoring typhoon, points. the whole flow field in the study area changed significantly. The water level distribution shows that the closer to the typhoon center, the Serial Number Longitude Latitude Bathymetry Offshore Distance higher the water level. Compared with normal wind, the water level increased. The water ◦ 0 ◦ 0 level inPoint the whole 1 sea area 116 52 increasedE by 23 more08 N than 0.3 m. 31.5 Under m the influence 12.5 of km the ty- Point 2 116◦560 E 23◦220 N 11.5 m 1.8 km phoon, the current direction in the study area was the far current pointing to the typhoon Point 3 116◦410 E 23◦060 N 21 m 7.8 km center, Pointwhile 4 the offshore 117◦ 15current0 E was similar 23◦180 N to the counterclockwise 30 m rotation 13.1 kmof wind field. ThePoint flow 5 velocity117 also◦04 increased0 E greatly, 23◦290 andN the main 4.5 driving m force of the 0.4 flow km field in the study area changed from tidal potential to wind field.

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0.08 m in the sea area around the Chaoyang area. It can be seen that the closer it is to the J. Mar. Sci. Eng. 2021, 9, 912 typhoon center, the more obvious the increase in water level. 17 of 28 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 18 of 29 Five typhoon monitoring points were selected to compare the different responses of the water surface height relative to the general state when the typhoon transited, and the different response of speeds when there was no wind, normal wind, and when the typhoon transited. See Table 5 and Figure 11 for its distribution and information.

Table 5. Information of typhoon monitoring points.

Serial Number Longitude Latitude Bathymetry Offshore Distance Point 1 116°52′ E 23°08′ N 31.5 m 12.5 km Point 2 116°56′ E 23°22′ N 11.5 m 1.8 km Point 3 116°41′ E 23°06′ N 21 m 7.8 km Point 4 117°15′ E 23°18′ N 30 m 13.1 km Point 5 117°04′ E 23°29′ N 4.5 m 0.4 km

FigureFigure 11. 11.Path Path of of typhoon typhoon “Mangkhut” “Mangkhut” in in model model and and location location of of typhoon typhoon monitoring monitoring points. points.

FigureFigure 12 12 shows shows the the water water level level when when the the typhoon typhoon was was passing passing through through minus minus the the waterwater level level of of the the normal normal wind wind field field at at the the same same time. time. Figure Figure 13 13 compares compares the the sea sea water water velocityvelocity in in the the windless windless state, state, the the normal normal wind wind field, field, and and the the typhoon typhoon transit. transit. Figure 12 shows the water level increase comparison of five points. The influence of wind fields is more obvious at the shallow water depth (see Figure 12b,e) than at the deep offshore water depth (see Figure 12a,c,d). When the typhoon passed through, the tide elevation generally increased significantly. Within 24 h, the average water increases at five points were 0.17 m, 0.20 m, 0.15 m, 0.16 m, and 0.45 m, respectively. The maximum water increases were 0.24 m, 0.35 m, 0.19 m, 0.23 m, and 0.75 m, respectively. In addition to the tide level, the typhoon also affects the velocity of the sea. In Figure 13, the red line means there is no wind in the area. The green line is the normal wind field and the blue line is the wind field of typhoon “Mangkhut”. Figure 13 shows the variation in velocity Figureunder 11. Path different of typhoon wind “Mangkhut” conditions. In in themodel region and oflocation high current of typhoon velocity, monitoring the sea points. current velocity is mainly controlled by tide, and the influence of the normal wind field is limited (see Figure 13a,c,d). In the area of low current velocity, the sea current velocity is less Figure(a )12 The shows water the level water rises duringlevel when the passage the typhoon of Typhoon was “Mangkhut” passing through at point minus 1 the affected by the tide. The normal wind can also cause a larger change in the velocity (see waterFigure level 13 of b,e the). Whennormal a typhoon wind field exists, at thethe sea same current time. velocity Figure is mainly13 compares determined the sea by the water velocitytyphoon in the at windless any control state, point the (see norm Figureal 13winda–e). field, and the typhoon transit.

(b) The water level rises during the passage of Typhoon “Mangkhut” at point 2

(a) The water level rises during the passage of Typhoon “Mangkhut” at point 1

Figure 12. Cont.

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(b) The water level rises during the passage of Typhoon “Mangkhut” at point 2

(d) The water level rises during the passage of Typhoon “Mangkhut” at point 4

(e) The water level rises during the passage of Typhoon “Mangkhut” at point 5

FigureFigure 12. The 12. The increment increment in inwater water level level during during the passage of of Typhoon Typhoon “Mangkhut”. “Mangkhut.”

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(a) variation in velocity at point 1

(b) variation in velocity at point 2

(d) variation in velocity at point 4

(e) variation in velocity at point 5

Figure 13. Variation in velocity under different wind conditions.

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The nearshore water depth is shallow, where the storm surge caused by the typhoon increases in wave height, the energy converges, and the water level overlaps online-early, showing a large-scale increase in water. Therefore, it is very necessary to protect the coast during the passage of the typhoon. In the presence of a typhoon, the speed can be increased by several times, so the influence of wind field should be especially considered when a typhoon is operated offshore. In areas far from the coast, the wind field changes the velocity equally dramatically. In the case of normal wind field, the flow velocity is still mainly controlled by the tide potential. In the case of a typhoon, the distribution of the flow velocity is completely different, which indicates that the wind dominates the flow field at this time. At the same time, it should be noted that in the case of the typhoon, the flow direction of sea current is also mainly controlled by the wind field (Figure 10). When there is no typhoon, there are four maximum values of sea water velocity in the study area in a day, while there are only two maximum values when the typhoon comes. The tide in the Shantou sea area is an irregular semidiurnal tide; there are two high tides and two low tides in a day, so there are four maxima of velocity in one day. When the typhoon comes, sea water flow is mainly controlled by the typhoon. When the sea current caused by the tide is in the same direction as the sea current caused by the typhoon, the flow velocity will overlap and increase in speed. However, when the current direction caused by the tide is opposite to that caused by the typhoon, the two offset each other, making the velocity lower than that without the typhoon. Therefore, when a typhoon passes through, the sea surface can only have two maximum velocity values in one day. Typhoon “Mangkhut” is a case of many typhoons, belonging to the intensity and harm of the greater typhoon. However, in most cases, the typhoon landing in Shantou was not a super typhoon. In order to better reflect the change in sea surface flow field and the increase in water level caused by typhoon storm surges of different levels, four typhoon scenes with different recurrence periods were defined. The central pressure of a typhoon was given by a hurricane model and the typhoon parameters were determined to correspond to the model to reconstruct the wind field. The most important parameters that determine the scale of the typhoon wind field are the minimum pressure of the typhoon center (P0), the radius of maximum wind speed (Rmax), the maximum wind speed (Vmax), and the moving path of the typhoon. Among them, the maximum wind radius of the typhoon can be estimated according to the empirical formula [29,30], and its maximum wind radius is related to the minimum pressure of the typhoon center: −0.806 Rmax = 1119.0 × (1010 − P0) (8) The maximum wind speed of the typhoon can also be calculated according to the empirical formula [31], which is expressed as follows:

0.6065 Vmax = 3.7237 × (1010 − P0) (9)

In this study, four typhoons with different central pressures were constructed, and each typhoon represented a different recurrence period. The four typhoons have the same path and all landed in Shantou. The typhoon path is shown in Figure 14. The information of each typhoon is shown in Table6:

Table 6. Information of each typhoon.

Number Central Pressure Maximum Wind Speed Maximum Wind Radius Recurrence Period 1 940 hPa 49 ms−1 36 km 10 yr 2 920 hPa 57 ms−1 30 km 50 yr 3 910 hPa 61 ms−1 27 km 100 yr 4 880 hPa 71 ms−1 22 km 1000 yr J.J. Mar. Mar. Sci. Sci. Eng. Eng. 20212021, ,99, ,x 912 FOR PEER REVIEW 2221 of of 29 28

(a) pressure difference and maximum wind speed of (b) typhoon path typhoon in different recurrence periods

FigureFigure 14. 14. TyphoonTyphoon intensity intensity and and its its path path (red (red circ circlesles are are the the selected selected control control points). points).

AsAs shown shown in in Figure Figure 14,14 ,the the typhoons typhoons enter enter the the sea sea area area of of Shantou Shantou City City from from the the outsideoutside sea and landland nearnear the the mouth mouth of of Rongjiang Rongjiang River. River. They They pass pass through through the main the urbanmain area of Shantou. Typhoons land at the time of astronomical spring tide, so the increase urban area of Shantou. Typhoons land at the time of astronomical spring tide, so the inin water level caused by the typhoon and the increase in water level caused by the tide crease in water level caused by the typhoon and the increase in water level caused by the are superimposed. The simulation time of each typhoon is 72 h. Figure 15 shows the tide are superimposed. The simulation time of each typhoon is 72 h. Figure 15 shows the distribution of isobathymetric lines and the change in flow field in the Shantou sea area distribution of isobathymetric lines and the change in flow field in the Shantou sea area during the passage of typhoons with different intensities. during the passage of typhoons with different intensities. Figure 15a shows the comparison when there is no typhoon. This moment is the high tide of the astronomical spring tide. The sea water flows from northeast to southwest. Figure 15b–e, respectively, show the situation of typhoons with recurrence periods of 10 years, 50 years, 100 years, and 1000 years. The central air pressure of the typhoon is set at 940 hPa, 920 hPa, 910 hPa, and 880 hPa, respectively. When a typhoon appears, the sea current in the study area is controlled by the typhoon. In the model, the typhoon center path roughly passes through the center of the study area, moves from southeast to northwest, and lands in Shantou City near the Rongjiang River estuary. The northeast side of the Rongjiang River estuary is on the right side of the typhoon path, while the southwest side of the Rongjiang River estuary is on the left side of the typhoon path. Shantou City is located near the Tropic of Cancer, belonging to the northern hemisphere, affected by the geotropic deflection force. The typhoon wind field has counterclockwise rotation. In (a)the normal study wind area, the water flow(b on) typhoon the northeast with a recurrenc side basicallye period flows of 10 from years south to north, while on the southwest side, the velocity vector is roughly the same as that of the typhoon, indicating that the direction of the water flow was controlled by the typhoon at this time. The flow direction of sea water is basically the same under typhoons of different intensities. In terms of velocity, the sea current is affected by both the wind field and the tide. On the left side of the typhoon path, the wind is in the same direction as the current caused by the tide. The superposition of the two makes the flow rate fast. On the right side of the typhoon path, the direction of wind is opposite to the direction of the tide. The velocity of the two is offset, so the velocity is not as fast as the sea water on the right side of the typhoon path. By comparing the distribution of sea level in the absence of typhoons, it can be found that the presence of typhoons will obviously cause a rise in sea level and a (c) typhoon with adecrease recurrenc ine period water of level 50 years gradient. (d) typhoon The more with intense a recurrenc the typhoonse period ofare, 100 theyears more the water level is increased. Four control points were selected, two on the right side of typhoon path and two on the left side of the typhoon path. Control points were used to compare the height and velocity of the water increase caused by the typhoon in different recurrence periods (see Figure 14). The information of control points is shown in Table7. Point 1 and point 2 are located on the left side of the typhoon path. Point 3 and point 4 are located on the right side of the typhoon path.

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(a) normal wind (b) typhoon with a recurrence period of 10 years

(e) typhoon with a recurrence period of 1000 years

FigureFigure 15. The 15. waterThe water level level distribution distribution and and fl ﬂowow field ﬁeld of of typhoon crossingcrossing with with different different recurrence recurrence periods. periods.

Table 7. Location, bathymetry, and offshore distance of 4 control points. Figure 15a shows the comparison when there is no typhoon. This moment is the high tide ofSerial the Number astronomical Longitude spring tide. The Latitude sea water Bathymetryflows from northeast Offshore Distanceto southwest. Figure 15b–e,Point 1 respectively, 116◦36 show0 E the situatio 23◦060 Nn of typhoons 21.2 mwith recurrence 2.4 km periods of 10 years, 50Point years, 2 100 years, 116 ◦and500 E 1000 years. 23◦14 The0 N central air 13.1 pressure m of the 3.2typhoon km is set at ◦ 0 ◦ 0 940 hPa,Point 920 3hPa, 910 hPa, 117 04 andE 880 hPa, 23 23respectively.N 16.4When m a typhoon 3.8appears, km the sea Point 4 116◦560 E 23◦300 N 6 m 1.8 km current in the study area is controlled by the typhoon. In the model, the typhoon center path roughly passes through the center of the study area, moves from southeast to north- Figure 16 shows the increment in water level in the typhoon with different recurrence west,periods and comparedlands in Shantou with the normalCity near wind the ﬁeld. Rongjiang Figure 16 a–d,River respectively, estuary. The correspond northeast to the side of thefour Rongjiang control pointsRiver selected.estuary Figureis on the17 shows right theside changes of the intyphoon sea water path, velocity while in the the passage southwest sideof of typhoons the Rongjiang in different River recurrence estuary periods is on the and le comparesft side of them the typhoon with those path. in the Shantou absence ofCity is locatedtyphoons. near Figurethe Tropic 17a–d, of respectively, Cancer, belonging correspond to to the the northern four control hemisphere, points selected. affected by the geotropic deflection force. The typhoon wind field has counterclockwise rotation. In the study area, the water flow on the northeast side basically flows from south to north, while on the southwest side, the velocity vector is roughly the same as that of the typhoon, indicating that the direction of the water flow was controlled by the typhoon at this time. The flow direction of sea water is basically the same under typhoons of different intensities. In terms of velocity, the sea current is affected by both the wind field and the tide. On the left side of the typhoon path, the wind is in the same direction as the current caused by the tide. The superposition of the two makes the flow rate fast. On the right side of the

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typhoon path, the direction of wind is opposite to the direction of the tide. The velocity of the two is offset, so the velocity is not as fast as the sea water on the right side of the typhoon path. By comparing the distribution of sea level in the absence of typhoons, it can be found that the presence of typhoons will obviously cause a rise in sea level and a decrease in water level gradient. The more intense the typhoons are, the more the water level is increased. Four control points were selected, two on the right side of typhoon path and two on the left side of the typhoon path. Control points were used to compare the height and velocity of the water increase caused by the typhoon in different recurrence periods (see Figure 14). The information of control points is shown in Table 7. Point 1 and point 2 are located on the left side of the typhoon path. Point 3 and point 4 are located on the right side of the typhoon path.

Table 7. Location, bathymetry, and offshore distance of 4 control points.

Serial number Longitude Latitude Bathymetry Offshore distance Point 1 116°36′ E 23°06′ N 21.2 m 2.4 km Point 2 116°50′ E 23°14′ N 13.1 m 3.2 km Point 3 117°04′ E 23°23′ N 16.4 m 3.8 km Point 4 116°56′ E 23°30′ N 6 m 1.8 km

Figure 16 shows the increment in water level in the typhoon with different recurrence periods compared with the normal wind field. Figure 16a–c and Figure 18d, respectively, correspond to the four control points selected. Figure 17 shows the changes in sea water velocity in the passage of typhoons in different recurrence periods and compares them J. Mar. Sci. Eng. 2021,with9, 912 those in the absence of typhoons. Figure 17a–d, respectively, correspond to the four23 of 28 control points selected.

(a) the water level increases in control point 1

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(b) the water level increases in control point 2

(d) the water level increases in control point 4

Figure 16. TheFigure increment 16. The in increment water level in water of typh leveloon of typhoonin different in different recurrence recurrence periods. periods.

(a) variation in sea water velocity in control point 1

(b) variation in sea water velocity in control point 2

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J. Mar. Sci. Eng. 2021, 9, 912 (d) the water level increases in control point 4 24 of 28 Figure 16. The increment in water level of typhoon in different recurrence periods.

(a) variation in sea water velocity in control point 1

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(b) variation in sea water velocity in control point 2

(d) variation in sea water velocity in control point 4

Figure 17. SeaFigure current 17. velocitySea current curves velocity induced curves by induced typhoons by typhoons in different in different recurrence recurrence periods. periods.

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In Figure 16a,b, the water increase at these two points is relatively small. In particular, at point 1 in Figure 16a, the water increase does not exceed 0.6 m. There is just a little difference in the water level increase caused by typhoons of different intensities. In Figure 16b, only typhoons with a recurrence period of 1000 years can increase the water by more than 1 m. However, the impact of typhoons with different intensities is quite different. The increase in water in the millennium-scale typhoons is twice that in the decade-scale typhoons. In Figure 16c,d, points 3 and 4 are located on the right side of the typhoon path. The typhoon-induced water increases are intense at these two control points. In particular, in Figure 16d, even a typhoon with a 10-year recurrence period can cause an increase of more than 1.2 m. The amplitude of the water increase caused by typhoons in different recurrence periods is different, but the process of water increase is roughly the same. As can be seen in Figure 17, the increase in sea water velocity caused by typhoons is shown. The stronger the typhoon, the greater the maximum velocity of sea water. However, the increase in velocity is not as obvious as the water level. In contrast to the increase in water, the velocity of sea water at points 1 and 2 is greater than that at points 3 and 4. It can be seen from the curve that the rise in water level in the whole sea area caused by the typhoon is oscillating. Point 1 is located on the left side of the typhoon path and on the southwest of the whole sea area. When there is no typhoon passing through, the tidal current is relatively simple and the water level is low. The intensity of increasing water caused by the typhoon is weak here. Even the water increase caused by a typhoon with a recurrence period of 1000 years is less than 0.7 m. The wind vector of a typhoon does not point to the coast here, so the water level increase caused by the typhoon is relatively small. Point 2 is located near the Rongjiang River estuary; it is very close to the typhoon center path. When the typhoon enters the study area, the effect of increasing water is obvious, but the range of increasing water decreases rapidly afterward. As the typhoon gets closer, the wind increases, and the water level begins to rise continuously. After that, the typhoon lands and gradually drifts away, and the rising water level begins to decline slowly. Point 3 is located on the south side of Nan’ao Island, on the upper right of the typhoon path. Its water increase curve is basically the same as that at Point 2. However, the wind vector of the typhoon points roughly to the coast here, which makes the sea water converge toward the coast. The water increasing effect of the storm surge is more obvious. Typhoons with a 50 year recurrence period can lead to a water increase of more than 1 m. The water level drops slowly here, and the water increase duration of the storm surge is long. Point 4 is located in the narrow sea area between the northwest side of Nan’ao Island and the land. The tidal range here is large with normal wind. When a typhoon passes through, it is affected by the wind field and blocked by the terrain, and a large amount of seawater overflows and gathers, resulting in a significant increase in the water level. Even a typhoon in the 10 year recurrence period caused the water level to rise by more than 1 m. The increase in water here is large and lasts for a long time. The large increase in water here will easily cause seawater to enter the land, causing serious harm. It is the key area for typhoon disasters prevention. When a typhoon is passing though, the velocity of each control point will be controlled by the wind field and tide at the same time. Taking Figure 17a as an example, the wind vector and tide current are in the same direction at about 18:00 on 7 June. The velocity of the water at this time was accelerated. The typhoon vector and tide current directions are opposite at about 23:00 on 7 June. This causes the velocity of the water to decrease instead. It is worth noting that the increase in water caused by the typhoon is not completely consistent with the change in velocity. For example, at point 4, the water increase range is highest, but the sea water velocity is not highest, which is less than 1 ms−1. In this section, we simulated the typhoon “Mangkhut” and four typhoons with different intensities. Typhoon “Mangkhut” belongs to the type of landfall from the east and the rest of the typhoons landed from the front. When Typhoon “Mankhut” landed, its central pressure was close to 910 hPa. Its intensity approached that of a typhoon with a recurrence period of 100 years. However, compared with typhoons with a recurrence J. Mar. Sci. Eng. 2021, 9, 912 26 of 28

period of 50 years, the water increase caused by Typhoon “Mangkhut” was small. This is a better indication that the damage was worse on the right side of the typhoon’s path. About 35% of the typhoons in Shantou belong to the easterly landing type. The increase in water brought by this kind of typhoon is smaller than that landing from the west and front. If the typhoons landing from the east are weak, simple countermeasures can be enough, such as stopping marine activities. Of course, when the typhoon intensity is large, it may still lead to a great water increase, especially in the northeast coast of Shantou city. The four typhoons constructed in the model are all landing from front. They all bring great storm surge disasters. Especially in the northeast coast of Shantou city, such as Chenghai District and Nan’ao Island, the storm surge disaster is huge. The tidal range in Shantou city is larger in the northeast than in the southeast. The key areas for storm surge prevention are the coastal areas in the northeast of Shantou and Nan’ao Island. Once-in-a-century typhoons are relatively rare, so our advice is focused on the more common typhoons. For the northeast coast of Shantou with a large tidal range and easily affected by storm surge, it is suggested to build high dikes to cope with the disaster. More adequate contingency plans, such as evacuation routes and shelters, should be made for these areas. The southeast of Shantou city has a small tidal range and is located on the left side of the path of the typhoons landing front, so the storm surge disaster is relatively small. However, it is necessary to make emergency plans well.

4. Conclusions The main conclusions of this paper are as follows: (1) The direction of tidal current in Shantou coastal area is basically parallel to the shore-line. It flows from southwest to northeast at high tide and the flow direction is opposite at low tide. The water level is high in the northeast and low in the southwest at high tide, but it is opposite at low tide. (2) Due to the many islands in the northeast area of the Rongjiang River estuary boundary, the flow field is complex, the velocity is fast, and the tidal range is large. The number of islands in the southwest part is small, the flow field is relatively simple, and the tidal range is small. (3) Tides in the study area are mainly controlled by four component tides: M2, S2, K1, and O1, but there is an error in the lower high tide per day if we only consider these four tidal components. The P1 tidal constituent can effectively reduce this error. (4) In general, the main driving force of the sea current in the study area is the tidal potential. When the typhoon passes through, the wind field also disturbs the current. The typhoon will cause a change in the flow field, so the distant sea water flows to the typhoon center. The sea water around the typhoon will rotate and the velocity will increase, bringing about an abnormal water increase. (5) When the typhoon directly attacks Shantou, the northeast side of the study area is greatly affected by the typhoon, and there is an obvious water increase, while the southwest side has a slightly small impact, and the water increase range is slightly low. Therefore, the northwest coastal areas of Nan’ao Island need to pay more attention to storm surge disaster prevention.

Author Contributions: Data curation, E.Z.; formal analysis, Y.T. and E.Z.; funding acquisition, Y.W.; investigation, J.Y., K.F., H.W. and W.W.; project administration, Y.W.; writing—original draft, Y.T.; writing—review and editing, Y.T. and E.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Comprehensive Geological Survey of Chaoshan Coastal Zone, grant number DD20208013. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. J. Mar. Sci. Eng. 2021, 9, 912 27 of 28

Data Availability Statement: Wind field data are from the European Centre for Medium-Range Weather Forecasts. Here is the link: https://cds.climate.copernicus.eu (accessed on 11 July 2021). Acknowledgments: European Centre for Medium-Range Weather Forecasts; the China Meteorologi- cal Administration. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References 1. Zhao, E.J.; Sun, J.K.; Tang, Y.Z.; Mu, L.; Jiang, H.Y. Numerical investigation of tsunami wave impacts on different coastal bridge decks using immersed boundary method. Ocean Eng. 2020, 201, 107132. [CrossRef] 2. Cai, W.T. Characteristics of storm surge along Shantou coast. Public Commun. Sci. Technol. 2013, 5, 111–113. 3. Liu, J. A High Resolution Numerical Forecast Model for Storm Surge Floodplain in Shantou Area. Master’s Thesis, Ocean University of China, Qingdao, Shandong, China, 2004. 4. Chan, J.C.L.; Shi, J.E.; Liu, K.S. Improvements in the seasonal forecasting of tropical cyclone activity over the western North Pacific. Wea Forecast. 1998, 13, 997–1004. [CrossRef] 5. Zhao, E.J.; Qu, K.; Mu, L. Numerical study of morphological response of the sandy bed after tsunami-like wave overtopping an impermeable seawall. Ocean Eng. 2019, 186, 106076. [CrossRef] 6. Zhou, B. An overview of the discretization methods of governing equations in computational fluid dynamics. Technol. Econ. Guide 2017, 21, 146. 7. Zhao, E.J.; Dong, Y.K.; Tang, Y.Z.; Xia, X.Y. Performance of Submerged Semi-circular Breakwater Under Solitary Wave in Consideration of Porous Media. Ocean Eng. 2021, 223, 108573. [CrossRef] 8. Lai, W.F.; Pan, J.Y.; Adam, T.D. Impact of tides and winds on estuarine circulation in the Pearl River Estuary. Cont. Shelf Res. 2018, 168, 68–82. [CrossRef] 9. Liu, R.Z.; Zhu, Q.Y.; Ni, P.T. Numerical simulation of storm surge in the Pearl River estuary area. Guangdong Water Resour. Hydropower 2018, 4, 6–10. 10. Xu, R.G.; Wang, S.C.; Xie, M.X. Research on the hydrodynamic and silt backsilting in western Shenzhen Port area. Port Eng. Technol. 2020, 31, 81–90. 11. Jia, R.P.; Dou, M.; Mi, Q.S. MIKE21 based 2d water environment numerical simulation of Marlboro Lake. Environ. Pollut. Control. 2018, 40, 527–532. 12. Xu, T. Overview and application examples of Danish MIKE21 model. Water Conserv. Sci. Technol. Econ. 2010, 16, 867–869. 13. Zheng, P.N.; Song, J.; Zhang, F.N. Introduction to common numerical models of the ocean. Mar. Forecast. 2008, 4, 108–120. 14. Chen, T.Q.; Zhang, Q.H.; Wu, Y.S. Development of a wave-current model through coupling of FVCOM and SWAN. Ocean Eng. 2018, 164, 443–454. [CrossRef] 15. Zhao, E.J.; Dong, Y.K.; Tang, Y.Z.; Sun, J.K. Numerical Investigation of Hydrodynamics and Local Scour around Submarine Pipeline under Joint Effect of Solitary Wave and Current. Ocean Eng. 2021, 222, 108553. [CrossRef] 16. Zhou, J.; Zeng, C.; Wang, L.L. The influence of drag coefficient of wind stress on numerical simulation of wind flow. J. Hydrodyn. 2009, 24, 440–441. 17. Wu, J. The sea surface is aerodynamically rough even under light winds. Bound. Layer Meteorol. 1994, 69, 149–158. [CrossRef] 18. Le, F. A new dynamic formula for determining the coefficient of Smagorinsky model. Theor. Appl. Mech. Lett. 2011, 1, 032002. 19. Chen, C.S.; Beardsley, R.C.; Cowles, G. An unstructured grid, finite-volume coastal ocean model (FVCOM) system. Special issue entitled “Advance in computational oceanography”. Oceanography 2006, 19, 78–89. [CrossRef] 20. Zhao, C.; Lv, X.G.; Qiao, F.L. Numerical study of tidal wave in Beibu Gulf. Acta Oceanol. Sin. 2010, 32, 1–11. 21. Wang, D.S.; Liu, X.D.; Zhuang, H.D. Three-dimensional tidal simulation and tidal flow numerical simulation in the Taiwan Strait based on FVCOM. Mar. Sci. 2016, 37, 10–19. 22. Fang, G.Y.; Kwok, K.; Zhu, Y. Numerical simulation of principal tidal constituents in the South China Sea, Gulf of Tonkin and Gulf of Thailand. Cont. Shelf Res. 1999, 19, 845–869. [CrossRef] 23. Wu, Z.; Tian, Z.W.; Zhao, Q. Three-dimensional tidal wave assimilation numerical simulation in the South China Sea. J. Hydrodyn. 2004, 19, 501–506. 24. Lu, M.; Ding, Y.; Si, M.C. Typhoon response in the northern part of the South China Sea. Trans. Oceanol. Limnol. 2019, 5, 9–19. 25. Tao, Y.J.; Zhang, L.; He, Q.Q. Numerical analysis of the hydrodynamic force of typical typhoons in Nan’ao area of Shantou. Pearl River 2019, 40, 48–55. 26. Hong, Y.J.; Lin, Y.S.; Chen, W.Z. China’s worst typhoon of the century caused a tidal disaster. J. Catastrophol. 1986, 1, 62–63. 27. He, Y.C.; He, J.Y.; Chen, W.C.; Chan, P.W. Insights from Super Typhoon Mangkhut (1822) for wind engineering practices. J. Wind Eng. Ind. Aerodyn. 2020, 203, 104238. [CrossRef] 28. Liu, Z.K.; Li, J.G.; Li, H. Analysis and suggestion of wave damage caused by Typhoon Manchu on the South China Sea platform. Shipbuild. China 2019, 60, 197–203. J. Mar. Sci. Eng. 2021, 9, 912 28 of 28

29. Vickery, P.J.; Skerlj, P.F.; Twisdale, L.A. Simulation of hurricane risk in the US using empirical track model. J. Struct. Eng. 2000, 126, 1222–1237. [CrossRef] 30. Cheung, K.F.; Tang, L.; Donnelly, J.P.; Scileppi, E.M.; Liu, K.B.; Mao, X.Z.; Houston, S.H.; Murnane, R.J. Numerical modeling and ﬁeld evidence of coastal overwash in southern New England from Hurricane Bob and implications for paleotempestology. J. Geophys. Res. Earth Surface 2007, 112, F3. [CrossRef] 31. Atkinson, G.D.; Holliday, C.R. Tropical cyclone minimum sea level pressure maximum sustained wind relationship for the western North Paciﬁc. Mon. Weather Rev. 1977, 105, 421–427. [CrossRef]