Risk assessment of wave hazard in Guangdong Province

In order to improve the prediction ability of wave hazard levels in the ocean under the jurisdiction of Guangdong Province, this paper uses the wave simulation data to evaluate the wave hazard index of the ocean under the jurisdiction of Guangdong Province, and analyses the risk assessment analysis of wave hazards in the Guangdong Province, which provides data support for future wave disaster prevention and mitigation, coastal development, and spatial planning in Guangdong Province. The significant wave height of the 50-year return period in the coastal areas of Guangdong Province is generally distributed within 9 meters of the territorial sea baseline. The intensity of the return period gradually increases with the distance from the mainland and decreases with the decrease of water depth. In the past 30 years, the sea areas under the jurisdiction of the Guangdong Province are mainly dominated by class III and IV wave heights, with class IV being higher than class III. The frequency of Class IV wave heights in the mainly marine area is more than 90%. The risk level of wave disasters in the coastal waters of Guangdong Province is mainly level IV. However, the risk level of wave disasters in the waters under the jurisdiction of Guangdong Province is mainly Level I.


Introduction
The distribution of wave elements significantly impacts the safety of coastal residents, fishing, oil and gas extraction, military engineering construction, and naval and landing operations.With the increasing demand for marine disaster prediction and analysis by the state, government, and the public, the requirements for marine disaster assessment and analysis are also increasing, and assessment results are being advocated more.According to the statistical analysis of the Guangdong Provincial Marine Disaster Bulletin, 52 wave disasters occurred along the coast of Guangdong Province in the past ten years, with an average of 5.2 occurrences per year.The leading causes of wave disasters in Guangdong Province are cold air, typhoons, combined effects of typhoons and cold air, strong convective weather, cyclones, combined effects of cold air and tropical depression, and combined waves of cold air and cyclones.Among these, cold air caused the highest number of wave disasters, with 33 times, accounting for 63.5%; secondly, there were 16 typhoon waves, accounting for 30.8%.From 2013 to 2022, the direct economic losses caused by coastal wave disasters in Guangdong Province amounted to approximately 24.3013 million yuan, with 85 deaths (including missing) and 34 damaged vessels.However, the Guangdong Provincial Marine Disaster Bulletin mainly reflects the direct economic losses, deaths (including missing persons), and ship damage caused by storm surges and waves during typhoons.It does not specifically distinguish between the losses caused by typhoon storm surges and those caused by typhoon waves, so the losses caused by wave disasters are much higher than the current statistics.Using the Wavewatch III and SWAN, Jiang et al. (2011, 2017)  analyzed the air-sea interaction on waves in the northern South China Sea during an intense cold air outbreak [1][2] .Based on 30-year simulated wave data, Jiang et al. (2018) analyzed the interannual changes of waves in the northern South China Sea [3] .Y Luo et al. (2021) Used the non-stationary generalized extreme value analysis, trends for a 100-year return period of significant wave height(SWH) in the South China Sea [4] .There are many studies on waves in the northern part of the South China Sea, but the assessment of wave disasters in the northern South China Sea has not yet been carried out.Based on the Technical Specification for Risk Assessment and Zoning of Wave Disasters (FXPC/ZRZY P-06), this paper evaluates and analyses the risk assessment and zoning of wave disasters in Guangdong Province(Figure 1), which can provide scientific support for the emergency decision-making on wave disasters by the coastal marine administrative department of Guangdong Province, effectively reduce the risk of marine disasters in coastal areas, and reduce casualties and property losses caused by wave disasters.

CCMP data
The conventional observations at sea include fixed ocean buoys and coastal stations.The subsurface conditions of coastal stations are very different from those at sea, so the station observations cannot fully represent the actual situation at sea.On the vast ocean surface, the conventional observations are scarce and lacking, so we can only rely on data from unconventional observations to analyze the ocean phenomenon.As an unconventional observation method, satellite remote sensing has been widely used by scholars at home and abroad to study ocean and atmospheric environments due to its timely, rapid observation data, long-term periodicity, and wide coverage(Zhan Siyu et al., 2017) [5] .The wind field data used in this paper are the Cross Calibrated Multi-Platform(CCMP), which is an integrated multiplatform wind field launched by the National Aeronautics and Space Administration(NASA) in 2009, with high spatiotemporal resolution and continuity, and the ability of global ocean coverage.The spatial resolution of the data is 0.25 ° × 0.25 °, with a temporal resolution of 6 hours and a time length from 1989 to 2018.The elements are wind field's meridional and latitudinal velocity components at a distance of 10 meters from the sea surface.

Wind field data
The typhoon wind field is usually highly variable.The large speed gradient and the rapid change of wind direction in a typhoon vortex generate very complex ocean wave fields.Since the NCEP/ NCAR reanalysis wind speed is lower than the observation during the typhoon period, an empirical typhoon model (Holland, 1980) [6] is considered in this paper(Jiang et al., 2018) [3] .In this study, we reconstructed all the wind fields of tropical cyclones with life-history processes between 120°-105°E, 0°-25°N and intensities above tropical storms from 1989 to 2018.The reconstructed wind field significantly improved the low wind speed at the center of the typhoon.The reconstructed wind speed at the typhoon's center agreed with the typhoon intensity from the JMA( Figure 2), with an average absolute error of 2.85m/s.The wind field during the typhoon period was reconstructed and merged with the CCMP wind field to form a new set of merged wind field data as the essential wind field for this study.(Tolman, 1989, 1991) and the WAVEWATCH II (Tolman, 1992) developed at NASA Goddard Space Flight Center [7][8] [9] .The WW3 model solves the spectral action density balance equations for directional wave number spectra with the full nonlinear physics, and it accounts for wind input, wave-wave interaction and dissipation due to white capping and wave-bottom interaction.

Verification of the wave data
Comparative analysis of wave simulation data was conducted using buoy data from the northern South China Sea in 2017 (see Figure 1 for buoy locations), with inspection indicators including bias, mean relative error, root mean square error, and correlation coefficient.Figure 3 shows the scatter plot of the buoy and wave mode data, while Table 1 shows the comparison and inspection indicators of buoy and wave mode data.From the inspection results, the results of the wave simulation are in good agreement with the buoy observations.The bias of each buoy station for the significant wave height is 0.04m, the mean relative error is 0.35m, the root mean square error is 17.78%, and the correlation coefficient is 0.91.

The return period of the significant wave height
Based on the wave model data set in the ocean of Guangdong Province from 1989 to 2018, the annual extreme value sequence of the wave elements at each grid point is statistically determined.The Pearson III extreme value calculation method is used to calculate and determine the significant wave height of the typical return period at each grid point.The distribution maps of significant wave heights for the 2-year, 5-year, 10-year, 20-year, 50-year, and 100-year return periods in the Guangdong Province are shown in Figure 4.The significant wave height of the 50-year return period along the coast of Guangdong Province is generally distributed within 9 meters of the territorial sea baseline.The intensity of the return period gradually increases with the increasing distance from the mainland and decreases with decreasing water depth.The 50-year return period wave heights within the sea baseline of the territorial waters in Dongguan, Guangzhou, and Zhongshan are generally distributed within 2.5m, while are generally distributed within 6m in Chaozhou, Shanwei, Jieyang, Huizhou, Shenzhen, Zhuhai, and Jiangmen.The 50-year return period wave heights below the 15m isobath are generally distributed within 4m, and it is mainly distributed between 4-5m in the surrounding ocean of Wailingding Island, Guishan Island and Dong'ao Island.The 50-year return period wave height below the 15m isobath in Yangjiang City and Maoming City is generally distributed within 4m, while it is generally distributed within 2.5m in Zhanjiang City.However, the highest value of the 50-year return period wave height within the territorial sea baseline of three cities and counties exceeds 9m.The spatial distribution of the 100-year return period significant wave height in the coastal waters of Guangdong Province is similar to that of the 50-year return period significant wave height.The significant wave heights with a 100-year return period in the territorial sea baseline are generally distributed within 12m, and the 100-year return period wave heights below the 15m isobath are generally distributed within 6m.Among them, the 100-year return period wave heights in the waters of Wailingding Island, Guishan Island, and Dong'ao Island are mainly distributed between 4m and 6m.

Statistics on the frequency of the wave heights
The technical guidelines for risk assessment and zoning of wave disasters stipulate the technical requirements for provincial-level risk assessment and zoning of wave disasters.The fourth-level wave height delineation standard in the wave intensity classification table (Table 2) for wave disaster risk assessment is equivalent to the four-color offshore wave warning release standard in China's current wave disaster warning release standard.Statistical analysis was performed on the hourly grid results of the northern South China Sea from 1988 to 2018 to obtain the hours of occurrence of fourth-level wave heights at each calculation grid point.Based on this, frequency statistics of fourth-level wave heights at each level were conducted.A pie-shaped distribution map of the frequency of fourth-level wave heights was drawn (Figure 5).According to the statistical results of the fourth-level wave heights based on the classification criteria, there has been no class I wave height in Guangdong Province, and class II wave heights have occurred.However, the frequency is generally not more than 2%.The sea areas under the jurisdiction of the whole Guangdong Province are mainly dominated by class III and IV wave heights, with class IV being obviously higher than class III.The frequency of class IV wave heights in the mainly marine area is more than 90%.

Distribution of wave hazard levels
According to the guidelines, evaluating the wave hazard level involves three steps.First, calculate the wave hazard index for each cell point using the following formula: Hwn is the hazard index after conversion, Hw is the hazard index before conversion, and are the maximum and minimum values of the sample, respectively.Thirdly, determine the risk level of wave disasters at each grid point according to Table 3.As the Figure 6 shows, the distribution of wave hazard zoning levels in Guangdong Province is parallel to the coastline of Guangdong, and the risk level of wave hazards increases with the distance from land.The risk level of wave disasters in the coastal waters of Guangdong Province is mainly Level IV.However, the risk level of wave disasters in the waters under the jurisdiction of Guangdong Province is mainly Level I.

Conclusion
The significant wave height of the 50-year return period in the coastal areas of Guangdong Province is generally distributed within 9 meters of the territorial sea baseline.The intensity of the return period gradually increases with the increasing distance from the mainland and decreases with the decreasing water depth.The spatial distribution pattern of the 100-year return period the significant wave height in the coastal ocean of Guangdong Province is similar to that of the 50-year return period.In the past 30 years, there has been no class I SWH in Guangdong Province, and class II wave heights have occurred, but the frequency is generally not more than 2%.The sea areas under the jurisdiction of the whole Guangdong Province are mainly dominated by class III and IV wave heights, with class IV being higher than class III.The frequency of class IV wave heights in the main sea area is more than 90%.The distribution of wave hazard zoning levels in the Guangdong Province is parallel to the coastline of Guangdong, and the risk level of wave hazards increases with distance from the land.The risk level of wave hazards in the coastal waters of Guangdong Province is mainly level IV.However, the risk level of wave hazards in the waters under the jurisdiction of Guangdong Province is mainly level I.At present, the classification level of wave intensity in the technical guidelines for wave disasters is the same as the publication standards for nearshore wave warnings.It is recommended that in the future, wave disaster assessment and risk assessment should be carried out with the release standards of nearshore wave warning, so that the classification of nearshore wave intensity is more recognizable.

Figure 1 .
Figure 1.Bathymetric chart of the jurisdiction of Guangdong Province(the red stars present the position of the buoys)

Figure 2 .
Figure 2. Comparison of the maximum wind speed between CCMP, JMA and reconstruction wind

Figure 3 .
Figure 3.Comparison of the SWH between wave model and buoy

Figure 4 .
Figure 4.The return period of the significant wave height in 50-year and 100-year (unit:m)

6 Figure 5 .
Figure 5. Frequency distribution of SWH in the jurisdiction of Guangdong Province

4 N
are the annual average occurrences of level I, II, III, and IV wave heights at each cell point.Second, the calculated hazard index Hw for each cell point is linearly normalized, and the normalized hazard index is represented as Hwn .The linear normalization method is as follows:

Figure 6 .
Figure 6.Distribution of wave hazard level in Guangdong Province

Table 1 .
Comparison of the SWH between wave model and buoy

Table 2 .
Classification of wave intensity levels

Table 3 .
Classification criteria for hazard levels of wave disasters