Effects of Storm Waves Caused by Typhoon Bolaven (1215) on Korean Coast: A Comparative Analysis with Deepwater Design Waves
Article information
Abstract
This paper employs the third-generation simulating waves nearshore (SWAN) ocean wave model to estimate and analyze storm waves induced by Typhoon Bolaven, focusing on its impact along the west coast and Jeju Island of Korea. Utilizing reanalyzed meteorological data from the Japan Meteorological Agency meso scale model (JMA-MSM), the study simulated storm waves from Typhoon Bolaven, which maintained its intensity up to high latitudes as it approached the Korean Peninsula in 2012. Validation of the SWAN model against observed wave data demonstrated a strong correlation, particularly in regions where wind speeds exceeded 20 m/s and wave heights surpassed 5 m. Results indicate significant storm wave heights across Jeju Island and Korea’s west and southwest seas, with coastal grid points near islands recording storm wave heights exceeding 90% of the 50-year return period design wave heights. Notably, specific grid points near islands in the northern West Sea and southwest Jeju Island estimated storm wave heights at 90.22% and 91.48% of the design values, respectively. The paper highlights the increased uncertainty and vulnerability in coastal disaster predictions due to event-driven typhoons and emphasizes the need for enhanced accuracy and speed in typhoon wave predictions amid the escalating climate crisis.
1. Introduction
According to the typhoon data provided by the Korea Meteorological Administration (KMA), the average number of typhoons occurring each year for 23 years until 2023 is 24.09. Among these, the average number of typhoons affecting the Korean Peninsula is 3.22. Typhoons affecting the Korean Peninsula typically move to the west or northwest along the edge of the North Pacific anticyclone, reaching their peak intensity. They then change direction, drawing a parabola as they move to the northeast, and eventually weaken and disappear. Due to these movement characteristics, typhoons often pass through the Korea Strait between the Korean Peninsula and Kyushu, Japan, into the East Sea, as shown in Fig. 1(a). The 14th typhoon, Maemi (0314), in 2003, was accompanied by high storm surge heights and strong winds, making it the largest in terms of size. Typhoon Maemi, classified as a super typhoon, followed the typical track described above. This same typical track was also predicted by the probabilistic cluster analysis by Kim et al. (2014), which used past typhoon data affecting the Korean Peninsula, as well as by the artificial intelligence (AI) analysis by Kim et al. (2019a), which applied neural network technology, and the AI analysis by Kim et al. (2019b), which applied deep learning technology. Consequently, typhoons affect the Korean Peninsula only from June to October (from early summer, when the North Pacific anticyclone begins to expand, to autumn, when its intensity weakens), with a concentration in July to September. The average number of typhoons invading the Korean Peninsula is largest (1.2) in August, but the most significant damage occurs in September. Most typhoons enter the Korean Peninsula in July and August due to the high number of typhoons during this period. Another reason for typhoons affecting Korea is the contraction of the North Pacific anticyclone, which opens a path toward the Korean Peninsula.
Historical typhoon tracks provided by JTWC, including (a) typhoons in 21st century with typical tracks that have affected Korean Peninsula and (b) typhoons that have significantly impacted west coast of Korea
Kang and Kim (2019) classified storm surges caused by large typhoons into three types to investigate their characteristics on the west coast of Korea, as shown in Fig. 1(b). Typical examples of steep surges are Typhoons Olga (9907) and Kompasu (1007). These typhoons approached and passed through the west coast at high speed, with the maximum surge height occurring regardless of the tide. Mild surges occur when typhoons, such as Prapiroon (0012) and Bolaven (1215), move slowly to the north at a distance from the west coast. These typhoons showed the characteristics of a tide-modulated surge, where the maximum surge height occurs mainly at low tide. Typhoons Muifa (1109) and Winnie (9713), also classified as mild surges, are considered surges propagated from the outside and do not show the characteristics of a tide-modulated surge. Based on these surge types, Kang and Kim (2019) analyzed the pattern of high water. They found that the sea level on the west coast was highest when a full-wave type surge, with a relatively low surge height, overlapped with the high tide level of the Baekjung spring tide.
Among the large typhoons that affected the west coast, Typhoon Bolaven was a powerful super typhoon that maintained its intensity to high latitudes. It caused severe damage across a wide range of areas, including Jeju Island, the southwest coast, and the west coast. A tropical depression formed in the western Pacific Ocean in early August 2012 gradually strengthened as it moved westward. According to the Joint Typhoon Warning Center (JTWC), when Typhoon Bolaven reached its peak strength, it exhibited the characteristics of a powerful super typhoon with a one-minute average maximum wind speed of 63.9 m/s and a central pressure of 910 hPa. It recorded a maximum instantaneous wind speed of 51.8 m/s in Wando-gun, Jeonnam, and a minimum pressure of 961.9 hPa in Heuksando Island, Sinan-gun. Typhoon Bolaven maintained its intensity as it moved to high latitudes, eventually landing in Hwanghae-do, North Korea, and then reaching the Russian Far East region with weakened intensity. In this process, it passed through the West Sea and caused storm surges. Surge heights of 1 m or higher were observed at 13 tidal stations, with maximum surge heights of 1.68 m in Goheung and 1.52 m in Incheon. After Typhoon Bolaven landed in Jangsu-ri, Gangnyeong-gun, Hwanghae-do, North Korea, at 16:00 on August 28, 2012, Typhoon Tembin (1214) reached the sea approximately 90 km west-southwest of Jeju Island 38 h later, at 6:00 on August 30. This left Jeju Island and the southwest coast under the direct influence of the typhoon again. Due to the unusual situation of two typhoons consecutively entering the Korean Peninsula, the property damage ranks fourth in the history of the Korean Peninsula.
As a result, the Jeju Naval Base, which was under construction, suffered severe damage, with most of the high-wave damage occurring along the coast of Seogwipo. As the typhoons moved north, high-wave damage occurred sequentially along the west coast. In particular, on Gageodo Island, the southwesternmost part of the Korean Peninsula and adjacent to the track of Typhoon Bolaven, a 280 m section of the breakwater, which had been completed over 30 years from 1979 to 2008 with a total length of 480 m, was severely damaged due to the aftermath of Typhoons Muifa and Bolaven. During the invasion of Typhoon Bolaven, 413 residents were isolated for five days on Gageodo Island. All wired and wireless communications and maritime transportation were cut off. Therefore, the construction of the world’s largest super breakwater commenced in 2013. The design wave height was increased from 8.3 m to 12.5 m for the 100-year return period, the width of the breakwater was expanded from 15 m to 108 m, and the crest height was raised from 8 m to 11 m.
In coastal areas, typhoon damage is caused by wind, waves, surges, and tides. Particular attention is given to storm surges caused by wind and low pressure, as they are the main cause of flooding and inundation in coastal lowlands. Therefore, it is important to predict storm surges to respond to typhoon invasions. However, it is also necessary to predict storm waves accompanied by typhoons, especially those with strong winds, as they cause significant high-wave damage. When the storm surge height is not very high, flooding due to high-wave overtopping can occur. This means that the increase in water level caused by wave action (wave setup) and the increase in water level on the coast due to seawater being pushed by the wind (wind setup), as well as storm waves, should not be overlooked.
In general, wind fields are estimated using several parameters, such as the typhoon track, central pressure, radius of maximum wind, and maximum wind speed. Since this parameter model considers only the wind field within the influence of the typhoon, it excludes the impact of the surrounding wind and weakens as the typhoon weakens (Kwon et al. 2020). Consequently, when the wind field of the parameter model is applied for the estimation of storm waves, it is highly likely to result in underestimation. Accordingly, studies have utilized reanalysis weather fields, benefiting from developments in weather forecasting and reanalysis technology. A representative case is the Japan Meteorological Agency - meso scale model (JMA-MSM) weather field provided by JMA. The JMA-MSM weather field is a regional model specialized in the surrounding waters, including Japan and Korea. It provides weather data at one-hour intervals using approximately 10 km grids since 2002 and more precise 5 km grids since 2006. This enhancement has significantly increased the accuracy of estimating storm surges and storm waves.
Kim et al. (2020) and Kwon et al. (2020) simulated storm surges and storm waves by applying the JMA-MSM weather fields for Typhoons Bolaven and Kong-rey (1825). Yoon et al. (2020) estimated the storm surges and storm waves caused by Typhoon Maemi, one of the strongest typhoons, using the ECMWF Re-Analysis 5th Generation (ERA5), the latest reanalysis data from JMA-MSM and the European Center for Medium-Range Weather Forecasts (ECMWF). Storm waves were simulated using the simulating waves nearshore (SWAN) model, which applied the sea surface wind data of RDAPS, an operational weather forecast model of KMA, for Typhoons Maysak (2009) and Haishen (2010), and JMA-MSM, a weather forecast model of JMA (Son and Do, 2022). Seo et al. (2023) also investigated storm waves that approached or exceeded the deepwater design wave height (MOF, 2019) of the 50-year return period at the coastal grid points (GPs) of the East Sea and Ulleungdo Island, where overtopping and flood damage caused by storm waves were severe, through wave simulations applying JMA-MSM data for Typhoons Maysak and Haishen. Moreover, studies on storm waves have been actively conducted even when typhoons do not invade. Chun et al. (2014) analyzed wintertime storm wave characteristics in the East Sea using the WAM (wave model). Kang et al. (2015) examined the weather fields from the National Centers for Environmental Prediction (NCEP), ECMWF, and JMA-MSM during the modeling process that applied SWAN in long-term wave research. Eum et al. (2016) also simulated waves in the waters around the Korean Peninsula by substituting ECMWF and JMA-MSM weather fields into the SWAN model. Do and Kim (2018) analyzed the spatiotemporal wind data from RDAPS and the Weather Research and Forecast (WRF; Park et al., 2015) models, contributing to an improvement in the accuracy of the SWAN model by adjusting the coefficient of the energy dissipation term by white capping, as proposed by Rogers et al. (2003). Son and Do (2021) optimized the SWAN model for its application to the east coast by simulating wintertime storm waves using the wave observation-based source term ST6 (Rogers et al., 2012).
In this study, storm waves, a major factor in coastal disasters caused by typhoons, are estimated using the third-generation wave model SWAN. The characteristics and effects of storm waves are analyzed from various perspectives. The storm waves are simulated using the reanalysis weather field provided by JMA-MSM for the 15th typhoon, Bolaven, in 2012, which maintained high intensity up to high latitudes and significantly affected the west coast of Korea. Furthermore, the size of the storm waves is evaluated by comparing them with the 50-year deepwater design wave specifications (MOF, 2019) at major GPs in the waters around the Korean Peninsula, as revised in 2019.
2. Numerical Analysis Method
2.1 SWAN Model
To estimate storm waves during typhoon invasions, the third-generation wave model SWAN ver. 41.31A, based on the wave action balance equation, is used to reproduce multi-directional irregular waves. The SWAN spectrum wave model can simulate the growth of wave energy by wind and wave propagation according to refraction, shallow water effects, reflection, and diffraction. It is divided into first, second, and third generations according to the source and sink terms. The governing equation is as follows:
2.2 Computational Domain
The computational domain of this study is set between latitudes 20° to 43° and longitudes 117° to 142°, considering the track of Typhoon Bolaven, to accurately estimate the waves generated and propagated by the typhoon wind. As shown in Fig. 2(a), the computational domain is composed of rectangular grids at 0.0417° (approximately 4.6 km) intervals. For the water depth of the computational domain, the underwater topography from SRTM5+V2.0 (Tozer et al., 2019), constructed based on 494 million depth survey data points selected from the latest global models CryoSat-2 and Jason-1, is applied as shown in Fig. 2(b). Spatial interpolation is performed according to the computational grid size (0.0417°) of SRTM5+V2.0, which has a resolution of 15” (approximately 450 m).
(a) Computational domain including grids and (b) bathymetric map applied to SWAN model for storm wave simulations
2.3 Numerical Conditions
In this study, the frequency of the wave energy spectrum (0.04 to 1.0 Hz) is divided into 34 sections to utilize the JONSWAP wave spectrum and to account for the period component of long-period waves, such as storm waves. In addition, the wave energy spectrum is calculated by dividing the wave direction into 36 sections at 10° intervals (see Table 1).
Coefficient values applied in SWAN model for wave simulation
The one-hour interval wind field with a resolution of 0.0625° × 0.05° provided by JMA-MSM is applied to the calculation of storm waves using the SWAN model. This field covers the area from the Shandong Peninsula, China to the west, Hokkaido, Japan to the east, Taiwan’s southern waters to the south, and Manchuria to the north, including Japan and Korea.
Typhoon Bolaven, as shown in Fig. 2(b), is considered for storm wave simulation. It represents a maximum wind speed of 36 m/s at 125.2°E and 33°N at 03:00 on August 28, 2012, as it passes through the western sea of Jeju Island. It also maintained strong winds of up to 28.29 m/s at 15:00 on August 28, 2012, when it moved north at a close distance from Taean.
2.4 Model Verification
SWAN ver. 41.31A was applied to estimate swell-like storm waves on the east coast of Korea in winter by Son and Do (2021). This model was found to be suitable for wave estimation in the waters around the Korean Peninsula through a comparison with observed waves. In this study, an attempt was made to verify the validity of the SWAN model and the effectiveness of the numerical results before estimating the storm waves generated by the wind of Typhoon Bolaven. The numerical results were compared with the data observed by KMA at the locations of nine wave OPs (see Fig. 3 and Table 2). Although the observation data from OP2, OP4, and OP5 had missing sections, the maximum wave height of storm waves was measured, except for OP2 and OP4.
Locations of wave observation points (OPs) used by KMA during Typhoon Bolaven
Comparison of observed and calculated maximum significant wave heights during Typhoon Bolaven
Fig. 4 and 5 compare the calculated values of the SWAN model with the time distribution of the significant wave height (Hs ) and significant wave period (Ts ) observed at the nine points from August 14 to September 5, 2012, when Typhoon Bolaven invaded the Korean Peninsula. Table 2 shows the maximum significant wave heights at the nine OPs during the invasion of Typhoon Bolaven. In addition, the quantitative accuracy of the storm wave estimation by the SWAN model is presented using R-squared (R2), root mean square error (RMSE), and normalized root mean square error (NRMSE) in Fig. 6.
Comparisons of estimated and observed time-domain significant wave heights by SWAN model and KMA
Comparisons of estimated and observed time-domain significant wave periods by SWAN model and KMA
Comparison between estimated and observed maximum significant wave heights by SWAN model and KMA during Typhoon Bolaven
From Fig. 4 and Fig. 5, it can be seen that Typhoon Bolaven had a direct impact on the southwest and west coasts and an indirect impact on other waters beginning from Jeju Island around August 28, 2012, when it approached the Korean Peninsula. A second peak was observed as the Korean Peninsula came under the direct influence of Typhoon Tembin on August 30, 2012. At each observation point, the trends in Hs and 2, as well as the tendencies of maximum Ts and Hs, were relatively similar. When examining the maximum Hs during the typhoon invasion as shown in Table 1, it was found that the estimated maximum Hs by Typhoon Bolaven differed by more than 1 m from the observed Hs at OP10 to OP13. Among these, the values at OP11 (Oeyeondo Island on the west coast) and OP13 (Marado, Jeju Island) were errors caused by missing observations of peak Hs . Excluding these, the maximum Hs at OP12 (Chilbaldo Islet) was 1.69 m higher than the observed value, resulting in a 31% overestimation. At OP14 (Geomundo Island), where the highest Hs was observed, Hs was 0.4 m lower, showing a 4% underestimation. At OP14 to OP18, located in the South Sea and the East Sea, the estimated Hs from the SWAN model reproduced the observed values with significantly high accuracy. On average, the maximum Hs estimated by the SWAN model overestimates the observed values by 7%. Excluding OP11 and OP13, where peak Hs was not observed, the estimated maximum Hs underestimated the observed values by 3%. Fig. 6 compares the estimated and observed values. The scatter plot and the quantitative indicators (R2, RMSE, and NRMSE) suggest that the validity and effectiveness of the SWAN model for storm wave analysis have been partially verified.
The accuracy of the SWAN model was secured to some extent in estimating the maximum storm wave height due to typhoon wind, but the reliability of the period is not as secured as the wave height. In this study, the maximum storm wave height is compared with the design wave height of the 50-year return period to discuss the storm wave characteristics of Typhoon Bolaven. Therefore, storm waves generated by the typhoon wind are simulated using the SWAN model, whose reliability in storm wave height prediction accuracy was confirmed through a comparison with observed values.
3. Storm Wave Characteristics
3.1 Wind Field
As shown in Fig. 2(b), which includes the track of Typhoon Bolaven, the typhoon moved to the north in the East China Sea, passed through the sea to the west of Jeju Island, moved to the north at a close distance from the west coast of Korea, and landed in Jangsu-ri, Gangnyeong-gu, Hwanghae-do (16:00 on August 28, 2012). During this period (00:00 to 21:00 on August 28, 2012), the intensity of Typhoon Bolaven did not significantly decrease before landing as can be seen from Fig. 7. Fig. 7 shows the wind fields (sea surface wind) at three-hour intervals based on the reanalysis weather fields provided by JMA-MSM for Typhoon Bolaven that directly affected the west coast of the Korean Peninsula. In the figure, the locations of the center of Typhoon Bolaven are (a) 170 km to the southwest of Seogwipo (sea), (b) 120 km to the west-southwest of Seogwipo (sea), (c) 120 km to the southwest of Mokpo (sea), (d) 100 km to the northwest of Mokpo (sea), (e) 80 km to the west of Seosan (sea), (f) 120 km to the west-northwest of Seoul (sea), (g) 120 km to the south of Pyongyang (land), and (h) 30 km to the southwest of Pyongyang (land). The typhoon information (central location, central pressure, maximum wind speed, radius of strong winds, and movement speed) provided by KMA is shown in Table 3.
Wind fields based on JMA-MSM during both direct and indirect impacts of Typhoon Bolaven on Korean Peninsula
Information of Typhoon Bolaven provided by KMA
Considering the radius of strong winds of Typhoon Bolaven from Table 3, Jeju Island was directly affected by the typhoon from Fig. 7(a) to 7(c). The southwest coast came under direct influence from Fig. 7(b), and the west coast was directly influenced in Fig. 7(c). As Typhoon Bolaven landed in Jangsu-ri, Gangnyeong-gu, Hwanghae-do while maintaining its strong winds, the entire west coast was directly affected as it moved north, as shown in Fig. 7(d) to 7(g). As shown in Fig. 7(h), the intensity of Typhoon Bolaven weakened after landing. It subsequently transitioned into an extratropical cyclone on land, approximately 220 km north of Ganggye, Jagang-do, North Korea, at 06:00 on August 29, 2012.
3.2 Wave Field
Fig. 8 demonstrates the storm wave height at three-hour intervals from 00:00 on August 28 when Jeju Island was under direct influence of Typhoon Bolaven to 16:00 when the typhoon landed in Jangsu-ri, Gangnyeong-gun, Hwanghae-do and 21:00 when it passed through the land. Table 4 shows maximum Hs and the wind speed based on JMA-MSM at time points that correspond to Fig. 7(a) to 7(h). Before the landing of Typhoon Bolaven, the location with maximum Hs was not significantly different from the center of the typhoon.
Significant wave field simulated by SWAN model during direct and indirect impacts of Typhoon Bolaven on Korean Peninsula
Maximum significant wave height and its location, along with wind speed at that location during Typhoon Bolaven
From Fig. 8, it can be seen that (a) Hs of more than 10 m occurred near the south coast of Jeju Island when Typhoon Bolaven passed through the sea at 170 km to the southwest of Seogwipo, and (b) Hs was more than 11 m when the typhoon passed through the sea at 120 km to the west-southwest of Seogwipo. (c) Hs on the south coast of Jeju Island slowly decreased as the typhoon passed through the west coast of the island and entered the sea at 120 km to the southwest of Mokpo. In addition, Hs of 7 to 8 m occurred in Shinan, an island included in Jindo, as the typhoon entered the West Sea. As the typhoon moved to the north along (d) the sea at 100 km to the northwest of Mokpo, (e) the sea at 80 km to the west of Seosan, and (f) the sea at 120 km to the west-northwest of Seoul, Hs of 6 to 7 m was observed on the adjacent west coast. After Typhoon Bolaven landed in Jangsu-ri, Gangnyeong-gun, Hwanghae-do at 16:00 on August 28, 2012, the wave height on the south coast of the Korean Peninsula, including Jeju Island, significantly decreased when the typhoon reached (g) the land at 120 km to the south of Pyongyang and (h) the land at 30 km to the southwest of Pyongyang. High waves, however, existed in the West Sea over a considerable period even after the landing of the typhoon. Meanwhile, as the typhoon slowly moved to the north, the storm wave height significantly increased on the southeast coast as well as the southwest coast. In addition, after the landing of Typhoon Bolaven, the wave height in the northern part of the East Sea tended to increase under the indirect influence of the typhoon. This is because wind waves continuously develop within the influence of Typhoon Bolaven, which has a large radius of strong winds as in Table 3.
3.3 Maximum Storm Wave Height
Fig. 9 shows the spatial distribution of (a) the maximum wind speeds obtained from the JMA-MSM wind field of Typhoon Bolaven and (b) the maximum significant wave heights of storm waves simulated in the SWAN model by applying the maximum wind speeds. In other words, the figure shows the maximum wind speeds and significant wave heights between 18:00 on August 26, 2012, when the typhoon directly or indirectly affected the Korean Peninsula, to 09:00 on August 29, when it transformed into an extratropical cyclone on land.
Spatial distributions of maximum (a) wind speeds and (b) significant wave heights around Korean Peninsula during Typhoon Bolaven
Fig. 9 suggests that the sphere of influence of the strong winds above 20 m/s by Typhoon Bolaven almost coincides with the area where high waves of more than 5 m are distributed. Hs of more than 6 m is distributed over a very wide area in Jeju Island, the Southwest Sea, and the West Sea, which are located in the track of the typhoon. Since the area to the north of Jeju Island is a shielded area, Hs is not relatively high in Chujado, Jindo, and Haenam. Although Typhoon Bolaven moved to the north with no significant decrease in intensity, Hs ranged from 6 to 8 m due to the relatively shallow depth of the West Sea. This is also because the fetch length, duration, and wind direction were changed under the influence of land on the eastern side of the typhoon as it moved north in the West Sea. Although the intensity of the typhoon significantly decreased when it landed in Jangsu-ri, Gangnyeong-gun, Hwanghae-do at 16:00 on August 28, 2012, Hs of 5 to 5.5 m was relatively widely distributed as the northern part of the East Sea entered the sphere of indirect influence.
4. Discussion
4.1 Deepwater Design Waves
To assess the size of the storm waves caused by Typhoon Bolaven, they are compared with the 50-year return period deepwater design waves proposed by the Ministry of Oceans and Fisheries (MOF, 2019). These design waves are used for coastal and port planning, as well as for structure design in Korea. MOF (2019) revised the deepwater design waves in 2019 to reflect marine characteristics such as sea level rise caused by global warming, extreme weather events, changes in typhoon intensity, and the occurrence of abnormal waves. Fig. 10 shows the major coastal GPs used by MOF (2019) to provide deepwater design wave specifications around the Korean Peninsula. MOF (2019) provides deepwater design wave specifications (wave height, period, and wave direction by frequency) at a total of 210 major coastal GPs in the East Sea, the West Sea, and the South Sea, including islands.
Distribution of major coastal grid points (GPs) around Korean Peninsula, which are used to provide design wave specifications in deepwater, as described in MOF (2019)
The design wave specifications (MOF, 2019) show that GPs corresponding to the South Sea, typically passed through by typhoons invading the Korean Peninsula, range from GP63 to GP109. The average maximum wave heights among the design waves for each GP in this region is 11.67 m, which is higher compared to the East Sea. In the East Sea, corresponding to GP110 to GP152, the average maximum wave height is 9.43 m. Overall, the wave directions for maximum wave heights at GP63 to GP91, located to the west of the South Sea, are predominantly S, SSW, ESE, and SE-series, due to the presence of Jeju Island to the south. At GP92 to GP109, in the remaining area of the South Sea (the eastern part), corresponding to the Korea Strait, the S and SSW series exhibit maximum wave heights. In the southern part of the East Sea, at GP110 to GP121, the S and SSW series also show maximum wave heights, similar to the eastern part of the South Sea. In the remaining part of the East Sea (the northern part), from GP122 to GP152, N and NNE-series design waves have the maximum wave heights. For Jeju Island, GP170 has the highest design wave height of 14.5 m among the GPs, while the northern points GP180 and GP181 have heights of 7.9 m, indicating significant differences by area. Various wave directions are also observed. For Ulleungdo, the average maximum wave heights at GP203 to GP206 is 10.25 m, with N and NNE-series wave directions being dominant. For Dokdo, the average maximum wave heights at GP207 to GP210 is 10.5 m, with SSW and N-series wave directions observed. In the West Sea, corresponding to GP1 to GP62, the average maximum wave height is 8.04 m, with GP62 on the southwest coast having the highest design wave height of 11.7 m. Overall, S, SSW, and SW-series wave directions have maximum wave heights. At GP1 to GP4 in the northern part of the west coast, the NW series show maximum wave heights. At GP187 to GP190, corresponding to the islands in the northern part of the West Sea, including Nando, Seokdo, Woobaedo, and Gyeokryeolbi Yeoldo, design waves with an average height of 8.7 m are observed, with the main wave directions being SSW series. At GP191 to GP202, corresponding to the islands in the southern part of the West Sea, including Heuksando, Hongdo, and Gageodo, the design wave height at GP197 in the south is the highest at 11.9 m, with S and SSE-series wave directions being dominant.
4.2 Comparison with Design Waves
Fig. 11 and 12 compare the maximum storm wave heights during the invasion of Typhoon Bolaven, as estimated from the SWAN model, with the design wave heights provided at the major coastal GPs around the Korean Peninsula. Fig. 11 shows the comparison results for the coastal GPs in (a) Jeju Island and (b) the South Sea of Korea, which are located in typical typhoon tracks. Fig. 12 shows the results for the coastal GPs in (a) the islands in the northern part of the West Sea, (b) the entire west coast, and (c) the islands in the southern part of the West Sea.
Comparison between design wave heights for 50-year return period and maximum storm wave heights at coastal GPs on (a) Jeju Island and in (b) South Sea of Korea
Comparison between design wave heights for 50-year return period and maximum storm wave heights at coastal GPs on (a) Northern islands, (b) Southern islands, and (c) West Sea of Korea
The storm wave heights at the coastal GPs of Jeju Island (GP153 to 186) and the South Sea (GP63 to 109) in Fig. 11 correspond to 70% to 94% and 47% to 89% of the design wave heights, respectively. In addition, the storm wave heights of Jeju Island and the south coast compared to the design waves are 83.49% and 71.14% on average. In Fig. 12, the storm surge heights for the islands in the northern part of the West Sea (GP187 to 190), the entire West Sea (GP1 to 62), and the islands in the southern part of the West Sea (GP191 to 202) are 88% to 91%, 42% to 86%, and 74% to 87%, respectively, compared to the design wave heights at the coastal GPs. The average values for these areas correspond to 89.55%, 79.55%, and 71.05% compared to the design wave heights, respectively.
4.3 Storm Wave Analysis
Fig. 11 and 12 compare the storm waves generated by Typhoon Bolaven with the design waves. Table 5 provides the wave heights, periods, and wave directions at coastal GPs where the storm waves correspond to more than 90% of the design wave heights.
Storm wave specifications at coastal GPs exceeded 90% of design wave heights for 50-year return period during Typhoon Bolaven
Typhoon Bolaven slowly moved north at a distance from the west coast while maintaining its strong intensity until it landed. Consequently, for the islands in the northern part of the West Sea, which have relatively low design wave heights, storm waves with an average height of 7.82 m, corresponding to approximately 90.22% of the design wave heights, occurred at GP187 to 190, except for GP188 (125.583°E, 36.5°N). During the invasion of Typhoon Bolaven, the average wave height reached 11.57 m at GP157 to 162, to the southwest of Jeju Island, which corresponds to 91.48% of the design wave heights.
The storm waves of Typhoon Bolaven that exhibit more than 90% of the design wave heights occur at the coastal GPs of the islands in the northern part of the West Sea and Jeju Island. Storm wave heights on the southern coast of Jeju Island are high because the coast is located in the dangerous semicircle of the typhoon, and thus the movement speed and rotation speed of the typhoon are combined, making the wind stronger. Moreover, the southern sea of Jeju Island has few obstacles such as islands. This further develops wind waves by ensuring the fetch length and duration of the typhoon wind. Historically, there have not been many typhoons that entered the West Sea of the Korean Peninsula, and their intensity rapidly decreased as they moved to higher latitudes. The design wave heights of the islands in the northern part of the West Sea are not high compared to the coastal GPs in other areas, possibly for this reason. Therefore, the storm waves of Typhoon Bolaven, which slowly moved north while maintaining its intensity, reached 90% of the design wave heights at the coastal GPs of the islands in the northern part of the West Sea.
Kossin (2018) reported that global warming has weakened tropical circulation in summer, thereby decreasing the movement speeds of typhoons by 10% worldwide. Accordingly, the movement speed of typhoons striking the Korean Peninsula has slowed compared to the past, leading to the development of more storm waves in the East China Sea. If these storm waves enter the waters of the Korean Peninsula, coastal areas will inevitably suffer increased typhoon damage. The World Meteorological Organization (WMO) revealed that four major climate change indicators―greenhouse gas concentrations, sea level rise, seawater temperature rise, and ocean acidification―set new records. Notably, the sea level rose by an average of 4.5 mm each year from 2013 to 2021, with the global mean sea level reaching its peak in 2021. The increase rate between 1993 and 2002 more than doubled, primarily due to the loss of ice from glaciers. This sea level rise significantly affects hundreds of millions of coastal residents and increases vulnerability to the storm surges caused by tropical cyclones (WMO, 2022). Therefore, the impact of climate change will significantly increase the vulnerability of coastal areas, considering the uncertainties of future predictions. This could have serious regional and global impacts from social and economic perspectives. Consequently, it is necessary for researchers specializing in coastal engineering to accurately analyze potential risks and develop countermeasures to reduce the damage caused by coastal disasters caused by abnormal climate conditions.
5. Conclusions
In this study, storm waves were estimated for Typhoon Bolaven, which damaged Jeju Island and the west coast of Korea among the large typhoons that invaded the west coast. Storm waves were simulated by applying the JMA-MSM sea surface wind, the reanalysis weather field of Typhoon Bolaven, to the third-generation wave model SWAN ver. 41.31A. Those reliability of this model was verified through a comparison with wave observation data. The storm wave characteristics of Typhoon Bolaven, which slowly moved north at a distance from the west coast while maintaining strong intensity before landing in Jangsu-ri, Gangnyeong-gun, Hwanghae-do at 16:00 on August 28, 2012, are as follows:
(1) Comparison of Calculated and Observed Values: A comparison between the calculated and observed values for the time distribution of significant wave height and significant wave period by Typhoon Bolaven showed that the trends in the increase and decrease of the significant wave height and period at each observation point were generally well reproduced. Although the maximum significant wave heights estimated at the points, other than the two points with missing peak values, slightly underestimated observed wave heights, they showed significant quantitative accuracy.
(2) Radius of Strong Winds: Considering the radius of strong winds of Typhoon Bolaven, the entire southwest and west coasts of Korea, beginning from Jeju Island, were under the direct influence of the typhoon as it approached the Korean Peninsula. After landing, the typhoon weakened into an extratropical cyclone on land, 220 km north of Ganggye, Jagang-do, North Korea, due to ground friction.
(3) Sphere of Influence: The sphere of influence of strong winds above 20 m/s by Typhoon Bolaven almost coincided with the area where high waves of more than 5 m were distributed. Maximum significant wave heights occupied a very wide area in Jeju Island, the Southwest Sea, and the west coast, which were located in the track of the typhoon. Although the intensity of the typhoon significantly decreased when it landed in Jangsu-ri, Gangnyeong-gun, Hwanghae-do, at 16:00 on August 28, 2012, significant wave heights of 5 to 5.5 m were distributed as the northern part of the East Sea entered the sphere of indirect influence.
(4) Comparison with Design Waves: At the major coastal GPs of islands, the storm wave heights by Typhoon Bolaven exceeded 90% of the deepwater design waves of the 50-year return period. Storm wave heights corresponding to 90.22% of the design wave heights occurred at GP187 to 190 (islands in the northern part of the West Sea), and those with an average of 11.57 m, corresponding to 91.48% of the design wave heights, occurred at GP157 to 162 (southwest of Jeju Island).
Global warming decreases the movement speed of typhoons worldwide by weakening tropical circulation in summer. An increase in the climate crisis and extreme weather events will increase the uncertainties of future predictions and significantly increase the vulnerability of coastal areas. Against this backdrop, it is necessary to improve the accuracy and speed of storm wave and storm surge predictions to reduce coastal disasters during typhoon invasions. In future research, the reliability of storm wave estimation will be improved through comparisons with wave observation data from past typhoon invasions into the waters around the Korean Peninsula. Moreover, the speed of wave prediction systems will be improved through machine learning techniques, as demonstrated by Kim and Lee (2023a; 2023b), by constructing big data on storm waves.
Notes
Woo-Dong Lee serves as an editorial board member of the Journal of Ocean Engineering and Technology. However, he was not involved in the decision-making process for the publication of this article. Additionally, no potential conflicts of interest related to this article have been reported.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (RS-2022-00144263).