Hindcasting of Storm Surge by Typhoon 0314(Maemi) in Masan Bay, Korea

Transcription

1 Hindcasting of Storm Surge by Typhoon 0314(Maemi) in Masan Bay, Korea See-Whan Kang 1 ;Kicheon Jun 1 ; Kwang-Soon Park 1 ; Sung-Dae Han 2 1 Coastal Engineering Research Division, Korea Ocean Research and Development Institute Ansan P.O. Box 29, Gyeonggi, Korea swkang@kordi.re.kr ; kcjun@kordi.re.kr; kspark@kordi.re.kr 2 Department of Civil Engineering, Kyoungnam University 449 Wolyong, Masan, Kyoungnam, Korea sdhan@kyungnam.ac.kr Typhoon 0314(Maemi) landed on the south coast of Korea at 21LST, 12 September, 2003, where its storm surge accompanying with high waves and astronomical high tide induced the most severe coastal disaster in Korea. Especially Masan City facing Masan Bay had the worst damages by the storm-surge flooding with the maximum surge height of ~2.3m recorded at Masan Port. In order to obtain the design surge-heights for the coastal structures in Masan City, a hindcasting study of storm surge for Typhoon 0314 were conducted using a fine-scaled regional surge model which was connected to the larger scaled surge models by a nesting grid system. The sea surface pressure and wind fields were computed by a primitive vortex wind model to obtain the input data for storm surge model simulation. The hindcasted surge heights were compared with the observed surges at Masan Port and also were compared with the surge data recorded at the major tide stations in the south coast of Korea. The result shows that the computed surges were in a good agreement with the observations. Keywords: Typhoon 0314(Maemi), storm surge, coastal disaster, Masan Bay, hindcasting, fine-scaled surge model, vortex wind model 1. INTRODUCTION Typhoon 0314(Maemi) landed on the south coast of Korea at 21LST, 12 September, 2003, where its storm surge accompanying with high waves and astronomical high tide induced the most severe coastal disaster in Korea, causing the loss of 130 lives and the property damage of 4.8 trillion won (~5 billion US dollars) which is the largest disaster since Typhoon Sarah in Especially Masan City facing Masan Bay had the worst damage by the storm-surge flooding, killing 32 peoples and inflicting severe coastal damage. During the typhoon passage with a central pressure of 950hPa and a progression speed of 45km/hr, the maximum surge height of ~2.3m was recorded at a tide gage of Masan Port. The residential and commercial area facing to the Masan Bay was heavily flooded and the almost underground facilities suffered from the inundation by the storm surge (Yasuda et al 2005).

2 A hindcasting study of storm surge for Typhoon 0314 was conducted by Kawai et al (2004). The hindcasted surges in Masan Bay were much underestimated comparing with observation. In the present study, a hindcasting study of storm surge for Typhoon 0314 were conducted using a fine-scaled regional surge model which was connected to 3 larger different scaled surge models by a nesting grid system. The sea-surface pressure and wind fields were computed by a primitive vortex wind model (Cardone et al, 1992; Kang et al. 2002) for the input data of storm surge simulation. The hindcasted surge heights were compared with the observed surges at Masan Port and also were compared with the surge heights recorded at the tide stations of Yeosu, Tongyoung, Busan and Ulsan in the south coast of Korea. 2. TYPHOON MAEMI AND MASAN BAY 2.1 CHARACTERISTICS OF TYPHOON MAEMI Figure 1 shows the track of typhoon Maemi. Typhoon Maemi was generated on 6 Sep., 2003 at 25 N and 140 E, and moved in the NW direction. Its direction course changed to NNE after passed Miyakojima Island, Okinawa on Sep.11, and approached to Korean Peninsula. The typhoon passed by Jeju Island and landed on the south coast at 21LST, 12 September, At the landing, the central atmospheric pressure was 950hPa and the progression speed was approximately 45km/h. Then, Typhoon Maemi passed across the Korean Peninsula to East Sea in the early morning on Sep. 13. Fig. 1 Typhoon No. 14 (Maemi) Track 2.2 MASAN BAY Masan Bay is located in about 50km west of Busan as shown in Figure 2, which is a long and slender bay with 2.5km in width and 8km in length, and a bay mouth opens to SSE

3 direction. An average depth of the water in the bay area is about 10 meters. The distance from the Gadeog Channel to Masan Bay mouth is about 25km. It is considered that waves are hard to progress into the bay because the bay mouth is narrower and Masan Port was considered as a good natural harbor. Actually, the Masan Bay had not encountered the damage so far since the opening of Masan Port in a century. An urban area of Masan City lies along the closed-off section of the bay, and harbors are constructed in the coastal area by reclamation. Tid l Fig. 2 Location of the Masan Bay 2.3 OBSERVED WATER-LEVEL DATA While Typhoon Maemi was passing by the southern coast near to Masan Bay, the level of the astronomical tide reached near the high tide of a spring tide. Figure 3 shows the time variation of water level measured at the Masan Port tide station. The tide record indicates that the water-level reached approximately 4.3m above the datum line (chart design level), at which the high tide and the storm surge occurred simultaneously. The astronomical tide level was ~2.0m and then the storm surge deviation became ~2.3m.

4 (C.D.L., m) Tide level tide (Astro.+Storm Surge) Astronomical tide Time (hr) (9/11/03) (9/12/03) (9/13/03) Fig. 3 Time Variation of Water-Level at the Masan Port Tide Station 3. HINDCASTING OF TYPHOON PRESSURE AND WIND FIELD The sea-surface wind and pressure fields for Typhoon Maemi were hindcasted by using the primitive vortex model (PVM) which was developed by Chow (1971) and modified by Cardone et al. (1992). This model is based on the equation of horizontal motion, vertically averaged through the depth of the PBL. Kang et al. (2002) conducted a comparison study of typhoon wind models for 64 typhoon cases for In this study the typhoon wind fields were simulated by the both of primitive vortex model (PVM) and typhoon parametric model (TPM). The hindcasted sea-surface winds of the two models were compared with the typhoon winds observed at JMA ocean buoys (22001 and 21002) and Kyushu ocean observation tower. The analysis of RMS and relative errors between hindcasted and observed winds was made to find the accuracy and sensitivity of the typhoon wind models. The hindcasted winds of TPM and PVM both underestimate the observed typhoon winds but PVM winds were much closer to the observations with less RMS and relative errors. To hindcast the pressure and wind fields of Typhoon Maemi, the computational grid of the PVM was used by a moving grid system (Thompson et al. 1996) of rectangular nests, which provides relatively fine grid spacing of 2km near the typhoon inner region and coarse spacing in the outer region. Fig. 4 shows the wind and pressure fields simulated by the PVM wind model for Typhoon Maemi. As shown in Fig. 5, the hindcasted winds were compared with the observed winds at Ieodo Ocean Station during the passage of Typhoon Maemi. They

5 are generally in a good agreement although the hindcasted winds were slightly underestimated in the peak wind region. Typhoon: Maemi(0314) Date: 2003/09/12/09(LST) Vector Scale 40m/s Typhoon: Maemi(0314) Date: 2003/09/12/15(LST) Vector Scale 40m/s Korea Korea Japan Japan (a) 2003/09/12 09LST (b) 2003/09/12 15LST Typhoon: Maemi(0314) Date: 2003/09/12/21(LST) Vector Scale 40m/s Typhoon: Maemi(0314) Date: 2003/09/13/03(LST) Vector Scale 30m/s Korea Japan Korea 980 Japan 980 (c) 2003/09/12 21LST (d) 2003/09/13 03LST Fig. 4 Surface Wind and Pressure Fields Simulated by PVM Wind

6 * Ieodo Wind Speed (m/sec) Wind Dir. (deg) /11 9/12 9/ /11 9/12 9/13 September, Fig. 5 Comparision of PVM Wind with Observation at Ieodo Station for Typhoon Maemi 4. STORM SURGE SIMULATION The storm surges during the passage of Typhoon Maemi were simulated by the KORDI's storm surge model (Park et al. 2000) including the terms of the curvature of the earth and Coriolis force, which was established on the spherical coordinate system to cover the whole region of typhoon past tracks. Assuming the vertical acceleration is negligible as if the vertical distribution of currents is uniform, the depth-integrated continuity and momentum equations can be expressed as follows: η 1 + t R cosφ λ U g η + = fv t R cosφ λ P τ bλ τ s φ λ ρh ( HU ) + ( HV cosφ) = 0 φ ρr 1 cos V g η P τ b + = fu 1 t R φ ρr φ φ τ ρh sφ λ (1) (2) (3) Where λ, φ and t are the longitude, latitude and time, respectively. η, U and V are the sea surface elevation, depth averaged velocities. R is the radius of the earth, H is total depth, ρ is density of sea water; is gravity, is pressure, is Coriolis g P f

7 parameter, τ s and τ b are stresses induced by wind and bottom friction which are set to be quadratic forms (Park et al. 2000). The governing equations, Eqs. (1) (3), are numerically analyzed by using the fractional step method (Chorin 1968). Those terms are finitely differentiated to the form of a tridiagonal matrix by using ADI implicit method on the staggered grid system. For initial conditions, all variables including the sea level and the current vectors are set to be zero. For boundary conditions, the normal components of current vectors are set to zero at the land boundaries. At open boundaries, the sea levels are calculated from the hydrostatic equation using sea surface pressure. The storm surge model was simulated by using a nested grid system of 4 different grid sizes(table 1). A large-scale model with the grid interval of 1/12 ( 10 km), a mid-scale model with the grid interval of 1/60 ( 2 km) and a small-scaled with 250m and a fine-scaled with 50m were established for the Korean seas including its coastal waters. Fig. 6 shows grid system of the seas around the Korean Peninsula for the storm surge model. Two fine-scaled grids were located on the mid-western and east-southern coasts of Korea. The nested grid system and boundary conditions for simulating storm surge using a regional fine-scaled surge model are shown in Table 1. This surge model has been developed to produce long-term estimation of storm surge in the Korean coast (Kang et al. 2003). Fig. 6 Nested Grid System and Depth Contour for Storm-Surge Hindcasting

8 Table. 1 Nested Grid System and Boundary Conditions for Fine-Scaled Surge Grid system Large-scale Mid-scale Small-scale Fine-scale Domain Size Domain ~143 E 20 ~50 N 124 ~132 E 32 ~40 N ~ ~ ~ E ~35.21 N Interval 1/12 1/60 250m 50m Number Boundary Condition Static Equation Large-scale result Mid-scale result Small-scale result The results of storm surge hindcasting were compared with the observed surges at the 5 different tidal stations in the south coast of Korea. Fig. 7 shows the locations of the tide stations. Fig. 8 shows the comparison of the hindcasted storm surges with the observations. The hindcasted surges were generally in a good agreement with the observations at the major tide stations of Yeosu, Tongyoung, Masan, Busan, and Ulsan. Fig. 9 shows the distribution of the maximum surge in Masan Bay which was hindcasted by the previously described procedures of typhoon wind field and storm surge simulations. The maximum surge height computed by the model simulation was 226cm at the Masan Port, which was a well agreement with the observation of ~230cm. Masan Busan Ulsan Yeosu TongYoung Ieodo Fig. 7 Major Tide Stations in the South Coast of Korea

9 Surge (cm) Ulsan Surge (cm) Surge (cm) Surge (cm) -20 9/12 9/ Busan -40 9/12 9/ Masan /12 9/ Tongyoung /12 9/13 Surge (cm) Yeosu /12 9/13 Fig. 8 Hindcasting Storm Surges with Observations at the Tide Stations of South Coast

10 (a) small-scaled model A B C (b) fine-scaled model Fig. 9 Maximum Surge Heights(in cm) in Masan Bay, Hindcasted for Typhoon Maemi

11 5. CONCLUSION Typhoon Maemi caused the most severe coastal disaster in Masan City due to storm surge on September 12, The coastal disaster occurred in Masan Bay was a historical record since the opening of Masan Port in a century. According to the field survey of storm surge traces, which was investigated by Yasuda et al. (2005), the inundation water depth on the coast of Masan City was found in the range of 4.1~4.4m. Also the tide record at Masan Port indicates that the water-level reached approximately 4.3m above the datum line, and at that time the spring high tide and the storm surge occurred simultaneously. The storm surge deviation became approximately ~2.3m because the astronomical tide level was 2.06m. The hindcasted result of the maximum storm surges along the Masan coast was in the range of 2.1~2.26m as shown in Fig. 9. This result was in quite a well agreement with the observed surge height in Masan Port. However, it is necessary to improve further the regional, fine scaled surge model to hindcast more accurately and also to understand in more detail on the dynamics of storm surge propagation in a long, narrow and slender coastal bay. ACKNOWLEDGMENTS This study was conducted by the support of Top Brand Project Safe Coast of Korea Ocean Research and Development Institute (KORDI), and also The investigation and mitigation of coastal hazards in Masan Bay supported by Masan City Government. REFERENCES 1. Cardone, V.J., C.V. Greenwood, and J.A. Greenwood, 1992: Unified program for the specification of hurricane boundary layer winds over surfaces of specified roughness, Contract Rep. CERC-92-1, U.S. Army Engrs. Wtrwy. Expreriment Station, Vicksburg, Miss. 2. Chorin, A. J., Vortex Sheet approx. of boundary layer, J. of Comput. Phys., 27: Chow, S.H, 1971: A study of the wind field in the planetary boundary layer of a moving tropical cyclone, MS thesis, School of Engrg. and Sci., New York Univ., N.Y. 4. Kang, S.W., K.C. Jun, K.S. Park, and G.H. Bang, 2002: A Comparison of Typhoon Wind s with Winds. J. Korean Soc. Oceano., 7(3): Kang, S.W., K.C. Jun, K.S. Park, and D.Y. Lee, 2003: Long-term Estimation of Storm Surge in the Korean Coast, Coastal Hazard 2003, Japan, G1-G8. 6. Kawai, H., T. Tomita, T., Hiraishi, D.S. Kim, and Y.K. Kang, 2004: Hindcasting of storm surge by typhoon 0314(Maemi), Waves and Storm Surges around Korean Penninsula, Workshop Proceeding on 19 August, 2004, KSCOE.

12 7. Park, K. S. et. al., 2000: Mitigation Engineering Development for sea and Coastal Hazards(II). Technical Report N A-02, Korea Ocean Research and Development Institute. p (in Korean). 8. Thompson, E. F., and V. J. Cardone, 1996: Practical modeling of hurricane surface wind fields, J. of Waterway, Port, Coastal and Ocean Engineering. 122(4): Yasuda, T., T. Hiraishi, H. Kawai, K. Nagase, S.W. Kang, and W.M. Jeong, 2005: Field survey and computation analysys of storm surge disaster in Masan due to Typhoon Maemi, Proceeding of Asian and Pacific Coasts 2005, Jeju, Korea.