Causes of the Unusual Coastal Flooding Generated by Typhoon Winnie on the West Coast of Korea

Natural Hazards 29: 485–500, 2003. 485 © 2003 Kluwer Academic Publishers. Printed in the Netherlands. Causes of the Unusual Coastal Flooding Generated by Typhoon Winnie on the West Coast of Korea I.-J. MOON1,I.S.OH2,T.MURTY3 and Y.-H. YOUN4 1University of Rhode Island, Graduate School of Oceanography, Narragansett, RI 02882, U.S.A. (E-mail: [email protected]); 2Seoul National University, Department of Oceanography and Research Institute of Oceanography, Seoul 151-742, Korea; 3W. F. Baird & Associates, Coastal Engineers Ltd., Ottawa, Canada; 4Korea Meteorological Administration, Meteorological Research Institute, Seoul 151-742, Korea (Received: 30 October 2000; accepted: 11 February 2002) Abstract. On 19 August 1997 Typhoon Winnie brought unusually strong and extensive coastal flooding from storm surges to the west coast of Korea, which was far enough from the typhoon’s center to lack significant local wind and pressure forcing. Sea levels at some tidal stations broke 36-year records and resulted in property damages of $18,000,000. This study investigated the causes of the unusual high sea levels by using an Astronomical-Meteorological Index (AMI) and a coupled ocean wave-circulation model developed by the present authors. The AMI analysis and the numerical simulation of the surge event showed that the major cause of the high sea levels was not the standard inverse barometric effect supplemented by water piling up along the coast by the wind field of the typhoon as is usual for a typical storm surge, but rather an enhanced tidal forcing from the perigean spring tide and water transported into the Yellow Sea by the currents generated by the typhoon. The numerical results also indicated that the transported water accounted for about 50% of the increased sea levels. Another cause for the coastal flooding was the resonance coupling of the Yellow Sea (with a natural normal mode period of 37.8 h) and the predominant period of the surge (36.5 h). Key words: coastal flooding, high sea levels, astronomical-meteorological index, coupled ocean wave-circulation model, tidal forcing, transported water, resonance coupling. 1. Introduction The number of typhoons passing adjacent to the Korean peninsula in summer of each year is about two or three on average. These events sometimes create great coastal flooding by storm surges, which has been one of the most serious threats to people living in these regions for the last half century or longer. Typhoon Sarah in 1959 attacked the south coast of the Korean peninsula and this disastrous event resulted in the loss of 849 lives and over $0.2 billion in property damage. (Oh et al., 1993). Further coastal flooding was experienced on these coasts on 30 August 1971 and 15 July 1987. All of these instances of coastal flooding were mainly caused by a pileup of sea water as a result of strong onshore wind and pressure effects due to the typhoon. However, the extensive coastal flooding of 19 August 1997 which happened along 486 I.-J. MOON ET AL. Figure 1. Track of Typhoon Winnie and location of tidal stations. the west coast of Korea was very unusual and not a typical event. The floods occurred at coastal regions not having any significant wind and pressure effects from the typhoon. Typhoon Winnie struck the east coast of China (Figure 1), but the inundated areas of the west coast of Korea were far enough from the typhoon’s center to lack significant local wind and pressure forcing. At 3:00 LST on 19 August, the sea levels at Kunsan and Mokpo, which are located along the west coast of Korea, recorded about 805 cm and 553 cm, respectively (Figure 2). The extraordinarily high values broke 36-year records for Kunsan and Mokpo, i.e., 783 cm and 516 cm (highest water levels prior to this event). The tidal levels, which are predicted by the National Oceanographic Research Institute (NORI), were 748 cm, and 503 cm at the same points. The residuals (surges) were 60 cm and 50 cm respectively at these two locations. The estimated damage from this event exceeded $18,000,000. Some farm land along the coast was submerged and a lot of seawalls and breakwaters were undermined. The damage was greater than usual in view of the unexpected nature and location of the flooding and no advance warning of potential flooding could be provided. Here we investigated the characteristics and causes of the unusual coastal flooding that occurred along the west coast of Korea on 19 August 1997. In Section 2, we evaluate the Astronomical-Meteorological Index (AMI), which is composed of an empirically obtained astronomical index and a meteorological UNUSUAL COASTAL FLOODING GENERATED BY TYPHOON WINNIE 487 Figure 2. Time series of (a) observed sea level, (b) predicted tidal level, and (c) residuals during the passage of Typhoon Winnie at Kunsan and Mokpo. index affecting coastal flooding as suggested by Wood (1978). The former includes the tidal effects of both the Moon and the Sun, and the latter, wind speed, its direction, and duration of overwater movement. In Section 3, a numerical simulation of the surge event was performed. As external forcing in the model, tides, winds, sea surface pressure, oceanic currents are considered. The numerical model used in the present study is a coupled ocean wave-circulation model, which is based on the synchronous two-way coupling of a third-generation wave model, WAVEWATCH-II and a three-dimensional Princeton Ocean Model, POM. The results of the simulation suggested that the volume transport into the Yellow Sea generated by Typhoon Winnie and resonance effects are the major causes of the coastal flooding and this has been discussed in Section 4. Summary and conclusion is given in the last section. 488 I.-J. MOON ET AL. Table I. The Astronomical-Meteorological Index (AMI) estimated at Inchon, Kinsan, Mokpo, and Cheju for 19 August 1997. Location Date Astronomical Meteorological Potential for (1977) parameters parameters tidal flooding ω−S V cos θD 34(±P) Index Intensity coefficient (kt) (h) AMI rating Inchon 8/19 82.187 4 0 23.8 62.387 Insignificant Kunsan 8/19 82.187 −3 0 0 79.187 Insignificant Mokpo 8/19 82.187 −2 0 13.6 66.587 Insignificant Cheju 8/19 82.187 −3 0 6.8 72.387 Insignificant Intensity rating scale: AMI ≥ 170 – Extreme AMI ≥ 120 – Moderate AMI ≥ 160 – Severe AMI ≥ 100 – Slight AMI ≥ 140 – Strong AMI < 100 – Insignificant 2. Astronomical and Meteorological Effects on the Flooding The astronomical-meteorological index (AMI) describing the active potential for coastal flooding can be used to investigate astronomical and meteorological causes of the flooding. AMI may be represented by the combination of the astronomical and the meteorological indexes (Wood, 1978). AMI = ω−S + V cos θ + D − 34(±P ). (1) (ω−S) is the astronomical index and the meteorological index is expressed by the wind speed V , its direction θ and duration D and the atmospheric pressure gradient P . The greater the amount by which the numerical value of this index exceeds 100 (representing an average condition), the greater is the potential for tidal flooding. Table 1 shows AMI calculated for 19 August 1997 at four coastal locations of Korea (Figure 1), which were inundated by high sea levels. For all locations, the astronomical index indicated very high values because this period corresponded to that of perigean spring tides, which have an enhanced tide-raising forcing resulting from the closest monthly approaach of the Moon to the Earth; the meteorological effects were weak even though Typhoon Winnie passed close to the adjacent seas of Korea. Typhoon Winnie moved from the north of Taiwan to the west and then ap- proached the east coast of China at 9:00 LST on August 18 as shown in Figure 1. Although the typhoon was strong enough to have wind velocities of 30–50 m/s, the wind speeds and pressure gradients recorded at coastal meteorological stations of Korea were not particularly high because the areas were far enough from the typhoon’s center. The maximum wind speeds during the passage of Typhoon Win- nie from 18 August to 21 August were 7 m/s, 6 m/s, 14 m/s, 9 m/s at Inchon, Kunsan, Mokpo, Cheju, respectively (Figure 3). The resultant meteorological effects on the high sea levels were not so significant as to produce the coastal flooding UNUSUAL COASTAL FLOODING GENERATED BY TYPHOON WINNIE 489 directly. Here we are particularly concerned with the indirect effects of the typhoon on the coastal flooding. This is achieved by a numerical simulation of the surge event in the next section. 3. Numerical Simulation of Storm Surge In this section we describe simulating storm surges generated by Typhoon Win- nie to investigate movement of seawater over the Yellow and East China Seas during the period of the coastal flooding. For this purpose a coupled ocean wave- circulation model is developed. The coupled model consists of a third-generation ocean wave model WAVEWATCH-II (Tolman, 1991) and a three-dimensional Princeton Ocean Model (POM). In the coupling scheme of the two models, WAVEWATCH-II uses the new currents and elevations fed back from POM to consider wave-current interactions (Tolman, 1990); POM uses a wave-dependent drag coefficient calculated from WAVEWATCH-II to consider the dependency of sea state (wave age) on the wind stress (Janssen, 1991; Mastenbroek et al., 1993; Zhang and Li, 1996). 3.1. MODEL DESCRIPTION WAVEWATCH-II is a third-generation wave model, which includes wave growth and decay, wind input, wave-wave interaction and dissipation due to whitecapping and wave-bottom interaction.

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