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. Furthermore it incorporates effects of unsteady and inhomogeneous currents on ocean waves. The model was developed at NASA Goddard Space Flight Center and the updated versions of the model have been used for research and operational applications worldwide. The wave-dependent stress, which is used for coupling process, is calculated by using the wind profiles in the presence of waves based on the theory of Janssen (1991). A detail description is given by Tolman (1992) and Moon (2000). The POM used in the coupled model to simulate storm surges is a sigma- coordinate, free-surface, primitive equation ocean model, which contains the Mellor and Yamada (1982) turbulent closure submodel to provide vertical mixing coefficients and a mode-split technique for computational efficiency (Blumberg and Mellor, 1987). The POM used in the present study has three open boundaries to consider realistic ocean currents, the shelf break east of Taiwan (the Kuroshio), the Taiwan Strait (Taiwan current) and the Yangtze River. The volume transports at these open boundaries are shown in Figure 4. Along the open lateral boundaries, real-time tidal forcing and meteorological influences are specified according to a radiation condition due originally to Flather (1976). c U = U (s, t) + U (s, t) + (U · n/|U|) [η − η (s, t)], (2) n mean An H A where n is a unit outward normal to boundary, Un is the depth-averaged velocity normal to boundary, U is the depth-averaged velocity vector, subscript ‘s’ measures 490 I.-J. MOON ET AL.

Figure 3. Observed wind and sea surface pressure during the passage of Typhoon Winnie at Inchon, Kunsan, Mokpo, and Cheju. The maximum wind speeds represent 7 m/s, 6 m/s, 14 m/s, 9 m/s at each stations, respectively.

1/2 distance along the open boundary, c is (g/H ) , ηA denotes the sum of the tidal effect (ηT ) and the meteorological effect (ηM ). To consider the tidal effect, the real-time tides (ηT ) containing eight tidal constituents (M2,S2,K1,O1,K2,N2,P1 and Q1) are used. The coamplitude and cophase lines for the constituents simulated by the present model and their comparison with observations are given by Moon et al. (2000). To calculate the meteorological effect, which is particularly induced by pressure gradients, the hydrostatic law is assumed to apply at the open boundary of the model giving

ηM (x,y,t) = (pn − Pa(x, y, y))/ρw g, (3) where Pn is the mean atmospheric pressure taken to be 1,012 hPa and Pa is the atmospheric pressure at the sea surface at point x, on the open boundary. The wind and pressure ﬁelds at the sea surface during the passage of Typhoon Winnie, as UNUSUAL COASTAL FLOODING GENERATED BY TYPHOON WINNIE 491

Figure 4. Bathymetry of the Yellow and East China Seas and volume transport at the open boundaries and Yangtze River mouth. The unit for volume transport is Sv (= 106 m3/s). shown in Figure 5, were obtained from an analytic model for the radial proﬁles of pressure and winds proposed by Holland (1980).

3.2. RESULTS OF THE SIMULATION For initialization of the model, the monthly mean diagnostic calculation is first performed for 60 days using the monthly mean data and the real-time tides. After that, the model is run for 7 days by using the wind and pressure fields of Typhoon Winnie. Figure 6 shows the distributions of the surface currents simulated by the coupled model considering real-time tides, wind forcing, and oceanic currents during the passage of Typhoon Winnie. The maximum speed of the currents reached 1.5 m/s over strong tidal areas and around the typhoon’s center. Figure 7(a) shows spatial distributions of the simulated surge at 9:00 LST on August 18, which is estimated by filtering out tides (Figure 7(c)) from the total sea levels (Figure 7(b)). To eliminate the tides, a tidal model is run separately. The maximum surge of more than 120 cm was found along the east coast of China where Typhoon Winnie was headed for. The time series of the simulated surge are compared with the observed surge at two tidal stations of the west coast of Korea, Kunsan and Mokpo (Fig- 492 I.-J. MOON ET AL.

Figure 5. The wind and pressure ﬁelds obtained by the Holland (1980) model, which uses locations, center pressure and maximum wind speed of typhoon calculated from GFDK typhoon model of KMA on August 17–19, 1997. The unit for wind and pressure are m/s and hPa, respectively. ure 8). They are in reasonable agreement with RMS errors of 7.8 cm and 8.5 cm at Kunsan and Mokpo, respectively.

4. Volume Transport and Resonance Coupling To investigate water movement during the passage of Typhoon Winnie, the tide- filtered and depth-averaged current vectors at five points in the entrance to the Yellow Sea (Figure 9) are plotted in Figure 10. The current vectors show that there are continuous inflows into the Yellow Sea by the northward flow at all points until Typhoon Winnie approaches China (August 19). To investigate how much the in- flow contributes to the rising of sea level in the Yellow Sea, the net volume transport along the line (33.3◦N) across Cheju Island and China is estimated from the model (Figure 9). The results indicate that the transport accounts for the maximum rise of sea level to 38 cm at Kunsan (Figure 11). The time of the highest enhanced sea level, August 19 at 3:00 LST, corresponds to that of the highest observed surge. For the three surge events of Kunsan when relatively high sea levels occur, the enhanced sea levels explain 45%, 63%, and 52% of the surge, respectively (Table 2). The residual part of the surge can be explained by the resonance coupling of the natural periods of the Yellow Sea and the predominant period in the surge. Although the primary storm surge itself is associated with the passage of the typhoon, this can be followed by edge waves, continental shelf waves or other trapped waves. If one of these should produce a resonance with the natural period of the underlying water, the damage can be greater than that caused by the original storm surges, even though the storm center has passed. In this study, it is hard to find what kinds of waves are generated from this typhoon event due to the limitation UNUSUAL COASTAL FLOODING GENERATED BY TYPHOON WINNIE 493

Figure 6. Distributions of surface currents considering tides, wind forcing and ocean currents at (a) 0:00 h, (b) 3:00 h, (c) 6:00 h and (d) 9:00 h LST, 18 August 1997. The unit for currents is m/s. 494 I.-J. MOON ET AL. Spatial distributions of (a) surge, (b) total sea level, and (c) tidal level. Figure 7. UNUSUAL COASTAL FLOODING GENERATED BY TYPHOON WINNIE 495

Figure 8. Comparison between observed and simulated surge at (a) Kunsan and (b) Mokpo along the west coast of Korea during the passage of Typhoon Winnie.

Figure 9. Locations of selected points for investigating currents generated by Typhoon Win- ◦ nie. The volume transport along the line (33.3 N), which is indicated by a dotted line in the ﬁgure, across Cheju Island and China is calculated from the model. 496 I.-J. MOON ET AL.

Figure 10. Time series of current vectors generated by Typhoon Winnie at ﬁve selected locations.

Table II. Effect of water inﬂow into the Yellow Sea at Kunsan

Date Enhanced sea level Observed surge Residual by water transport

18 August 3:00 a.m. 20 cm (45%) 44 cm 24 cm 19 August 3:00 a.m. 38 cm (63%) 60 cm 22 cm 20 August 3:00 a.m. 26 cm (52%) 50 cm 24 cm

Average 28cm(53%) 51cm 23cm

of observed data. However, we can assume that the predominant periods of such waves strongly depend on the scale of typhoon, the speed of typhoon transition, and topography. By this assumption, we can simply estimate the predominant period of the surge generated from the passage of a typhoon as being expressed by

L T = sur , (4) sur V UNUSUAL COASTAL FLOODING GENERATED BY TYPHOON WINNIE 497 N) across Cheju Island and China and enhanced sea levels. The maximum ◦ The net volume transport estimated by the model along the line (33.3 Figure 11. rising of 38 cm (Figure 11) occurred when the surge was a maximum. 498 I.-J. MOON ET AL.

Figure 12. Time series of the surge simulated at point A1 by the model. This shows a period of about 38 hours.

where Lsur is the length scale of the typhoon and V is the speed of the typhoon transition. For the case of Typhoon Winnie, if we consider that Lsur is 750 km (based on mean distributions of wind and air pressure) and V is 20.5 km/hour (based on mean transition speed of Typhoon Winnie), the predominant period of the surge will be about 36.5 hours. The time series of the surge (Figure 12) simulated by the coupled model at point A1 also shows a dominant period of 38 hours similar to those of the surge calculated from Equation (4). On the other hand, the Yellow Sea and East China Sea (YECS) are semi- enclosed marginal seas of the northwestern Paciﬁc Ocean, surrounded by the Korean Peninsula, Chinese coast, and Ryukyu Islands. The YECS have a shallow and wide continental shelf and a steep continental slope adjacent to the Okinawa Trough in the southern part of the East China Sea (Figure 4). If we assume that the Yellow Sea is an open channel closed at one end with a length of 820 km and depth of 60 m (Figure 9), the natural period of the channel, Tn,aregivenby

4L 4 · 820000 Tn = √ = √ , (5) n gh n · 9.8 · 60 where n is the number of nodes (n = 1,3,...),L is the length of the channel, g is the gravity acceleration, and h is the water depth. For n = 1andn = 3, the natural period, T1 and T3, are 37.8 hours and 12.6 hours, respectively. Here, the period (T3) having three nodes is known to reinforce tides by a resonance with semi-tidal period, resulting in producing strong tides in this area (Choi, 1980). The natural period (T1) having one node is very close to the predominant period (Tsur) of the surge generated from Typhoon Winnie. This resonance phenomenon between the surge and the natural period could enhance the sea levels along the west coast of Korea. UNUSUAL COASTAL FLOODING GENERATED BY TYPHOON WINNIE 499

5. Summary and Conclusion The causes of the coastal flooding that occurred along the west coast of Korea on 19 August 1997 were investigated by using an Astronomical-Meteorological Index (AMI) and a coupled ocean wave-circulation model. The AMI analysis and the numerical simulation of the surge event showed that the major cause of the flooding was not the primary storm surge itself associated with the passage of Typhoon Winnie, but rather the enhanced tidal forcing from the perigean spring tide, coupled with the water transported into the Yellow Sea by the currents generated by the typhoon, and the resonance coupling of the natural period of the Yellow Sea (37.8 h) and the surge (36.5 h). The west coast of Korea is one of the strongest tidal areas in the world. The tidal range is about 4 m on the southern part and it increases to about 10 m on the northern part. During the perigean spring tides, the tidal level is significantly enhanced. If the astronomically enhanced tide levels coincide with the passage of a cyclone, whether the storm center has passed or not, the sea levels can be higher than when the tides are average. This coastal flooding on Korea’s West Coast is a good example of the extent of serious damage which can occur due to combination of strong tidal forcing, water transport, and resonance effects at regions being far from the typhoon’s center.

Acknowledgement This work was supported by postdoctoral fellowship program from Korea Science & Engineering Foundation (KOSEF).

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