TropicalCyclonesAffectingPohang

TropicalCyclonesAffectingPohang


POHANG

TROPICAL CYCLONES

For the purpose of this study, any tropical cyclone approaching within 180 nmi of Pohang is considered to represent a threat to the Port. Table 1 contains a descriptive history of all tropical storms and typhoons passing within 180 nmi of Pohang during the 63-year period 1945-2007. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Pohang are based on the SAIC generated data set used to compile Table 1.

Tropical cyclones which affect Korea generally have the same genesis area as those affecting Japan: 5°N to 30°N and 120°E to 165°E. The latitudinal boundaries shift poleward during the summer months and then equatorward in winter in response to the seasonal changes of the synoptic environment. Most of the storms affecting Pohang move northwestward from the genesis area to the East China Sea or western Philippine Sea south of the island of Kyushu, Japan before recurving north or northeastward to within 180 nmi of Pohang. Note that two typhoons (Typhoon Sarah in September 1959 and Typhoon Maemi in September 2003) each passed within 15 nmi of Pohang when at CPA. Typhoon Sarah had a CPA of 8 nmi to the south-southeast and Typhoon Maemi had a CPA 15 nmi to the northwest. The tropical cyclones that have the most impact to the Pohang area are those tropical cyclones that pass just to the west, such as Typhoon Maemi. However, storms passing east of Pohang can cause damage to the Port by wave action and swell.

An examination of Table 2 shows that June through September is the primary season for tropical cyclone activity at Pohang, with 96% (101 of 105) occurring during that period. Of those, 89% (93 of 105) occurred from July through September. Although storms have occurred as early as May (one tropical storm on May 28th) and as late as October (three tropical cyclones), the overwhelming majority of the storms are confined to the June through September period. August is the month of greatest activity, with 45% (47 of 105) occurring during that month. August is also the month of greatest threat from typhoon-strength storms, as it has 52% (13 of 25) of the total typhoon occurrences.

Table 2 also shows the motion history of the 105 tropical storms and typhoons that passed within 180 nmi of Pohang during the 63-year period 1945-2007. The average movement for all storms when at CPA to the Port of Pohang is 031° at 21 kt. Average storm speeds are quite high at CPA and vary from 16 to 27 kt. June's 27 kt average speeds are the fastest.

Figure 30 shows the annual distribution in seven-day increments of the 105 storms by 7-day periods. The period of activity extends from 28 May until 12 October with peak in mid-August.

Figure 31 shows a chronology of the 105 tropical cyclones that passed within 180 nmi of the Pohang area in the 1945-2007 period. An average of 1.67 tropical cyclones per year passes within 180 nmi of Pohang. Historically, there were 25 tropical cyclones at typhoon strength while within the 180 nmi radius which equates to one every 2.5 years. Out of a total of 63 years there were only 10 years which had no tropical cyclones and a total of 12 years which had three or four tropical cyclones.

Figure 32 depicts the octants from which the 105 tropical cyclones in the data set approached the Port of Pohang. As the figure shows, a total of 67 (63.8%) approached from the south-southwest octant. This was largely due to Korea's position relative to the primary tropical cyclone storm track, and Pohang's position just north of the primary storm recurvature area on the southeast coast of Korea. Eighty-seven of the 105 storms (83%) were moving in a direction toward the northeast quadrant when at CPA which indicates that they had already recurved. Sixteen of the 105 storms were still moving towards the northwest quadrant when at CPA. It should be noted that the approach direction is determined at CPA, and may not represent the initial approach direction of the tropical cyclone toward the Port of Pohang.

Figure 33 and Figure 34 show the tracks of the 105 tropical cyclones split into two periods of time: July through August, and September through June. Be aware that the total number of storms on the two track charts adds up to 114 because some of the tropical cyclones overlap. A comparison of Figure 33 and Figure 34 shows that there is a distinct difference in the number of storms that pass east and west of the Port. During the September through June period there are a greater percentage of tropical cyclones, particularly typhoons, which pass to the east of the Port. Note that most of the storms have recurved or are in the process of recurving when they are at CPA to Pohang.

Figure 35 and Figure 36 are the statistical summaries of threat probability based on tropical cyclone tracks for the period 1945-2007. The summaries coincide with the tracks presented in Figure 33 and Figure 34. The data is presented with solid thin lines representing "percent threat" for the 180 nmi radius circle surrounding Pohang. The solid dashed lines represent approximate approach times to Pohang based on the climatological speeds of movement. In Figure 35 for example, a tropical cyclone located near 26°N 139°E has approximately a 20 % probability of passing within 180 nmi of Pohang during the July through August period. In addition, this tropical cyclone would reach the Port of Pohang in 3-4 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of the Port of Pohang. A comparison of Figure 35 and Figure 36 shows that there is a significant difference in threat axes according to time of year. The months of July and August ( Figure 35) have an axis that extends southward from Pohang to about 28°N near the Ryukyu Islands before slowly turning southeastward to the more tropical latitudes. The threat axis for the period from September through June, however, extends southwestward from Pohang to the east coast of the People's Republic of China at about 28°N before turning southeastward to the tropics.

Figure 37 depicts the primary threat axis of the 105 tropical storms and typhoons that passed within 180 nmi of Pohang during the period 1945-2007. The shaded arrow, pointing to the average location of CPA to the site, is a composite track zone for the 65 tropical storms and typhoons that crossed the dashed arc before entering Pohang's 180 nmi radius. The mean transit time from the arc to CPA for these 65 storms was 77 hours. However, the inset in the figure depicts a wide variation from the mean. The minimum transit time was 25 hours for Tropical Cyclone Ellis (1989) and the maximum was 146 hours for Tropical Cyclone Marge (1951).

Figure 38 shows the tracks of the 105 tropical cyclones that passed within 180 nmi of Pohang over the years 1945-2007 (all months combined). Circular markers along the track indicate positions of tropical cyclones 72 hours before reaching the closest point of approach to Pohang. Note that only two of the tracks did not extend back 72 hours.

WIND

In the vicinity of Pohang, the hills of Yeongil Peninsula (Figure 29) provide limited protection from easterly winds, and the Taebaek Mountains provide protection from strong west to northwesterly wind flow. The port is exposed and vulnerable to winds from north through northeast, and southeast through southwest.

The Pohang Vessel Traffic Service (VTS) Center located in the vicinity of Pier #6 has an anemometer and the wind measurements (Figure 26) are representative of those in the Port and the anchorage areas.

Due to the location of Pohang on the east coast of the rugged terrain of the Korean Peninsula, most of the tropical cyclones causing strong winds at Pohang pass east of the port, either through the Tsushima (or Korea) Strait or over the islands of southwestern Japan. As can be seen in Figure 39 completed during the previous port study, 15 of the 21 storms causing winds 22 kt at Pohang passed east of the port. The beginning and end points of the arrows in Figure 39 give the positions of tropical cyclone centers when sustained winds 22 kt began and ended at Pohang for tropical cyclones that passed within 180 nmi during the period 1973-1992. Pohang Weather Station personnel indicated that the maximum wind recorded from a tropical cyclone was 20 m/s (38.9 kt) sustained with gusts to 31.8 m/s (61.8 kt). Typhoon Thelma (July 1987), which was identified as one of the storms with the highest gusts, had a CPA of 72 nmi to the west-northwest and was heading 020 at 25.2 kt.

The Korean Meteorological Association and Busan Regional Maritime Affairs and Port Office have Severe Weather Alerts Criteria for issuing inshore and offshore advisories and warnings (Figure 40). Wind and Wave Advisories are issued with sustained winds 27.2-38.9 kt (14-20 m/s) for a duration of 3 hours on the sea surface or a wave height greater than 9.8-13.1 ft (3-4 m). Wind and Wave Warnings are issued with sustained winds 40.8 kt (21 m/s) or greater for a duration of 3 hours on the sea surface or a wave height greater than 16.4 ft (5 m). The Busan Regional Maritime Affairs and Port (Fisheries) Office will provide this info to the Pohang VTS who will pass the information to vessels initially on VHF Channel 16 and subsequently on VHF Channel 12.

WAVES

Ships in the anchorage are exposed to any wave motion from north through northeast. Local port authorities state that waves over 5 m have been noted outside the entrance to the man-made harbor, and waves of 6.5 to 10 ft (2 to 3 m) have been experienced inside the harbor entrance. The landmass of the Korean Peninsula effectively precludes the approach of any significant waves from east clockwise through northwest at Pohang.

The pilots indicated during the Site Visit in April 2010 that open ocean wave motion and long period swells have had a major impact at the piers. The pilots specifically stated that surging with up and down motion is a concern at the piers with a strong northeast swell entering the harbor. Although rare, the situation has occurred in which lines were parted and ships sustained damage at the piers due to tropical cyclones.

Additionally, the pilots have difficulties boarding vessels with waves over 6.6-9.8 ft (2-3 m). Generally, this occurs during late winter or early spring.

STORM SURGE

Storm surge may be visualized as a raised dome of water, moving with the storm, and centered a few miles to the right of its path. The dome height is related to local pressure (i.e., a barometric effect dependent on the intensity of the storm) and to wind stress on the water caused by local winds. Other significant contributing factors are storm speed, direction of approach, bottom topography, and coincidence with astronomical tide.

The worst combination of circumstances (Harris, 1963, and Pore and Barrientos, 1976) would include:
(1)An intense storm approaching perpendicular to the coast with the Harbor within 30 nmi to the right of the storm's track.
(2)Broad, shallow, slowly shoaling bathymetry.
(3)Coincidence with high astronomical tide.

The potential vulnerability of Pohang to storm surge is uncertain. Pohang's location relative to the normal track for tropical cyclone movement through the Tsushima (or Korea) Strait or across southwestern Japan would place the Port on the weaker, left side (with respect to the storm's direction of movement) of the storm's circulation, and reduce the duration and impact of the wind and its effects. Consequently, the criterion stated in paragraph (1) above would not likely be met. Also, the location of the Port on the relatively deep Sea of Japan precludes the broad, shallow, slowly shoaling bathymetry criterion stated in paragraph (2) above.

Local port authorities state that a minimal storm surge, in the range of 12 to 15 inches (30 to 40 cm), has been noted in the past.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/korea/pohang/text/tropical_cyclones.htm