TropicalCycloneAffectingBucknerBayNaha
NAHA, OKINAWA
TROPICAL CYCLONE AFFECTING BUCKNER BAY/NAHA
Tropical Cyclone Climatology for Buckner Bay/Naha Harbors
Refer to paragraph 7.3.1 of the Buckner Bay Port Study for the tropical cyclone climatology of Buckner Bay/Naha harbors.
Wind and Topographical Effects
Maximum winds can be expected from the northwest at Naha since the Harbor opens to the ocean in this direction. Thus, tropical cyclones to the north of Okinawa can cause severe problems for Naha.
Wind measuring instruments are locally located on the northwest corner of the channel entrance. Winds are accurately represented with the exception of easterly winds that are blocked by high rise buildings.
To determine the extent to which threat tropical cyclones produced strong winds (22 kt) or gale force winds (34 kt) in Naha Harbor, the wind observations from Kadena Air Base (26°21'N, 127°45'E) at an elevation of 152 ft were analyzed (refer to paragraph 7.3.2 of the Buckner Bay Port Study). Since both Naha Harbor and Kadena Air Base are located on Okinawa's western coastline and the surrounding topography is similar, winds recorded at Kadena Air Base are representative of wind conditions experienced in Naha Harbor.
Wave Action
Wave action in Naha Harbor area is severe enough to halt all traffic with the onset of 25 kt or greater winds. Although ships have been moved in winds up to 50 kt during emergency conditions, wave action in the harbor can be destructive enough to necessitate clearing the port of all vessels when winds of 50 kt or greater are expected within 24 hours. The pilots stated during our site visit in June 2000 that they could normally board ships with winds less than or equal to 30 kt.
The wave heights that can be expected with typhoon strength winds (64 kt) in Naha Harbor are presented as Table V-27.
According to Naha Military Port officials, average wave heights at the anchorages are 3-5 ft (0.9 - 1.5 m) during favorable conditions. During unfavorable weather conditions wave heights have been observed as high as 20 ft (6.1 m).
Storm Surge and Tides
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. Storm surge is caused by wind stress on the water surface and the effects of atmospheric pressure reduction. The piling up of water on a coast ahead of a tropical storm or typhoon is more apparent in the dangerous semicircle, the region of most intense winds which is located to the right of the storm's direction of movement. The speed of the storm adds to the wind velocity generated by the mechanics of the storm itself. For storms approaching Okinawa from the south, this surge effect will be maximum in bays that open to the south and east if the Harbor is located in the dangerous semicircle. The Port of Naha will be placed in the dangerous semicircle when a typhoon passes to the west of the area.
During periods of moderate to strong northwesterly winds, a surge effect of 2-3 ft (0.6 - 0.9 m) is evident in the Main Inner Harbor. This is caused by wind stress on the water surface and the effects of atmospheric pressure reduction. When this surge effect coincides with high tide, an abnormal rise in water level occurs. There are no recorded instances of the effects of storm surge causing damage to vessels in the Port or to Port facilities.
The tidal range in the harbor is 6.6 - 9.8 ft (2-3 m) while mean currents do not exceed 2.5 kt. A tide gauge is located at the northwest corner of the Port near the channel entrance.
The tidal currents on the rising and falling tides, respectively, flow northeast and southwest in the approach to the Harbor, and north and south across its entrance; they turn from one and a half to two hours after HW and LW. Westward of the Harbor limits the flood current sets north-northeast with a velocity of 2.3 kt, and the ebb current sets south-southwest with a velocity of 2 kt. In To-kuchi entrance, the flood current sets east with a velocity of .3 kt two hours after LW. The ebb current sets southwest with a velocity of 1.3 kt four hours after HW.
Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/japan/naha/text/sect3.htm
