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BUNBURY

TROPICAL CYCLONES AFFECTING BUNBURY

Historical tropical cyclone information of a general nature is available for Bunbury as early as 1910, but specific tropical cyclone data, containing storm tracks, wind and pressure data, etc., are only available starting in 1961. Table XI-31 contains a descriptive history of all 10 tropical cyclones and hurricanes passing within 180 nmi of Bunbury during the 38-year period 1961-1998. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Bunbury are based on the data set used to compile Table XI-31.

The majority of tropical cyclones with a "classical track" have originated in the area to the north of Western Australia, have followed a west/southwest track parallel to or just off the northwest coast, and then have turned to the south or southeast. A few of the tropical cyclones have actually moved out of the central Indian Ocean on a more east-southeastward track toward Bunbury. Very few tropical storms keep intensifying after moving past 20° S. Note that tropical cyclones in the Southern Hemisphere have a clockwise wind circulation, and at the latitude of Bunbury, have a much higher speed of advance than in the tropics.

Although the tropical cyclone season for Australia is considered to extend from November through April, Table XI-32 shows that Bunbury's season is essentially limited to March and April. January and May each had one tropical cyclone recorded throughout the 38-year period 1961-1998 and no tropical cyclones were recorded during November, December, and February. March and April had 8 of the 10 storms (80%) of record and only one of the 10 storms in the data set was of hurricane strength ( 64 kt) within 180 nmi of Bunbury. Table XI-32 also shows the average direction towards which tropical cyclones were moving when they were at CPA to Bunbury. The average movement for all tropical cyclones is 124° at 29 kt. This speed of advance is two to three times faster than typically experienced in the tropics.

Figure XI-66 shows in seven-day increments the annual distribution of the 10 tropical cyclones that entered the 180 nmi threat radius around Bunbury. The period of activity, from January 22nd through May 13th, is clearly shown.

Figure XI-67 depicts the annual chronology of the 10 tropical cyclones that passed within 180 nmi of Bunbury during the 38-year period 1961-1998. As shown in the figure, there is an average occurrence rate of 0.26 storms per year for all tropical cyclones, regardless of intensity. The recurrence interval of all tropical cyclones is 3.8 years. There have been several multiple-year periods since 1961 when no tropical cyclone entered Bunbury's 180 nmi threat radius. The eight-year period from 1961 through 1968, seven-year period from 1992 through 1998, and six-year period from 1983 through 1988 had no tropical cyclones passing within 180 nmi of Bunbury.

Figure XI-68 shows the octants from which the 10 tropical cyclones were approaching when they were at CPA to Bunbury. As shown in the figure, the predominant threat direction is from the west through northwest. It must be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the storm's initial movement towards Bunbury. Note that both the threat direction and the high speed of movement are important considerations for evasion at sea.

As shown in Figure XI-69, the movement of the 10 tropical cyclones that entered Bunbury's 180 nmi radius can be described as fairly consistent. The dark circles in the figure indicate the locations of the 10 tropical cyclones when they were 72 hours from CPA to Bunbury. Note that the tracks of 8 tropical cyclones follow a similar pattern approaching from the northwest and two tropical cyclones approached from the west.

Figure XI-70 is a subset of Figure XI-69, and shows the track of tropical cyclone Alby, the one tropical cyclone during the 1961-1998 period that contained at least hurricane force ( 64 kt) sustained winds while within Bunbury's 180 nmi threat radius. The dark circle in the figure indicates the location of the storm when it was 72 hours from CPA to Bunbury.

Figure XI-71 is the statistical summary of threat probability based on tropical cyclone tracks for the period 1961-1998. The data is presented with solid lines representing "percent threat" for the 180 nmi radius circle surrounding Bunbury. The heavy dashed lines represent approximate approach times to Bunbury based on the climatological speeds of movement. In Figure XI-71 for example, a tropical cyclone located near 28°S 97°E has approximately a 10 % probability of passing within 180 nmi of Bunbury. In addition, this tropical cyclone would reach Bunbury in 1½ - 2 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of Bunbury. Note the acceleration of the speed of advance as tropical cyclones approach Bunbury, as indicated by the increasing distance covered between approximate time to CPA lines.

Wind

Wind measuring instruments are located at the Bunbury Port Authority Office at about 33°19'S 115°38.3'E. The instruments are not blocked or affected by other buildings, and indicate winds that are representative throughout the port area.

The sequence of local wind directions from a tropical cyclone passing to the east is first from the east, then south, and then west. For a tropical cyclone passing to the west it is first from the north and then west.

Locations most vulnerable to wind damage are the anchorage areas and the berths in the Outer Harbor. The sea-breeze effect is evident during Australian Summer on practically a daily basis. From approximately sunrise the wind blows off the land (easterly wind) until about 1000 to 1200. Then the sea breeze normally comes in from the south-southwest at speeds averaging 19-26 knots by mid-afternoon. Extremes can be greater than 30 knots. This sea breeze usually begins to dissipate around midnight.

Waves

The two Outer Harbor berths can be affected by swell, and this is a problem particularly during the Australian winter months. Waves have very little impact on ships in the Inner Harbor.

Storm Surge

Local authorities reported that there was a surge of 6.6 ft (2 m) during tropical cyclone Alby in 1978.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/australi/bunbury/text/sect7.htm