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TROPICAL CYCLONES AFFECTING TOWNSVILLE

Australia's tropical cyclone season extends from November through April. Historical tropical cyclone information of a general nature is available for Townsville as early as 1867, but specific tropical cyclone data, containing storm tracks, wind and pressure data, etc., are only available starting in 1958. Table XI-11 contains a descriptive history of all 36 tropical storms and hurricanes passing within 180 nmi of Townsville during the 40-year period 1958-1997. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Townsville are based on the data set used to compile Table XI-11.

Although the official tropical cyclone season for Australia is considered to extend from November through April, Table XI-12 shows that Townsville's season is effectively limited to the period of December through March. No storms were recorded during November, and only four storms were recorded during April during the 40-year period 1958-1997. January is the month of greatest tropical cyclone activity for Townsville, with 11 of the 36 storms (31%) occurring during that month. Of the 36 storms in the data set, six (17%) were of hurricane strength (64 kt) at their closest point of approach (CPA). Table XI-12 also shows the average direction towards which storms were moving when they were at CPA to Townsville. The average movement for all storms is 166° at 11 kt, but there are considerable differences in movement directions and speeds during the months represented in the table. Average monthly directions vary from southeast in January to southwest in March. Average speeds vary from a low of 8 kt in March to 14 kt in January.

During the 40-year period 1958-1997 there were 36 tropical cyclones that entered Townsville's 180 nmi threat radius. Figure XI-19 shows the annual distribution of the storms in seven-day increments. The figure clearly shows the well-defined tropical cyclone season, with maximum activity occurring during the period from December 17th through March 4th.

Figure XI-20 depicts the chronology of the 36 tropical cyclones that passed within 180 nmi of Townsville during the 40-year period 1958-1997. As shown in the figure, there is an average occurrence rate of 0.9 storms per year for all tropical cyclones, regardless of intensity. The recurrence interval of all tropical cyclones is 1.1 years. For hurricane strength (64 kt) storms, the average occurrence rate is 0.15 storms per year, with a recurrence rate of 6.7 years. There have been several multiple-year periods since 1958 when no tropical cyclone entered Townsville's 180 nmi threat radius. The four-year period from the 1991-1992 season through the 1994-1995 season had no storms passing within 180 nmi of Townsville.

Figure XI-21 shows the octants from which the 36 tropical cyclones were moving when they were at CPA to Townsville. As shown in the figure, the predominant threat direction is the northwest octant, although a similar number of storms approached Townsville from the north and northeast octants combined. It must be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the storm's initial movement toward Townsville. The fact that seven of the tropical cyclones approached Townsville from the west octant is significant. It represents storms that likely originated over or near to the Gulf of Carpentaria and moved across the Cape York Peninsula to the Townsville area. Although these storms generally weaken as they cross the Peninsula, some regenerate over the waters of the Coral Sea and pose a threat to the Port of Townsville.

The movement of the 36 tropical cyclones that entered Townsville's 180 nmi radius can only be described as inconsistent. The lack of steering flow is apparent in many of the cases, and as a result many storms took meandering, and in some cases, looping tracks. Figure XI-22, which depicts the tracks of the 10 storms that had hurricane force winds (64 kt) at some time while within Townsville's 180 nmi threat radius, shows the erratic movement typical of many of the storms affecting Townsville. The dark circles in the figure indicate the locations of eight of the storms when they were 72 hours from CPA to Townsville. Two storms were either not detected or were too weak to be tracked 72 hours prior to CPA.

Figure XI-10 of the Brisbane evaluation depicts the track of Tropical Cyclone Justin and illustrates the meandering nature of some of the tropical cyclones occurring over the Coral Sea. Justin occurred in March 1977, and is included in Townsville's 36-storm data set. The text accompanying Figure XI-10 discusses the 17-day life of Justin.

Wind

The prevailing wind directions at Townsville are northwesterly during the summer, and southeasterly during the winter (Townsville Port Authority, 1997b).

Historical records indicate that Townsville has experienced several periods of strong winds associated with a tropical cyclone passage. According to Callaghan (1997b), in March 1903 Tropical Cyclone Leonta brought winds of hurricane force (64 kt) to the area. Wind gusts of 73 kt were recorded during Tropical Cyclone Agnes in March 1956. According to the Australian Bureau of Meteorology, the strongest winds on record occurred during the passage of Tropical Cyclone Althea in December 1971, when a 106 kt gust was recorded.

Wind records are available for Mackay Airport for the 24 tropical cyclone seasons occurring during the period from December 1973 through March 1997. During that time, only four tropical cyclones brought sustained winds 22 kt to Townsville. None caused sustained winds of gale force (34 kt); the maximum wind speed recorded was 28 kt.

Waves

The only waves that could significantly impact the facilities inside the breakwaters at the Port of Townsville are those propagating across Cleveland Bay from approximately 030° to 070°. Except for those waves that might refract around it, wave motion from the north would be blocked by Magnetic Island. Cape Cleveland would block any wave motion with a strong easterly component from reaching the port. Waves from the northwest, passing through West Channel between Magnetic Island and the mainland, would be blocked by the breakwater on the northwest side of the harbor. Deep ocean swell waves generated by a tropical cyclone in the Coral Sea would be dampened by the relatively shallow water covering the Great Barrier Reef northeast of Townsville.

Storm Surge

Historical records indicate that Townsville is vulnerable to storm surge. The most notable event occurred in 1971, when a surge height of 9.5 ft (2.9 m) was recorded in Townsville Harbor during the passage of Tropical Cyclone Althea. The storm surge coincided with low tide, so the effect of the water rise was minimized. If the surge had coincided with an astronomical high tide, the water level would have been higher.

Townsville is also subject to flooding due to precipitation runoff. Widespread flooding is not uncommon when a tropical cyclone's circulation reaches the watershed that affects the Townsville area.

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