The Northern Territory news Sat 12 Dec 2009
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20 Northern Territory News, Saturday, December 12, 2009 www.ntnews.com.au P U B : N T N E W S D A T E : 1 2 -D E C -2 0 0 9 P A G E : 2 0 C O L O R : K Cat 5 cyclonewould BEFORE: House before Cyclone Monica AFTER: And house after Cyclone Monica INCREASING TREND: Nhulunbuy gets belted by 100+km/h winds, rain and big surf as Cyclone Monica looms over the Northern Territory in April 2006. Picture: BRAD FLEET New report reveals building code at pre-Tracy levels LESSONS FROM THE PAST: Prime minister Gough Whitlam (in the first white car) inspects the wrecked suburb of Casuarina after Cyclone Tracy hit Darwin in 1974 N O ONE knows what wind speeds Cyclone Tracy was producing when it hit Darwin in 1974. The anemometer which measures wind speed at the airport blew out at 217km/h. Accepted wisdom is that the northern suburbs were wiped out by winds of 280km/h, if not more. Is that where the wisdom ended? Prior to Tracy, Darwin homes had to be designed to withstand what in contemporary terms are wind speeds of 248km/h. In 1975, authorities reacted to the Tracy disaster by decreeing that houses had to be able to withstand 277km/h wind speeds. By 1989, three separate studies had convinced wind standards experts that there had been an overreaction to Tracy and the wind-load requirement slipped back to 252km/h. By 2002, it had dropped back to the same pre-Tracy levels of 248km/h. Darwin has had near misses by three of the maximum Category 5 cyclones in the past 11 years, being Thelma in 1998, Ingrid in 2005 and Monica in 2006. Thelma and Monica were capable of producing wind speeds of more than 350km/h and all three cyclones were bigger and stronger than Tracy, which was a high-end Category 4 cyclone. Monica made landfall near Maningrida with maximum wind speeds of 365km/h, making it the most intense tropical cyclone in recorded history to have hit the Australian mainland. Thelma, Ingrid and Monica caused only minor damage. In no case did any of the cyclones eyewalls the central tower of winds around a cyclones clear eye directly hit a city or town. But Monicas eyewall did pass over a newly refurbished house, built to cyclone code, on the tiny Matjanba outstation on the Wessel Islands, off Arnhem Land. Fortunately, no one was home at the time. They never did find that house. Darwin structural engineer, Michael Nicholls, and Garry Cook, from CSIRO, last month had a report published in the American Meteorological Societys Journal of Applied Meteorology and Climatology, warning that Darwin and all Top End coastal towns would suffer badly in the event of a Category 5 cyclone passing directly over. Mr Nicholls, who lived through Tracy, has been warning for years that Darwin is not ready for the kinds of Category 5 cyclones that have been increasingly creeping closer to the capital. His message is not popular with politicians who, if they accepted the research, would need to make costly decisions about upgrading existing buildings, including cyclone shelters, and would be forced to place extra demands on major projects. Most painful of all, extra steel reinforcement would push up the cost of new residential housing. Mr Nicholls has been easy to dismiss, but the publication of his and Dr Cooks study in the prestigious, peer-reviewed American journal suggests he might be given a proper hearing. The study finds that during December the potential intensity of tropical cyclones forming in seas near Darwin is the highest of any location in the world. This, they say, is because of a clash between some of the warmest sea temperatures in the world with the freezing skies in the tropopause the last layer at the top of the atmosphere which is affected by weather. The tropopause over Darwin is 18-22km above sea level and is, surprisingly, much colder that the tropopause over the poles, which are only 12km above sea level and much warmer. Added to that, the waters off Darwin are shallow due to the continental shelf. This means there is no cold water to be pulled from the depths to cool down and reduce the intensity of a cyclone. Such extremes explain the powerful cyclonic energy experienced off Darwin. Monica, Ingrid and Thelma represent a new force in cyclonic events, yet Darwins building wind code has not been adjusted upwards. It allows for the same wind hazards experienced in Townsville and Bundaberg, which are further south in latitude and experiences less intense cyclones. Mr Nicholls and Dr Cook claim historically inaccurate modelling of cyclone data has led to Darwin building codes being the same as the Queensland towns, which the report says appears to substantially underestimate the tropical cyclone wind hazard for Darwin. Standards Australia is the government-accredited organisation with committees that set national safety standards on all things to do with buildings, including electrical wiring, plumbing and concrete strength. One of its committees classifies windrisk factors.
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