PDF | 2 MB - Australian Building Codes Board

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PDF | 2 MB - Australian Building Codes Board

9. Observations and projections for the Northern Territory

9.1 Introduction

This Chapter reviews some studies by individuals and groups for cyclonic wind

speeds in Darwin and the Northern Territory. Comments are made about the

appropriate zoning of Darwin and other parts of the Northern Territory, and the need

for more studies are discussed.

9.2 Reports by M. Nicholls

A community Group for the ‘Review of NT Cyclone Risks’ was formed in 2005 and

received a grant from Emergency Management Australia in 2006 to study cyclone

risks in the Northern Territory. Mike Nicholls was the Secretary of the Group and

also the subcontractor for most of the work. The work was completed in early 2007

and a web site (www.cyclone.org.au) contains the report of the work consisting of a

main report and fifteen lengthy Appendices (Nicholls et al, 2007).

A two-page summary of the report was supplied to Sub-Committee BD006-02 of

Standards Australia in August 2007. The following comments relate primarily to that

summary.

The findings of the report were summarized as follows (with wording only slightly

altered):

a) Three of the most intense cyclones that have been observed in Australian waters

since satellite observations began in 1960 (‘Thelma’ 1998, ‘Ingrid’ 2005 and

‘Monica’ 2006) all came within 350 km of Darwin when they were at maximum

intensity and within a nine-year period.

b) These ‘TIM’ cyclones may have been the result of global warming or they may

signal the return of a more active period similar to one that appears to have existed in

the first 90 years of (European) NT settlement.

c) NT buildings should probably be designed for wind loads that are at least 60%

higher than the minimum loads permitted under the current AS/NZS1170.2.

a) is generally not disputed, although the exact strength of the three storms is subject

to speculation. All three were rated Category 5 at some point in their lives by the

Bureau of Meteorology, but so also was Cyclone ‘Larry’ in Queensland in 2006.

However, on landfall at Innisfail, the latter produced damage commensurate with a

high Category 3 or a low Category 4 event.

b) The first 90 years of European settlement produced a number of severe cyclones

but without the benefit of modern satellites and ground-based instrumentation it is

difficult to be categorical about their intensities. It is possible that the intensity of

TIM cyclones was related to global warming.

c) This conclusion is justified by some ‘ball-park’ probabilistic estimates. These

are discussed in the following.

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