What Would Happen If Cyclone Tracy Hit Darwin in 2008?

The Cyclone

Tropical Cyclone Tracy hit Darwin early on 25 December 1974 with wind speeds of up to 250km/hr. The cyclone caused 71 deaths, the destruction of thousands of homes, the evacuation of over 35 000 people and around A$800M in damages.

It was believed that 80 per cent of residential buildings were either destroyed or rendered unliveable. Several factors contributed to the widespread destruction, including the intensity of the cyclone, vegetation overhanging buildings and construction materials employed in Darwin at the time.

Tropical Cyclone Tracy remains one of the smallest tropical cyclones on record, with a radius of gale force winds of less than 50km. This means that while the cyclone had very high wind speeds, its spatial impact was restricted to a narrow swath. This unusual characteristic posed a significant challenge to wind field modelling.

What have we learned?

Since 1974, the population of Darwin has grown rapidly, from 46 000 to 115 000 in 2006. If a similar cyclone to Tracy were to strike Darwin in 2008, the impacts could be catastrophic. Tools developed at Geoscience Australia can be used to allow emergency managers to plan for such a scenario, such as the Tropical Cyclone Risk Model (TCRM) outlined here.

TCRM is a statistical model of tropical cyclone activity which is used to assess hazard and risk associated with tropical cyclones. TCRM can also be used to simulate individual scenarios to estimate the impact of severe winds on a community. When analysing a scenario, TCRM provides estimates of the maximum wind speeds generated by a tropical cyclone over a region. This analysis incorporates the influences of terrain, topography and shielding from other structures.

Applying the model to Cyclone Tracy

Geoscience Australia modelled the impacts of the 1974 event using TCRM and compared it with data acquired following the cyclone. The Cyclone Tracy scenario was then modelled using 2008 residential building data, which was then compared to the estimated and observed damage of the actual impact in 1974.

Damage inflicted to residential structures was estimated by utilising a set of vulnerability curves, specially selected for the classes of building located in the survey area. The buildings in Darwin and the standards to which they were built differed significantly between 1974 and 2008. This was due to the reconstruction of buildings following Cyclone Tracy and the revision of building codes over time (Nicholls, 2007).

The results of the analysis

When TCRM was run for Cyclone Tracy, it produced a maximum gust wind speed of 260km/h at the Darwin Airport wind sensor site, which agrees well with estimates of the maximum wind speed (Bureau of Meteorology, 1977). Qualitatively, TCRM could spatially replicate the damage inflicted on Darwin by the small cyclone. The distribution and magnitude of wind-related damage was well captured - this could allow emergency managers to identify likely areas of significant damage at the suburb level.

When the damage for 1974 was assessed, TCRM estimated the damage to residential structures at 36 per cent of the full reconstruction costs of those structures, compared to post-Tracy survey estimates of 50 - 60 per cent of full reconstruction costs. Some of this deficit was believed to be due to the effects of very large debris, such as roof trusses. The significant loss of shielding due to the total destruction of upwind buildings is another contributing factor.

For the 2008 analysis, Geoscience Australia estimated the damage to Darwin residential structures at 3.5 per cent of full reconstruction costs. This reflects a 90 per cent reduction in mean losses compared to the 1974 impact. In contrast to the 1974 simulation, the improved building standards mean a vast reduction in the number of buildings that would suffer complete destruction.

Benefits of modelling to emergency managers

Emergency managers preparing for, or recovering from, an impact can use the information provided by Geoscience Australia through the use of TCRM. This information can guide decisions on the deployment of resources or identify areas that may require evacuation before the onset of gale-force winds.

TCRM can be used to identify areas likely to suffer significant severe wind damage due to the impact of a tropical cyclone, providing invaluable information to emergency managers involved in the preparation and recovery phases.

TCRM was designed to allow probabilistic and scenario modelling. A set of pre-calculated scenarios may be of great benefit to emergency managers for training and demonstration purposes.

References

  • Bureau of Meteorology (1977): Report on Cyclone Tracy, December 1974, Australia Government Publishing Service, Canberra, Australia
  • Nicholls, M., (2007): Review on NT Cyclone Risks, 79pp.

Figure 1: An analysis of the impacts of Cyclone Tracy on Darwin. The left shows the estimated damage (Estimated mean residential building stock loss (percentage Replacement Cost) surveyed in the immediate aftermath of Cyclone Tracy.

Figure 2: on the right, the impact (percentage Replacement Cost) Cyclone Tracy would have on Darwin in 2008.

Impacts of Cyclone Tracy on Darwin

Figure 1: Impacts of
Cyclone Tracy on Darwin
© Geoscience Australia

Impacts of Cyclone Tracy on Darwin

Figure 2: Impacts of
Cyclone Tracy on Darwin
© Geoscience Australia

Topic contact: hazards@ga.gov.au Last updated: January 12, 2011