Assessing the impacts of tropical cyclone Tracy on residential building stock – 1974 and 2008

Tropical cyclone Tracy (Tracy) remains one of the most destructive natural hazard events in Australia’s history. Growth in the population and size of Darwin since 1974 makes it desirable to know what impact an event similar to Tracy would have on the present day built environment. To assess the impacts in 1974 and the present day, we apply the Tropical Cyclone Risk Model (TCRM) developed at Geoscience Australia. A parametric wind field generated by TCRM is applied to building damage models in an attempt to reproduce the widespread damage to residential structures associated with Tracy in 1974. Employing these models yields a mean damage estimate of 36 per cent of replacement cost across all residential building stock in Darwin — a figure lower than that determined by post-event damage assessments. The unaccounted impact of large windborne debris is one possible explanation for the discrepancy between the observed and simulated damage. Based on the satisfactory replication of the damage associated with the historical impacts of Tracy, the wind field is then applied to the current day residential building database in order to assess the impact of Tracy were it to strike Darwin in 2008. We find that the mean damage to Darwin for the same urban footprint as the 1974 analysis in the present day would be around five per cent. This represents an approximately 90 per cent reduction in the modelled damage, and a significant portion of this reduction can be attributed to building code improvements.