Destructive power: stakes high for changing cyclone risks

Australia has so far this season avoided the extensive wind-driven destruction that tropical cyclones can bring but the impact of Gabrielle in New Zealand has again shown the devastating impacts of such storms even as they move south and change.

Gabrielle has become the first event where cover will be provided under the Australian Reinsurance Pool Corporation (ARPC) cyclone scheme, as a result of bringing severe weather to Norfolk Island from February 11. For pool purposes the claims period ended on February 14 at 3am, 48 hours after the Bureau of Meteorology downgraded Gabrielle.

Norfolk Island lies directly east of Evans Head in northern NSW, and while cyclones rarely cross the Australian mainland coast below central Queensland, a recent report highlights that events tracking further south are a real and less well understood risk.

The report published by Geoscience Australia in December, titled Severe Wind Hazard Assessment for South East Queensland: Technical Report, looks at dangers from tropical cyclones tracking further south than usual.

Tropical cyclones have made landfall in Queensland’s south east, and numerous cases exist of them passing close by while remaining offshore.

Climate change is expected to influence the behaviour, frequency and intensity of tropical cyclones, and while they may become less frequent in the Queensland region, a greater proportion of more intense events may develop. The report highlights that more research is needed.

The stakes are high. South East Queensland’s population is predicted to increase by about 40% to 5.3 million residents by 2041, up from an estimated 3.8 million last year, yet the report says the risk posed by severe wind “is not well understood, nor are the optimal strategies for managing and potentially reducing this risk”.

The two-year Severe Wind Hazard Assessment for South East Queensland project was started in 2020 in conjunction with Queensland Fire and Emergency Services, six local councils and James Cook University. Other partners included University of Queensland, Insurance Council of Australia (ICA), IAG, Suncorp and CoreLogic.

Alarms bells were further raised by Tropical Cyclone Seroja, which crossed the coastline at category three strength on April 11 2021 between Kalbarri and Geraldton in WA, an area at similar latitude to South East Queensland.

The event, which generated $404 million in claims according to ICA data, resulted from the rare Fujiwhara Effect, where two cyclones interact with each other, but it highlighted potential vulnerabilities on the other side of the country.

Older homes in the past have been shown to be the most vulnerable to damage, but the damage in WA has indicated modern homes also have issues.

“While the design winds speeds for modern buildings are identical between the two regions, the population of the study area is significantly higher than the area impacted by TC Seroja and hence, if a similar cyclone were to cross the study area significantly greater damage would be expected,” the Geoscience Australia report says.

The report looks at residential housing stock across the South East Queensland study region and at vulnerabilities and retrofit strategies to improve resilience, and examines five tropical cyclone scenarios.

In scenario two, a cyclone crosses the coast near Mooloolaba at category 3 intensity, then decays as the centre passes over Ipswich and into northern NSW, before moving offshore near Ballina.

Across the six local government areas of Noosa, Sunshine Coast, Moreton Bay, Brisbane, Redland and Gold Coast, 114 buildings are completely destroyed, 170,770 are extensively damaged and 179,800 sustain moderate damage.

The impact causes more than 200,000 houses to sustain either moderate or extensive damage in Brisbane, while 43% of homes in Moreton Bay sustain moderate or higher damage, but Gold Coast is relatively unscathed.

In scenario four, the cyclone makes landfall around Coolum, weakens as it tracks over Caboolture, Brisbane and Redland, briefly moves offshore, then tracks southwards past Coolangatta before moving offshore again in northern NSW. The result is moderate damage to 44,367 properties and extensive damage to 3936 buildings.

Scenario one in the report was used for an exercise that took place in June last year to examine challenges facing local and state governments and critical infrastructure operators if a severe tropical cyclone were to impact the region.

In that scenario, the cyclone enters Moreton Bay at category three strength, with strongest winds across the bay and through the Gold Coast, with steep topography over the hinterland meaning extreme wind speeds are possible well inland. The Gold Coast is severely impacted with around 100,000 houses extensively damaged and another 37,000 moderately damaged.

Whether future wind hazards in the region will increase remains uncertain, but the population in that part of the state will continue to rise, increasing potential risks to be borne by communities, insurers and governments.

The report says the research outcomes are expected to be a valuable resource for all the project partners and stakeholders in planning, preparation, response, recovery and strategic mitigation, and it includes areas for further action and research.

“Just as government has a concentration of exposure in [South East Queensland] from the perspective of disaster management and recovery, the insurance sector has an increasingly concentrated portfolio exposure that could deliver significant portfolio losses in a single storm event,” the report says.

The report is available here.