Australian National Wind Risk Assessment (NWRA): Quantifying wind hazard and risk under current and future climate

Severe wind is one of the major hazards facing Australia. While cyclonic winds are the major source of wind hazard in the northern states, non-cyclonic winds driven by synoptic lows, thunderstorms and tornadoes affect the southern states. Severe winds are responsible for about 40% of damage (structural loss) to Australian residential buildings (Chen 2004). Tropical cyclones pose a significant threat to life and property around Australia's northern coastline; average annual losses are estimated at around A$260 million (BTE 2001) and nearly A$700 million (Waters et al., 2010). Over 80% of the nation's population reside within 25 km of the coastline (Chen and McAneney 2006) where extreme winds are most prevalent. Continued rapid growth in the northern and coastal parts of the continent is increasing the population exposed to severe thunderstorms and tropical cyclones in particular. Failure to address issues such as climate change adds pressure on the development in this zone, with consequences on the social, economic and environmental sustainability.

Climate change is expected to increase severe wind hazard in many regions of the Australian continent with consequences for exposed infrastructure and human populations (Stewart and Wang 2011). Geoscience Australia, assisted by funding from the Federal Department of Climate Change and Energy Efficiency (now Department of Environment), has conducted a preliminary study to investigate severe wind risk to residential buildings in the Australian region both under current climate and also for a range of climate change scenarios. The study is known as the National Wind Risk Assessment (NWRA) and it has been established to identify communities subject to high wind risk under present climate.

Australian Bureau of Statistics (ABS) population projections have also been employed to estimate how residential building exposure may change in the future. During 2009, the Australian Prime Minister and the Commonwealth Treasury made a number of announcements that the likely population of Australia in 2050 might be about 35 million (currently about 21 million), based on projections released by the ABS. The present regional distribution of population is expected to continue with a drift to the north, especially to coastal Queensland and New South Wales (east Australian coast). This “Sea Change” has been identified by the Australian House of Representatives Standing Committee on Climate Change, Water, Environment and the Arts inquiry in their October 2009 report (SCCCWEA, 2009), as an important feature of the future.

The NWRA has developed a computational framework to evaluate both the wind hazard and risk due to severe wind gusts (based on modelling techniques and application of the National Exposure Information System; NEXIS; Nadimpalli et al., 2007). A combination of tropical cyclone, synoptic and thunderstorm wind hazard estimates is used to provide an estimate of climatology of severe wind hazard across Australia. This hazard modelling utilises both current-climate information and also simulations forced by IPCC SRES climate change scenarios, which are employed to determine how wind hazard will be influenced by climate change. The NWRA assessed severe wind hazard in both current and future climate, providing the following outputs: • Firstly, the NWRA provides a baseline assessment of current climate severe wind risk across the continent at the regional scale. The approach estimates risk based on aggregated datasets of hazard, exposure and vulnerability. The baseline wind hazard in terms of likelihood and severity was developed based on the regional gust wind speeds as provided by the Australian – New Zealand wind loadings standard for building design (AS/NZS 1170.2, 2011). The NWRA has developed an understanding of wind vulnerability (how severe wind hazard relates to impact/damage) for each of the residential structural types and ages considered. NEXIS was used for the understanding of residential building exposure. These vulnerability relationships were developed through a series of expert workshops, analysis of wind damage data, and a survey of skilled damage assessors within the Australian wind engineering community. The baseline wind risk assessment provides wind risk estimates (expressed as annualised loss) for each statistical local area (SLA), considering 10 year to 2000 year return period hazard.

• The aggregate data used in the risk quantification in the above phase introduces a level of error in the quantification of risk. To identify the magnitude of the error in the national regional-scale benchmark estimate, the second output of the study provides risk estimates using locally-defined wind hazard, exposure and vulnerability information. The local risk estimates are generated for four case study regions: Hobart, Perth, Southeast Queensland and Cairns/Innisfail.

• Thirdly, the NWRA provides a modelled estimate of the wind gust hazard across the continent, at regional scale, which has been validated by statistical analysis of observations where possible. This was done by combining separate models for tropical cyclone, thunderstorm and synoptic winds, utilising a range of statistical, dynamical and stochastic approaches. This work demonstrates techniques that can be applied to climate change projections to evaluate possible future changes in the wind hazard.

• Fourthly, the NWRA provides a measure of regional severe wind risk under future climate. For this purpose, the NWRA has developed an understanding of severe wind hazard for a range of climate change. A preliminary understanding of how residential building exposure may change for the four case study regions in the timeframes associated with those scenarios (2060 and 2100, respectively), and NEXIS to project current trends in occupancy statistics.

The NWRA has identified those communities which will be most susceptible to any climate change related exacerbation of local wind hazard, requiring an adaptation response. The NWRA has also examined trends in severe wind hazard and risk, as well as laying the foundation for the exploration of a range of adaptation strategies. The outputs of the NWRA will be crucial for informing the “next steps” for climate change adaptation options with regards to severe winds. Damage to residential buildings from severe winds is a significant concern to decision-makers in planning, building codes, construction, emergency services, and the insurance industry, as well as the community as a whole.