An assessment of severe wind hazard and risk for Queensland’s Sunshine Coast region

Abstract

Understanding how regional climate, and the extremes associated with it, will change over the coming decades is a vital step towards effective local adaptation. understanding of the vulnerability and risk from severe winds is derived from a number of factors, including the frequency and intensity of the hazard, community exposure and the relationship between gust wind speed and impact/loss associated with residential structures. Australian Government, through the Australian Building Codes Board (ABCB), has responsibility for maintenance of the Building Code of Australia (BCA), which establishes performance requirements that buildings must satisfy when subjected to the actions of, among other hazards, severe winds. Australian /New Zealand Wind Loadings Standard (AS/NZS 1170.2, 2011) specifies the gust wind speeds used in calculating loads on structures in different of the country. Design wind speeds are similar to wind hazard (both described in terms of return period gust wind speeds), and the levels are set in an effort to equalise the risk between the low and high hazard across the country. Areas around the northern coastline are subject to the impact of tropical cyclones, and as such are deemed to experience the highest wind hazard. This results in Australia being divided into four regions, within which the wind speed is broadly similar. Regions C and D are those where the hazard is dominated by tropical cyclones. A recent report of climate change on design wind speeds in cyclonic regions (Holmes, 2008) prepared for the Australian Building Codes Board recommended that: • wind Region C should be extended south on the Queensland coast to 27 o S. This is justified based on the recent simulation studies on the effects of climate change, and restores the boundary to that in the Standard between 1975 and 1989. • The existing uncertainty factor for Region C, FC, should be increased to 1.10 (+5%), primarily in response to predictions of increased Category 3-5 cyclones by climate model simulations. An area where the impact of such regulatory changes would be significant is the Sunshine Coast region in Queensland. Changing the AS/NZS 1170.2 region from B to C would render most of the existing homes non- compliant, hence increasing the risk by virtue of increased hazard without reduction in vulnerability via adaptation. This study seeks to develop the underpinning science that could be used to evaluate the need for adaptation (re-classification and retrofit of existing housing to comply with revised classification). Cyclonic wind hazard for the Sunshine Coast region has been estimated using Geoscience Australia's Tropical Cyclone Risk Model, utilising synthetic tropical cyclone event sets derived from observations (1981-2006) as well as IPCC AR4 climate models. regional wind hazard levels have been modified for terrain, topographic and shielding effects to reflect localised variations in wind hazard. resulting local wind hazard maps depicting annual exceedence probabilities for severe wind gusts have been integrated with the National Exposure Information System (NEXIS) to provide an analysis of impact, in terms of residential buildings affected and the cost of replacement. Return periods (annual exceedence probabilities) associated with severe wind gust hazard were evaluated for the Sunshine Coast and Southeast Queensland for both current climate and future climate scenarios. This wind gust hazard was subsequently used to evaluate annualised loss, which represents the average annual cost to the region of exposure to wind hazard if viewed through a very wide window of time. costs and benefits of potential changes to wind loading for the Sunshine Coast area have been analysed in a Regulation Impact Statement (RIS) prepared by the Australian Building Codes Board (ABCB, 2010). RIS focuses on changes to standards for new buildings and houses; it does not address the impacts on existing buildings. This study presents analyses of risk for both existing and new buildings considering the variation in structural vulnerability within the building stock.