Wildfire house loss hazard mapping and fuel management scenario planning on a Tasmanian wildland-urban interface using radiant heat and firebrand exposure modelling

Grant James Williamson , Amila Wickramasinghe, Stefania Ondei, Owen Price, David Bowman

Globally, wildfire disasters are increasing in frequency through a combination of urban expansion into flammable wildlands and climate change. Accordingly, the wildland urban interface (WUI) is a crucial geographic domain for disaster risk reduction. Accurate mapping of wildfire house loss hazard is a basic requirement for effective wildfire risk management. We developed a novel geographic analytical framework for mapping wildfire house loss hazard based on exposure to radiant heat load and firebrand density constrained by the biophysical environment of individual properties including vegetation type, fuel load, topography, and likely fire weather. We created a radiant heat exposure model based on physical principles and developed a statistical firebrand exposure model derived from detailed fluid dynamics modelling of firebrand transport. Combining outputs from these models we estimate total number of houses vulnerable to wildfire house loss for the WUI of Hobart, Tasmania, an extremely fire prone Australian capital city. We found our hazard mapping was more spatially nuanced that current official Tasmanian bushfire hazard mapping that broadly delineates areas with 100 m proximity to bushland >1 ha. Using our hazard mapping framework, we explored the potential benefits of shaded firebreaks and prescribed burning fuel treatment scenarios in reducing wildfire house loss hazard, finding that exposure to firebrands were more important than exposure to radiative heat, particularly at greater distances from the interface. Further, our analyses suggest that mechanical thinning of bushland to increase the distance from homes by 50 m, combined with reducing bushland fuel loads by half, could substantially reduce by 68.3% the number of houses in mapped hazardous land (from 8,939 to 2,836 houses) in the study area. With acquisition of relevant fuel load and topographic inputs our wildfire house loss hazard mapping framework can be applied to flammable landscapes elsewhere in the world. Building on our findings and previous research of garden wildfire hazards we propose a conceptual model of wildfire house loss hazard that can be used to further design wildfire house loss mitigation strategies on the WUI.