@article {bnh-7499, title = {Determining threshold conditions for extreme fire behaviour - annual report 2019-2020}, number = {626}, year = {2020}, month = {11/2020}, institution = {Bushfire and Natural Hazards CRC}, address = {MELBOURNE}, abstract = {

At this phase the project was focused on development of a new method to test flammability of live vegetation in dynamic conditions and understanding influence of climatic changes on the 2019/20 bushfire season in New South Wales (NSW), Victoria, and South Australia (SA).

Understanding live vegetation fuel properties and how they behave when exposed to radiant heat and flame allows us to better predict fire behaviour in forested areas. This study aims to determine a more effective, replicable and accurate method of testing flammability in live vegetation by comparing the impact different radiant heating regimes have on the ignitibility of live vegetation samples. Current methodologies are limited in their ability to provide accurate quantification of flammability due to their reliance on static heat flux exposure, which does not accurately replicate how live plants experience radiative heat flux during a wildfire in their natural environment. Two heating regimes were tested for this study {\textendash} a static heat flux to reflect current methods and a dynamic (increasing) heat flux to more accurately replicate real conditions of an approaching fire front. Piloted-ignition and unpiloted-ignition were also tested for both of these heating regimes. A Variable Heat Flux (VHFlux) Apparatus was used to study flammability of Acacia floribunda, Cassinia arcuata, Pinus radiata and bark from Eucalyptus obliqua. Time to pyrolysis (production of volatile products), smouldering, flaming ignition, complete consumption and radiant exposure (the radiant energy received by a sample over a time of heating, He) were used as ignitability measures. It was observed that time and radiant exposure required to reach flaming ignition (and the other ignitibility metrics) was higher under a dynamic heating regime. It was also observed that the presence of a pilot igniter greatly increased the number of samples that reached flaming ignition, and decreased the time and He required to reach flaming ignition (and the other ignitibility metrics). These results suggest clear differences observed between heating regimes for time and Herequired for ignition and other ignitibility measures, which supports the validity of using dynamic heating regimes and the VHFlux apparatus as a standardised method. Adoption of this methodology is recommended to ensure more realistic data on flammability of individual plant species and plant communities, which will ultimately lead to better informed and more accurate wildfire behaviour modelling.

There is no doubt that the fire season of 2019/20 was extraordinary. A total of 18,983,588hectares were burned, 3113 houses and 33 lives lost in 15,344 bushfires in Black Summer fires. NSW had the highest number of fires, area burned, houses and lives lost for the last 20 years. Two mega-blazes occurred in NSW and burned more than in any fire season during the last 20 years. Victoria had the highest number of fires, area burned, and houses lost (except for the Black Saturday fires). SA had the highest number of houses lost in the last 20 years. Relationships between the burned area and number of fires, the houses and lives lost had positive trend for all states irrespective of the dataset. A negative relationship between the houses and lives lost for SA was the only exception. Multiple studies show that fire weather will become more severe in many regions around the world. Based on this and observed positive trends for all categories for NSW and Victoria, it is likely that the values will continue to increase in these states in the future. SA before 2019/20 was in a relatively good position showing negative trends for almost all categories. However, the 2019/20 fire season changed that for the worse. The magnitude of effect from increased fire weather may depend on how these conditions alter vegetation across Australia, however the indications shown in this analysis are concerning for fire managers.

Smoke from bushfires significantly impacted on people with cardiovascular and respiratory problems and increased mortality. It also had indirect impact on the economy by disrupting communities. The total impact of the 2019/20 bushfire season to the economy is estimated to be as much as A$40 billion. Due to the record burned area, at least 1 billion vertebrate animals were lost. It will take many years to restore the economy in impacted areas, and for animal and vegetation biodiversity to recover. Understanding of high-level trends of number of fires, area burned, houses and lives lost for the last two decades in south-eastern Australia will provide useful insights to fire managers for future strategies and policies.

}, keywords = {Behaviour, conditions, extreme fire, threshold}, issn = {626}, author = {Alex Filkov and Thomas Duff and Trent Penman} }