EXISTING fire behaviour guides under-predict the rate of spread and intensity of bushfires that burn long enough to develop headfires more than 100m wide, according to a new study.
The finding from Project Vesta, a collaboration between WA’s Department of Environment and Conservation, the University of New South Wales, Bushfire Cooperative Research Centre, CSIRO Ecosystem Sciences and the CSIRO Climate Adaptation Flagship.
Predicting fire behaviour in dry eucalypt forest in southern Australia details the results of experimental fires conducted at two locations in jarrah forest in WA’s South West—and found the rate of fire spread was under-predicted by as much as three times.
DEC Manjimup Landscape Conservation Program Leader Dr Lachie McCaw, one of the paper’s authors, says existing fire behaviour guides were developed using data derived from small scale experiments mostly lit with point ignitions.
“The fires had not reached their potential rate of spread by the time the experiments were terminated,” he says.
“But our experiments also showed a line of fire can accelerate to its potential rate of spread very rapidly, within 1–2 minutes.
“This is important because it means a long flank of a fire can rapidly change direction to become a new headfire following a wind change.
“This has important implications for the safety of firefighters and other people who may be placed in the path of a fire following a wind change.”
By comparing the rate of spread of Project Vesta’s fire experiments—with observations from well-documented bushfires—researchers were confident their experimental method produced fires comparable to real bushfires that had been burning for at least a few hours.
“But we also recognised very large fires burning under extreme weather conditions can interact with the atmosphere in ways impossible to replicate safely under experimental conditions,” Dr McCaw says.
“So there will always be some level of uncertainty about how fires will behave under these conditions.
“Our experiments also showed fires spread faster in fuels that were older and had higher ratings for a variety of attributes that reflect flammability.
“Fires in older fuels also produced a much higher density of firebrands capable of initiating spot fires downwind of the main fire front.”
Research findings have been used to improve safety awareness for firefighters, emphasising the risk of unburnt fuel between them and the flank of a fire which may become active if there is a change in wind direction.
The concept is known as the Dead Man Zone and has been included in a training DVD.