Fighting bushfires is dynamic, dirty and dangerous.
For firefighters trying to determine how to best contain their spread, there’s a number of factors that need to be considered. Weather, vegetation and terrain all play a major role in determining a fire’s behaviour.
But even then, fires do not move uniformly. From ember storms to pyrocumulonimbus (bushfire-generated cloud) lightning strikes, bushfires have any number of tricks to further complicate their spread and behaviour.
What is a spotfire?
Spotfires are fires that ignite ahead, sometimes many kilometres, of the main bushfire. They usually occur as a result of firebrands (burning bits of bark or debris) being blown downwind of the main fire. Here they land in suitable fuel and start a new fire.
Spotfires are the major cause of the loss of control of fires, and are often how fires breach containment lines. They can start well ahead of the main front, and are a serious hazard to the safety of firefighters and others downwind of a bushfire. Firebrands and embers blown by a fire are also the primary cause of damage to the built environment.
Dr Andrew Sullivan leads our Bushfire Behaviour and Risks team. He says spotting is a key feature of Australian native forests. Many major fire events in the past, such as those on Ash Wednesday in 1983 and Black Saturday in 2009, were characterised by large zones of mass spotfires. These spotfires formed multiple ‘pseudo’ fronts. Spotting enables many forest fires to sustain very high rates of spread – sometimes spreading at up to 10 km per hour.
“Very often people caught in a bushfire say they were ‘surrounded by fire’. In a sense this can be quite true,” he says.
Depending on their size and the local conditions, embers and firebrands can be blown kilometres downwind from the main fire zone. Streamers of bark, such as from smooth-barked gum trees, may be lofted to great heights in the plume. These can travel distances of 30 km or more.
“Major fires can throw so many firebrands that can quickly start spotfires. These then develop into new fires that themselves begin to throw firebrands. The number of density of fires burning makes it appear that fire is everywhere. And that there is no easily identifiable way to escape.”
Predicting when, where and how these spotfires can break out is difficult. But understanding when spotfires are likely to occur is essential. Especially for suppression planning and determining the potential impact on fire behaviour and firefighter safety.
The probability that a firebrand will start a spotfire depends on a range of factors. These include how long it flames and smoulders, and the condition of the fuel on which it lands.
Spotting the threat
Carried out under controlled conditions in a small wind tunnel, our bushfire research replicated the type of conditions found during bushfire events. Such as in the dry eucalypt forests of southern Australia. We looked at the probability of successful spotfire ignition on fuel beds of different moisture contents in varying wind conditions.
Under most wildfire conditions, we found that a flaming firebrand is certain of igniting a spotfire.
For glowing firebrands the likelihood of ignition depends on the moisture content of the fuel and the presence of wind. These firebrands are where the flaming has ceased but is still smouldering.
These triggers for spotfire potential will help fire managers to better determine when spotting will be a hazard. And to provide a basis for prediction of its occurrence.
Further research is needed to build robust models for the full range of possible firebrand aerodynamic and combustion characteristics as well as environmental conditions, including wind gusts.
To learn more about bushfire prevention and response in your area, contact your local fire authority. If you are in an emergency situation, please call 000.