Whirls or “fire tornadoes” can occur during extreme forest fire events. Perhaps unsurprisingly, they have been difficult to measure, leaving researchers to try to extrapolate their behaviour into real-life situations – with some difficulty.
But a world-first effort near Twizel in late May has provided a treasure trove of sensor and monitoring data for the researchers to pore over, and given them some revelatory insights into how these events are created and behave.
Hugh Wallace, one of Scion’s lead fire scientists, says Twizel proved the ideal field trial site for the work partly thanks to a highly supportive community whose understanding of fire’s impact has been sharpened by events in recent years, including the Pukaki and Ōhau burns in 2020.
A strong relationship with Fire and Emergency New Zealand personnel and volunteer brigades also played a big part in the location.
Wallace says fire vortexes are of particular interest because of their ability to take on a velocity of their own and to throw burning material ahead of their path. They pose particular risks to fire crews.
Interest in the work from overseas, including the United States and Australia, has been intense, with US Forest Service lead researcher Jason Forthofer engaged in the work.
“NZ is in quite a unique situation compared to these countries right now where authorities are really under the pump just containing fires, whereas we have some luxury of time to prepare and understand what could happen here,” Wallace says.
This month parts of Canada are being seared by massive bush fires, four of them exceeding 120,000ha each and eight over 50,000ha each.
And the number of extreme fire events that can spawn whirls are only increasing.
In NZ, estimates are that the $142 million that wildfires cost the country in 2020 will have ballooned to $547m by 2050 due in part to the changing climate.
In the Twizel trial, once a slash pile 20m in diameter was ignited, the team generated a “smoke devil” that captured the flame and created the fire whirl, with a wide array of infrared and remote sensors capturing data.
This included recording sustained whirl revolutions of between 10 and 100 times per second.
“The whirls do take a bit of effort to spin up. They require a very well-constructed and large pile to create, and it is about the ratio of fuel to pile size, shape and energy within that fuel.”
The whirls developed by trial and error could last 5-30 seconds, and proved largely stationary.
The study is adding to Scion’s growing level of understanding on forest fire behaviour, with earlier work in 2019 helping with the development of a NZ real-time fire and smoke modelling framework capturing multiple data sources to provide useful information on how smoke will behave in certain conditions.
Wallace says it is possible the data will better inform firefighters who may often have developed a strongly intuitive recognition of fire whirls over time, if not knowledge of the underlying science.
“If we know the conditions that create them we can support their decisions, proving a situation they may not intuitively have felt comfortable with,” he says.
The work has provided insights for the team’s next big project, a large-scale outdoor fire over 12ha involving standing timber and trees, planned for late summer/ early autumn 2025 in the district.
Data from that trial will prove invaluable for understanding real fire behaviour, when much of the past work has been based on extrapolating up, based on often conservative modelled behaviour.
Wallace says the researchers were humbled by the time volunteers and landowners offered them during the trials.
“The work can also help prove when fire may be an appropriate tool to use. This research will help us better understand how fire behaves and how it might be more safely used in the future.”
