New understanding about the behaviour of 'firebrands' from ribbon bark eucalypts suggests a change in thinking may be needed to fight fires in extreme conditions.

The challenge

Firebrands add fuel to the bushfire

It's the stuff of spectacular and terrifying TV footage, fierce fireballs and flames leaping from burning tree canopies. Potentially more dangerous than these crown fires and one thing TV cameras can't catch, however, are firebrands. They are the strips of bark, ignited in the firefront and lofted up to immense heights in the convection column above the fire, transported long distances and that start new fires downwind of the originating fire.

Our response

Barking up the right tree

Our scientists investigated the various shapes of the most likely type of bark under wildfire conditions to demonstrate how they can travel so far and remain viable firebrands. Their findings indicate that managing firebrands, and the forests prone to produce them, is critical in the suppression of high intensity wildfires that may be prone to long-distance and mass fire spotting.

ribbons of bark hanging from the high branches of a gum tree

Long ribbons of bark hanging high in the branches of manna gums like this are key to understanding the real risk of firebrands.  © James Hall/CSIRO Publishing

Bark characteristics (including ease of ignition and ease of detachment) are now considered to be a key component of the fuel complex and in understanding likely fire behaviour. Our Bushfire Behaviour and Risks team and researchers from the ANU, studied how firebrand behaviour contributed to fire spotting during the Black Saturday bushfires, with particular attention to the unkempt ribbon bark eucalypts such as Eucalyptus viminalis (manna gum) and E. rubida (candlebark). Ribbon barks typically shed their bark in hot weather, producing strips several metres long that have a tendency to curl longitudinally.

The team used our Vertical Wind Tunnel to investigate just how strips of burning bark from these trees combust under the conditions of terminal velocity (the maximum speed at which an object travels through the air). From this they could deduce how far they could travel as viable firebrands.

They tested flat, simple cylinders and curled cylinders of bark, oven-dried to mimic wildfire conditions. The tests showed that the average burnt out time for short tightly curled cylinders was more than seven minutes. This suggests that a similar firebrand 2.7 m long would be capable of remaining alight for more than 30 minutes. Under conditions typical of a high intensity forest fire, the firebrand would be capable of travelling and causing a spotfire 37 km away.

The research concluded that it is both the length and, importantly, also the way the strips curl in on themselves which make them so successful as long distance firebrands.

Of course, spotfires are not just the result of the firebrand itself. The combustibility of the fuel in which the firebrand lands provides the potential for the firebrand to start a spotfire. Short to medium distance spotting may extend the fire front and exacerbate the behaviour of the fire. Long distance spotting creates whole new fires that can complicate fire suppression activities.

The enormous distance firebrands can travel and start spotfires has, before now, been the subject of anecdote.

The results

Fighting the firebrands

The tragedy of Black Saturday has brought new and more sophisticated understanding of fire behaviour as a function of forest type. This can now be used to improve fire-fighting strategies. Spotting is a very complex phenomenon. We try to predict where fires might spread – how far, how fast, how intense and what they might impact. Any science that improves our ability to predict spreading will help us plan strategies and tactic to fight fires.

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