A hot bushfire, a strong wind, and some species of gum tree can send flying sparks more than twenty-five kilometres ahead of the main blaze.

CSIRO's Peter Ellis is using a novel, specially-built 12-metre tall vertical wind tunnel at CSIRO Forestry and Forest Products in Canberra to investigate the hazards of flying sparks and embers.

His data will help fire managers predict how far ahead of a blaze 'spot fires' can break out.

"In the first study of eucalypt bark firebrands, I used the wind tunnel to study the behaviour of pieces of burning bark from messmate stringybark (Eucalyptus obliqua)," says Peter Ellis. "This species, widely distributed through hilly country in south-eastern Australia, has a reputation for extensive spotting up to five kilometres out. The manna gum (E. viminalis) can send sparks five times this distance."

The research project started after the devastating 1994 bushfires in Sydney's Como-Jannali region, where pieces of burning bark are reported to have travelled more than a kilometre to start a new fire, which destroyed a hundred houses.

"There are many factors which affect the maximum spotting distance from a blaze," says Mr Ellis. "These include the fire intensity, the wind speed, the initial size of the ember and how rapidly it is burning.

"A key variable is the ember's 'terminal velocity', the speed at which it falls in the absence of an updraft. This decreases as the ember burns away," he says.

Mr Ellis has studied the behaviour of hundreds of pieces of burning bark in the vertical wind tunnel.

"To find out the terminal velocity, we adjusted the upward air flow in the tunnel until the ember stopped rising or falling," he says. "At this point, the air speed equals the terminal velocity. The rate at which the terminal velocity declines can be worked out from a series of these measurements."

The length of time a piece of bark is exposed to flame — the ignition time — has a marked effect on how quickly it burns, and therefore how long it remains capable of starting a spot fire, says Mr Ellis. Smouldering embers can also sometimes burst into flame, in one case 6 minutes after ignition. This is important in terms of the probability of flying sparks igniting fuel. 

Using a bushfire plume model developed by Dr Mike Raupach of CSIRO Land and Water, Mr Ellis has calculated the likely paths of embers from fires of varying intensity in different wind conditions.

"The important things are how high they are carried in the updraft, and the strength of the wind driving them forward," he says. "The rate of combustion can have a big impact as well, because of its effect on terminal velocity."

Mr Ellis says that bushfire fighters need to be aware of different patterns of flying ember behaviour under different fire and wind conditions.

"When conditions are extreme, pieces of fast burning bark will be carried high in the plume and are likely to burn out before landing; a typical combustion time is 2-3 minutes," he says. "Pieces of slowly burning bark are a greater risk, as these can exit the plume and a longer burn-out time of up to 6 minutes or more means they may still be burning when they land. This means they can start spotfires several kilometres ahead of the fire.

"Under milder conditions, fast burning embers are the main concern as they may be quickly carried to a height from which they can descend and land perhaps hundreds of metres ahead of the fire before burning out," he says. "Slowly burning pieces, because of their slow loss of terminal velocity, aren't carried very high and so come to ground not far ahead of the fire."

The next research goal is to apply the wind tunnel results to other eucalypt species, and the development of practical fire spotting guidelines for fire managers and bushfire fighters.

The spot fire research project is sponsored by Mayne Nickless Ltd.

More information from:
Mr Peter Ellis 02 6281 8386
Mr Mick Crowe 02 6281 8357
0419 696 184

Mick.Crowe@ffp.csiro.au

Vertical wind tunnel - when the firebrand (insert) appears stationary its terminal velocity is equal to the vertical air velocity which can be measured.