25 January 2000

Science comes to the help of bush fire fighers

by Kate Melville

While this writer freezes to death in New York, it's nice to see that not everyone is preoccupied with frozen pavements and snowstorms.

In Australia it's mid summer and the peak of the bushfire season. For most people the risks posed by bush fires seem very remote, but in a continent as dry as Australia they pose a very real threat even to suburban dwellers.

Over a decade ago a huge bushfire on Ash Wednesday burnt out several towns along the famous Ocean road, including a farm where this writer was working. The heat was so intense that many trees and houses simply exploded in the path of the fire. Luckily on that particular weekday most holidaymakers had returned to the capital, Melbourne, so luckily while property damage was extensive not too many lives were lost.

Protection for such bush fires comes primarily from local Country Fire Authority (CFA) brigades, staffed by volunteers like my friend Angus McKenzie. It's a pretty thankless job, with lots of boring but necessary tasks like telling landowners to clear vegetation around their houses. However year in and year Angus and people just like him put in 20 hours per week into an unpaid role where they are on call 24 hours a day, seven days a week all year long.

Fortunately the Commonwealth Scientific and Industrial Research Organisation (CSIRO) are constantly working to see how science can help fire fighters prevent or manage bush fires.

Peter Ellis of the CSIRO Forestry and Forest Products division has conducted one new piece of interesting research. What he has found is that during a hot bushfire with strong winds some species of gum tree can send flying sparks more than twenty-five kilometres ahead of the main blaze!

Ellis results came from experiments he carried out using a custom built 12-metre tall vertical wind tunnel to investigate the hazards of flying sparks and embers.

His new data will help fire fighters predict how far ahead of a blaze 'spot fires' can break out. Spot fires are both a risk to property but also to firefighters who can suddenly find themselves trapped by fires that have jumped over them.

According to Peter Ellis, "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). 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."

There are many factors that can affect the maximum distance a spot fire can occur from a blaze and these include the wind speed fire intensity, initial ember size and how rapidly it is burning. However a key variable seems to be the ember's 'terminal velocity', the speed at which it falls in the absence of an updraft (this decreases as the ember burns away).

Using his special wind tunnel Peter Ellis studied the behaviour of hundreds of pieces of burning bark. "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."

What he found was the length of time a piece of bark is exposed to flame (the ignition time) was a key factor on how quickly it burns, and therefore how long it remains capable of starting another fire. In one test a smouldering embers burst into flame, 6 minutes after the initial ignition! Once the burn results were in Ellis added these to a bushfire plume model also developed by the CSIRO to calculate 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," Pete Ellis said. "The rate of combustion can have a big impact as well, because of its effect on terminal velocity. 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 spot fires several kilometres ahead of the fire.

Fire fighters like Angus need to know what are the likely different patterns of flying ember behaviour under different fire and wind conditions?

In California authorities may also be interested in this work as their state has many eucalypts in fire prone areas. Peter Ellis is now carrying out further work on the characteristics of other eucalypt species.