Anatomy of a Bushfire

The following article appeared on the ABC News after the 2009 Victorian fires and is reproduced here with the permission of the ABC and the author. It refers to Victorian conditions but the principles apply anywhere in Australia. The fire danger rating system was altered since this article was published. Note that the term controlled burn used here is the same as prescribed burn or fuel reduction burn.
Some formatting changes have been made from the original article.

Anatomy of a bushfire

By Peter Attiwill, Principal Fellow in Botany, Senior Fellow, The Australian Centre, The University of Melbourne

The start of a bushfire is no different from the start of any other fire. The ‘fire triangle’ is elementary.
Quite simply, a fire must have fuel to burn, heat to ignite the fuel, and oxygen.
Remove any one of these parts of the triangle, and the fire will go out.

About half of the bushfires in Victoria are ignited either by lightning or are deliberately lit. However, lightning strikes alone account for 50 per cent of the total area burnt each year.

Oxygen makes up about 21 per cent by volume of the atmosphere, and so it is always in good supply. As the fuels burn, oxygen is used in converting organic carbon in the fuel to carbon dioxide (the process of oxidation). As wind speed increases (up to certain limits), fresh air is brought in, thereby increasing the supply of oxygen and hence increasing the rate of oxidation.

Given that we have both uncontrollable sources of ignition and an atmosphere rich in oxygen, it is obvious that the availability of fuel is the most critical factor in determining the intensity and behaviour of a bushfire.

How does a small fire become a bushfire?

A bushfire that can be attacked very soon after it starts can usually be controlled. Fuel can be removed by raking around the fire, or by creating a fire line around the fire with bulldozers. Heat can be removed by spraying with water. The supply of oxygen is always difficult to control, except when the fire is very small – for example, a campfire can be extinguished by covering it with soil, thereby depriving it of oxygen.

A bushfire that is not attacked soon after it starts can quickly get out of control. The two key factors are:

  • The quantity and quality of fuel. Fine, loose fuels burn more rapidly than large, compacted fuels), and drier fuels burn more rapidly than wetter fuels; and
  • Weather conditions

The forward rate of spread of a bushfire is almost directly proportional to the fuel load for given conditions of moisture content of the fuel and wind speed. These two last variables-moisture content of the fuel and wind speed-have major effects on the rate of forward spread. For example, as moisture content decreases from 20 per cent to 7 per cent, the rate of forward spread increases ten-fold; the rate of forward spread increases exponentially as wind speed increases.

At low intensities, fires spread largely by convection. At higher intensities, direct ignition dominates, both by direct flame contact and by ‘firebrands’ of burning eucalypt bark carried by the wind can ignite new fires (‘spotting’) at great distances ahead of the fire: up to 30 kilometres.

Bushfires start as ground fires, burning the fine fuels on the forest floor. Heat from the ground fire can be carried by convective and radiative forces up to the tree crowns, and the oil-rich leaves of the eucalypt leaves are highly flammable under dry conditions. The ground fire thus creates and supports a running crown fire.

How do we measure the intensity of a bushfire?

The intensity of a fire, or the heat released per unit length of fire edge, depends on the amount of fuel consumed every second, which in turn depends on the speed of the fire and the amount of fuel available for combustion. Fire intensity (I) has the units watts per metre, (W/m). The range of intensities for forest fires is enormous – from 20 to 100,000 kilowatts per metre (kW/m). It is difficult to imagine an intensity of 100,000 kW/m; however, a large, household radiator emits about 1 kW, so imagine 100,000 large radiators, powered and stacked up along every metre!

Predicting bushfire danger

The Forest Fire Danger Rating (FFDR) is calculated from predictions of air temperature, relative humidity, rainfall and number of days since rain, a drought index and wind speed. Fire Danger Rating varies from 1-100±; the fire danger classes represent the degree of difficulty of fire suppression in a dry eucalypt forest with a fine fuel load of 12.5 tonnes per hectare:

  Low <5  Almost no danger of a fire starting, little effort needed by fire-fighters to put the fire out;

Moderate 5-12  Moderate possibility of a fire starting, slightly increased effort needed by fire-fighters to put out fire. In other words, on days of low or moderate risk a fire will either go out or be easily put out;

High 12-24  Increasing possibility of a fire starting, more effort needed by fire-fighters. Fires can be either put out rapidly or managed in cooler weather in the evening or on the next day. Houses could be threatened;

Very High 24-50   Authorities will consider a total fire ban, high chance of fire starting and expanding quickly, fire-fighters need strong efforts. The fire generates sparks and embers, and property can be threatened. Proper safeguards will enable a house to be protected if the householder stays on the scene.

Extreme 50-100  Total fire ban, very high chance of fire starting. On these days, a fire becomes uncontrollable very quickly and is impossible to put out, even in low-fuel areas. Fire-fighting resources are stretched to the limit, and the support and preparedness of householders is crucial to the protection of life and property.

The fire danger rating of 100 was based on the weather conditions measured in Melbourne on the fateful day, Black Friday, 13 January 1939, when bushfires raged through the mountains of Victoria and southern Australia. However, we now have many more weather recordings around the state than in 1939 and these stations may record much higher fire dangers at other locations around the State when the fire danger in Melbourne is rated at 100. For example, Ash Wednesday, 1983, saw the rating reach an astonishing 140 in parts of Victoria and South Australia, and 160 in the Yarra Valley a week after Black Saturday, 2009. It is highly likely that similar fire dangers were experienced through rural Victoria in 1939.

Both the rates of spread and spotting distance ahead of the fire increase more or less linearly with Fire Danger rating and with the load of fine fuels. Increases in fuel load rapidly take the flames up into the crowns of the trees when the Fire Danger rating is greater than 20.

At what intensities do we have any hope of controlling a bushfire? A fire of less than 1,000 kW/m can be suppressed by crews working on the ground with hand tools. Bulldozers and aircraft can be used to suppress fires of slightly higher intensity (about 2,500 kW/m ) and are effective at low fuel loads. As the load of fine fuels increases, intensities become too great, even at low to moderate fire danger, for direct suppression by any means. In Phil Cheney’s words, ‘our ability to quell this heat is puny. Under favorable conditions we can put out a fire of 2,000 kW/m or 2 per cent of the maximum intensity. This means that under extreme conditions we simply cannot put out a bushfire unless we catch it within minutes of ignition or until it runs out of fuel’.

In short, given the right combination of climate and fuel, bushfires can become so intense over such an extensive area that ‘no fire fighting capabilities of any nation’ could stop them.

Controlled burning for fuel reduction

The only way that we can minimize fires of high intensity, such as those that have ravaged south-eastern Australia over the past 6 summers (2002/03-2008/09) is to reduce the fuel loads in our forests by controlled burns. There is a wealth of evidence that controlled burns reduce both the extent and intensity of bushfires. The relative effects of regular controlled burns and high intensity bushfires on biodiversity and ecosystem processes are matters for continuing research to produce optimum outcomes for both biodiversity and fire control.

However, the argument presented here is that megafires are emerging as more of a land management issue than the more commonly perceived fire issue. Forests that have been protected by minimum disturbance and by fire suppression over the years have built up huge fuel-loads that are now, together with increasingly hotter and drier conditions, fuelling the hottest fires. The very values that we set out to protect-species habitat, catchments for water supply, and many others-are being destroyed by land- and fire-management policies, the consequences of which are high-intensity bushfires.


Further reading

Chapters in ‘Fire and the Australian Biota’ Eds AM Gill, RH Groves, IR Noble. Australian Academy of Science: Canberra, 1983.

‘Burning Bush: A Fire History of Australia’. Stephen J Pyne. Allen & Unwin: North Sydney, 1992.

‘Bushfires in Australia’. RH Luke, AG McArthur. Australian Government Publishing Service: Canberra 1978.

‘Prescribed Burning in California Wildlands Vegetation Management’. Harold H Biswell. University of California Press; Berkeley, 1989.

The first step in combating forest fires is to understand their behaviour. NP Cheney. In ‘In the Living Forest’ (Ed. John Keeney). ForestsNSW: Sydney.


 

About the Author

Peter Attiwill (BSc, PhD) has 45 years of teaching and research experience in plant ecology. He has more than 170 publications, particularly in eucalypt and bushfire ecology. He is co-author of a number of books including Forest Soils and Nutrient Cycles (MUP, 1987) and co-editor of Nutrition of Eucalypts (CSIRO Publishing, 1996) and Ecology: An Australian Perspective (Oxford University Press, 2003, 2006). He was among the pioneers of research on soils and nutrition in Australia’s native forests, and his many PhD students now hold major positions in universities, CSIRO and state departments. He has been involved with the boards of a number of international journals such as Plant and Soil and Biogeochemistry; he was Chairman of the Board of Australian Journal of Botany and is currently Editor-in-Chief of the foremost international forestry journal, Forest Ecology and Management.

Attiwill retired from The University of Melbourne in 2001, and is now Principal Fellow in Botany and Senior Fellow in The Australian Centre, The University of Melbourne. He is a member of the Stretton Group, a small group that aims to bring some commonsense and action to the management of fire in Victoria. He is a Director of the Natural Resources Conservation League.