Basics

What is a bushfire?

Bushfires and grassfires are common throughout Australia. Grassfires are fast moving, passing in five to ten seconds and smouldering for minutes. They have a low to medium intensity and primarily damage crops, livestock and farming infrastructure, such as fences. Bushfires are generally slower moving, but have a higher heat output. This means they pass in two to five minutes, but they can smoulder for days. Fire in the crown of the tree canopy can move rapidly.

Bushfires are an intrinsic part of Australia's environment. Natural ecosystems have evolved with fire, and the landscape, along with its biological diversity, has been shaped by both historic and recent fires. Many of Australia's native plants are fire prone and very combustible, while numerous species depend on fire to regenerate. Indigenous Australians have long used fire as a land management tool and it continues to be used to clear land for agricultural purposes and to protect properties from intense, uncontrolled fires.

Historically, bushfires have caused loss of life and significant damage to property. While naturally occurring bushfires cannot be averted, their consequences can be minimised by implementing mitigation strategies and reducing the potential impact to areas which are most vulnerable

What factors create a favourable environment for bushfires to occur?

The basic factors which determine whether a bushfire will occur include the presence of fuel, oxygen and an ignition source. The fire intensity and speed at which a bushfire spreads will depend on ambient temperature, fuel load, fuel moisture, wind speed and slope angle.

Fuel load

Fuel load describes the amount of fallen bark, leaf litter and small branches accumulating in the landscape. Generally speaking, the greater the fuel load, the hotter and more intense the fire. Fuel which is concentrated but loosely compacted will burn faster than heavily compacted or scattered fuel sources. Smaller pieces of fuel such as twigs, leaf litter and branches burn quickly, particularly when they are dry and loosely arranged and will burn quickly in the fire front. Larger fuels, such as tree trunks often burn later after the fire front has passed. The natural oil within eucalypt trees promotes the combustion of fuel.

Fuel moisture

Dry fuel will burn quickly, but damp or wet fuel may not burn at all. As a consequence, the time since rainfall and the amount of rain received is an important consideration in assessing bushfire danger. Often a measure of the drought factor, or moisture deficit, will be used as an indicator of extreme bushfire weather conditions.

Wind speed

Wind acts to drive a fire by blowing the flames into fresh fuel, bringing it to ignition point and providing a continuous supply of oxygen. Wind also promotes the rapid spread of fire by spotting, which is the ignition of new fires by burning embers lofted into the air by wind. Spotting can occur up to 30km downwind from the fire front.

There is a threshold wind speed of around 12 to 15km/h which makes a significant difference in the behaviour of bushfires in the open. When wind speeds are below this threshold, fires with heavy fuel loads burn slowly. However, even a slight increase in wind speed above this threshold results in a significant increase in fire behaviour and advancement. The width of a fire front also has an influence on the rate of spread and a wind shift can immediately widen the forward edge of a fire.

Ambient temperature

The higher the temperature the more likely it is that a fire will start or continue to burn. This is because the fuel is closer to its ignition point at high temperatures and pre-heated fuel loads burn faster.

Relative humidity

Dry air promotes a greater intensity fire than moist air. Plants become more flammable at a low humidity because they release their moisture more easily.

Slope angle

Fires pre-heat their fuel source through radiation and convection. As a result, fires accelerate when travelling uphill and decelerate travelling downhill. The steepness of the slope plays an important role in the rate of fire spread. The speed of a fire front advancing will double with every 10 degree increase in slope, so that on a 20 degree slope, its speed of advance is four times greater than on flat ground.

Ignition Source

Bushfires can originate from both human activity and natural causes with lightning the predominant natural source, accounting for about half of all ignitions in Australia. Fires of human origin currently account for the remainder and are classified as accidental or deliberate. Fires lit deliberately can be the result of arson or might be designed to achieve a beneficial outcome but conditions have changed, resulting in uncontrollable spread.

Unfortunately deliberate and accidentally lit fires are more prevalent near populated areas and have a disproportionately higher risk of infrastructure impact. Arsonists place people and property at serious and unnecessary risk, particularly when igniting fires on extreme fire weather days.

Where do bushfires occur?

The Australian climate is generally hot, dry and prone to drought. At any time of the year, some parts of Australia are prone to bushfires. The widely varied fire seasons are reflected in the continent's different weather patterns. For most of southern Australia, the danger period is summer and autumn. For New South Wales and southern Queensland, the peak risk usually occurs in spring and early summer. The Northern Territory experiences most of its fires in winter and spring.

Grassland fires frequently occur after good periods of rainfall which result in abundant growth that dries out in hot weather. Bushfires tend to occur when light and heavy fuel loads in Eucalypt forests have dried out, usually following periods of low rainfall.

The potential for extreme fire weather varies greatly throughout Australia, both in frequency and severity. When potential extreme fire weather is experienced close to populated areas, significant loss is possible. In terms of the total area burnt, the largest fires are in the Northern Territory and northern areas of Western Australia and Queensland. Most loss of life and economic damage occurs around the fringes of cities where homes are commonly in close proximity to flammable vegetation.

  • In the years between 1967 and 2013, major Australian bushfires have resulted in over 8000 injuries and 433 fatalities, close to 50 per cent of all deaths from major Australian natural disasters in the period (excluding heatwaves). Over this same period, bushfires cost approximately A$4.7 billion (2013 Australian dollars, including deaths and injuries but excluding most indirect losses).
  • A fire front advances more quickly when travelling upslope and slows travelling down slope. The speed of a fire front advancing will double for every 10 degree increase in slope, so that on a 20 degree slope, its speed is four times greater than over flat ground.

References:

Ladds M, Keating A, Handmer J and Magee L (2017)How much do disasters cost? A comparison of disaster cost estimates in Australia. International J of DRR.  10.1016/j.ijdrr.2017.01.004

Ladds, MA, Magee, L, Handmer, J (2015) AUS:DIS - Database of losses from disasters in Australia 1967-2013. https://github.com/liammagee/sealand

John Handmer, Monique Ladds and Liam Magee (December 2016), Disaster losses from natural hazards in Australia, 1967–2013. (Report with AGD)