Low intensity fire for managing fuel.
Low intensity fire for managing fuel.
Photo © Parks and Wildlife 

Managing fuel

The severity of a bushfire largely depends on the weather, topography and the condition of the vegetation, which becomes the fuel. Of these, the condition of the fuel is the only factor that land managers can influence. Regular prescribed burns are the most effective way to manage fuel loads and fuel flammability or structure. Bushfires can usually be successfully and safely controlled when they burn into low fuel areas. Other ways to restrict the available fuel in particular situations, such as around valuable assets, include dousing the fuel with water or fire retardants, removing the fuel on fire breaks, slashing or crushing vegetation and grazing.

What is fuel?

Fuel is live and dead vegetation that accumulates over time. Dead leaves, twigs and bark build up as they fall from trees and shrubs, but some of this material is decomposed by insects, micro flora and fungi. Fuel can be characterised by type, size, quantity, arrangement and moisture content.

Common types of fuels include:

Comfortable distance from flames Illustration
Comfortable distance from flames illustration
.(Select image for larger view)
  • grass
  • forest litter lying on the ground
  • small shrubs and scrub
  • trees, logs, stumps and bark and
  • decomposing humus and duff (fine ground fuel).

Fuel size is classified as fine or heavy. Fine fuels, such as grasses and small twigs, are less than 6mm in diameter. Heavy (or coarse) fuels are greater than 6mm in diameter and include branches, logs and stumps. Heavy fuels do not contribute to rate of spread or flame size, but add substantially to the total amount of heat released and make fire suppression more difficult.

The arrangement of fuel can affect fire behaviour. Fuel that is tightly packed is less likely to burn and will smoulder due to lack of oxygen, whereas loosely arranged fuel will burn with more ferocity due to more oxygen. Fuel that is separated is less likely to carry fire than fuel that is continuous. In addition, more fuel means larger flames and greater fire intensity.

Fire behaviour is affected by the moisture content of fuels, which in turn depends on a number of factors such as weather conditions, vegetation types and whether the fuel is dead or living. When fuel moisture is high there is less chance of a fire igniting than if the fuel moisture was low.

Fire fact

Fuels in jarrah forests:

  • accumulate at 1-2 tonnes/ha each year
  • reach a maximum of about 20 tonnes/ha in 20 years.

Fuels in karri forests:

  • accumulate at 3-4 tonnes/ha each year
  • reach a maximum of about 60 tonnes/ha in 20 years.
fire safe distance2
Fire intensity table.
(Select image for larger view)

Fire intensity, expressed in kilowatts per metre (kW/m), is the amount of energy released from each metre of headfire edge. One kW/m is equivalent to the energy released by a small bar radiator. Fire intensity depends upon how much fuel is burnt and how fast it burns. Severe bushfires, such as the Victorian Black Saturday fires, can generate intensities in excess of 100,000 kW/m, whereas prescribed fires are usually less than 500 kW/m. Only bushfires of less than 2,000 kW/m can be safely and effectively suppressed by people and machinery working directly on the flaming edge of the fire.

  • High intensity bushfire.
    Photo © Parks and Wildlife
  • High impact resulting from an intense bushfire. Photo © Parks and Wildlife

Fire fact

Recommended limits of head fire behaviour and suppression strategies.

  • Fire intensity less than 800 kW/m can be suppressed with hand tools with water support as a direct attack.
  • Fire intensity less than 2000 kW/m can be suppressed by machines, tankers and water bombers as a direct attack.
  • Fire intensity greater than 2000 kW/m may be suppressed by machines, tankers and water bombers using an indirect attack.
  • Fire intensity greater than 3000 kW/m is unlikely to be suppressed.

It only takes between five and eight years for fuel loads in most forest types to build up to a point where fire intensity is likely to exceed 3000kW/m under summer conditions. Creating a mosaic of fuel loads across the landscape – including low fuel areas where bushfires can be easily controlled and to minimise fire runs in high fuel areas – enables safe and effective fire suppression operations to occur in these forests.

Low intensity, mosaic or patch burning is designed to provide protection to the environment and to communities, and is usually conducted in spring and autumn and autumn-like conditions when weather is mild and fire behaviour is moderate and easier to manage. Science-based fire behaviour models assist fire managers to plan and implement prescribed burns.

Seasonal burning

In southern parts of Western Australia, spring burning is undertaken when fuels are still reasonably moist from winter rains. The moisture content of the fuels affects fire behaviour, resulting in mild, slow moving fire with low flame height and low fire intensities. Flames are usually extinguished overnight when the air gets cooler and the moisture in the air (relative humidity) increases. Burning under these conditions results in a patchy burn with some wetter areas left unburnt. These areas act as refuges for flora and fauna and provide reservoirs for recolonisation into the burnt areas. As spring progresses towards summer and the sun and wind continue to dry the fuels, it makes them more flammable, and the opportunity to undertake safe burning decreases.

Autumn burning is undertaken when the dry summer fuels increase in moisture content as a result of the opening seasonal rains. Cooler weather and damper fuels after the first rains in autumn modify the behaviour and intensity of fire. However, the landscape is drier than in spring. Bark and dead woody material above ground can catch alight and scorching of tree canopies is more common than in spring. Larger fuels such as limbs and logs may continue to smoulder for days. Burning under these conditions usually results in a more uniform burn, with larger areas within the burn area being burnt and fewer, smaller unburnt patches. Autumn burns tend to promote regeneration and resprouting of vegetation, release of nutrients and reinvigoration of local habitats. As autumn progresses, temperatures decrease and rainfall continues to wet the fuels, making them less flammable. Due to the smaller window of opportunity to safely carry out autumn burns, before winter rain makes the fuel to wet to burn, in most years most burning is conducted in spring. See prescribed burning.

A fire regime that includes a mix of spring and autumn burns is preferred for maintaining healthy habitats and protecting communities and the environment from bushfires.