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Rate of spread increase up slope
Wind direction, spot fires.
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Fire behaviour depends on fuel type, fuel quantity, fuel moisture content, topography and ambient weather conditions, especially wind speed.
Fires can be described in terms of:

  • flame height
  • flame length
  • rate of spread
  • spotting distance
  • fire intensity (in kiloWatts of energy released per metre of fire line).
Flame height and flame length measurement.
Flame height and flame length measurement.
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Rate of spread increase up slope
Rate of spread increase up slope.
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A detailed understanding of fire behaviour is required to manage fire safely and effectively.
Fire behaviour is affected by the following.

  • Fuel moisture content affects the rate at which fuels burn. The drier the fuels, the more fiercely they burn. Fuel moisture content is influenced by the humidity and temperature of the surrounding air, by solar radiation and prior rainfall.
  • The amount and arrangement of fuel influences fire intensity, flame length and the duration of flaming. More fuel means more intense fire, larger flames and longer burn time. Fine, aerated fuels, such as grass, burn much faster than larger fuels such as branches and logs.
  • Terrain affects the speed at which a fire travels. Fire moves much faster upslope than downslope. North facing slopes tend to be drier and have less vegetation growth and therefore lower fuel loads. South facing slopes tend to be wetter and have higher fuel loads.
  • Wind speed affects the rate of spread of a fire and therefore its intensity. Higher wind speeds tilt the flames forward to pre-heat the fuel ahead of the fire and push the fire along increasing the rate of spread.

 

 

The fire triangle
The fire triangle.
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Fire requires heat, oxygen and fuel. Land managers cannot influence the heat (weather) nor can they influence the availability of oxygen. The only factor that land managers can influence is the amount of fuel. Reducing the fuel loads by prescribed burning does not prevent bushfires, but it significantly reduces their severity and damage potential.

 

Fire behaviour on coastal flats
This video was taken in the coastal flats and paperbark country south of Nannup during a bushfire in summer 2000. The fire behaviour is typical for this area due to the influence that wind has in this open vegetation structure.

 

Spot fires
This short video shows spot fires burning in grassland in Wooroloo. The spot fires were caused by burning embers being thrown in front of an approaching bushfire. Spot fires can present a dangerous situation for people, especially if they find themselves between the head fire and the spot fires or between multiple spots.

 

  • Fire behaviour typical of open savannah woodland in the Kimberley. Note this is still within the early dry season.
  • Fire behaviour in open woodland with high sorghum grasses.
     
  • Fire behaviour in karri country during the Northcliffe bushfire, 2012.
  • Spinifex mosaic burn, Karijini National Park.

 

Crowning fire in a pine plantation
This short video shows a fire crowning (burning into the tops [crowns] of the trees) in a pine plantation. Initially it is an intense surface fire. Within 10 seconds it begins to climb further into the crowns of the trees. The radiant and conducted heat from the fire preheats the pine needles ahead of the flames, enabling it to climb into the crowns. The suspended dead pine needles and the live needles with a high oil content act as a ladder, assisting the flames to move into the crowns. At about 13 seconds the video footage shows the fire in the crowns. The very strong convective winds caused by the fire draw the flames further up into the tree crowns.

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