One of the first concepts taught in Bush Firefighter (BF) is the fire triangle. Made up of heat, oxygen and fuel, it is a fundamental to the way we understand and extinguish fires. For Wildfire Behaviour (WFB), the fire triangle is enhanced to become a fire intensity triangle.

Each side of the triangle is layered with a new term, building on the one beneath. The prediction of fire behaviour and its intensity then becomes a relationship between the fuel moisture content; the wind influencing the fire; and the quantity, type and arrangement of the fuel. It is important to assess all of these elements to produce a reasonable estimate of fire behaviour. A brief description of each follows.


Fire Intensity Triangle

Fuel moisture content (FMC)

Is a measure of the amount of moisture (water) that is present in a fuel, expressed as a percentage. The moisture content of a fuel source affects how readily it will burn as moisture will absorb heat. Therefore, moisture content is an important factor in predicting the behaviour of bushfires..

Wind

Is one of the main variables that influences fire behaviour, it has an exponential relationship with rate of spread. Wind blows burning debris or firebrands ahead of the fire front, starting spot fires and increasing the difficulty of suppressing the fire.
Wind decreases the angle between flames and fuel, and increases the radiant pre-heating of the unburnt fuel ahead of the flame front.

Quite simply, the faster the wind blows, the faster a fire spreads.

Fuel type, load and arrangement

Influence 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.

Taking into account all of the weather and fuel information, fire behaviour is then calculated using a “McArthur Meter” and reported using the following key parameters:

  • Rate of spread – measured in kilometres per hour (km/h)
  • Flame height – measured in metres (m)
  • Spotting distance – measured in kilometres (km)

Each of these parameters once entered, provide an estimate of how fast, how high and how far the fire is likely travel (with some margin of error) and is vitally important in the safe prediction of a fire’s intended path and how to manage it.

Other aspects of fire behaviour of importance are:

  • Intensity – measured in kilowatts per metre (kW/m or KWm-1)
  • Flame length and depth (grass fires)

In this manual, each of these concepts is explored and presented so as to be practical on the fireground.