How a fire behaves
The combustion process
Before discussing fire behaviour we need to know how fires ignite. There are three basic components that are required for a fire to ignite, burn and continue to burn. These are oxygen, heat and fuel and are described in the fire triangle. The fuel can be any material that can be burnt, oxygen (O2) is an essential part of the chemical reaction needed to create fire, and heat is needed for ignition.
Fire behaviour is how fast a fire spreads and how intensely a fire burns and is determined by fuel, topography and weather (predominantly wind and temperature).
Fire spreads by a process called heat transfer. This is when the material immediately next to a fire is preheated to point where it gets hot enough to ignite.
The process of heat transfer is influenced by topography (slope and aspect). Because heat rises (convection) fuel above the fire is preheated more than fuel bellow a fire. Heat transfer therefore occurs more rapidly through fuels up a slope causing a fire to travel more quickly upslope than downslope. Aspect will influence the type of vegetation and fuel moisture. In NSW west facing slopes are usually hotter and dryer and support more fire tolerant (therefore more flammable) vegetation. South facing slopes however, are usually cooler and wetter and support more fire intolerant (less flammable) vegetation.
The amount of fuel available to burn is defined in terms of low, moderate, high, very high or extreme overall fuel hazard. Not all vegetation is fuel that burns. The important fuel is dead vegetation that is thinner than a pencil called fine fuels and the type of bark on trees. Fine fuels comprise surface fine fuels (leaves, fallen bark etc.. in the litter layer on the ground) and elevated fine fuels (twigs, leaves and grasses just above the ground surface).
How much fuel builds up in a given area depends upon how much the local vegetation 'sheds' dead fine fuel litter and how quickly it rots. The overall fuel hazard is measured by assessing the influences or hazard of the type of bark on trees, the amount of elevated fuel such as grasses, ferns and shrubs and the amount of fine fuel on the surface of the ground.
Overall Fuel Hazard = Bark Hazard + Elevated Fuel Hazard + Surface Fine Fuel Hazard
This approach is current best practice developed by the then Victorian Department of Natural Resources and Environment (now Department of Environment and Primary Industries), and represents a significant change in the philosophy of assessing fuel factors affecting fire behaviour. Rather than simply considering surface fine fuel loads (in tonnes per hectare) as in the past, it shifts the emphasis to considering the whole fuel complex, and particularly the bark and elevated fuels (fuel arrangement).
Research into the fuel accumulation and rotting rates of different vegetation communities have been used to develop fuel accumulation models. Office of Environment and Heritage uses fuel accumulation models as one of the indicators for prescribed burning.
Weather influences fire behaviour by creating conditions suitable for burning. Obviously it is harder for a fire to burn in high humidity or rain however, wind and temperature are the predominant drivers of fire behaviour. Hot temperatures will speed up the process of preheating and heat transfer and allow a fire to spread more quickly. Wind also speeds up the process of heat transfer by pushing flames and heat sideways to preheat unburnt areas. Wind can also change the direction of a fire and turn a fire flank (the side of a fire - lower intensity) into a fire front (the head of the fire - highest intensity).
More information on how weather can affect fire behaviour can be found on the 'Bushfire Weather' web page maintained by the Bureau of Meteorology. This site describes how weather systems like highs, lows and cold fronts affect fire behaviour.
Types of bushfires
Fire behaviour will determine what type of bushfire occurs. There are three types of bushfire; ground fire, surface fire and crown fire and one, two or all of these types of fire may make up a fire event.
Ground fire surface fire crown fire (images)
A ground fire can occur in any conditions and is where peat, coal, tree roots or other materials ignite and burn under the ground. Ground fires can burn through to the surface and become surface fires.
Surface fires are low to high intensity fires that burn on the surface of the ground. The tree canopy may be scorched but does not burn to the extent that it will carry a fire.
A crown fire occurs during fires of extreme intensity. A crown fire is when fire burns and spreads through the crown or canopy of trees. The influence of wind is greater in the tree canopy and where this canopy is interconnected or continuous, fires can spread incredibly quickly.
Page last updated: 07 December 2015