Bushfire fuel, behaviour and risk

When people talk about the risk of bushfire they often mention fuel, but what is it? Which fuels are the most hazardous, how do they contribute to fire behaviour (the speed that a fire spreads, how much heat is released, and how long the fire burns) and what does that mean in terms of bushfire risk? The way fire behaves is determined by a combination of weather, topography and fuel. The only element of this bushfire triangle that we can influence as land managers, is fuel.

What is bushfire fuel?

Bushfire fuel is living and dead vegetation that influences the behaviour of a bushfire. It is characterised by its:

  • size (fine or coarse)
  • arrangement (horizontal or vertical orientation of the fuel)
  • flammability (influenced by vegetation type and moisture content)
  • quantity (the amount of fuel available for a fire to consume).

Fuel size and fire behaviour

How does fuel carry fire?

The fuel size determines a fire’s behaviour and level of risk.

Fine fuel

Fine fuels are less than 6 mm wide, or smaller than your little finger. The finer (and drier) the fuel, the more easily it will burn. This type of fuel is made up of dead grass, leaf litter, twigs, bark or some live vegetation, similar to what you would use to light a campfire or a wood fire in your lounge room. Fine fuels dry out fast and heat up quickly as a fire approaches, which means they catch alight and burn easily.

Fine fuel carries the fire front, making it the most dangerous of all fuel types, which is why it is the main one targeted for fuel reduction activities such as prescribed burning.

Coarse fuel

Larger fuels such as tree branches and fallen logs typically don’t burn in the fire front or carry the fire, however, they will generally burn for some time after a fire front has passed. This visible coarse fuel is far less combustible than fine fuels. It also doesn’t contribute to the rate of fire spread or flame size but does add substantially to the amount of heat released which makes fire suppression more difficult.

Bark fuel

Strong winds can carry burning bark fragments long distances and create spot fires ahead of the main fire. Burning the bark of rough-barked trees during a prescribed burn can reduce the risk of spotting and intense bushfires for a longer period than just burning fine fuels.

What is fuel arrangement?

The arrangement of fine fuel significantly affects how a fire behaves. The horizontal and vertical arrangement largely determines the rate of spread and intensity of fires.

There are 4 important fuel arrangement layers that influence fire behaviour:

  1. surface fuel (ground leaf litter and loose bark, measured by its depth)
  2. near-surface fuel (low bushy shrubs, clumps of grass and dead leaf material that rests on this type of vegetation, and is usually burnt in a fire)
  3. elevated fuel (includes taller shrubs and young trees)
  4. bark fuel (rough or loose bark on tree trunks and branches e.g. the bark fuel of stringybark trees are generally extreme hazard, whereas smooth bark gums will be low to moderate).

Fine fuel that is tightly packed together is less likely to burn and will smoulder due to lack of oxygen, whereas loosely arranged fuel will burn with more ferocity. Fine fuel that has gaps or spaces between it is less likely to carry fire than fine fuel that is continuously connected. More fine fuel means larger flames and greater fire intensity.

When shrubs and bark provide a continuous ladder of fuel into the tree canopy, a bushfire can burn high in the trees and produce a lot of heat.

assessing fuel hazards diagram

How is a fuel hazard assessed?

National Parks and Wildlife Service (NPWS) staff use this Overall fuel hazard guide for South Australia to assess the hazards posed by fuel layers. They regularly monitor fine fuel levels in parks to ensure any risk is managed in line with zone standards set by the State Bushfire Coordination Committee.

Once fuel hazards are assessed, this information is used:

  • in fire management planning to assess bushfire risk
  • to identify fuel hazards before and after a prescribed burn
  • to get an idea of how difficult it might be to control particular vegetation during a bushfire, taking into account the Fire Danger Index and the overall fuel hazard rating for the area.

Fires which are generally easy to suppress and are less likely to impact on assets have low fuels (see Photo A below). These fires will still ignite but their rate of spread will be slower and their intensity (measured in kilowatts per metre) will be lower. They have smaller flames, short-lived embers and are unlikely to develop into crown fires due to the absence of a fuel ladder. When fuels are extreme (Photo B) fires will burn at a greater intensity with larger flames and long-distance spotting. They are harder to suppress and have greater potential to cause significant damage.

Fire fuel, fire behaviour and bushfire risk fuel types

How do we manage fuel hazard?

Reducing the amount of bushfire fuel in an area could be the difference between 2m and 50m flames on a day of heightened fire danger. In other words, a fire we can work on or one where the only option is to retreat to a safe distance and to protect assets where we can.

There are 3 main methods of reducing fuel to suit different situations: mechanical removal of vegetation (such as slashing, thinning or mulching), chemical knockdown (such as weed spraying), and prescribed burning. NPWS Fire Management takes care in choosing the most appropriate and environmentally sensitive method for the area.