Factors that create bushfire risk

A number of factors can affect the risk of a bushfire.

These include:

• fuel types and conditions
• weather
• topography.

The intensity of a bushfire and its rate of spread are influenced by the fuel or vegetation's:

• quantity
• type
• curing state (when grass dies and dries out)
• continuity.

Fuels

In the Northern Territory, fuel types consist of:

• dried grass fuels, including naturally occurring, invasive and pastoral, native sorghum species, spinifex species, tully (Urochloa humidicola), pangola (Digitaria eriantha) and invasive fire weed species such as:
  • gamba grass (Andropogon gayanus)
  • mission grass (Cenchrus sp.)
  • buffel grass (Cenchrus ciliaris)
• agricultural and pastoral feed crops - mango, banana and market gardens
• understorey shrub, herb and forb species.

Under the right circumstances, most of these fuels can readily ignite and keep burning. However, the intensity of a fire will vary depending on the vegetation type.

Gamba grass

Gamba grass causes very high fuel loads resulting in intensive fires destroying native plant communities.

It poses a threat to lives and property in the greater Darwin region.

Read the gamba management guide PDF (1.1 MB).

Read more about gamba grass.

Buffel grass

Buffel grass can create a fire hazard for infrastructure, animals and people.

As it dries off between periods of growth, a high volume of dry plant matter can accumulate, which can fuel intense fires.

Resilience to fire enables buffel grass to survive and quickly produce new growth after burning. This provides fuel for more fires.

Read the buffel management guide PDF (3.5 MB).

Read more about buffel grass.

Weather conditions

Bushfires can be unpredictable and vary greatly according to local weather conditions. Bushfires frequently start on days marked by hot, dry and windy conditions.

Relative humidity

Relative humidity refers to the percentage of water vapour in the atmosphere.

A reduced relative humidity signifies that the lower atmosphere is very dry. This happens during the Top End dry season and throughout spring/summer in semi-arid/arid regions.

When it falls below 30%, the risk of fire steadily increases.

Lower relative humidity causes vegetation and flammable materials to lose moisture through evaporation. This makes them more prone to ignition.

Wind

During large bushfires, strong winds typically occur. They can pose significant challenges for firefighters as they try to control the blaze.

Winds can influence:

• fire spread - the speed at which a fire moves through a landscape
  • the higher the wind speed, the greater the fire danger
• fire direction - the direction in which a fire travels and the size of the fire front
  • a shift in wind direction can rapidly change the fire front and direction
• fire intensity - wind gusts can increase a fire’s intensity by providing more oxygen
• spotting likelihood - spot fires can happen when winds carry burning leaf and organic matter (embers) ahead of the main fire.

Wind direction and speed

A change in wind direction is one of the most dangerous influences on fire behaviour.

Many people who have lost their life to bushfires in the past have been caught out during or just after a change in wind direction or speed.

Rapid changes in wind direction and speed can cause the flank of a fire to become the head of the fire. This rapidly alters the fire direction and intensity.

Lightning

Dry lightning refers to thunderstorms where thunder and lightning occurs with minimal rainfall (patchy, isolated showers accompanying these storms).

Thunderstorms during the October to December 'build-up' period in the Top End/Savanna/Arnhem Land can spark bushfires across the landscape. Similar fire risks extend to the Tanami, Barkly, and Central Australia regions in spring.

Topography

Fire will typically burn at a faster rate uphill as flames reach out for unburnt fuel and the fuel is preheated ahead of the front.

For every 10 degree increase in slope, a fire front will double its speed. As a fire front accelerates, its intensity also increases. This results in a significantly higher convective temperature and rate of spread.

The opposite applies to a fire front travelling downhill. The flames have less of a reach toward fuels and there is less radiant heat to pre-heat the fuel ahead of the fire front. This results in a decreased rate of spread.

How bushfires spread

Bushfires spread across the ground surface in 3 ways:

• Direct flame contact - when flames touch unburnt fuels, they raise their temperature and cause them to catch fire. This can happen faster when the wind blows the flames ahead or when the fire is travelling uphill.
• Radiant heat - this is the heat you feel from a bushfire and it can cause the temperature of fuel to increase enough for it to catch fire. This can happen even before the fire reaches the fuel.
• Burning embers - embers ignite when they land on fine fuels and can start spot fires. If the fire isn't put out, it can smoulder, grow and spread. Embers are typically carried by wind ahead of the actual front, sometimes landing in areas several hundred metres away.

Bushfire seasonal outlook

Each quarter, the Australasian Fire Authorities Council (AFAC) releases the seasonal outlook to help communities:

• better understand their bushfire risk
• take appropriate action to prepare to protect people and property.

To find out more, go to the AFAC website.