FIRE BEHAVIOUR – The Bushfire Foundation Inc

Victoria is one of the most fire-prone areas in the world. Bushfires and grassfires start quickly, often without warning and they can threaten lives and properties within minutes.

There’s no questioning the facts. During a fire, embers can travel up to 40 kilometres ahead of the fire front and fire speeds can reach over 25 kilometres per hour.

Remember, on high-risk fire days, leaving early before a fire starts, is always the safest option.

Terrain (topography)

A fire will burn faster uphill. This is because the flames can easily reach more unburnt fuel in front of the fire. Radiant heat pre-heats the fuel in front of the fire, making the fuel even more flammable.

For every 10˚ slope, the fire will double its speed. For example, if a fire is traveling at 5 km per hour along flat ground and it hits a 10˚ slope it will double in speed to 10 km per hour up the hill.

By increasing in speed the fire also increases in intensity, becoming even hotter.

The opposite applies to a fire traveling downhill. The flames reach less fuel, and less radiant heat pre-heats the fuel in front of the fire. For every 10˚ of downhill slope, the fire will halve its speed.

Fires tend to move more slowly as the slope decreases.

Vegetation (Fuel)

Grass can burn early and quickly on hot, dry, windy days.
Branches, twigs and leaves dropped from shrubs and trees become fine fuels, which burn easily. These can give off far more heat when they burn
Fibrous and dry tree bark can carry fire to treetops. The fire can then break away and spread further
Dry branches, twigs and leaves and other fine fuels found on the ground can also burn easily.

Weather Conditions

Bushfires are unpredictable and vary greatly according to weather conditions. They often start on hot, dry, windy days.

Temperature

A string of hot days dries out vegetation, making it easier to burn. This can be made even worse by underlying dry conditions caused by lack of rain.

The drier the vegetation the easier it will burn.

A fire spreads as a result of burning embers, radiant heat and direct flame contact.

Wind

Wind influences the:

speed at which a fire spreads
direction in which a fire travels and the size of the fire front
intensity of a fire – wind provides more oxygen
likelihood of spotting – burning pieces of leaves, twigs and bark (embers) that the wind carries ahead of the fire. These cause new ‘spot fires’ to ignite.

Wind Change

A change in wind direction is one of the most dangerous influences on fire behaviour. Many people who die in bushfires get caught during or after a wind change.

In Victoria, hot, dry winds typically come from the north and northwest and are often followed by a southwest wind change. In this situation the side of the fire can quickly become a much larger fire front.

What is an Ember Attack?

Embers are burning twigs, leaves and pieces of debris
Ember attack occurs when twigs and leaves are carried by the wind and land on or around houses
Ember attack is the most common way houses catch fire during bushfires
Embers can land on top of debris in your gutters and set fire to your house
Ember attack can happen before, during and after the bushfire.
Fire embers are glowing hot and remain after or precede a fire. Embers are often as hot as the fire which created them.
During a fire, embers can travel up to 40 kilometres, starting spot fires well ahead of the fire-front – often without warning.
There’s no questioning the facts, leaving a high fire-risk area early is always the safest option.

Spot Fires

The term spot fire describes a fire ignited outside of a burning unit as a result of a windborne ember (sometimes referred to as a firebrand).

Spot fires can start new bushfires well ahead of the main fire front.
In large fires, there’s the potential for many spot fires to start and quickly join together, blocking a safe escape.
There’s no questioning the facts, your garden hose is no match for spot fires so leaving a high fire-risk area early is always the safest option.

Radiant Heat

Radiant heat is the heat you feel from a fire. It is the biggest killer in a fire. The best protection is distance.

Protection from radiant heat

Make sure all skin is covered.
Do not wear shorts, t-shirt and thongs.
Cover up as soon as you are aware of a fire in your area.
A solid object, such as a brick wall, can provide some protection from radiant heat.
Distance is the best protection from radiant heat. Move as far away from the fire as you can, don’t get caught out in the open.
During a bushfire, the atmosphere will literally feel like hell on earth.
Flame temperatures can reach up to 1100⁰C and radiant heat fluxes high enough to vaporise vegetation, only adding speed to the scorching hot flames.
There’s no questioning the facts, if you are stuck in a bushfire, your chances of survival are slim. Leaving early is always the safest option.

Clothes to Wear:

a long-sleeved, collared shirt made from cotton or wool
pants made from cotton or some other natural fibre
sturdy boots and woollen socks
tough leather garden gloves – not rubber or synthetic
a wide-brimmed hat to protect your head
a face mask (P2 type) or towel to cover your mouth and nose
eye protection such as smoke goggles to shield your eyes.

Protective clothing and a solid shield may NOT save your life.

If you’re in a car

Do not travel on roads when there is a fire in your area. Late evacuation is extremely dangerous and can result in serious injury or death – always plan to leave early to avoid this situation. If you encounter smoke or flames and are not able to turn around and drive to safety, as a last resort:

1. Position the car to minimise exposure to radiant heat:

Park away from dense bush – try to find a clearing
If possible, park behind a barrier such as a wall or rocky outcrop
The car should ideally face towards the oncoming fire front
Park off the roadway and turn hazard lights on. Car crashes are common in bushfires due to poor visibility.

2. To increase your chances of survival:

Stay in the car and tightly close windows and doors
Cover up with woollen blankets and get down below window level – this is your highest priority
Drink water to prevent dehydration

3. As soon as you become aware that the fire front is close by:

Shut all vents and turn the air conditioning off.
Turn engine off
Be prepared: if you drive in high-risk areas, keep woollen blankets in your car. This is an essential precaution during the warmer months