WHAT DRIVES BUSHFIRES

It is not necessary for everyone to become a fire expert, but it always helps to know a bit more. If we understand the factors that can alter fire risks we can make more informed decisions about what we should do to reduce that risk.

Fire ignition. Fire behaviour.

Fire ignition and behaviour

Two concepts are always closely considered by bushfire managers. They are often conveyed as simple triangles. The first is the fire ignition triangle, which shows the three factors involved in any fire – fuel, oxygen and heat or a source of ignition. Only one of these three factors can be changed through bushfire management, the availability of fuel. The second concept is fire behaviour, which again is often conveyed as a triangle. Bushfire managers cannot change the weather or topography, but they can influence fuel loads. When predicting fire behaviour the amount and type of fuel as well as how it sits in the landscape becomes very important. For example, lots of shrubs and young trees or trees with fibrous bark can act as ladder fuels, helping fire climb into forest canopies. Weather

Weather is a critical factor for when and how an area can burn. Bushfires tend to escalate when the temperature increases above 30°C, the humidity drops below 30% and when wind speeds increase above 12km/h. We cannot control the weather but we can take advantage of the right weather conditions to safely reduce the build-up of flammable materials in our forests through prescribed burns. The City of Hobart carries out prescribed burns across small, easily managed burn areas when the weather is mild, usually between autumn and spring. Prescribed burns are ideally carried out when the temperature is under 25°C, the humidity greater than 40% and winds are below 10km/h. Vegetation on the ground should be dry enough to burn and the soil needs to contain enough moisture to prevent the fire from burning too fiercely. Forecast rain in the days following the burn create the safest conditions.

Topography

The other key factor that affects bushfire behaviour is topography. The angle of slope, slope height and the shape of valleys all influence fire behaviour and intensity. Valleys and peaks can direct wind in ways that are quite unpredictable. The angle of a slope is a critical factor in how a fire travels – how fast it spreads as well as its intensity. The steeper the slope the faster the fire will move up it. A fire moving in a straight line across flat land

at 2km/h will double its speed almost immediately if it hits a slope angled upwards at 10 degrees. For every additional 10 degrees in slope the fire will double its speed. If our 2km/h fire hits a slope angled upwards at 20 degrees it would start travelling at 8km/h. If the slope was angled upwards at 30 degrees the fire would race ahead at 16km/h. How fast can you run?

What can we change?

We cannot remove oxygen from the air, or prevent all fire ignitions whether sparked through lightning strikes, lit by fire bugs or ignited by accident, but we can reduce the build-up of a continuous layer of flammable vegetation in our forests, woodlands and grasslands through prescribed burns. The creation of a network of fuel breaks is another tool used to change the landscape and manage the threat of bushfire.