Bushfire behaviour
Understanding fire behaviour helps you predict what a bushfire may do. A bushfire needs four things to start:
- fuel (particularly dry, fine fuels like grasses, twigs, bark and fallen leaves)
- oxygen (air and wind)
- heat (hot, dry weather)
- source of ignition (lightning, faulty electrical wires or an accidental ignition such as hitting a rock when slashing and creating a spark).
Once a fire starts, bushfire behaviour is affected by several factors:
- amount and dryness of fuel (doubling the dry vegetation per hectare will increase fire intensity four-fold)
- wind (fire speed and intensity increases as wind speed increases)
- geography (for example, under mild conditions, a slope of 10 degrees upwards doubles the speed of a fire)
- temperature (the warmer the temperature, the greater the likelihood of ignition and the greater the speed of fire)
- humidity (low humidity contributes to greater potential intensity and speed of fire).
Although you cannot control the weather on your farm, you can control the fuel present (amount and dryness of vegetation). Any more than 8 tonnes of fuel per hectare will result in a fire of such intensity that it cannot be extinguished or controlled.
Managing fuel loads is the major way you can reduce your fire risk. Use your time and resources effectively by prioritising key places on the farm where you will manage fuel loads, remembering some vegetation will still be required for feeding livestock during a bushfire risk period.
Safety of houses close to bush
The location of your home on the farm is important for fire risk. The CSIRO analysed Victorian fires and found that for houses at least 90 metres away from the nearest forest, 90% of people sheltering in homes survived. Ideally, you need at least 100 metres of open pasture with no forest or bush around your house and major infrastructure. For your garden, choose low flammability species that tend to stay green in summer.
Houses on farms often accumulate fuel around them. This could include vegetation (forest, scrub or pasture growth) or build-up of machinery or farm consumables close to the house. Routinely remove vegetation and flammable materials from close to the house before every fire season: slash unnecessary vegetation or clean up and take a trip to the recycling centre. If you have a choice, plant green crops like lucerne next to the house. If possible, avoid highly flammable crops like canola near homes or important infrastructure.
Vegetation management
The two main things to do around houses and infrastructure are:
- Control vegetation around infrastructure (e.g. house, sheds) and in paddocks close to infrastructure.
- Prevent woody weed infestation of paddocks – use herbicides and slashing.
- Graze down paddocks close to infrastructure during the fire season.
- Remove long grass and regrowth from infrastructure – slash, mow or have a paddock structure that allows you to routinely graze stock as close to infrastructure as possible to avoid fuel build-up in the most efficient way.
- Have zero fuel (‘bare earth’) against important infrastructure such as wooden fencing strainers and stays, tanks, troughs, plastic piping, sheds, feed silos and critical boundary This could be achieved with herbicide application twice a year or by more permanent means such as concrete or gravel.
- Conduct controlled burns (with approval) to avoid fuel build-up in any bush.
- Regularly burn forest and scrub, especially close to critical infrastructure in the cool This will reduce the intensity of a bushfire or sometimes even prevent a bushfire if burning was done recently.
- You will need permits and approvals – talk to your local fire service. There are rules around safely containing a fire, the environmental impacts of controlled burns (e.g. regularity of burning) and what is allowed to be burnt (e.g. not rainforest).
- Two useful sources of advice are the WA Department of Fire and Emergency Services (DFES) website for planned burning to help landholders ‘burn smart’, and the NSW RFS standard for hazard reduction burning.
