What Conditions are Survivable During a Burnover?

The principle threat to survival during a bushfire is radiant heat. Actual air temperatures are often quite low, particularly near the ground (Cheney 1972). The maximum conditions survivable depends on the duration of the exposure and the protection available. One model suggests a maximum air temperature of 100 degrees C and a maximum body temp rise of 2 degrees C as survivable (Knight 1988). Another (Bond 1986) suggested a maximum 15 minute exposure of 120 deg C. Higher air temperatures may be survivable for shorter periods during a vehicle burnover provided there is adequate radiant heat protection.

A typical low to moderate intensity grass fire may produce flame heights of 2-3m, intensities of around 1500 kW/m and pass by in less than a minute. The Florida Burn (Mangan 1997) suggested that such a fire is easily survivable in either a fire shelter or a vehicle. There are also reports of survival in such fires by firefighters who laid on the ground or ran back through the flames onto the black. Fatalities have however occurred amongst crews who abandoned their vehicles or were caught in the open. A more intense grass or brush fire, producing 6m flames, and intensities of 3000-4000kW/m, similar to the Los Angeles Burns, (Mangan 1997) passes by in less than 2 minutes and may be survivable in shelters or vehicles. In one of these burns, conditions remained survivable in one vehicle, though ignition of the door liner might have forced a crew to evacuate the other during the burnover.

Cheney has suggested that 2000-3000kW/m is the maximum intensity fire a tanker crew will survive in their truck during a burnover. This may however be an underestimate, for some crew survived in each of the Carey Gully, Linton and Wingello burnovers, all of which occurred on narrow firetrails in heavy bush and may have been more severe than this (Figure 3, Figure 5, Figure 6).

A more intense fire, such as a forest or bush fire may involve flame heights of more than 10m, intensities of more than 5000-10,000kW/m and take 3-6 minutes to pass by (Bond 1986). The surrounds may then remain hostile for another half hour as the larger fuels burn out. Survival may still be possible with suitable protection in an appropriate clearing. Certainly there are numerous reports of survival in shelters in clearings during US forest fires.

Most of the Australian appliances destroyed by burnover have been lost on narrow trails in bushland and some crew have survived. The Australian tests indicated that conventional passenger cars provided survivable conditions for approximately 4 minutes in a severe fire, equivalent to a forest fire. Following experiments carried out in Australia in 1988, Knight (Knight 1988) estimated that the maximum fire intensity survivable in a personal shelter in a 10m clearing was 5600kW /m.