Murrumbidgee - Ginninderra Gorges National Park: Information Note 63

Here is a brief summary of the talk by Assoc Prof Jason Sharples to our recent public meeting.

Bushfires are commonly accepted as a natural hazard but, according to the ACT Strategic Bushfire Management Plan, Australia has experienced a growing number of extreme bushfires since 2001. Last century, the focus was on the more common form of bushfire that occurs on slopes up to 20 degrees and is affected mainly by the type and proximity of vegetation. These bushfires tend to burn along the ground in a long, thin front which exhibits a relatively constant rate of spread. Property tends to catch fire by radiant heat from one burning item igniting another and a 100m setback from vegetation (Asset Protection Zone - APZ) is generally adequate protection for property in this situation. Australian Standard 3959 covers the bushfire safety requirements of building in a bushfire prone area, along with the methodology for calculating the relevant bushfire attack level (BAL) under these conditions.

In 2003, however, half of the 500 houses lost in Duffy were more than 100m away from the forest edge. Further, the majority were lost to ember attack. Recent research has shown that dynamic fire behaviour occurs where slopes are over 24-26 degrees, as shown below.

Left: Schematic diagram of a fire burning up a positive slope towards a house. The orange and yellow flames represent the case of eruptive fire spread, in which the flames have attached to the surface. The black outlined flames represent the case of a separated plume, which is the situation depicted in AS 3959.

The 2003 Canberra fires revealed another unusual mode of fire spread in which the interaction of the winds, terrain and fire generates significant fire whirls on leefacing slopes, which carry the fire across the wind instead of with it. This can result in extreme wildfires, with a large area of land being alight at any one time, in contrast to the relatively thin line of a normal fire.[1]

Bushfire containment and determining the appropriate size of APZs under these conditions is more difficult. Further, the strong winds blow embers long distances, causing many spot fires that coalesce to form an extensive fire front. Fire intensities increase greatly when spot fires join together. Dynamic interactions between different parts of the fire and the atmosphere cause the individual fire fronts to accelerate, with a consequent increase in fire line intensity. AS 3959 for building in bushfire areas does not currently consider these increases in intensity nor the resultant behaviour.

Pyrocumulonimbus cloud over the ACT, Jan 2003 Credit: NSW Rural Fire Service

Under conditions of extreme and dynamic fire behaviour, the very large amounts of heat and moisture released from a fire can cause its plume to rise high into the atmosphere – up to several kilometres. The plume can become a towering cumulus cloud or even develop into a fire thunderstorm which can cause more damage than the fire itself. The combination of strong winds and dynamic fire behaviour drives embers vast distances that make 100m APZs totally ineffective. This begs the question – how close to areas with dynamic fire characteristics is it responsible to establish large settlements?

[1] Sharples, J.J. Risk Implications of Dynamic Fire Propagation: A case study, Preliminary Report (June 2017)

Cheers,

Darryl Seto
(for the Ginninderra Falls Committee)

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