Bushfire explained - Part 1

In my 16 February (click here) and 17 March 2014 (click here) postings I explained some of the basics of grass fires: how they spread across the landscape, how they impact on a building and surviving the passage of a grass fire using the family home as a bushfire shelter.

Considering how BMO bushfire management statements are received by some decision makers in Victoria, there appears to be a serious lack of understanding of the characteristics of bushfire as they move across the landscape and affect a dwelling, with a seemingly automatic assumption of a catastrophic result leading to inappropriate decisions in several of the cases in which I’ve been involved or observed from a distance. Consequently, it’s reasonable to assume that this is an indicator of bushfire knowledge, or lack of it, in the broader community.  

Bushfire spreads across the land in one or more forms:

Sparks, ember or firebrand attack

Fresh ignitions ahead of the main fire front due to “spotting” activity, being wind-borne embers or larger firebrands starting new fires in unburnt fuel (vegetation), sometimes out to many kilometres ahead of the main fire front depending on the type fuel available to be picked up by the wind or carried up in the convection column.*

The types of fuel that can “spot” ahead of a fire range from leaves and small pieces of tree bark on the ground within relatively close proximity of a “receiver” such as the family home; the bark of a stringybark species eucalypt tree for example Messmate that when burning is liberated by strong wind and blown horizontally at the “receiver” at distances possibly out to 100 metres; and the bark of certain of the gum species eucalypts such as Manna Gum the bark of which is sufficiently light to be carried up in a convection column and continue to burn before it returns to earth and lights a new fire, sometimes many kilometres ahead of the main fire.

This photograph is of eucalypt leaves and other dead material at the base of a stringybark on land at Toolern Vale on 12 September 2012. Leaves and twigs up to pencil size carry the fire front and are susceptible to being carried forward by wind close to the ground. The vegetation has reached "steady state" the land not having been burnt for greater than 30 years.

The following two photographs are of stringybark eucalypts on land at Buttermans Track, Christmas Hills that slopes up from the north towards the south. Taken on 4 July 2013 the trees show little sign of fire. The second of the two shows scattered small black spots on bark that is becoming deeply furrowed and exposing fine fibres of bark that will readily ignite if exposed to fire. Clearly the vegetation on this land has reached "steady state", an issue I'll return to in a future posting dealing with fire behaviour.

The following three photographs are of stringybark species eucalypts on land at Hobbs Road, Bullengarook. Photographed on 26 April 2013 the trees show diminishing signs of being involved in the Ash Wednesday 1983 fire as it approached Gisborne after the wind changed to the southwest. Given the small burn scars on the Buttermans Track trees their involvement in fire would have been pre-1983. The third photograph shows no sign of fire with the bark becoming deeply furrowed. In the 30 years since the Ash Wednesday fire the vegetation on this land has returned to "steady state".

Stringybarks photographed on 21 December 2009 on land at Aubrey Cuzens Drive, Marysville and showing signs of recovering from the Black Saturday fire 10 months earlier. Note how the bark is completely black with the fine fuel gone leaving the dense bark that ceased burning after the fire front had passed. Over time the burnt bark will be replaced as shown in the above photos.

The following two photographs are of gum eucalypts to illustrate the bark form mentioned above. The first photograph is of a gum in a Sunbury park on 14 February 2014 and the second on unmanaged land adjoining the Yarra River at Wonga Park on 3 September 2013. The pile of bark around the base of the tree will burn vigorously when dry with the resulting convection heating aiding the ignition of the bark suspended above and if the heating is sufficient send some of the suspended bark on a journey to possibly light a fire elsewhere in unburnt vegetation. It is this long range form of "spotting" that is responsible for the sometimes dramatic spread of forest fire in the summer months.

Radiant heat

Ignition due to high level radiant heat from the fire — the closer the fire front the greater the heat level to which the “receiver” is exposed. The less dense building materials such as Western Red Cedar weather boards can be expected to ignite and continue to burn if the radiant heat level is sufficiently high. Unprotected ‘ordinary’ window glass can be expected to fail early thereby allowing sparks and embers to penetrate the interior of the building. Table 1 on page 8 in the CFA's "Planning for Bushfire Victoria" provides information on the effect of radiant heat.

Flame contact

Concerning direct flame contact, the flammable nature of the “receiver” and the intensity and duration of the flame contact on the receiver influences its susceptibility to ignition. For example Western Red Cedar will readily ignite and likely continue to burn after the igniting (pilot) flame ceases as the fuel sustaining that flame is consumed whereas a much denser hardwood will likely cease to burn when the pilot flame is removed. In a direct flame contact situation unprotected windows are vulnerable to breaching, thereby allowing flames to penetrate into the interior of the building.

On 17 March 2014 (click here) I posted photographs of buildings vulnerable to ignition through flame contact, particularly the first two showing grass up to the edge of unprotected timber at ground level and shrubs against a window or timber.

Convection heating

While not a direct source of ignition, preheating of the “receiver” occurs when vulnerable surfaces are exposed to several hours of hot wind consistent with a north wind during a period of Total Fire Ban or are subjected to hot air generated by a fire. Simplistically, this preheating serves to reduce the temperature at which a pilot flame will ignite the “receiver”.

In the second paragraph (above) I’ve commented on what seems to be automatic assumptions by some decision makers that a proposed building would be subjected to fire of such intensity that it would not survive. The experience of severe bushfire events in Australia prove that’s largely not the situation.

Overwhelming evidence gathered in Victoria since the Beaumaris fire of 14 January 1944 (click here) proves that the vast majority of dwellings lost or damaged in a bushfire are ignited by ember attack. This was certainly the situation in the Ash Wednesday 1983 fires and the 2009 Black Saturday Bushfires in Victoria. Media photographs of the 2013 Sydney, NSW and Dunalley, Tasmania fires show dwellings that succumbed to ember attack and/or flammable vegetation too close and not being constructed to resist bushfire.

Below is an ABC News photograph of buildings that have succumbed to ember attack from the Blue Mountains, NSW fires earlier this year. The unburnt condition of the vegetation around the buildings is evidence of ember attack on unprotected and probably undefended buildings.

Below is a News Limited photograph of buildings in Winmalee, NSW that have succumbed to ember attack from the Blue Mountains, NSW fires earlier this year. Again, the unburnt condition of the vegetation around the buildings is evidence of ember attack on unprotected and probably undefended buildings.

The condition of the vegetation in both the above photographs are important indicators of how bushfire travels across the landscape that I will address in greater detail in "Bushfires explained - Part 2".

Australian Standard AS 3959 construction of buildings in bushfire-prone areas, first published in the aftermath of the Ash Wednesday 1983 fires in Victoria and South Australia, recommends design and construction measures to protect dwellings from bushfires at increasing levels of radiant heat exposure and ultimately flame contact.

All levels of design and construction in the current version of AS 3959 published in 2009 include measures to protect against ember attack. Of course AS 3959 is not the sole solution, with corresponding vegetation management (fuel reduction), no flammable rubbish, fire wood or similar in a position that if ignited will threaten the dwelling, and the occupants having a bushfire survival plan individualised to suit their situation while allowing for their physical and emotional strengths or limitations.

* Ahern, A. and Chladil, M. 1999, How far do bushfires penetrate urban areas? Disaster Prevention for the 21st Century. Proceedings of the Australian Disaster Conference, Canberra.