Bushfire explained - Part 2: How bushfire moves across the landscape

From continuing observation of how bushfire protection issues are dealt with in Victoria, it seems the broad community is largely unaware of how bushfire moves across the landscape, with one of the great fallacies being that trees always burn as part of a bushfire.

I regularly encounter people addressing bushfire risk on their land who believe that large scale tree removal is required, which is unnecessary in the majority of situations.

Towards the end of my 21 April 2014 posting “Bushfires explained – Part 1” (click here) are two media photographs of unburnt vegetation around houses lost in the Blue Mountains, NSW, fires earlier this year. In my 17 March 2014 posting “Grassfires, a simple truth” (click here) there is a link to a video clip available at The Age website (click here) showing the effect of the Mickleham fire travelling north towards Darraweit Guim on 8 February 2014 after the wind change. The unburnt vegetation around buildings lost or damaged can readily be seen.

The following three photographs concern a dwelling at Humevale that succumbed to the headlong rush of the Kilmore East fire on 7 February 2009.

Situated on predominantly grassland that slopes down towards the north and northwest at 10 degrees, the nearest significant group of trees was a relatively narrow band of eucalypts along a natural drainage line aligned northeast–southwest approximately 230 metres north of northwest from the dwelling. The nearest forest was approximately 450 metres northwest of the dwelling.

Approximately 25 metres east of the dwelling was a treed fenceline approximately 15 metres wide that can be seen behind the dwelling. Beyond that fenceline the vegetation is predominantly grassland that burning under the effect of the strong north to north to north-westerly wind at the time would not have contributed to fire behaviour affecting the dwelling.

Note the proximity of the shrubs and trees around the dwelling.

Grassland immediately north to northwest of the dwelling, with the tree line mentioned above visible in the centre of the photograph and grassland beyond also visible.

The preceding two photographs were taken on 31 January 2008.

As can be seen in the above photograph taken on 11 March 2009, it was a brick veneer building with short lawn and well maintained and separated trees and shrubs around it.

Compare the effect of the fire on the leaves on the trees and shrubs in this photograph with the same vegetation in the first photograph — much of the leaf damage is scorching rather than total incineration. At the extreme left of the dwelling is scorched thick creeper that would not have contributed to a fire involving the dwellings. The scorching of the Pine tree behind the dwelling is also significant; it would not have contributed to loss of the dwelling.

Also significant is the obviously undamaged condition — even at ground level — of the blue steel shed to the right of the dwelling and a large steel garage that can just be seen at left background of the dwelling. This lockup garage, with door opening facing the south, and its contents were not affected by the fire.

Why was this dwelling lost? Probably because it was unattended at the time the fire went through and not been constructed to withstand ember attack — BAL-12.5 according to AS 3959 Construction of buildings in bushfire-prone areas.

A major contributor to ignition of this dwelling would have the failure of a timber door over the underfloor entrance.

Located just right of centre in the close up of the northern end of the dwelling this door went to ground level and had gaps around it that would have allowed ember penetration into the underfloor area. The timber door and its timber frame were largely destroyed, the combustion of which would have been assisted by ‘convection heating’ prior to the arrival of the fire front and fire burning up to it through dry grass at its base.

No doubt in my mind that the dwelling could have been used to safely shelter from the passing fire front, then a well-prepared person able to safely go outside and look for and extinguish ignitions — the dwelling would have taken time to reach total involvement.

In my next posting I will give more examples of what does not necessarily burn when a bushfire travels across the landscape and hopefully further counter some of the 'mythology' of bushfire in Australia.