Bushfire explained – Part 3: What burns and what does not burn as bushfire moves across the landscape

To properly address bushfire risk it’s important to understand what actually burns as fire moves across the landscape, especially as the Victorian Fire Risk Register – Bushfire (VFRR-B) is being raised by the CFA and certain municipal councils to ‘influence’ planning permit application decisions. I’ll come back to this later as not properly understood the Risk Register has the potential to unnecessarily frighten people and lead to poor decision-making.

Extending on from my 15 June posting “Bushfire explained – Part 2” (click here) following are some examples of unburnt vegetation in the path of bushfire, beginning with trees in the vicinity of the Lancefield–Kilmore Road, mid-way between Lancefield and Kilmore, as the 9 February Mickleham fire (click here) travelled north.  

Roadside eucalypts on the sides of the Lancefield–Kilmore Road showing predominantly only canopy or “crown” scorch. It would have been a ‘hot’ fire that passed beneath those trees, but not hot enough to ignite the canopies. Remember, this is the fire that took several days to control.

Below are trees in open fields south of the Lancefield–Kilmore Road showing only “crown” scorch.

Below are trees and shrubs in the path of the 9 February 2014 Gisborne fire as it travelled northeast towards Riddells Creek after the wind change. The fire weather conditions this day were described as being the worst since 7 February 2009 (Black Saturday).

Above, trees and low vegetation "scorched" as the fire continued its rush towards Riddells Creek having crossed the Sunbury-Riddells Creek Road just visible at right.

In the photographs immediately above and below this text observe how the fire only "scorched" the base of these tree trunks, except for where it took hold on an open section of the trunk in a tree in the lower photograph. This burning section of trunk would not have been part of the fire front.

Finally, some well established Pines near Gisborne (courtesy Haydn Bishop Electrics, Gisborne) that did not succumb to crown fire. Insufficient fine fuel around the base of these trees and lack of trunk fine fuel (needles) kept the fire at ground level. Relevant to earlier photographs of "scorched" Pines.

I’m hoping by now that it’s becoming apparent that trees don’t automatically burn and thus don’t contribute significantly to the intensity of bushfire; indeed in some circumstances trees can actually help shield a building from radiant heat and reduce ember attack. I’ll continue this theme in my next posting as it’s important, particularly in understanding how the Victorian Fire Risk Register – Bushfire (VFRR-B) seems to deal with bushfire and how that Risk Register is being applied.

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.