Having regard to the reconstruction bushfire attack levels (BAL) recently imposed on the people of Wye River–Separation Creek, my opinion that the Victorian government and some councils only reinforce bushfire mythology is strengthened.
BAL–FZ and BAL–40 in a Township Zone??? What was the government thinking when it accepted that bushfire attack level assessment or was it a case of "pay the piper and call the tune"?
Why this was done needs consideration, including the adequacy of a risk assessment that should be the basis of the town protection plan, but first some facts about bushfire behaviour — if the community is to challenge government decisions they consider unfair, dictatorial or just require more explanation at least a basic knowledge of bushfire behaviour will help.
Knowledge of bushfire behaviour is also important in deciding whether to stay and defend a property or leave early and risk losing the family home or not being allowed back to deal with ember attack immediately after the fire has lost its intensity — see the pecuniary interest provision in Section 31(4) Country Fire Authority Act 1958.
Fire Triangle
When considering the threat of bushfire it is important to understand how fire occurs and spreads.
Of the three components required to have fire: fuel, oxygen and heat, in the bushfire environment the availability of fuel (vegetation) is the only component of the “triangle” that can be managed to inhibit the spread of fire and reduce its intensity.
I will refer to the availability of “bushfire fuel” as it will affect the threat to a building in the next posting.
Duration of the passage of the fire front – how bushfires spread
To introduce realism when considering the exposure of a building to bushfire it is important to be aware of how long the building will be exposed to an actual fire front.
Basically, bushfire spreads through an environment by flames at the fire front directly igniting fresh fuel (unburnt vegetation) in its path or in the case of a building, fracturing window glass thereby allowing the fire, usually in the form of sparks and embers, to enter the interior of the building, radiation from the fire front sufficiently high enough to ignite fresh fuel, including the exterior of vulnerable buildings due to a low flammability rating of the exposed materials.
The diagram below from Flame temperature and residence time of fires in dry eucalypt forest by B M Wotton, J S Gould, W L McCaw, N P Cheney and S W Taylor, 2011, International Journal of Wildland Fire that published a report on CSIRO Project Vesta, bushfire research in Western Australia that is relevant to Victorian conditions.
The trace in the diagram shows the very limited time that the fire peaks as it passes a given point. The time it takes for the temperature to fall behind the fire front depends on the amount of heavier fuel available to burn out.
Significant in assessing the bushfire threat to the dwelling it is important to understand that a fire only lasts as long as the fuel (vegetation) available to burn — grassland will burn out much faster than forest having a heavy understory of shrubs, ferns, grasses and fallen leaves and twigs.
Ignitability of the fuel is an issue; the fine fuel such as dead fallen leaves, twigs, dry grass and fine dead shrubbery close to the ground generally contribute to the passage of the fire front — the higher relative humidity (atmospheric moisture) and lower temperature that prevail over much of the summer months will inhibit the ignition of fine fuels and subsequent fire development by increasing its moisture content and raising the ignition temperature in a run to the building.
The heavier fuels such as sticks greater than six (6) mm in diameter, fallen branches, logs and hollow trees may be ignited by the fire front but don’t actually contribute to the fire front.
Again, the trace in the diagram above shows the very limited time that the fire peaks as it passes a given point. The time it takes for the temperature to fall behind the fire front depends on the amount of heavier fuel to burn out.
Effect of slope influencing fire intensity at the building site
Slope is a significant influence on fire behaviour. Simply, fire travels upslope faster than over flat ground. Conversely, a fire will be slowed as it traverses a lengthy downslope. The diagram below from Bushfires in Australia, Luke, R.H and McArthur, A.G 1978, Australian Government Publishing Service, Canberra, illustrates the effect of slope on rate of spread of a fire.
Other contributors to bushfire behaviour
Sparks, Embers and Firebrands
The most significant method of fire spread is the ignition of fresh fuel ahead of the main front due to burning wind borne material — known as “spotting”, as in the lighting of “spot fires”. To be a threat, this burning material must be sufficiently light in weight to be carried by wind while continuing to flame or smoulder sufficiently long enough to ignite fresh fuel.
I categorise “spotting” as follows:
Embers
The transmission of light burning or smouldering material generally horizontally above ground out to a distance of 100 metres and usually in the form of burning dead eucalypt leaves and other material of a similar weight picked up off the ground, and possibly the larger forms of grass seed heads or burning stringybark from tree trunks.
Stringybark eucalypts (below), where the bark has not been given a recent “haircut” — singed by fire and leaving the bark tight against the trunk with little or no fine fibres available to be ignited — will shed sparks and embers as fire travels up the bark. In strong wind stringybarks can be a prolific source of embers over a relatively short distance until all the aerated bark has been consumed.
Firebrands
Spotting over greater distances commonly involves the bark from gum species eucalypts. Some of the gum species shed their bark in the form of long ribbons (below). Some of these ribbons hang over higher branches, are relatively light and will burn readily. As they are relatively light they are susceptible to being carried aloft in the convection column — updraft above the fire and remain alight in the column for some time before falling to the ground and starting new fires, sometimes several kilometres distant from the main fire.
In my next posting I will cover how fire entered Wye River–Separation Creek, building loss and the "adequacy" of the township bushfire protection plan.
Meanwhile the the Government ramps up its political damage control and the economic damage to the State continues — now an "expert panel" appointed, so who signed off on accepting the bushfire attack level (BAL) assessment the cause of much distress in the community? And, the economics of the Victorian government's current approach to bushfire management is an important subject in itself.
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