The nature of bushfire Part 2 ... reality versus mythology and fuel reduced buffer zones

Continuing on from my Sunday, 16 April 2016 blog posting, but first a clarification of that post: my comment about variations to temperature and relative humidity during the summer months in the third paragraph below the “flame temperature and residence time of fire” diagram is deliberately broad. Important that we all understand and acknowledge that there are many days during the summer months when weather conditions are not conducive to the occurrence and spread of bushfire.

Convection Heating

While not usually a direct source of ignition, preheating of flammable materials occurs when those 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 gases generated by a fire. Simplistically, this preheating — hot air — serves to aid ignition of an unprotected building.

The diagram below from Bushfires in Australia, Luke, R.H and McArthur, A.G 1978, Australian Government Publishing Service, Canberra, illustrates flames, radiant heat from those flames, and convection beyond the radiation from the flames.

An example of convection heating is a gas fired domestic heater in the family home. The heater below from the Jetmaster website utilises two methods to heat the building, radiation from the gas flames and glowing coals and heating of fan-forced air passing over a register or heat exchanger inside the body of the heater.

The temperature of bushfire flame is finite — 1,000° to 1,100° Celsius according to Project Vesta — and the radiation from those flames decreases over distance. The distance from the predominant vegetation (defendable space) tables in Australian Standard AS 3959—2009 Construction of buildings in bushfire-prone areas provide separation from harmful radiant heat.

The temperature of the air above a bushfire, convective heating, depends on the size of the fire, the larger the fire the warmer the air. Convective heating (warm or hot air) likely to be felt at ground level requires very strong wind to keep it down and alone will not be of a temperature that exceeds the fire resistance capability of a building constructed the requirements of AS 3959—2009.

Convective heat compared with radiant heat

When considering the bushfire threat to the safety of the occupants of a building it is important to understand the difference between radiant heat and convective heating or “hot air”.

Under the heading “Duration of the passage of the fire front – how bushfires spread”, I sought to explain how fire moves across the land. The diagram is there to illustrate how quickly the fire intensity rise and fall occurs with the passing of a typical bushfire front. As I comment in the fourth paragraph, the time it takes for “the temperature to fall behind the fire front depends on the amount of heavier fuel available to burn out”.

Radiant heat

Radiant heat decreases over the distance that the heavier fuels behind the fire front are from a building and will normally not be sustained at the temperature associated with the passing of the actual fire front through surface fine fuel comprised of leaves, twigs and dry grass.

Below is an extract from the Manual of Firemanship Book 1, Elements of combustion and extinction, on radiation:

All forms of radiant energy travel in straight lines at the speed of light. The intensity [heat] falls off inversely as the square of the distance from the source of radiation [in this case fire]. This means at twice the distance the intensity is one quarter; at three times the distance, the intensity is one-ninth, and so on. The inverse square law can be understood by looking at Fig. 4.5.

The square with 1 metres sides is placed at, say, 2 metres from the source will throw a shadow with 2 metres sides on a second sheet placed 4 metres from the source. Thus the energy falling on 1 m² is the same as that which would have fallen on an area of 2 metres x 2 metres = 4 m² at a distance of 4 metres. So the energy per square metre at 4 metres is one quarter that at 2 metres, i.e. that is one quarter at twice the distance. This is important when considering the effect of radiation from the heat source such as a fire.

Manual of Firemanship, Home Office (Fire Department)1974, HMSO, London

An example of the reduction of radiant heat over distance is the depth of defendable space required to achieve a particular bushfire attack level (BAL), in this case not exceeding 29 kW/m² at the outer edge of the building across 26 metres on a downslope not exceeding 10 degrees below the dwelling to satisfy BAL–29 according to Table 2 Defendable space and construction, Clause 52.47 Victoria Planning Provisions (below) with “woodland” as the predominant vegetation.

Following the arrival of the fire front the level of radiant heat behind the fire front will rapidly become less than 29 kW/m² over the width of the defendable space to achieve BAL–29 in this case. Of course this is dependent on effective implementation of the specified defendable space vegetation management requirements for the subject land.

Referring to the explanation beneath the heading “Convection Heating”, regardless of the size of the fire further back in the “heavy fuel”, in the aftermath of the arrival of the fire at the dwelling it will increasingly become convective heat — hot air — that reaches the building. And, that “hot air” will dissipate at a rate influenced by that hot air normally rising close to where it is being generated, but of course dependent, too, on the slope of the ground involved — see the slope diagram under the heading “Convection Heating”.

Finally, referring to my 24 March 2016 posting "Bushfire attack levels and windfall financial gains" there are two photos associated with a BAL–29 dwelling on the high side of Karingal Drive, Wye River, showing dry but unburnt shrubs and tree canopy.

Following are two other examples in Wye River in the aftermath of the fire that are consistent with the effect of convective heat or radiant heat from low level flames, but certainly not sufficiently severe to warrant BAL–FZ or BAL–40 throughout the Township Zone if the Colac Otway Shire Council and the CFA had met their statutory fire prevention responsibilities.

Fuel reduced buffer zones

Questions remain to be answered by the Victorian government. Why was the privately owned land around much of the settled areas of Wye River–Separation Creek ignored pre-fire and continue to be ignored in township redevelopment arrangements? The fire hazard removal provisions in the Country Fire Authority Act 1958 exist to help achieve the fuel reduced buffer zone sought by the property owners, and earlier put to the Colac Otway Shire Council.

Why didn’t the Municipal Fire Prevention Officer Colac Otway Shire Council or the CFA Chief Officer "form the opinion" that the serving of fire prevention notices was necessary to protect life or property from the bushfire threat to Wye River–Separation Creek?

What losses and suppression costs could have been avoided if the powers available in the Country Fire Authority Act had been exercised?

Again, BAL–FZ and BAL–40 in a Township Zone as protection from ember attack and "hot air"??? 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"?

The nature of bushfire

As stated in the introduction my objective is to raise bushfire knowledge and assist Victorians seeking approval to build in a so-called bushfire prone area.

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.

Australia's bushfire management agencies ... do they deliver value for the dollars we are compelled to contribute?

A paper of relevance by Michael Lonergan as I contemplate the 'financial needs' bleating of the State Premiers at yesterday's COAG meeting and think about the total long term cost associated with the Christmas Day 2015 bushfire that devastated Wye River–Separation Creek.

The failure of the bushfire management agencies to implement an aggressive attack to make safe a lightning strike that should have been a 'bread and butter' issue for DELWP cost and continues to cost Victorians dearly, and not only financially. And what was government's response? A naked attempt to cover up failure due to mismanagement that can easily be seen through.

A case of subterfuge?

Or is there more to it? Was the fire started by the lightning strike seen by the land manager as an opportunity to undertake some fuel reduction burning under the guise of work to make safe a going fire, but not really necessary if an aggressive and well-resourced attack had been mounted on the initially relatively small fire from its very beginning on 19 December?

A disturbing thought given the government bluster over the October 2015 breakaway from the Cobaw Ranges fuel reduction burning undertaken north of Lancefield.

At the time of the fire, fuel reduction burning planned by DELWP for the Otways between Lorne and Wye River–Separation Creek had not been undertaken and maybe the lightning strike fire was seen as an opportunity to sneak in some otherwise contentious fuel reduction burning. Is this what the Victorian government is trying to hide?

And what of their future as the Wye River–Separation Creek community face yet another hardship inflicted on them by the emergency management bureaucracy and the insurance industry?

Where will be next in Victoria to be abandoned to the ravages of bushfire under the 'leave and lose' policy: Lorne, Daylesford, Hepburn, Eltham North–Eltham–Research–North Warrandyte–Warandyte and further south as wind-driven embers from house-to-house spread a fire further south with no one at home to save their homes?

When I hear utterances from the Victorian emergency management bureaucracy about the need for "surge capacity", I think of the costly failure to recognise that a potentially enormous "surge capacity" exists in the form of citizens, who with some training, could safely defend their homes from ember attack that if left unattended grow to cause damage and loss as experienced at Wye River–Separation Creek, Vic. And no doubt the same can be said for Yarloop, WA, in the aftermath of the fire that devastated that community.

Unfortunately, in Victoria, and it seems in Western Australia, the bushfire emergency management authorities prefer to frighten people rather than properly resource and implement an ongoing community partnership-based prevention/mitigation approach to minimising bushfire loss, with emergency response one aspect of a balanced approach to loss prevention. Here, I find the following from Mr Lonergan's paper particularly appealing:

It would recommend to the Federal government what conditions should be placed on funding for the States and Territories for their bushfire administrations. It would achieve the application of Force to bring those administrations to heel.

Food for thought, are the public servants who head up our fire and emergency management agencies and those who are supposed to monitor performance suitably qualified and experienced for the positions they hold? Could they lead firefighters at the fire front to achieve the performance we their clients should be able to expect? If they can't then they should go.

Then there’s the Colac Otway Shire Council and the CFA Board when it comes to neglecting statutory fire prevention responsibilities that increased the threat faced by Wye River–Separation Creek.

More to address at CFA than just internal bullying. Will Minister Garrett be up to this on our behalf?

High time for change folks, and it won't come easily from the fire and emergency services industry or the politicians that industry has in tow.