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"?