Bushfire explained - Part 1

In my 16 February (click here) and 17 March 2014 (click here) postings I explained some of the basics of grass fires: how they spread across the landscape, how they impact on a building and surviving the passage of a grass fire using the family home as a bushfire shelter.

Considering how BMO bushfire management statements are received by some decision makers in Victoria, there appears to be a serious lack of understanding of the characteristics of bushfire as they move across the landscape and affect a dwelling, with a seemingly automatic assumption of a catastrophic result leading to inappropriate decisions in several of the cases in which I’ve been involved or observed from a distance. Consequently, it’s reasonable to assume that this is an indicator of bushfire knowledge, or lack of it, in the broader community.  

Bushfire spreads across the land in one or more forms:

Sparks, ember or firebrand attack

Fresh ignitions ahead of the main fire front due to “spotting” activity, being wind-borne embers or larger firebrands starting new fires in unburnt fuel (vegetation), sometimes out to many kilometres ahead of the main fire front depending on the type fuel available to be picked up by the wind or carried up in the convection column.*

The types of fuel that can “spot” ahead of a fire range from leaves and small pieces of tree bark on the ground within relatively close proximity of a “receiver” such as the family home; the bark of a stringybark species eucalypt tree for example Messmate that when burning is liberated by strong wind and blown horizontally at the “receiver” at distances possibly out to 100 metres; and the bark of certain of the gum species eucalypts such as Manna Gum the bark of which is sufficiently light to be carried up in a convection column and continue to burn before it returns to earth and lights a new fire, sometimes many kilometres ahead of the main fire.

This photograph is of eucalypt leaves and other dead material at the base of a stringybark on land at Toolern Vale on 12 September 2012. Leaves and twigs up to pencil size carry the fire front and are susceptible to being carried forward by wind close to the ground. The vegetation has reached "steady state" the land not having been burnt for greater than 30 years.

The following two photographs are of stringybark eucalypts on land at Buttermans Track, Christmas Hills that slopes up from the north towards the south. Taken on 4 July 2013 the trees show little sign of fire. The second of the two shows scattered small black spots on bark that is becoming deeply furrowed and exposing fine fibres of bark that will readily ignite if exposed to fire. Clearly the vegetation on this land has reached "steady state", an issue I'll return to in a future posting dealing with fire behaviour.

The following three photographs are of stringybark species eucalypts on land at Hobbs Road, Bullengarook. Photographed on 26 April 2013 the trees show diminishing signs of being involved in the Ash Wednesday 1983 fire as it approached Gisborne after the wind changed to the southwest. Given the small burn scars on the Buttermans Track trees their involvement in fire would have been pre-1983. The third photograph shows no sign of fire with the bark becoming deeply furrowed. In the 30 years since the Ash Wednesday fire the vegetation on this land has returned to "steady state".

Stringybarks photographed on 21 December 2009 on land at Aubrey Cuzens Drive, Marysville and showing signs of recovering from the Black Saturday fire 10 months earlier. Note how the bark is completely black with the fine fuel gone leaving the dense bark that ceased burning after the fire front had passed. Over time the burnt bark will be replaced as shown in the above photos.

The following two photographs are of gum eucalypts to illustrate the bark form mentioned above. The first photograph is of a gum in a Sunbury park on 14 February 2014 and the second on unmanaged land adjoining the Yarra River at Wonga Park on 3 September 2013. The pile of bark around the base of the tree will burn vigorously when dry with the resulting convection heating aiding the ignition of the bark suspended above and if the heating is sufficient send some of the suspended bark on a journey to possibly light a fire elsewhere in unburnt vegetation. It is this long range form of "spotting" that is responsible for the sometimes dramatic spread of forest fire in the summer months.

Radiant heat

Ignition due to high level radiant heat from the fire — the closer the fire front the greater the heat level to which the “receiver” is exposed. The less dense building materials such as Western Red Cedar weather boards can be expected to ignite and continue to burn if the radiant heat level is sufficiently high. Unprotected ‘ordinary’ window glass can be expected to fail early thereby allowing sparks and embers to penetrate the interior of the building. Table 1 on page 8 in the CFA's "Planning for Bushfire Victoria" provides information on the effect of radiant heat.

Flame contact

Concerning direct flame contact, the flammable nature of the “receiver” and the intensity and duration of the flame contact on the receiver influences its susceptibility to ignition. For example Western Red Cedar will readily ignite and likely continue to burn after the igniting (pilot) flame ceases as the fuel sustaining that flame is consumed whereas a much denser hardwood will likely cease to burn when the pilot flame is removed. In a direct flame contact situation unprotected windows are vulnerable to breaching, thereby allowing flames to penetrate into the interior of the building.

On 17 March 2014 (click here) I posted photographs of buildings vulnerable to ignition through flame contact, particularly the first two showing grass up to the edge of unprotected timber at ground level and shrubs against a window or timber.

Convection heating

While not a direct source of ignition, preheating of the “receiver” occurs when vulnerable 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 air generated by a fire. Simplistically, this preheating serves to reduce the temperature at which a pilot flame will ignite the “receiver”.

In the second paragraph (above) I’ve commented on what seems to be automatic assumptions by some decision makers that a proposed building would be subjected to fire of such intensity that it would not survive. The experience of severe bushfire events in Australia prove that’s largely not the situation.

Overwhelming evidence gathered in Victoria since the Beaumaris fire of 14 January 1944 (click here) proves that the vast majority of dwellings lost or damaged in a bushfire are ignited by ember attack. This was certainly the situation in the Ash Wednesday 1983 fires and the 2009 Black Saturday Bushfires in Victoria. Media photographs of the 2013 Sydney, NSW and Dunalley, Tasmania fires show dwellings that succumbed to ember attack and/or flammable vegetation too close and not being constructed to resist bushfire.

Below is an ABC News photograph of buildings that have succumbed to ember attack from the Blue Mountains, NSW fires earlier this year. The unburnt condition of the vegetation around the buildings is evidence of ember attack on unprotected and probably undefended buildings.

Below is a News Limited photograph of buildings in Winmalee, NSW that have succumbed to ember attack from the Blue Mountains, NSW fires earlier this year. Again, the unburnt condition of the vegetation around the buildings is evidence of ember attack on unprotected and probably undefended buildings.

The condition of the vegetation in both the above photographs are important indicators of how bushfire travels across the landscape that I will address in greater detail in "Bushfires explained - Part 2".

Australian Standard AS 3959 construction of buildings in bushfire-prone areas, first published in the aftermath of the Ash Wednesday 1983 fires in Victoria and South Australia, recommends design and construction measures to protect dwellings from bushfires at increasing levels of radiant heat exposure and ultimately flame contact.

All levels of design and construction in the current version of AS 3959 published in 2009 include measures to protect against ember attack. Of course AS 3959 is not the sole solution, with corresponding vegetation management (fuel reduction), no flammable rubbish, fire wood or similar in a position that if ignited will threaten the dwelling, and the occupants having a bushfire survival plan individualised to suit their situation while allowing for their physical and emotional strengths or limitations.

* Ahern, A. and Chladil, M. 1999, How far do bushfires penetrate urban areas? Disaster Prevention for the 21st Century. Proceedings of the Australian Disaster Conference, Canberra.

Grass fires, a simple truth

My blog posting “Knowledge is power” (click here) on 16 February began to deal with fire in grasslands and commenced with the statement:

Unlike a scrub or forest fire that liberates burning leaves and bark to be carried ahead of the main fire front to start new fires known as spot fires, grass fires generally don’t have the spotting problem, except where they involve trees or shrubs in their path.

The simple truth is that grass fire intensity can be dramatically reduced by managing the fuel (grass) to near ground level by mowing or using horses, cattle, sheep or goats or creating bare earth by ploughing, rotary hoeing, use of a grader blade or herbicide. Concerning distance, in the 16 February posting I mentioned at least 30 metres radius around the dwelling to be protected.

The page below is an extract from Planning Scheme Clause 52.47 Table 1 (click here) . I have applied yellow highlighter to indicate defendable space (vegetation management) for grassland. The grassland distances are lifted from Australian Standard AS 3959—2009 construction of buildings in bushfire-prone areas and calculated at Grassland Fire Danger Index (FDI) 130 and flame temperature 1090 Kelvin (click here).

The second column in the Table 1 extract marked with a “red X” is extra defendable space required due to a Victorian government decision to use flame temperature 1200K rather than adhere to 1090K used in AS 3959—2009. I will discuss use of flame temperature 1200K in a future posting.

The video clip available at The Age (click here) web site shows the effect of the 9 February 2014 Mickleham fire travelling north towards Darraweit Guim after the wind change. As can be seen some houses survived and others were lost.

Towards the end of my “Knowledge is power” posting I mentioned ember attack as a major cause — actually the main cause — of housing loss.

Again, the simple truth, keep the grass short, prevent a build-up of leaves, bark and twigs that will carry a fire up to or into a dwelling and bring that dwelling up to the bushfire fire resistance capability of at least bushfire attack level BAL–12.5 according to Australian Standard AS 3959—2009 Construction of buildings in bushfire-prone areas, which is basically measures to prevent ember penetration and protection against low-level fires burning up to the walls of a dwelling in a grassland environment. The result intended is a well-prepared dwelling and well-prepared occupants choosing to stay and defend should at least be able to stay inside the dwelling until after the passage of the fire past that dwelling, even on a day like that which drove the Mickleham fire.

This video posted on You Tube (click here) is useful in gaining an understanding of the burning characteristics of a grass fire.

Features to watch for in the video:

• How quickly the fire front moves across the ground. Note the very short period of time that the fire front is actually flaming in any one location as it moves forward.
• The black ground immediately behind the flame front, with virtually nothing left to burn — somewhat different to a fire in scrub or forest, the characteristics of which I’ll deal with in a future posting.
• The significant reduction of flame height when the fire front or edge reaches the short grass of the vehicle track.
• How the fire front comes to a halt when it reaches a fence and how long it takes to overcome that obstruction, however flimsy that timber fence may be. A steel fence can be very effective in halting the spread of fire in grassland, particularly with shortening or removing the grass on the side of the fence exposed to the fire. I have seen a wind-driven grass fire halted by simple stone fence long enough for a pursuing fire tanker to catch and stop the fire at the fence.

This photograph taken in the Bonang area of East Gippsland around 14 February 2014 shows the aftermath of a grass fire disappearing into the distance. A good example of how fires in grassland generally do not involve the canopy of eucalypt trees other than scorching some of the leaves that eventually fall but do not contribute to the fire itself. (thanks to Mark Gunning for the photograph)

The following four photographs are examples of buildings that are at extreme risk of ignition due to ember attack and fire burning up to the base of unprotected combustible walls.The third photograph shows a gap between a softwood window ledge and softwood decking a prime location to trap wind-borne burning leaves and bark.

Rubbish around the yard will likely be ignited by the passing grass fire and subject nearby buildings to extreme heat and long lasting ember and spark attack.

Poly-plastic water tanks at risk from the long grass at their base.

A Building Commission publication (click here) "A guide to retrofit your home for better protection from a bushfire” provides advice on how to increase the bushfire resistance capability of a dwelling. Amongst other matters, it explains bushfire attack levels (BAL) to be used as a guide to levels of construction for various vegetation environments and threat levels expressed in kilowatts/m² — the further from the fire the lower the kilowatts/m² at the receiver.

The CFA provides useful information on dealing with bushfire (click here) and specifically grass fires (click here).

Finally, the second last paragraph from my blog posting “Knowledge is power” on 16 February 2014:

Knowledge of fire behaviour is a critical factor to consider when contemplating evacuation — ‘knowledge is power’ and we should never consider ourselves powerless when dealing with bushfire. When fire does threaten, to understand what is really happening we need to disregard the awesome smoke column and the dramatics of the media and make our own assessment of the threat. And be aware of our own emotional and physical limitations.

Understanding the fire danger rating system

As we travel Victoria’s roads many of us will have seen an adjustable fire danger rating sign (click here) used by the CFA to warn us of the bushfire danger in the area.

The Australasian Fire and Emergency Services Authorities Council (AFAC) defines “fire danger” as:

Sum of constant danger and variable danger factors affecting the inception, spread, and resistance to control, and subsequent fire damage; often expressed as an index.

and “fire danger rating” as:

A relative class denoting the potential rates of spread, or suppression difficulty for specific combinations of temperature, relative humidity, drought effects and wind speed, indicating the relative evaluation of fire danger.

Extracts from Bushfires in Australia, by R H Luke and A G McArthur considered relevant to this posting:

Fire danger rating systems serve two major purposes. The first is to provide the basis for the Bureau of Meteorology fire weather forecasts, which are issued through the media as a public information service. When warnings of increasing fire danger are included, such messages may be associated with or contain announcements of fire restrictions [e.g. periods of total fire ban]. General terms must be used as climatic regions cover large areas and many fuel types.

The second major purpose of a fire danger forecast is to provide fire control managers and other responsible persons with reliable daily or even hourly information on which to base their assessment of fire risk, likely fire behaviour and many other important matters such as those relating to the issue of burning-off permits, detection services [e.g. activating fire lookout towers] and the location [pre-positioning] of initial [firefighting] attack crews.   

If a precise prognosis of likely fire behaviour in a specific fuel type is required for a running fire the fire danger rating system should be capable of enabling the fire boss [incident controller] to answer such questions as:

• the rate of spread of the fire;

• difficulty of control; will mechanical equipment [including water bombing aircraft] be required or can it be handled by initial attack forces?

• will it be a high intensity or low intensity fire?

• will it produce a towering convection column or have a wind-driven smoke plume?

• will it be a crown [sustained tree canopy] fire or a ground [or surface] fire?

• what is the probability of it ‘blowing up’ [developing into an especially violent fire] in forest fuels?

• what will be the spotting potential of the fire and the likely distance that spot fires will be thrown?

Referring back to my posting of 16 February (click here) where I mentioned FDI (fire danger index) read this to mean “fire danger rating” in this posting.

The Fire Danger Rating sign (click here) mentioned earlier is graduated according to the FDI:

		Rating                                       FDI
	Code Red                                 100 + Extreme                                   75 – 100 Severe                                     50 – 75 Very High                                 25 – 50 High                                         12 – 25 Low – Moderate                         0 – 12

AFAC defines FDI (Fire Danger Index) as:

A relative number denoting an evaluation of rate of spread, or suppression difficulty for specific combinations of temperature, relative humidity, drought effects and wind speed.

So, what does this “fire danger rating” and ‘fire danger index” advice to the community mean and how should it be used? An issue touched on in my 16 February (click here) posting that I’ll come back to in a future posting, as I believe that in the manner it’s currently being used by the CFA to warn the public can be misleading, instil panic and is potentially dangerous.

The FDI/fire danger rating is not static or “one-size-fits-all” and can vary upwards according to a dynamic weather environment as the recorded weather data on my 16 February posting illustrates. And can vary downwards where land is subjected to onshore wind in coastal environments or the blocking influence of high ground.

To conclude this posting, the default FDI used by the CFA in its approach to dealing with Bushfire Management Overlay Bushfire Management Statements is FDI 120, which is based on weather recorded at the Bureau of Meteorology Recording Station at Melbourne Airport during the afternoon of the 1983 “Ash Wednesday Fires: 41degrees; Relative Humidity 5 per cent ; wind velocity of 45 k/hr and a drought factor of 10 (0 to 10).

One example of an on-line Forest Fire Danger Index calculator (click here).

Key inputs:

• Temperature

• Relative humidity

• Wind speed

• Drought factor