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.
