Monday 17 March 2014

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.

2 comments:

  1. Hi again John, Our block of land is zoned low density residential and has a BMO and VPO1. What is your opinion on being unable to reduce fuel loads in areas at risk due to Vegetation Protection Overlays?
    Jacqui

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  2. Hello Jacqui

    My opinion on removal of vegetation from land covered by a Vegetation Protection Overlay (VPO) in a planning scheme is somewhat influenced by the level of bushfire risk addressed in the Municipal Fire Prevention Plan (MFPP). It may be that the MFPP identifies a parcel of land as a threat to adjoining properties and thus requires mitigation works (fuel reduction) to protect those adjoining properties.

    This is the link to the Country Fire Authority Act and it’s section 55A that addresses the MFPP and section 55B that deals with auditing the MFPP http://www.legislation.vic.gov.au/domino/Web_Notes/LDMS/LTObject_Store/LTObjSt7.nsf/DDE300B846EED9C7CA257616000A3571/5F472CF4FFDF8A46CA257B96001459D4/$FILE/58-6228aa145%20authorised.pdf

    Access to Vegetation Protection Overlay is here http://planningschemes.dpcd.vic.gov.au/schemes/vpps/42_02.pdf

    Concerning any Schedules to the VPO covering your land, this link takes you to Planning Schemes Online http://planningschemes.dpcd.vic.gov.au/schemes/vpps/42_02.pdf Find your municipality then find your location on the map, ascertain if there’s a VPO Schedule that covers your land then bring up the Ordinances and go to the Overlays.

    You’ve raised an issue of concern to clients of mine at Wye River and Lorne who find themselves confronted by bushfire attack level at “flame zone” (BAL–FZ) due to the two councils involved: Colac Otway Shire and Surf Coast Shire failing to protect the dwellings either side of land owned by my clients through the MFPP, and the respective Municipal Fire Prevention Officers obviously failing to serve “fire prevention notices” according to Section 41 Country Fire Authority Act. Section 41F Country Fire Authority Act empowers the CFA Chief Officer to serve a “fire prevention notice”.

    While I can’t comment on your land as I’ve not inspected it, you could contact the Municipal Fire Prevention Officer to discuss your situation and ask to view the MFPP to see how it affects your land, if at all.

    I expect there are others who find themselves in similar situations across Victoria so will make this matter the subject of my next blog posting.

    I hope the above helps.

    John Nicholson

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