Above photograph is of an area of mulched or masticated vegetation starting to appear on the edge of some urban areas and elsewhere, apparently to help protect the communities from bushfire and act as firebreaks.
The yellow arrow points to a garden rake placed to bring scale to the photograph.
The red arrow points to a small excavation approximately 100 mm deep to ground level to illustrate the composition of the litter bed.
Close up of the photograph of mulched or masticated vegetation. The red arrows indicate the finer of the fuel available to contribute to ember attack.
Below, two more examples of mulching or mastication of vegetation on the edge of a township .
The Overall fuel hazard assessment guide 4th edition July 2010 by the then Department of Sustainability and Environment, defines “surface fine fuel” as:
Significant in the definition of “surface fine fuel” is the statement “usually contributes the most to fuel load or quantity”. While elevated fine fuel and tree bark can contribute to fire spread, it is principally the “surface fine fuel” that carries fire forward from the fire front as embers by wind at ground level.
The vast majority of buildings lost to bushfire are ignited by embers.
An example of windborne embers igniting a dwelling, the four photographs immediately above were taken shortly after the October 2015 Lancefield fire. The top photograph was captured from Google Earth Street View as it was prior to the fire. The next two are the remains of the dwelling after the fire.
The final photograph is of the forest further to the north, showing the tree canopies only scorched by the heat of the fire passing beneath them.
Highly likely that it was the impact of embers against an unprotected building, given the survival of the the trees and shrubs somewhat removed from the building — the skeletons of the two shrubs against the building were likely consumed by the heat of the building fire.
Of course, for a fire to reach the dwelling site so far removed from the forest, it was highly likely that rather than embers it was heavier fuel, such as initially airborne burning gum bark known as firebrands that ignited a fire closer to the dwelling.
If a building is not kept free of flammable vegetation close to or at its base, see “Part 5.1 Near-surface fine fuel” Overall fuel hazard assessment guide 4th edition July 2010 then, regardless of lawns that would act as a firebreak to protect the building, embers can reach and ignite that vegetation which in turn ignites the building.
As one of many examples, below is a News Limited photograph of buildings in Winmalee, NSW that have succumbed to ember attack from the Blue Mountains, NSW fires in 2014. The unburnt condition of the vegetation around the buildings is evidence of ember attack on unprotected and probably undefended buildings.
Important to understand that embers or firebrands are not confined to ground level attack, recognised in AS 3959 Construction of buildings in bushfire-prone areas that includes measures to prevent embers entering roof spaces.
Embers are insidious and will find their way into a building through the smallest of openings — it is said that many buildings lost to bushfire burn down from inside!
And another from the Christmas 2015 Wye River fire, note the unburnt tree canopies behind the remains of the dwellings — was it ember attack or dry vegetation against an unprotected building, we’ll never know, but another example of the much-maligned tree only being scorched by a relatively low level fire driven beneath them by strong wind.
What then of mulching or mastication as a method of bushfire fuel reduction?
Unlike fuel reduction by burning, mulching or mastication is used to bring elevated and near-surface fuel to ground level, thereby resulting in an increase of “surface fine fuel” reaching 50 mm deep — classified as “Extreme” in the Overall fuel hazard assessment guide” — and possibly deeper depending on the bulk of the vegetation being treated in this manner.