The Defendable Space Zones of the CFA
The CFA use a 1-in-50 year Fire Danger Index (FDI) of 120 (M & B). This equates to 40C, 5% RH and 40 kph wind speed at 10 m in the open.
In the following scenario, the terrain is flat.
CFA requires the following designs by law for new developments within the Bushfire Prone Areas of each Municipality.
The source data for this study is the CFA Applicants Kit for Building in a Wildfire Management Overlay and a recent reference by Maughan DPC and Bosomworth K (2004), Theory into practice: A case study of vegetation management and house bushfire survival [Proceedings:Earth, Wind and Fire – Fusing the Elements, May 2004] .. (Dept of Environment and Heritage, South Australia)
The CFA bases its design specs on Australian Standard 3959 and CSIRO research, both of which are guided by McArthur’s theory and use of Byram’s intensity.
CFA identifies that the danger to the house is the vegetation type that the bushfire runs through. Vegetation type in this study is medium forest. CFA does not specify fire behaviour “givens”, but they Byram’s intensity as an input for bushfire risk determinations. For illustration purposes, we will use the same figures as the RFS, ie, fuel load = 25 t / ha, rate of spread = 3.84 kph. Thus Byram’s fire intensity = 48,000 kW / m.
CFA assumes the forest produces a wall of flame and therefore requires a separation distance between the flame and the house so that radiation levels on the house will be less than 29 kW / sq m. The separation distance is called defendable space. Defendable space is comprised of two parts, Outer Zone and Inner Zone. This study will consider the separation distances on the danger side of the house.
For medium forest vegetation, the Outer Zone on the danger side (NW) is 50m, and the Inner Zone is 10m.
See Diagrams 1 and 2 below for the features of Defendable Space.
The stated aims of the Outer Zone are to moderate fire behaviour, to reduce radiant heat on the building, to eliminate flame contact on the building and to reduce ember attack on the building. The source of this heat and flame is the wall of flame at the forest edge.
What are the specifications for the Outer Zone?
The CFA document specifies that on the danger side, the Outer Zone is 50m wide, and maximum fuel loads for two fuel bed types. If it is a litter bed, there must be no more than 10mm depth of litter, which equates to 3 t / ha fuel load. If it is a grass bed, grass height must be below 10cm height.
[There are no specifications for other fuel bed types.] EXPECTED FIRE BEHAVIOUR IN LITTER FUEL: – 3 t / ha is 0.12 times 25 t / ha, so according to McArthur theory, rate of spread is 0.12 times 3.84kph = 0.46 kph. Therefore, fire intensity = 500 x 3 x 0.46 = 690 kW / m.
EXPECTED FIRE BEHAVIOUR IN GRASS FUEL BED: – 10cm height is approx 1 t / ha fuel load. In these weather conditions, the CSIRO Grassland Meter estimates rate of spread of around 14 kph. Therefore, Byram’s fire intensity = 500 x 1x 14 = 7,000 kW / m
The stated aim of the Inner Zone is to reduce radiant heat on the building, eliminate direct flame contact on the building and reduce ember attack on the building. The source of this heat and flame is the wall of flame at the forest edge.
What are the specifications for the Inner Zone?
The Inner Zone is 10m wide has the same maximum fuel loading as the Outer Zone. This means expected fire behaviour is the same in both zones.
FUEL-FREE BUFFER? The Applicant’s kit states that non flammable features such as tennis courts, swimming pools and driveways should be located on the danger side of the building.
WHAT DO THESE INTENSITIES MEAN?
The McArthur Meter formula used by the RFS suggests that flame length of the forest fire is 30m. This scenario therefore suggests that there will be a wall of flame at the forest edge at least 30m tall. Byram’s intensity at this point is 48,000 kW / m.
For a litter fuel bed, the fire intensity in the 60m wide Defendable Space is 690 kW / m. Fire fighters would regard that this intensity can be controlled (This statement assumes that on-site resources are adequate.). Luke and McArthur say a 450kW / m fire has a flame height of 1.2m, so this flame might be around 1.5m.
For a grass fuel bed, the fire intensity in the 60m wide Space is 7,000 kW / sq m. Fire fighters believe they can control grass fires up to 10,000 kW / m (This statement assumes that on-site resources are adequate.). The CSIRO Grassland Meter estimates a flame height of approx 1.5m.
WHAT DO THE CFA SEPARATION DISTANCES MEAN?
The documents imply that CFA tables have been designed to protect the building from the radiant heat of the wall of flame in the medium forest at the edge of the Outer Zone. A distance of 60m seems to achieve this objective comfortably.
The house will be subjected to non damaging radiation from the wall of flame at the forest edge. But the flame within the Inner Zone, which abuts the house, is of much more concern. Flame height in a litter fuel bed could be up to 1.5m and in a grass bed could be up to 1.2m. This means these documented fuel management prescriptions provide the house with zero protection / buffer from flame contact in a severe bushfire.
In regard to embers, these prescriptions do not address the ember issue.
THEREFORE, THESE PRESCRIPTIONS PER SE ARE UNABLE TO PROTECT THE HOUSE IN SEVERE FIRE CONDITIONS, AND THE ONLY CHANCE OF THE HOUSE SURVIVING DEPENDS ON THE PRESENCE AND THE ABILITY OF FIRE FIGHTERS. UNFORTUNATELY, EXPECTED FIRE BEHAVIOUR WITHIN THE DEFENDABLE SPACE IS NOT A PROTECTED ENVIRONMENT FOR FIRE FIGHTERS.
FURTHERMORE, BECAUSE THE PRESCRIPTIONS IGNORE THE FLAME WITHIN THE INNER ZONE, THERE WILL BE FLAME CONTACT ONTO THE HOUSE.
THIS MEANS THE STATED AIMS OF THE INNER AND OUER ZONES CANNOT BE ACHIEVED IN A SEVERE BUSHFIRE.
The reader should note a few key points:
*** These protection prescriptions are required by government legislation in new housing development areas within bushfire prone areas. The major source material for these prescriptions is Australian Standard 3959 and CSIRO research.
*** There is no documented requirement for existing housing estates.
*** The prescriptions are based on the assumption that the forest carries maximum fuel load. There is no option within the prescriptions that allows fire danger to be reduced by managing fuel load in the forest.