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Appendix D - Bushfire Risk Assessment and Management Plan
Bushfire Risk Assessment
and
Bushfire Risk Management Plan
for
A “Community housing” development
at
55 Carpenters Rd, Cooroy, Queensland
Lot 2 on RP 200519
Revision 1
February 2022
Prepared by
Ecological Natural Area Management
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Document details
Author details
The author of this bushfire risk assessment and bushfire risk management plan is Adrian Hansen,
proprietor of Ecological Natural Area Management. Note that the author is a Fire Protection
Association of Australia Accredited Practitioner Level 3 in Bushfire Planning and Design. See
Appendix 2: Qualifications and experience of the author of this report for demonstration that the
author is “a competent person [and] is an appropriately qualified and experienced consultant with
appropriate and proven technical expertise in the preparation of bushfire hazard assessment
reports”.
Contact details
Adrian Hansen
Ecological Natural Area Management
0400 132 634
[email protected]
Document history
Revision Date Reviewed
Revision 1 25 February 2022_ Adrian Hansen
Document certification
25 February 2022……………
Adrian Hansen Date
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Table of Contents
1 Executive Summary.................................................................................................................................... 5
2 Introduction................................................................................................................................................. 8
2.1 Commissioning of the Bushfire Risk Assessment and Management Plan........................................8
2.2 The aim of the Bushfire Risk Assessment and Management Plan....................................................8
2.3 Legislative considerations................................................................................................................. 8
2.4 Other regulatory considerations........................................................................................................ 8
3 Site description........................................................................................................................................... 9
3.1 Location............................................................................................................................................ 9
3.2 Zoning............................................................................................................................................. 10
3.3 Applicable development codes....................................................................................................... 10
3.4 Topography..................................................................................................................................... 10
3.5 Mapped Bushfire Hazard................................................................................................................ 10
4 Site vegetation assessment..................................................................................................................... 11
4.1 Drone aerial photography................................................................................................................ 11
4.2 Mapped Remnant Regional Ecosystems........................................................................................ 12
4.3 Visually assessed vegetation ......................................................................................................... 13
4.4 Map of the visually assessed vegetation.........................................................................................14
5 Bushfire risk assessment........................................................................................................................ 15
5.1 Assessment of the Potential Bushfire Intensity Class.....................................................................15
5.2 Derivation of inputs for assessment of the Potential Bushfire Intensity Class.................................16
5.3 Potential Bushfire Intensity Class for ranges of bushfire fireline intensities....................................16
5.4 Tabulation of the Potential Bushfire Intensity Class........................................................................18
5.5 Potential Impact Buffer.................................................................................................................... 18
5.6 Map of the areas of Potential Bushfire Impact classes...................................................................19
5.7 Assessment of Bushfire Prone Area............................................................................................... 19
6 Assessment of Asset Protection Zones, Radiant Heat Loads and Bushfire Attack Level..................20
6.1 Assessment of Asset Protection Zones and Radiant Heat Loads...................................................20
6.2 Modelled bushfire attacks............................................................................................................... 21
6.3 Assessment of radiant heat shielding.............................................................................................22
6.4 Bushfire parameters used in the bushfire behaviour modelling......................................................22
6.5 Map of the assessed Bushfire Attack Levels...................................................................................24
6.6 Assessment of the minimum Asset Protection Zone ......................................................................24
7 Bushfire risk management solutions and recommendations...............................................................25
7.1 Bushfire hazard mitigation recommendations: Considerations.......................................................25
7.2 Recommendations for the Noosa Plan Bushfire Overlay Code......................................................25
7.3 Recommendations for Australian Standard AS3959.......................................................................25
7.4 Recommendations for siting and landscaping................................................................................26
7.5 Recommendations for lodgement points for burning debris............................................................26
7.6 Recommendations for on-going management of the buildings & landscaping................................27
8 References................................................................................................................................................ 28
9 Appendices................................................................................................................................................ 29
9.1 Appendix 1: Regional Ecosystem full descriptions..........................................................................29
9.2 Appendix 2: Qualifications and experience of the author................................................................31
9.3 Appendix 3: The fire run paper by Tan (2006).................................................................................32
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List of Figures
Figure 1: Proposed site plan for the “Community housing” development..........................................................9
Figure 2: Excerpt from Noosa Plan Bushfire Overlay showing the areas mapped as Medium Potential
Bushfire Intensity or Potential Bushfire Impact Buffer.....................................................................................10
Figure 3: Drone aerial photography acquired on 16 February 2022 showing the proposed development in
relation to the native vegetation, suburban residential areas, 5m contour lines and mapped watercourses...11
Figure 4: Map of the Qld VMA conservation status of the Remnant Vegetation near the development..........12
Figure 5: View of west side of the central parkland showing Eucalyptus trees, mown underneath.................13
Figure 6: View of the north east corner showing rainforest vegetation............................................................13
Figure 7: View of the northern boundary showing E.ucalyptus resinifera, C. intermedia open forest..............13
Figure 8: View of east side of the central parkland showing Eucalyptus trees, mown underneath.................13
Figure 9: Map of the locations of the visually assessed vegetation, suburban residential areas, , 5m contour
lines and mapped watercourses . Note a 2.1m fence along the north side of the development which will
afford radiant heat shielding............................................................................................................................ 14
Figure 10: Map of vegetation patches used for patch filtering........................................................................17
Figure 11: Map showing that the house is in an area assessed as Potential Bushfire Impact Buffer Class....19
Figure 12: Map showing the modelled bushfire attack directions....................................................................21
Figure 13: Diagram showing how a radiant heat shield such as a fence shields part of the bushfire flame
reducing the view factor of the flame.............................................................................................................. 22
Figure 14: Map of the assessed Bushfire Attack Levels for lots in the development. All houses in the
development can be built in the green areas which have a bushfire construction requirement of BAL Areas in
the centre of the development have a radiant heat load less than 10kW/m2 and are safe areas...................24
Figure 15: Methodologies for minimising the lodgement points for burning debris around the house.............26
List of Tables
Table 1: Mapped Remnant Regional Ecosystems near the development.......................................................12
Table 2: Potential fuel loads for the assessed vegetation communities based on their applicable Vegetation
Hazard Classes.............................................................................................................................................. 16
Table 3: Table of Potential Bushfire Intensity Class derived from Potential Fire line Intensity.........................16
Table 4: Tabulation of the potential bushfire intensity classes for each of the patches ...................................18
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1 Executive Summary
Most of the proposed houses in a “Community housing” development “ at 55 Carpenters Rd, Cooroy
in the Noosa Council jurisdiction are mapped in Potential Bushfire Impact Buffer area on Queensland
Potential Bushfire Intensity mapping and Noosa Council Bushfire Overlay. Therefore, the location of
the proposed houses is deemed to potentially be in a Bushfire Prone Area and an assessment of the
bushfire risk to the development and a bushfire risk management plan is required.
Site specific assessment of the Potential Bushfire Intensity Class
A site specific assessment of the Potential Bushfire Intensity Class using the protocol of A new
methodology for state-wide mapping of bushfire prone areas assessed that many of the houses
are in an area assessed as Potential Bushfire Impact Buffer Class as shown on the map below.
Assessment of Bushfire Prone Area
From the above site specific assessment, many of the houses in the development are in a Bushfire
Prone Area. Potential houses in lots assessed as a Bushfire Prone Area are required by the
Queensland Building Act and the bushfire requirements of the National Construction Code to have a
Bushfire Attack Level assessment conducted using the protocol of Australian Standard AS3959.
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Assessment of Bushfire Attack Levels and escape routes
This map shows the assessed Bushfire Attack Level areas in the lots and potential escape routes.
• All houses in the development can be built in the green areas which have a bushfire
construction requirement of BAL 12.5,
• Areas in the centre of the development have a radiant heat load less than 10kW/m2 and are
safe areas but”
• The escape route is through a BAL 19 area.
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Recommendations
Recommendations for the Noosa Plan Bushfire Overlay Code
It is recommended that notwithstanding that the development is located in a Potential Bushfire Impact
area, the development maintains the safety of people and property from the adverse impacts of
bushfire by maximising setbacks from hazardous vegetation to achieve an acceptable risk as:
• Both new lots could have houses constructed to BAL 12.5 construction standard,
• Houses can be sited in a low hazard area in the lot and achieve a setback of 10 metres
from any retained vegetation,
• Houses will have a reliable reticulated water supply for fire fighting purposes
• Expected Radiant Heat loads at each house is less than 10kW/m2 and
• The escape route is through a safe area with expected radiant heat less then 10kW/m2,
• The development does not result in a material increase in the extent and severity of
bushfire hazard,
• The development supports and does not unduly burden disaster management response or
recovery capacity and capabilities as there are escape routes through a safe areas with
expected radiant heat less then 10kW/m2,
• No bushfire mitigation measures such as vegetation clearing that could adversely impact
biodiversity values are required.
Recommendations for Australian Standard AS3959
Australian Standard AS3959 Construction of buildings in bushfire-prone areas specifies Deemed to
Satisfy bushfire resistant methodologies based on the assessed Bushfire Attack Level.
It is recommended that:
• The building solution for all building elevations for any houses in the lots to address the
Deemed to Satisfy requirements of Sections 3 and 5 of Australian Standard AS3959-2018
Construction of buildings in bushfire-prone areas
Other recommendations for building siting and landscaping
It is recommended that:
• The buildings to have a bushfire fuel free zone around the house,
• The buildings be constructed with a minimum of lodgement points for burning debris and
• On-going maintenance of the buildings be conducted.
See Sections 8.4, 8.5 and 8.6 of this report
Recommendations for Part E: Natural hazards of the Queensland State Planning Policy 2017
It is recommended that, notwithstanding the developmenti in an area of Potential Bushfire Hazard:
• The building siting, construction, firefighting resources etc appropriately mitigates the
potential bushfire Hazard as required by the SPP
• The development does not unduly burden emergency services as there is good access for
firefighting appliances, good fire fighting space, good water supply and contributes only one
extra building in the emergency services area
• The building and the cleared and hard-standing areas decreases the severity of a bushfire
on the lot and therefore does not increase severity of the bushfire Hazard and,
• There will be no hazardous material stored on the lot.
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2 Introduction
Most of the proposed houses in a “Community housing” development “ at 55 Carpenters Rd, Cooroy
in the Noosa Council jurisdiction are mapped in Potential Bushfire Impact Buffer area on Queensland
Potential Bushfire Intensity mapping and Noosa Council Bushfire Overlay. Therefore, the location of
the proposed houses is deemed to potentially be in a Bushfire Prone Area and an assessment of the
bushfire risk to the development and a bushfire risk management plan is required.
2.1 Commissioning of the Bushfire Risk Assessment and Management Plan
Ecological Natural Area Management has been commissioned to prepare a Bushfire Risk
Assessment and Bushfire Risk Management Plan to fulfil the above requirements. See Appendix 2
for demonstration that the author is a “qualified and experienced consultant with appropriate and
proven technical expertise in the preparation of bushfire hazard assessment reports and
management plans”.
2.2 The aim of the Bushfire Risk Assessment and Management Plan
The aim of this bushfire impact assessment report is to achieve appropriate levels of bushfire
protection for the developed properties while maintaining ecological sustainability.
Bushfire protection for buildings in bushfire prone areas is best achieved using a mix of:
•Fire resistant building technologies in the structures.
•Fire resistant methodologies in the structures such as minimising ember lodgement points
•Providing and appropriate fire break between the developed properties and flammable
vegetation.
• Appropriate building siting for bushfire protection
• Fuel management in the flammable vegetation near the developed properties while
maintaining ecological values of the vegetation habitat.
• Fuel management between the flammable vegetation and the developed properties.
• Maintenance of ecological and habitat values of any vegetation subject to vegetation
management for bushfire risk management.
To achieve the aim, the Plan will conduct bushfire assessments against the applicable legislative
requirements and make recommendations that address the requirements of the legislation.
2.3 Legislative considerations
• Queensland Planning Act,
• Queensland Building Act,
• Queensland Vegetation Management Act,
• Queensland State Planning Policy 2017 and
• The Noosa Plan 2020.
2.4 Other regulatory considerations
• The National Construction Code and
• Australian Standard AS 3959 - 2018: Construction of buildings in bushfire-prone areas.
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3 Site description
3.1 Location
The site, on Lot 2 on RP 200519 is at 55 Carpenters Rd, Cooroy, Queensland in the Noosa Council
jurisdiction. The proposal is a “Community housing” development.
Figure 1: Proposed site plan for the “Community housing” development
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3.2 Zoning
The Rural Residential Zone on The Noosa Plan 2020 Zone map.
3.3 Applicable development codes
Development codes addressed in this report are:
• The Noosa Plan Bushfire Hazard overlay code and
• Queensland State Planning Policy 2017,
3.4 Topography
Site topography is “Plain” <5% slope.
3.5 Mapped Bushfire Hazard
The site has areas mapped on Queensland Potential Bushfire Intensity mapping and Noosa Plan
Bushfire Overlay as:
• Medium Potential Bushfire Intensity or Potential Bushfire Impact Buffer
Figure 2: Excerpt from Noosa Plan Bushfire Overlay showing the areas mapped as Medium Potential
Bushfire Intensity or Potential Bushfire Impact Buffer
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4 Site vegetation assessment
4.1 Drone aerial photography
Drone aerial photography acquired on 16 February 2022 shows areas of native forest vegetation near
the development which is in a suburban residential area.
Figure 3: Drone aerial photography acquired on 16 February 2022 showing the proposed development in
relation to the native vegetation, suburban residential areas, 5m contour lines and mapped watercourses
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4.2 Mapped Remnant Regional Ecosystems
Areas of the vegetation near the development have been mapped as Remnant Regional Ecosystem.
The Remnant Vegetation map below will be used as an aid to identifying the vegetation..
Figure 4: Map of the Qld VMA conservation status of the Remnant Vegetation near the development.
Table 1: Mapped Remnant Regional Ecosystems near the development
Regional Ecosystem Description
RE 12.3.2 Eucalyptus grandis open forest on alluvial plains
RE 12.3.1 Gallery rainforest (notophyll vine forest) on alluvial plains
RE 12.9-10.1 Eucalyptus resinifera, E. grandis, E. robusta and Corymbia intermedia
open forest on sedimentary rocks
(Source: Department of Environment and Heritage Protection, 2022)
See Appendix 1 for the full descriptions of these Regional Ecosystems.
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4.3 Visually assessed vegetation
The vegetation near the development was visually assessed and shown on the photographs below.
Figure 5: View of west side of the central parkland
showing Eucalyptus trees, mown underneath
Figure 6: View of the north east corner showing
rainforest vegetation
Figure 7: View of the northern boundary showing
E.ucalyptus resinifera, C. intermedia open forest
Figure 8: View of east side of the central parkland
showing Eucalyptus trees, mown underneath
The map on the next page shows the locations of the vegetation based on the visual assessment
shown in these photos.
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4.4 Map of the visually assessed vegetation
Based on the visual assessment of the vegetation, inspection of aerial photography, assessment of
vegetation mapping, the map below shows the locations of the visually assessed vegetation which are.
• Cleared urban residential areas with cleared house lots, gardens, roads, driveways etc,
• A central park land with scattered trees that are mown underneath,
• Notophyll Vine Forest (rainforest) and
• Eucalyptus spp, Corymbia spp woodland - Canopy height = 20 m
Note a 2.1m fence along the north side of the development which will afford radiant heat shielding
Figure 9: Map of the locations of the visually assessed vegetation, suburban residential areas, , 5m contour
lines and mapped watercourses . Note a 2.1m fence along the north side of the development which will
afford radiant heat shielding
These visually assessed vegetation communities will be used for subsequent bushfire risk
assessments in this report.
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5 Bushfire risk assessment
5.1 Assessment of the Potential Bushfire Intensity Class
As required by State Planning Policy 2017, this report will use the the protocol specified in A new
methodology for state-wide mapping of bushfire prone areas (Leonard et al, 2014) to assess the
bushfire prone area status of the development.
This methodology uses the following parameters.
• Potential Fuel Load
This is derived from Vegetation Hazard Classes table in the protocol. The table was derived
by relating vegetation community derived by combining Remnant Vegetation Regional
Ecosystem types and their Fire Regime Groups developed by the Queensland Parks and
Wildlife Service (Neldner et al, 2012) to a potential fuel load using the methodology of the
Overall Fuel Hazard Assessment Guideline .
• Patch and corridor filtering
Patch and corridor filtering can be used to remove or downgrade hazard levels of small
patches of vegetation and narrow corridors.
• Maximum Landscape Slope
The Maximum Landscape Slope is derived for 25m pixel separations.
• Fire Weather Severity
An expected McArthur Forest Fire Danger Index using a 1:20 year or 5% annual
exceedance probability was derived for all areas in Queensland to reflect the severity of
bushfire events suited to mitigation through land use planning and this expected maximum
FDI is presented in as a map in Figure 16: Fire weather severity for Queensland in A new
methodology for state-wide mapping of bushfire prone areas (Leonard et al, 2014). The
Forest Fire Danger Index for this site in Figure 16 is FFDI = 50.
• Potential Fire-line Intensity
With the above inputs, a Potential Fire-line Intensity can be calculated using the formula:
PFI = 0.62 x PFL2 x FFDI x exp(0.069 x slope)
Where:
PFI = Potential Fireline Intensity
PFL = Potential Fuel Load
FFDI = Expected maximum Forest Fire Danger Index
Slope = Maximum 25m pixel slope in degrees
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5.2 Derivation of inputs for assessment of the Potential Bushfire Intensity Class
The assessment of the Potential Bushfire Intensity Class using the protocol of A new methodology
for state-wide mapping of bushfire prone areas (Leonard et al, 2014) requires the following inputs.
Potential Fuel Load
Based on the map of the visually assessed vegetation communities shown in Figure 9, the
Potential Fuel Loads can be derived by relating the assessed vegetation to the Vegetation Hazard
Classes in Bushfire Resilient Communities.
Table 2: Potential fuel loads for the assessed vegetation communities based on their applicable
Vegetation Hazard Classes
Vegetation community Vegetation Hazard Class Potential fuel load
Eucalyptus spp, Corymbia VHC 9.1 Moist to dry eucalypt open forests on 24.1 t/ha
intermedia open forest coastal lowlands and ranges
Notophyll Vine Forest VHC 3.1 Notophyll vine forest 4.5 t/ha
Treed parkland VHC 39.2 Low to moderate tree cover in built- 8 t/ha
up areas
Suburban areas VHC 41.4 Discontinuous low grass or tree 3 t/ha
cover
Fire Weather Severity
The Fire Weather Severity used was FFDI 53 as required by Bushfire Resilient Communities
5.3 Potential Bushfire Intensity Class for ranges of bushfire fireline intensities
Using the inputs derived above, the Potential Bushfire Intensity Classes for the patches of
vegetation are derived from bands of fireline intensities using the table below.
Table 3: Table of Potential Bushfire Intensity Class derived from Potential Fire line Intensity
Potential Bushfire Intensity Class Potential Fire-line Intensity
Very high >40,000 kW/m
High 20,000 to 40,000 kW/m
Medium 4,000 to 20,000 kW/m
Low Less than 4000 kW/m
(Source: Leonard et al, 2014, page 20)
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Patch and corridor filtering
Leonard et al (2014) specify a
filtering process to remove or
downgrade hazard levels of small
patches and narrow corridors of
hazardous vegetation as fire-line
intensity in these small patches is
unlikely to reach the level
predicted for large contiguous
patches of vegetation (Leonard et
al (2014), page 5). Using the map
below, patch and corridor filtering
was conducted.
Figure 10: Map of vegetation patches used for patch filtering
Patch filtering tabulation
Patch Initial class Merge class Reason for merge Potential load
Patch 1 VHC 9.1 Moist to dry VHC 9.1 Moist to dry No merge as patch is large. Remains as 24.1 t/ha
eucalypt open forests eucalypt open forests VHC 9.1 Moist to dry eucalypt open forests
on coastal lowlands on coastal lowlands on coastal lowlands
Patch 2 Discontinuous low Discontinuous low No merge as patch is large. Remains as 3 t/ha
grass or tree cover grass or tree cover Discontinuous low grass or tree cover
Patch 3 VHC 9.1 Moist to dry Discontinuous low Patch 3 (Moist to dry eucalypt open forests) 3 t/ha
eucalypt open forests grass or tree cover allocated to Discontinuous low grass or tree
on coastal lowlands cover through the “filtering process to
remove or downgrade hazard levels of
vegetation less than 100m wide through
patch erosion and dilation...allocating it to
the class most common to the patch
boundary (Leonard et al, 2014), page 5.
Patch 4 VHC 9.1 Moist to dry Discontinuous low Patch 3 allocated to Discontinuous low 3 t/ha
eucalypt open forests grass or tree cover grass or tree cover through the “filtering
on coastal lowlands process
Patch 5 Discontinuous low Discontinuous low No merge as patch is large. Remains as 3 t/ha
grass or tree cover grass or tree cover Discontinuous low grass or tree cover
Patch 6 VHC 9.1 Moist to dry VHC 9.1 Moist to dry No merge as patch is large. Remains as 24.1 t/ha
eucalypt open forests eucalypt open forests VHC 9.1 Moist to dry eucalypt open forests
on coastal lowlands on coastal lowlands on coastal lowlands
Patch 7 VHC 39.2 Low to VHC 39.2 Low to No merge as patch is large. Remains as 8 t/ha
moderate tree cover in moderate tree cover in VHC 39.2 Low to moderate tree cover in
built-up areas built-up areas built-up areas
Patch 8 VHC 3.1 Notophyll vine VHC 3.1 Notophyll vine No merge as patch is large. Remains as 4.5 t/ha
forest forest VHC 3.1 Notophyll vine forest
Patch 9 VHC 9.1 Moist to dry VHC 9.1 Moist to dry No merge as patch is large. Remains as 24.1 t/ha
eucalypt open forests eucalypt open forests VHC 9.1 Moist to dry eucalypt open forests
on coastal lowlands on coastal lowlands on coastal lowlands
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5.4 Tabulation of the Potential Bushfire Intensity Class
Table 4: Tabulation of the potential bushfire intensity classes for each of the patches
Patch Vegetation class Fuel load Slope FDDI Fireline Intensity Potential Bushfire
Intensity Class
Patch 1 VHC 9.1 Moist to dry 24.2 t/ha 5.7 0 53 28538 kW/m High
eucalypt open forests
on coastal lowlands
Patch 2 Discontinuous low 3 t/ha 3.4 0 53 374 kW/m Low
grass or tree cover
Patch 3 Discontinuous low 3 t/ha 3.4 0 53 374 kW/m Low
grass or tree cover
Patch 4 Discontinuous low 3 t/ha 4.3 0 53 397 kW/m Low
grass or tree cover
Patch 5 Discontinuous low 3 t/ha 2.5 0 53 351 kW/m Low
grass or tree cover
Patch 6 VHC 9.1 Moist to dry 24.2 t/ha 2.5 0 53 22885 kW/m High
eucalypt open forests
Patch 7 on
VHCcoastal lowlands
39.2 Low to 8 t/ha 3.9 0 53 2756 kW/m Low
moderate tree cover in
Patch 8 built-up
VHC 3.1areas
Notophyll vine 4.5 t/ha 1.4 0 53 733 kW/m Low
forest
Patch 9 VHC 9.1 Moist to dry 24.2 t/ha 1.4 0 53 21202 kW/m High
eucalypt open forests
on coastal lowlands
5.5 Potential Impact Buffer
As analysis of heat radiation decay curves and national research indicating that in most fires over
80% of housing loss and 80% of human life loss has occurred within 100m of bushland (Leonard et
al., 2014, page 7) specify that a default 100 meter buffer from the edge of hazardous vegetation
which has a Medium, High or Very High Potential Bushfire Intensity Class.
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5.6 Map of the areas of Potential Bushfire Impact classes
The map below shows the areas of the Potential Bushfire Impact classes. The house is in an area
of Potential Bushfire Impact Buffer Class.
Figure 11: Map showing that the house is in an area assessed as Potential Bushfire Impact Buffer Class
5.7 Assessment of Bushfire Prone Area
From the above site specific assessment, many of the houses in the development are in a
Bushfire Prone Area.
Potential houses in lots assessed as a Bushfire Prone Area are required by the Queensland
Building Act and the bushfire requirements of the National Construction Code to have a Bushfire
Attack Level assessment conducted using the protocol of Australian Standard AS3959:
Construction of buildings in bushfire prone areas to be conducted.
Such a Bushfire Attack Level assessment is conducted on subsequent pages of this report.
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6 Assessment of Asset Protection Zones, Radiant Heat Loads
and Bushfire Attack Level
An assessment of Asset Protection Zone Widths, associated Radiant Heat Loads and a Bushfire
Attack Level as related to Radiant Heat Loads will be conducted using the protocol in Bushfire
Resilient Communities as it supersedes Sunshine Coast Planning Scheme Planning scheme policy
for the bushfire hazard overlay code
6.1 Assessment of Asset Protection Zones and Radiant Heat Loads
The Building Code of Australia references Australian Standard AS3959 Construction of buildings in
bushfire-prone areas for Deemed to Satisfy building requirements for bushfire risk management
construction of buildings.
This section assesses Asset Protection Zone Widths, associated Radiant Heat Loads and Bushfire
Attack Level as related to Radiant Heat Loads for the house to determine a level of construction
required to achieve the objective of the Standard which is to “prescribe particular construction
details for buildings to reduce the risk of ignition from a bushfire while the fire front passes”.
(Standards Australia, 2009).
Bushfire Resilient Communities allows the use of Method 2 of Australian Standard AS3959, subject
to the adoption of site-specific values of FFDI, site-specific vegetation hazard classes, modified
modelling of surface fuel loads and effective and site slopes
This report uses software developed by Newcastle Bushfire Consulting which models the Bushfire
Attack Level using Method 2 of Australian Standard AS3959.
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6.2 Modelled bushfire attacks
Six bushfire attacks were modelled based on the proximity of classified vegetation. Bushfire attack
attenuation was included in the bushfire behaviour modelling due to:
• Constrained fire runs caused by fragmentation of the vegetation from urban development.
(A short fire run can be used to attenuate bushfire attack parameters as it has been shown
that it takes a period of time and therefore a fire run distance for a bushfire to reach it full
quasi-steady state rate of spread (Cheney and Gould, 1997). The effect of a reduced
bushfire forward rate of spread on the reduction of assessed bushfire attack parameters
was modelled using the method of Tan (2006) and
• Radiant heat shielding for the north bushfire attacks afforded by the the 2m fence
Figure 12: Map showing the modelled bushfire attack directions
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6.3 Assessment of radiant heat shielding
The 2m fence will afford radiant heat shielding from the flame reducing the view factor that a house
has of the flame.
The method for calculating this radiant heat shielding uses Couch (2010) is as follows:
• Determine the view factor of the unshielded flame,
• Determine the view factor for a flame length equivalent to the effective height of the shield
and holding flame angle constant,
• Calculate the difference in the two view factors,
• Set a flame length which gives a view factor equal to the difference in view factors
calculated above; again holding the flame angle constant and
• With this shielded view of the flame, determine the expected radiant heat load.
Figure 13: Diagram showing how a radiant heat shield such as a fence shields part of the bushfire flame
reducing the view factor of the flame
6.4 Bushfire parameters used in the bushfire behaviour modelling
These bushfire attacks use bushfire attack parameters as specified by Bushfire resilient Communities
North west bushfire attack
This modelled fire burns up-slope through Eucalyptus spp, Corymbia intermedia open forest to
attack from the north west.
• This attack uses fuel loads specified for VHC 9.1 Moist to dry eucalypt open forests on
coastal lowlands of 17.5 t/ha surface fuel & 24.2 t/ha total fuel ,
• Separation of the house from the hazard as required for an appropriate Asset Protection
Zone and to assess radiant heat loads,
• Slope in the vegetation = 1.60
• Site slope = 1.60
• Fire run = 40m leading to a reduction of the bushfire rate of spread to 0.40 of the full quasi-
steady state rate of spread
• Radiant heat shield height of 2m and the assessment uses radiant heat loads of greater
than 40kW/m2 as the distance for BAL FZ as allowed in NSW RFS Planning for Bushfire
Protection to take account of the shield and
• FFDI = 53
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West bushfire attack
This modelled fire burns up-slope through Eucalyptus spp, Corymbia intermedia open forest to
attack from the west.
• This attack uses fuel loads specified for VHC 9.1 Moist to dry eucalypt open forests on
coastal lowlands of 17.5 t/ha surface fuel & 24.2 t/ha total fuel ,
• Separation of the house from the hazard as required for an appropriate Asset Protection
Zone and to assess radiant heat loads,
• Slope in the vegetation = 1.60
• Site slope = 1.60
• Fire run = 40m leading to a reduction of the bushfire rate of spread to 0.40 of the full quasi-
steady state rate of spread
• Radiant heat shield height of 2m and the assessment uses radiant heat loads of greater
than 40kW/m2 as the distance for BAL FZ as allowed in NSW RFS Planning for Bushfire
Protection to take account of the shield and
• FFDI = 53
North east bushfire attack
This modelled fire burns up-slope through Eucalyptus spp, Corymbia intermedia open forest to
attack from the north west.
• This attack uses fuel loads specified for VHC 9.1 Moist to dry eucalypt open forests on
coastal lowlands of 17.5 t/ha surface fuel & 24.2 t/ha total fuel ,
• Separation of the house from the hazard as required for an appropriate Asset Protection
Zone and to assess radiant heat loads,
• Slope in the vegetation = 1.30
• Site slope = 1.30
• Fire run = 80m leading to a reduction of the bushfire rate of spread to 0.50 of the full quasi-
steady state rate of spread
• FFDI = 53
East bushfire attack from the treed parkland
This modelled fire burns up-slope through the treed parkland to attack from the north west.
• This attack uses fuel loads specified for VHC 39.2 Low to moderate tree cover in built-up
areas of 8 t/ha surface fuel & 8 t/ha total fuel ,
• Separation of the house from the hazard as required for an appropriate Asset Protection
Zone and to assess radiant heat loads,
• Slope in the vegetation = 0.50
• Site slope = 9.50
• Fire run = 40m leading to a reduction of the bushfire rate of spread to 0.40 of the full quasi-
steady state rate of spread
• FFDI = 53
East bushfire attack from Notophyll Vine Forest
This modelled fire burns up-slope through the Notophyll Vine Forest to attack from the north west.
• This attack uses fuel loads specified for VHC 3.1 Notophyll vine forest of 4.5 t/ha surface
fuel & 4.5 t/ha total fuel ,
• Separation of the house from the hazard as required for an appropriate Asset Protection
Zone and to assess radiant heat loads,
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• Slope in the vegetation = 1.30
• Site slope = 1.30
• Fire run = 80m leading to a reduction of the bushfire rate of spread to 0.50 of the full quasi-
steady state rate of spread
• FFDI = 53
6.5 Map of the assessed Bushfire Attack Levels
This map of the assessed Bushfire Attack Levels for lots in the development shows that:
• All houses in the development can be built in the green areas which have a bushfire
construction requirement of BAL 12.5,
• Areas in the centre of the development have a radiant heat load less than 10kW/m2 and are
safe areas but”
• The escape route is through a BAL 19 area.
Figure 14: Map of the assessed Bushfire Attack Levels for lots in the development. All houses in the
development can be built in the green areas which have a bushfire construction requirement of BAL Areas in
the centre of the development have a radiant heat load less than 10kW/m2 and are safe areas.
6.6 Assessment of the minimum Asset Protection Zone
The minimum Asset Protection Zone that allows radiant heat loads less than 10kW/m2 and are
safe areas as required by Bushfire Resilient Communities
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7 Bushfire risk management solutions and recommendations
7.1 Bushfire hazard mitigation recommendations: Considerations
As shown above, many of the the proposed houses:
• Are located in an area assessed as Potential Bushfire Impact Buffer Class with a site
specific assessment,
• Are confirmed as being in a Bushfire Prone Area,
• Are assessed as having a construction requirement of BAL 12.5 and
• The areas in the centre of the development have a radiant heat load less than 10kW/m2
Accordingly, this section will recommend bushfire mitigation requirements for the above
assessments with reference to legislative requirements detailed in:
• The Noosa Plan Bushfire Hazard overlay code,
• Australian Standard AS 3959 Construction of Buildings in Bushfire Prone Areas and
• State Planning Policy 2017.
7.2 Recommendations for the Noosa Plan Bushfire Overlay Code
It is recommended that notwithstanding that the development is located in a Potential Bushfire Impact
area, the development maintains the safety of people and property from the adverse impacts of
bushfire by maximising setbacks from hazardous vegetation to achieve an acceptable risk as:
• Both new lots could have houses constructed to BAL 12.5 construction standard,
• Houses can be sited in a low hazard area in the lot and achieve a setback of 10 metres
from any retained vegetation,
• Houses will have a reliable reticulated water supply for fire fighting purposes
• Expected Radiant Heat loads at each house is less than 10kW/m2 and
• The escape route is through a safe area with expected radiant heat less then 10kW/m2,
• The development does not result in a material increase in the extent and severity of
bushfire hazard,
• The development supports and does not unduly burden disaster management response or
recovery capacity and capabilities as there are escape routes through a safe areas with
expected radiant heat less then 10kW/m2,
• No bushfire mitigation measures such as vegetation clearing that could adversely impact
biodiversity values are required.
7.3 Recommendations for Australian Standard AS3959
Australian Standard AS3959 Construction of buildings in bushfire-prone areas specifies Deemed to
Satisfy bushfire resistant methodologies based on the assessed Bushfire Attack Level.
It is recommended that:
• The building solution for all houses in lots assessed as being in a Bushfire Prone Area to
address the Deemed to Satisfy requirements of Sections 3 and 5 of of Australian Standard
AS3959 -2018,
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7.4 Recommendations for siting and landscaping
It is recommended that:the buildings have a bushfire fuel free zone. The fuel free zone to have no
overhanging tree branches that can:
• Break off in the high winds associated with bushfire weather and damage property or cause
injury to residents or fire fighting personnel
• Be a lodgement point for wind blown burning debris from the fire front which can then drop
burning debris onto the buildings
It is further recommended that landscaping around the buildings such as planting low, fire resistant
shrubs or building permanent barriers such as low stone or masonry walls to:
• Create barriers which slow down and cool the fire front.
Such methodologies help to protect buildings from possible attack by burning debris, heat radiation
and direct flame contact. See Figure 10 below for an example of the creation of a buffer zone.
7.5 Recommendations for lodgement points for burning debris
The Queensland Fire and Rescue Service recommend that buildings be constructed with a
minimum of lodgement points for burning debris.
Figure 15: Methodologies for minimising the lodgement points for burning debris around the house
(Source: Queensland Government, 2000)
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7.6 Recommendations for on-going management of the buildings & landscaping
On-going maintenance procedures will ensure that the buildings will maintain the highest levels of
bushfire protection afforded by the siting, landscaping and construction recommendations in this
report. Maintenance strategies include:
Cleaning the roof and gutters of all flammable material on a regular basis, especially in the lead up
to the fire season
• Keeping the lawn buffer zone mown and watered
• Keeping the lawn buffer zone clear of all flammable material
• Clearing away any flammable material lodged against the structure
• Repairing and close any exposed lodgement points for burning debris
• Ensuring all painted surfaces are in good condition
• Maintaining drive ways
• Maintaining screens in good condition
• Maintaining roof in good condition
• Maintaining doors in good condition
• Keeping flammable material away from the buildings
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8 References
Cheney, N.P. and Gould, J.S. (1997). Letter to the editor – fire growth and acceleration.
International Journal Wildland Fire 7(1):1-5
Couch, P, (2010). The Urban Fringe Under Pressure, in Proceedings of Fire Australia 2010, Fire
Protection association of Australia, Melbourne.
Department of Environment and Heritage Protection, (2020). Regional ecosystem descriptions.
Available from http://www.ehp.qld.gov.au/ecosystems/biodiversity/regional-ecosystems, accessed
February 2022.
Department of Environment and Heritage Protection, (2020). Regional Ecosystem Maps /
Regrowth Vegetation Maps and PMAVs, Available from
<http://www.ehp.qld.gov.au/ecosystems/biodiversity/regional-ecosystems/maps/index.php>,
accessed February 2022
McCarthy, GJ, Tolhurst, KG & Chatto K, (1999), Overall Fuel Hazard Guide: 3rd Edition,
Department of Natural Resources and Environment, Victoria
Noosa Council, (2020). The Noosa Plan 2020.Noosa Council, Tewantin.
NSW Rural Fire Service, (2015). Short fire run: a risk perspective for the bushland interface.
Policy document on bushfire behaviour affected by constrained fire runs. NSW Rural Fire Service,
Sydney.
Queensland Fire and Emergency Services, 2019. Bushfire Resilient Communities - Technical
Reference Guide for the State Planning Policy State Interest ‘Natural Hazards, Risk and
Resilience – Bushfire. Queensland Government, Brisbane.
Queensland Government, (1994). Queensland Vegetation Management Act 2003, The Office of
the Queensland Parliamentary Counsel, Brisbane, Queensland
Queensland Government, (2000). Protecting your home against bushfire attack, Queensland
Government
Queensland Government, (2017). State planning policy 2017 guideline. Department of Local
Government and Planning, Brisbane
Standards Australia, (2009). Australian Standard™ AS 3959-2009: Construction of buildings in
bushfire-prone areas, Standards Australia, Sydney
Tan, Z, (2006). Effect of fire run distance on bushfire behaviour and its implications for bushfire
hazard rating and prescription of asset protection zones, NSW Rural Fire Service Technical Paper
002/2006, NSW Rural Fire Service, Sydney.
Webster J, (2000). The complete bushfire safety book, Random House Australia Pty Ltd, Milsons
Point, NSW
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9 Appendices
9.1 Appendix 1: Regional Ecosystem full descriptions
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9.2 Appendix 2: Qualifications and experience of the author
This appendix to demonstrate that the author is an “appropriately qualified and experienced
consultant with appropriate and proven technical expertise in the preparation of bushfire hazard
assessment reports and management plans” (Sunshine Coast Regional Council, 2014b).
Author: Adrian Hansen
Relevant qualifications:
• Master of Environmental Management (with Distinction)
• Graduate Diploma in Design for Bushfire Prone Areas
• Diploma in Community Natural Resource Management
• Certificate IV Applied Environmental Management
• Certificate II in Bush Regeneration (with Distinction)
Relevant certifications:
•FPAA Accredited Practitioner Level 3 in Bushfire Planning and Design.
(See FPAA website (http://www.fpaa.com.au/bpad.aspx) for details of the certification)
Other relevant qualifications
• Certificate II in Public Safety (Fire fighting Operations)
• Certificate in Bushfire Management
• Certificates in Fire Fighting - Advanced skills & Crew leadership
• Certificate in Property Fire Management Planning
• Certificate in Fire and Biodiversity Monitoring
• Certificate in Regional Ecosystems Frameworks
• Bachelor of Applied Science (Majors in botany and ecology)
Relevant experience:
• 14 years experience preparing bushfire risk assessments and bushfire risk management
plans (Plans have included individual lots, sub-divisions, eco-cabins developments, schools
etc including both deemed to satisfy and alternate solutions)
• 15 years experience in bush fire fighting
• 15 years experience in bushfire ecology (includes fire and biodiversity monitoring and
ecological fire management plans for conservation areas)
• 16 years experience in natural resource management, ecology, ecological restoration,
environmental management planning etc
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9.3 Appendix 3: The fire run paper by Tan (2006)
As the paper by Tan (2006): Effect of fire run distance on bushfire behaviour and its implications
for bushfire hazard rating and prescription of asset protection zones, is an unpublished document
held be the NSW Rural Fire Service, this appendix is a full printout of the paper.
EFFECT OF FIRE RUN DISTANCE ON BUSHFIRE BEHAVIOUR
AND ITS IMPLICATIONS FOR BUSHFIRE HAZARD RATING AND
PRESCRIPTION OF ASSET PROTECTION ZONES
Technical Paper 002/2006
By Dr Zhenxiang Tan
RISK MANAGEMENT PERFORMANCE
9 March 2006
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EFFECT OF FIRE RUN DISTANCE ON BUSHFIRE
BEHAVIOUR AND ITS IMPLICATIONS FOR BUSHFIRE
HAZARD RATING AND PRESCRIPTION OF ASSET
PROTECTION ZONES
By Dr Zhenxiang Tan
1. Introduction
In Australia, the bushfire behaviour prediction system used by Australian land
management/fire agencies is based on McArthur Fire Danger Meter (McArthur 1973)
developed on the basis of over 5000 documented wildfires and 500 prescribed fires
(Chandler et al. 1983). Potential fire behaviour is predicted from fire danger ratings
derived from meteorological data (McArthur 1966, 1967, Noble et al. 1980, Luke and
McArthur 1978). The predicted behaviour by the meter is only for quasi-steady or
established fires with no provision for initiating fire behaviour. Therefore the potential
bushfire behaviour based on the meter may be over-predicted in the case of an
unestablished bush fire. The hazard ratings and the prescribed widths of asset
protection zones based on the over predicted bushfire behaviour will then be over
estimated in such a case. In order to rate bushfire hazard or prescribe APZs properly,
it needs to determine whether a block of bushfire prone land will allow for a fire to
develop to a quasi-steady state or not and take the effect of an unsteady fire on hazard
rating or APZ prescription into account accordingly.
In theory, a bushfire will eventually develop to a quasi-steady state if the distance of fire
run is long enough. This means that there must exist a relationship between bushfire
belabour and fire run distance. This paper is aimed to establish such a relationship and
analyse the implications for rating bushfire hazard and prescribing APZs. To do this, a
theoretical model developed by Cheney and Barry (1969) for predicting the rate of fire
spread at its early stage has been used to derive the corresponding equation of fire run
distance. Based on these equations, an approach for predicting the effect of fire run
distance on rate of fire spread has been established and the resultant implications for
rating bushfire hazard and prescribing APZs have also been studied.
2. Mathematical Model
It has been identified that bush fires from a point source ignition increase their spread
rates as they grow and eventually develop to the so-called quasi-steady state. The
equation predicting fire growth in forest fires given by Cheney and Barry (1969) was:
R = Rss e-a/t ……(1)
Where
R = rate of spread at time t
Rss= equilibrium spread rate
t = elapsed time since ignition
a is a constant.
The integrated form of Equation (1) is:
t t
S= R
0
SS e -a/t dt =Rss 0
e -a/t dt ……(2)
Where
S = fire run distance
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Equation (1) and Equation (2) can be used in two different ways. Firstly, it can be used
to predict the rate of fire spread at a given time and the resultant fire run distance up to
this point of time. Secondly, they can be used to estimate the maximum rate of fire
spread which can be reached for a given fire run distance. As shown in Equation (1)
and (2), the use of them requires the constant a. This constant value depends on the
time which a fire requires to develop to a quasi-steady state. The required time for a
point ignition fire under fairly stable fuel moisture and wind conditions is in 20 – 30
minutes (Luke and McArthur 1978). According to Cheney and Gould (1997), the more
frequently the direction of wind switches the shorter a fire will take to reach its quasi-
steady state. With respect to the magnitude of wind, the opposite is true. In addition,
fire intensity also influences how soon a fire can reach its quasi-steady state. Low
intensity fires tend to reach its quasi-steady state faster than high intensity fires
(McAlpine and Wakimoto 1991). Cheney and Gould (1995) found that low intensity
grass and woodland fires lit from point sources reached their maximum rate of spread
in 5-10 minutes while medium to high intensity fires in winds above 4m/s took 12 -45
minutes. In some circumstances, it may take many hours for high intensity forest fires
to reach a quasi-steady state.
Considering the time which a fire reaches a quasi-steady state is highly variable, the
value of constant a in Equation (1) and (2) also varies within a certain range. In order to
obtain meaningful predictions with these equations, a proper value of constant a needs
to be used according to the expected fire scenarios such as fast growth fires, medium
fast growth fires and slow growth fires. If the times for fast growth fires, medium fast
growth fires and slow growth fires develop to 90% of their quasi-steady rate of spread
are assumed to be 10 minutes, 30 minutes and 60 minutes respectively, then the
corresponding values of constant a can be determined by using Equation (1) being
1.05, 3.16 and 6.32 respectively. Therefore, the variants of Equations (1) and (2) for
the three different fire scenarios are:
(1) Fast growth fires
R = Rss e-1.05/t ……(3)
t
e
-1.05/t
S=Rss dt ……(4)
0
(2) Medium fast growth fires
R = Rss e-3.16/t ……(5)
t
e
-3.16/t
S=Rss dt ……(6)
0
(3) Slow growth fires
R = Rss e-6.32/t ……(7)
t
e
- 6.32/t
S = Rss dt ……(8)
0
The relative rate of spread for a given time for a given fire scenario can then be directly
predicted by utilising the analytical equations or the graphic forms of these equations
shown in Figure 1. For instance, the relative rate of spread at time = 30 minutes for
fast, medium and slow growth fires are determined to be 0.96, 0.90 and 0.81
respectively in accordance with Figure 1.
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1.00
0.90
Relative Rate of Spread (R/Rss)
0.80
0.70
0.60
0.50 Fas t
Medium Fas t
0.40
Slow
0.30
0.20
0.10
0.00
0 5 10 15 20 25 30 35 40 45 50 55 60
Elapsed Time (min)
Figure 1. Graphic Formats of Equations (3), (5) and (7)
To predict the actual rate of fire spread for a given time or the fire run distance up to
this point of time, the quasi-steady rate of fire spread R ss needs to be determined first.
For forest and woodlands, the rate of spread equation derived from McArthur meter
can be used for this purpose and its mathematical expression is (Nobel et al. 1980):
Rss= 0.0012 FFDI w e0.069θ ……(9)
Where
Rss = quasi-steady rate of fire spread (km/h)
FFDI = Forest Fire Danger Index
w= fine fuel load (t/ha)
θ = slope (degrees)
For FFDI =80, w=25 t/ha and θ =0, R ss will take a value of 2.4 km/h which is equivalent
to 40 m/min. Therefore, the actual rates of spread will be 38, 35 and 30 m/min at time
=30 minutes after a point ignition for the above three fire scenarios. The fire run
distances can also be determined by doing numerical integrations of the fire run
distance equations or using the curves shown in Figure 2.
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2000
1800
1600
Fire Run Distance (m)
1400
1200
1000
800 Fas t
600 Medium Fas t
Slow
400
200
0
0 5 10 15 20 25 30 35 40 45 50 55 60
Elapsed Time (min)
Figure 2. Graphic Formats of Equations (4), (6) and (8)
In addition, one can predict the relative rate of spread for a given fire run distance by
utilizing the flowing two step approach:
(1) Estimate the elapsed time corresponding to a given fire run distance by performing
iterative calculations with Equation (2) or its graphic format (Figure 2);
(2) Determine the relative rate of spread corresponding to the estimated time by using
the Equation (1) or its graphic format (Figure 1).
For instance, the times corresponding to fire run distances of 100m, 200m, 300m,
400m and 500m are determined to be 4.6 minutes, 7.6 minutes, 10.4 minutes, 13.2
minutes and 15.8 minutes for a fast growth fire in accordance with Figure 2. The
relative rate of spread corresponding to these fire run distances are then be determined
being 0.795, 0.871, 0.904, 0.923 and 0.935 for the same type of fire in accordance
with Figure 1.
By applying the approach described above, the direct relationship between fire run
distance and relative rate of spread can then be established. The curves shown in
Figure 3 is the graphic representation of the relationship between fire run distance and
relative rate of spread under the three different fire growth scenarios. With this graph,
the relative rate of spread can be directly determined for a given fire run distance under
a given fire scenario. For instance, for a block of bushfire prone land with a maximum
fire run distance of 300m the maximum relative rates of fire spread for a fire within this
block of land are determined to be 0.90, 0.80 and 0.70 respectively for fast, medium
and slow growth fires.
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1.0
0.9
0.8
Relative Rate of Spread (R/Rss)
0.7
0.6 Fast
Medium
0.5
Slow
0.4
0.3
0.2
0.1
0.0
Fire Run Distance (m)
Figure 3. Relative Rate of Spread and Fire Run Distance
3. Implication for Rating Bushfire Hazard
Fire run distance influences the rate of fire spread and therefore fire hazard. The
hazard associated with a given vegetation type is usually measured by fire intensity
which is expressed by (Byram 1959):
I = H W R/36 ……(10)
Where
I = fire intensity (kW/m)
H= heat yield of the fuel, fuel related constant
W = Available fuel load (tonnes/hectare)
R = Rate of spread (km/h)
Based on Equation (10), the fire intensities of steady fires and non-steady fires can be
determined by:
I = H W R/36 ……(11)
Iss = H W Rss/36 ……(12)
Divide Equation (11) by Equation (12), we have
I/ Iss =R/Rss ……(13)
Equation (13) suggests that the relative rate of spread is equivalent to the relative
hazard rating for a given fuel type. The relative rate of spread for a given fire run
distance can be predicted with Equation (1) and consequently its implication for hazard
rating. If fire run distance is grouped into the categories of <=50m, 50-100m, 100-
200m, 200-500 and 500-1000m, then the relative hazard rating for a given vegetation
type can be determined for the three different fire scenarios ( See Table 1).
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Table 1 Relative Hazard Ratings of Fire Run Distances
Relative Hazard Rating
Fire Run Distance (m) Medium growth
Fast growth fire Slow growth fire
fire
<=50 0.42 0.25 0.18
50-100 0.75 0.59 0.47
100-200 0.84 0.70 0.59
200-500 0.91 0.81 0.72
500-1000 0.95 0.89 0.82
The relative hazard ratings shown in Table 1 can be used to quantify the effect of fire
run distance on overall hazard level by factoring the overall hazard index/score by a
corresponding relative rating of a give fire run distance. By doing so, the potential over
- prediction of hazard level for a block of bushfire prone land with a small fire run
distance can be avoided and therefore more cost-effective mitigation measures can be
chosen and implemented.
4. Implication for Prescribing APZs
The objective of APZs is to mitigate the impact of bushfire by providing a buffer zone
between a development and surrounding bushfire hazard. Apparently the wider the
APZ prescribed for a site, the less impact the site will be subject to. However, if an
APZ is over prescribed, then developments on the subject site may become more
costly. In view of this, it is necessary to accurately quantify the level of attack from the
bushfire hazard for a given site. To do this, NSW RFS has developed a systematic
methodology which comprises a set of equations (Tan et al. 2005). The principle of the
methodology is to predict the potential bushfire behaviour and the resultant radiant heat
flux to which a site at a given distance from fire front may be exposed. With the NSW
RFS methodology, the required minium width of APZ can be determined by finding the
distance at which the maximum radiant heat flux to which the subject site may be
exposed is equal to the predefined radiant heat flux threshold.
The rate of fire spread equation used in the methodology predicts the quasi-steady rate
of spread and therefore the APZs prescribed by using this methodology might be more
than adequate in the cases of bushfire prone lands with small fire run distances. In
order to prescribe cost-effective APZs, it is necessary to take the effect of fire run
distance on bushfire behaviour and the resultant impact into account. This can be
done by incorporating Equation (1) and Equation (2) into the current methodology. For
instance, the APZ for forest under FDI = 80 and slope = 0 degree is determined to be
28 m. when taking fire run distance into account, the corresponding APZs under the
same conditions are determined and shown in Table 2.
As shown in Table 2, the modelled widths of APZs for three different fire scenarios vary
from fire run distance and will tend to be a constant when fire run distances become
large enough. This means that a smaller APZ needs to be prescribed if the maximum
fire run distance is less than a threshold distance. For instance, if the maximum fire
run distance of a block of land covered with forest is 100m, then the corresponding
APZs for fast, medium and slow growth fires will be 24m, 21m and 19m compared with
a APZ of 28m for a steady state fire.
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Table 2 Modelled APZs for forest (FDI=80, Level Ground)
Asset protection Zones (m)
Fire Run Distance (m) Fast Growth Medium Growth Slow Groth
10 16 13 11
20 19 15 13
30 20 17 14
40 21 18 15
50 22 19 16
100 24 21 19
150 25 22 20
200 26 23 21
250 26 24 22
300 26 24 22
350 27 25 23
400 27 25 23
450 27 25 24
500 27 25 24
550 27 26 24
600 28 26 24
650 28 26 25
700 28 26 25
750 28 26 25
800 28 26 25
850 28 26 25
900 28 26 25
950 28 27 25
1000 28 27 25
1500 28 27 26
2000 28 27 27
2500 28 28 27
5000 28 28 28
5. Conclusions
To sum up, the following conclusions can be made:
(1) An approach for predicting the effect of fire run distance on bushfire behaviour has
been developed on the basis of the existing time dependant rate of spread equation
developed by Cheney.
(2) By utilizing the established approach, the implications of fire run distance for rating
bushfire hazard and prescribing APZs have been analysed for different fire
scenarios.
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(3) The analysis results reveal that the bushfire hazard ratings and the APZs for a
given bushfire hazard vary with fire run distances and will become constant when
fire run distance reach a threshold value. This means that it is necessary to take
into fire run distance into account in order to prevent from over rating bushfire
hazard or over prescribing APZs for bushfire prone lands with small fire run
distances.
6. References
Byram GM 1959. Forest fire behaviour. In Forest fire control and use. (Ed. KP Davis),
McGraw – Hill, New York, 90-123 pp.
Chandler, C., et al. 1983. Fire in forestry. Vol. I: Forest fire behaviour and effects. Wiley,
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