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Reference Document - Bushfire Risk Assessment & Bushfire Risk Management Plan
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REFERENCE DOCUMENT
MCU21/0036
11 MAY 2021
-
Bushfire Risk Assessment
and
Bushfire Risk Management Plan
for
A house
at
64 Pheasant Lane, Doonan, Queensland
Lot 36 on SP139155
Revision 2
April 2021
Prepared by:
Ecological Natural Area Management
for
Scott Elliott
C/- 64 Pheasant Lane
Doonan
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Ecological Natural Area Management
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 3: 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 14 September 2020_ Adrian Hansen
Revision 2 08 April 2021_ Adrian Hansen
Document certification
08 April 2021 ……………
Adrian Hansen Date
Bushfire Risk Assessment and Management Plan for a house at 64 Pheasant Lane, Doonan September 2020
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Table of Contents
1 Executive Summary.................................................................................................................................... 5
2 Introduction................................................................................................................................................. 7
2.1 The reasons for the Bushfire Risk Assessment and Management Plan............................................7
2.2 Commissioning of the Bushfire Risk Assessment and Management Plan........................................7
2.3 The aim of the Bushfire Risk Assessment and Management Plan....................................................7
2.4 Legislative considerations................................................................................................................. 7
2.5 Other regulatory considerations........................................................................................................ 7
3 Site description........................................................................................................................................... 8
3.1 Location............................................................................................................................................ 8
3.2 Zoning............................................................................................................................................... 9
3.3 Area of the lot.................................................................................................................................... 9
3.4 Building Code of Australia building class.......................................................................................... 9
3.5 Applicable development codes......................................................................................................... 9
3.6 Topography....................................................................................................................................... 9
3.7 Mapped bushfire hazard................................................................................................................... 9
3.8 Climate.............................................................................................................................................. 9
4 Site vegetation.......................................................................................................................................... 10
4.1 Queensland Globe aerial photography........................................................................................... 10
4.2 Mapped Remnant Regional Ecosystems........................................................................................ 11
4.3 Visually assessed vegetation communities ....................................................................................12
4.4 Map of the visually assessed vegetation.........................................................................................13
5 Conservation significance of the vegetation.......................................................................................... 14
5.1 Conservation significance of the vegetation from Queensland Vegetation Management Act..........14
5.2 Conservation significance of the vegetation from the The Noosa Plan...........................................14
6 Bushfire risk assessment........................................................................................................................ 15
6.1 Assessment of the Potential Bushfire Intensity Class.....................................................................15
6.2 Derivation of inputs for assessment of the Potential Bushfire Intensity Class.................................16
6.3 Tabulation of the Potential Bushfire Intensity Class........................................................................18
6.4 Potential Impact Buffer.................................................................................................................... 19
6.5 Map of Potential Bushfire Impact classes and class buffers...........................................................19
6.6 Assessment of bushfire Prone Area................................................................................................ 19
7 Assessment of the Bushfire Attack Level...............................................................................................20
7.1 Assessment of the Bushfire Attack Level: Methodology..................................................................20
7.2 Modelled bushfire attacks............................................................................................................... 20
7.3 Assessment of fuel loads................................................................................................................ 21
7.4 Forest Fire Danger Index................................................................................................................ 22
7.5 Parameters used in the bushfire behaviour modelling....................................................................22
7.6 Results of bushfire attack modelling...............................................................................................23
7.7 Bushfire Attack Levels for each building elevation..........................................................................23
7.8 Assessment of vegetation clearing.................................................................................................23
8 Bushfire risk management solutions and recommendations...............................................................24
8.1 Bushfire hazard mitigation recommendations: Considerations.......................................................24
8.2 Recommendations for the Noosa Plan Bushfire Overlay Code......................................................24
8.3 Recommendations for Australian Standard AS3959.......................................................................24
8.4 Recommendations for siting and landscaping................................................................................25
8.5 Recommendations for lodgement points for burning debris............................................................25
8.6 Recommendations for on-going management of buildings & landscaping......................................26
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8.7 Recommendations for Part E: Natural hazards of Qld State Planning Policy 2017........................26
9 References................................................................................................................................................ 27
10 Appendices.............................................................................................................................................. 28
10.1 Appendix 1: Regional Ecosystem full descriptions........................................................................28
10.2 Appendix 2: Software report from the bushfire behaviour modelling.............................................29
10.3 Appendix 3: Qualifications and experience of the author..............................................................32
10.4 Appendix 4: The fire run paper by Tan (2006)...............................................................................33
List of Figures
Figure 1: Site plan showing the location of the house in the lot........................................................................8
Figure 2: Excerpt from The Noosa Plan Bushfire Overlay map showing the house in an area mapped as High
Potential Bushfire Intensity............................................................................................................................... 9
Figure 3: Excerpt from Qld Potential Bushfire Intensity mapping showing the house in an area mapped as
Very High Potential Bushfire Intensity............................................................................................................... 9
Figure 4: Aerial photo showing Lot configuration, house location, approved building envelope, vegetated
areas, rural residential areas, mapped watercourses and 5m contour lines...................................................10
Figure 5: Map of the Queensland Vegetation Management Act conservation status of the mapped Remnant
regional Ecosystems near the house.............................................................................................................. 11
Figure 6: View the lot confirming the vegetation is Eucalyptus spp open forest..............................................12
Figure 7: Map of the visually assessed vegetation.........................................................................................13
Figure 8: Map of vegetation patches used for assessment of patch and corridor filtering...............................17
Figure 9: Map of the assessed Bushfire Intensity Classes showing that all of the the house in an area
assessed as Potential Bushfire Impact Buffer Class.......................................................................................19
Figure 10: Map showing the modelled bushfire attacks..................................................................................20
Figure 11: Fuel accumulation curves for open forests in eastern Australia.....................................................21
Figure 12: Methodologies for minimising the lodgement points for burning debris around the house.............25
List of Tables
Table 1: Regional Ecosystems near the house............................................................................................... 11
Table 2: Conservation significance of the Remnant Regional Ecosystems derived from the Queensland
Vegetation Act................................................................................................................................................. 14
Table 3: Potential fuel loads for the assessed vegetation communities based on their applicable potential fuel
hazard classes................................................................................................................................................ 16
Table 4: Patch filtering tabulation.................................................................................................................... 17
Table 5: Table of Potential Bushfire Intensity Class derived from Potential Fire line Intensity.........................18
Table 6: Tabulation of the potential bushfire intensity classes for each of the patches ...................................18
Table 7: Surface and total fuel loads for use in AS3959 Method 2 assessments of BALs...............................21
Table 8: Excerpt from State Planning Policy Natural Hazards, Risk and Resilience Technical Manual of
potential maximum fuel accumulation for the subject vegetation communities...............................................22
Table 9: Assessed Bushfire Attack Levels (BAL) for the modelled bushfire attacks........................................23
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1 Executive Summary
Introduction
A house to be constructed at 64 Pheasant Lane, Doonan, Queensland in the Noosa Council
jurisdiction is mapped in a High Potential Bushfire Intensity area on the Noosa Plan Bushfire
Overlay map in a Very High Potential Bushfire Intensity area on the Queensland Potential Bushfire
Intensity mapping. Therefore, the house is deemed to potentially be in a Designated Bushfire
Prone Area and an assessment of the bushfire risk to the building and a bushfire risk management
solution is required. A Noosa Council Information Request of 30 March 2021 in respect of
MCU21/0036 specifies that the Bushfire Attack Level Assessment for the house use a Forest Fire
Danger Index of FDDI 50.
This report is to fulfil the above requirements.
Site specific assessment of the Potential Bushfire Intensity Class
A site specific assessment of the Potential Bushfire Intensity Class using the protocol specified in A
new methodology for state-wide mapping of bushfire prone areas (Leonard et al, 2014) as
specified by Queensland State Planning Policy 2017 to assess the bushfire prone area status of
the development assessed that:
• The house is assessed as being in a Potential Bushfire Impact Buffer Class area.
Assessment of Bushfire Prone Area
As the house is assessed as Potential Bushfire Impact Buffer Class, this confirms that:
• The house is deemed to be in a Bushfire Prone Area.
As the house is in a Bushfire Prone Area, an assessment of the Bushfire Attack Level using
Australian Standard AS3959 Construction of buildings in bushfire prone areas is required to be
conducted as specified by the Queensland Building Act and the bushfire requirements of the
National Construction Code.
Assessment of the Bushfire Attack Level
The assessed Bushfire Attack Level (BAL) using Method 2 of Australian Standard AS3959 is:
• BAL 19 for all building elevations
Recommendations
Recommendations for the Noosa Plan Bushfire Overlay Code
It is recommended that:
• The level of bushfire hazard shown on a Bushfire Hazard Overlay Map is confirmed through
the preparation of a site-specific bushfire hazard assessment prepared in accordance with
the Planning scheme policy for the bushfire hazard overlay code and
• The house is sited in a low hazard area in the lot and achieves a setback of 10 metres from
any retained vegetation.
As the house location is in a Potential bushfire Impact Buffer Class area and a reliable reticulated
water supply is not available,
It is recommended that:
• The premises to have a minimum water supply capacity of 5,000 litres dedicated for fire
fighting purposes which is sourced from a separate tank, a reserve section in the bottom
part of the main water supply tank, a swimming pool installed immediately upon
construction or a permanent dam.
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Recommendations for Australian Standard AS3959
It is recommended that
• The building solution for all building elevations address the Deemed to Satisfy requirements
of Sections 3 and 6 of Australian Standard AS3959 -2018: Construction of buildings in
bushfire-prone areas.
Other recommendations for building siting, construction & landscaping
It is recommended that:
• The building have a bushfire fuel free zone around the house,
• The building be constructed with a minimum of lodgement points for burning debris and
• On-going maintenance of the building 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 that the proposed building location is in an area of
Potential Bushfire Hazard:
• The building siting, construction, firefighting resources 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
2.1 The reasons for the Bushfire Risk Assessment and Management Plan
A house to be constructed at 64 Pheasant Lane, Doonan, Queensland in the Noosa Council
jurisdiction is mapped in a High Potential Bushfire Intensity area on the Noosa Plan Bushfire
Overlay map in a Very High Potential Bushfire Intensity area on the Queensland Potential Bushfire
Intensity mapping. Therefore, the house is deemed to potentially be in a Designated Bushfire
Prone Area and an assessment of the bushfire risk to the building and a bushfire risk management
solution is required. A Noosa Council Information Request of 30 March 2021 in respect of
MCU21/0036 specifies that the Bushfire Attack Level Assessment for the house use a Forest Fire
Danger Index of FDDI 50.
This report is to fulfil the above requirements.
2.2 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 3:
Qualifications and experience of the author of this report for demonstration that the author is a “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”.
2.3 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 development 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 legislative requirements
and make recommendations that address the requirements of the legislation.
2.4 Legislative considerations
• Queensland Planning Act,
• Queensland Building Act,
• Queensland Vegetation Management Act,
• Queensland State Planning Policy 2017 and,
• The Noosa Plan 2020
2.5 Other regulatory considerations
• The National Construction Code of Australia 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, described as Lot 36 on SP139155 is at 64 Pheasant Lane, Doonan, Queensland in the
Noosa Council jurisdiction. The development is a new house.
Figure 1: Site plan showing the location of the house in the lot.
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3.2 Zoning
The Zoning Class of the site is mapped as Rural Residential on the Noosa Plan Zone Map
3.3 Area of the lot
The area of the lot = 4,000m2
3.4 Building Code of Australia building class
The building is a Class 1a building under the National Construction Code of Australia.
3.5 Applicable development codes
The Noosa Plan: Bushfire Overlay code,
3.6 Topography
Site topography is “Undulating” 5% to 10% slope.
3.7 Mapped bushfire hazard
The house is mapped in an area of:
• High Potential Bushfire Intensity on the Noosa Plan Bushfire Overlay map and
• Very High Potential Bushfire Intensity on Queensland Potential Bushfire Intensity mapping.
Figure 2: Excerpt from The Noosa Plan Bushfire Figure 3: Excerpt from Qld Potential Bushfire
Overlay map showing the house in an area Intensity mapping showing the house in an area
mapped as High Potential Bushfire Intensity mapped as Very High Potential Bushfire
Intensity
3.8 Climate
The climate is subtropical. That is, the area is influenced by monsoonal climate systems in summer
but also from temperate climate systems in winter. Mean rainfall is approximately 1700.mm per
year. (Bureau of Meteorology, 2004) The “wet” season runs from December to April.
Severe bushfire weather can occur. This is characterised by dry windy days, winds north to north
west for 2-4 days and temperatures 350C or over. These conditions occur from September to
December; worst month, November. (Webster J, 2000)
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4 Site vegetation
4.1 Queensland Globe aerial photography
Queensland Globe aerial photography shows the lot in a rural residential area with native forest
near the house
Figure 4: Aerial photo showing Lot configuration, house location, approved building envelope,
vegetated areas, rural residential areas, mapped watercourses and 5m contour lines
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4.2 Mapped Remnant Regional Ecosystems
Some of the vegetation has been mapped as Remnant Regional Ecosystems. The map of
Remnant Vegetation will be used as an aid to identifying the vegetation
Figure 5: Map of the Queensland Vegetation Management Act conservation status of the mapped
Remnant regional Ecosystems near the house
Table 1: Regional Ecosystems near the house
Regional Ecosystem Description
RE 12.9-10.1 Eucalyptus resinifera, E. grandis, E. robusta, Corymbia intermedia open
forest on sedimentary rocks. Coastal
(Source: Department of Environment and Heritage Protection, 2018)
See Appendix 1 for the full descriptions of these Regional Ecosystems
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4.3 Visually assessed vegetation communities
The vegetation of the lot was visually assessed. The visually assessed vegetation is confirmed as
Eucalyptus spp open forest.
Figure 6: View the lot confirming the vegetation is Eucalyptus spp open forest
See the next page for a map showing the extent of the visually assessed vegetation.
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4.4 Map of the visually assessed vegetation
Based on the visual assessment of the vegetation shown in the photographs on the previous page,
inspection of aerial photography, assessment of Remnant vegetation mapping and field
verification, the map below shows the locations of the visually assessed vegetation communities.
The visually assessed vegetation is:
• Rural residential lots with mown lawns, gardens, houses roads, driveways etc and,
• Eucalyptus spp open forest – canopy height = 30m.
Figure 7: Map of the visually assessed vegetation
This map of the visually assessed vegetation will be used in subsequent assessments of the
bushfire risk in this report.
Note the approved building envelope to establish the house.
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5 Conservation significance of the vegetation
5.1 Conservation significance of the vegetation from Queensland Vegetation
Management Act
Some of the vegetation on the lot derives enhanced conservation significance from the
Queensland Vegetation Act as it is mapped as Remnant Regional Ecosystem. See Figure 5 of this
report for the map showing the areas in the lot mapped as Remnant Regional Ecosystem.
Table 2: Conservation significance of the Remnant Regional Ecosystems derived from the
Queensland Vegetation Act
Regional Regional Ecosystem short description VMA conservation VMA biodiversity
Ecosystem class status
RE 12.9-10.1 Eucalyptus resinifera, E. grandis, E. robusta, Corymbia Of concern Of concern
intermedia open forest on sedimentary rocks. Coastal
5.2 Conservation significance of the vegetation from the The Noosa Plan
All of the vegetation on the derives enhanced conservation significance from The Noosa Plan 2020
Biodiversity Overlay as it is mapped as:
• An Area of Biodiversity Significance and
• MSES Environmental Values
Note that there is an approved building envelope to establish the house.
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6 Bushfire risk assessment
As the house is mapped in a High Potential Bushfire Intensity area on the Noosa Plan Bushfire
hazard Overlay Map, this report will detail bushfire hazard assessments for:
• The Noosa Plan
• Queensland State Planning Policy 2017 and
• Australian Standard AS 3959 as referenced by the National Construction Code of Australia.
6.1 Assessment of the Potential Bushfire Intensity Class
As required by The Noosa Plan, this report will use the the protocol specified in A new methodology
for state-wide mapping of bushfire prone areas (Leonard et al, 2014) as specified by Queensland
State Planning Policy 2017 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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6.2 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 7 above, the
Potential Fuel Loads can be derived by relating the assessed vegetation to Table 1: Vegetation
Hazard Classes (VHC) and associated Potential Fuel Loads in Risk and Resilience Technical
Manual: A ‘fit for purpose’ approach in undertaking natural hazard studies and risk assessments
Table 3: Potential fuel loads for the assessed vegetation communities based on their applicable
potential fuel hazard classes
Vegetation community Vegetation Hazard Class Potential fuel load
Eucalyptus spp tall open forest 9.1 Moist to dry eucalypt open forests 24.1 t/ha
on coastal lowlands and ranges
Rural residential lots with 40.4 Low grass or tree cover in rural 5 t/ha
gardens, mown grass etc areas
Fire Weather Severity
The applicable Fire Weather Severity for the subject lot was drawn from the map of Queensland
Fire Weather Severities on Figure 16: Fire weather severity for Queensland in Leonard et al
(2014), page 17.
The applicable Fire Weather severity for the subject area = FDI 50
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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 8: Map of vegetation patches used for
assessment of patch and corridor filtering
Table 4: Patch filtering tabulation
Patch Initial class Merge class Reason for merge Potential load
Patch 1 Moist to dry eucalypt Moist to dry eucalypt No merge as patch is large. Remains as 24.1 t/ha
open forests open forests Moist to dry eucalypt open forests
Patch 2 Low grass or tree Low grass or tree Remains as Low grass or tree cover in rural 5 t/ha
cover in rural areas cover in rural areas areas
Patch 3 Moist to dry eucalypt Low grass or tree Allocated to Low grass or tree cover in rural 5 t/ha
open forests cover in rural areas areas 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 Moist to dry eucalypt Low grass or tree Allocated to Low grass or tree cover in rural 5 t/ha
open forests cover in rural areas areas through the “filtering process above
Patch 5 Moist to dry eucalypt Low grass or tree Allocated to Low grass or tree cover in rural 5 t/ha
open forests cover in rural areas areas through the “filtering process above
Patch 6 Moist to dry eucalypt Low grass or tree Allocated to Low grass or tree cover in rural 5 t/ha
open forests cover in rural areas areas through the “filtering process above
Patch 7 Low grass or tree Low grass or tree Remains as Low grass or tree cover in rural 5 t/ha
cover in rural areas cover in rural areas areas
Patch 8 Moist to dry eucalypt Low grass or tree Allocated to Low grass or tree cover in rural 5 t/ha
open forests cover in rural areas areas through the “filtering process above
Patch 9 Low grass or tree Low grass or tree Remains as Low grass or tree cover in rural 5 t/ha
cover in rural areas cover in rural areas areas
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6.3 Tabulation of the Potential Bushfire Intensity Class
Using the inputs derived above, the Potential Bushfire Intensity Classes for the patches of
vegetation are tabulated below.
Table 5: 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)
Table 6: 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 Moist to dry eucalypt 24.1 t/ha 2.8 0 50 21771 kW/m High
open forests
Patch 2 Low grass or tree cover 5 t/ha 3.1 0 50 963 kW/m Low
in rural areas
Patch 3 Low grass or tree cover 5 t/ha 3.7 1 51 1020 kW/m Low
in rural areas
Patch 4 Low grass or tree cover 5 t/ha 4.4 2 52 1092 kW/m Low
in rural areas
Patch 5 Low grass or tree cover 5 t/ha 4.9 3 53 1153 kW/m Low
in rural areas
Patch 6 Low grass or tree cover 5 t/ha 5.8 4 54 1247 kW/m Low
in rural areas
Patch 7 Low grass or tree cover 5 t/ha 6.3 5 55 1314 kW/m Low
in rural areas
Patch 8 Low grass or tree cover 5 t/ha 3.1 6 56 1073 kW/m Low
in rural areas
Patch 9 Low grass or tree cover 5 t/ha 3.0 0 50 954 kW/m Low
in rural areas
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6.4 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. The areas of Medium,
High or Very High Potential Bushfire Intensity Class and Medium, High or Very High Potential
Bushfire Intensity Class Buffer are shown on the map on the next page.
6.5 Map of Potential Bushfire Impact classes and class buffers.
The map below shows the areas of the Potential Bushfire Impact classes and class buffers. The
location of the house is in an area assessed as Potential Bushfire Impact Buffer Class.
Figure 9: Map of the assessed Bushfire Intensity Classes showing
that all of the the house in an area assessed as Potential Bushfire
Impact Buffer Class.
6.6 Assessment of bushfire Prone Area
The assessment of Potential Bushfire Impact Buffer Class confirms that:
• The location of the house is deemed to be in a Bushfire Prone Area.
As the house is in a Bushfire Prone Area, an assessment of the Bushfire Attack Level using the
protocol of Australian Standard AS3959 Construction of buildings in bushfire prone areas is
required to be conducted as specified by the Queensland Building Act and the bushfire
requirements of the National Construction Code.
Such a Bushfire Attack Level assessment is conducted on subsequent pages of this report.
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7 Assessment of the Bushfire Attack Level
7.1 Assessment of the Bushfire Attack Level: Methodology
The National Construction 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 the construction Bushfire Attack
Level (BAL) for the house using Method 2 of Australian Standard AS3959 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, 2018). This report uses software developed by Newcastle Bushfire
Consulting which models the Bushfire Attack Level using Method 2 of Australian Standard AS3959.
7.2 Modelled bushfire attacks
Three bushfire attacks were modelled based on the proximity of the hazardous vegetation.
Bushfire attack attenuation was included in the bushfire behaviour modelling due to:
• Constrained fire runs of bushfires due to a short fire run from residential areas leading to a
reduced bushfire rate of spread.
(A reduced bushfire rate of spread attenuates bushfire attack parameters such as bushfire
intensity, flame length and radiant heat. The assessment of reduction of the rate of spread
from the quasi-steady state rate of spread used the method of Cheney & 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)
• The flame width for the north west bushfire attack constrained to 30m due to cleared rural
residential lots and the sheds and tanks on the subject lot
Figure 10: Map showing the modelled bushfire attacks
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7.3 Assessment of fuel loads
Australian Standard AS3959 allows for the use of two fuel loads for assessing Bushfire Attack
Levels for bushfire attacks from open forest and woodland vegetation communities. The two fuel
loads allow for worst-case scenarios for woodland and open forest vegetation associations.
Table 7: Surface and total fuel loads for use in AS3959 Method 2 assessments of BALs
AS3959 vegetation descriptor Surface fuel load Total fuel load
Forest 25 t/ha 35 t/ha
Woodland 15 t/ha 25 t/ha
Notwithstanding the names of the two fuel loads are “woodland” and “forest”, the names are
AS3959 descriptors for fuel loads and do not relate to the Specht vegetation formation names of
open forest and woodland.
The reasons that the two fuel loads are specified instead of allowing the consultant to input any fuel
loads based on an assessment of the extent vegetation is to ensure that the fuel loads used reflect
the maximum potential fuel accumulation and to allow for a worst case scenario. An assessment of
the fuel loads in the extant vegetation could potentially be less than the maximum fuel accumulation
if the assessment is conducted after vegetation management works such as prescribed burning.
Figure 15 below shows fuel accumulation curves for open forests in eastern Australia which indicate
that none exceeds 20 t/ha maximum fuel accumulation.
30
Fuel load (t/ha)
20
10
0
30
Fuel load (t/ha)
20
10
0
0 5 10 15 20 30 0 5 10 15 20 30
Time since fire (years) Time since fire (years)
Figure 11: Fuel accumulation curves for open forests in eastern Australia
It is to be noted that, Gilroy & Tran (2006) showed that open forest and woodland ecosystems in
coastal south-east Queensland do not achieve the high fuel loads for equivalent hinterland
vegetation communities in New South Wales and Victoria due to higher humidity leading to a
higher rate of breakdown of leaf litter and so reducing the potential maximum fuel accumulation in
these ecosystems.
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State Planning Policy Natural Hazards, Risk and Resilience Technical Manual: A ‘fit for purpose’
approach in undertaking natural hazard studies and risk assessments (Queensland Department of
Infrastructure, Local Government and Planning, 2016) also specify potential maximum fuel
accumulation for vegetation communities in Queensland. The vegetation community in the
technical manual that is specific for the subject vegetation is shown on Table 8 below.
Table 8: Excerpt from State Planning Policy Natural Hazards, Risk and Resilience Technical
Manual of potential maximum fuel accumulation for the subject vegetation communities.
Vegetation Hazard Class Potential Fuel Load
9.2 Moist to dry eucalypt open forests on coastal lowlands and ranges 24.1 t/ha
(Queensland Department of Infrastructure, Local Government and Planning, 2016, page 21)
From Table 8 above, the vegetation community described in the technical manual has a potential
maximum fuel accumulation less than 25 t/ha.
Methods that can be used for for the assessing fuel loads include:
• Vegetation sampling and oven drying to derive weight of dry fuel and
• Estimating fuel load using the Overall Fuel Hazard Guide (McCarthy et. al., 1999).
The actual fuel loads of the impacting vegetation were not assessed, however, based on the above
published potential fuel accumulations from the published literature shown above, this report will
use the next highest acceptable fuel load from AS3959 which is the “Woodland” fuel loads which
are 15 t/ha surface fuel load and 25 t/ha total fuel.
7.4 Forest Fire Danger Index
This report will use the specified Forest Fire Danger Index of FFDI 50. as required by The Noosa
plan 2020.
7.5 Parameters used in the bushfire behaviour modelling
North bushfire attack
This modelled fire burns up-slope through Eucalyptus spp open forest to attack from the north.
• Fuel loads used are the “woodland” fuel loads of Australian Standard AS3959 of 15t/ha
surface fuel & 25t/ha total fuel.
• Separation of the house from the hazard: 16m
• Slope in the vegetation = 5.30.
• Site slope = 5.30
• Flame width = 100m (the default value)
• Fire run = 350m reducing the rate of spread to 0.69 of the quasi-steady state ROS
East bushfire attack
This modelled fire burns up-slope through Eucalyptus spp open forest to attack from the east.
• Fuel loads used are the “woodland” fuel loads of Australian Standard AS3959 of 15t/ha
surface fuel & 25t/ha total fuel.
• Separation of the house from the hazard: 14m
• Slope in the vegetation = 0.40.
• Site slope = 0.40
• Flame width = 100m (the default value)
• Fire run = 500m reducing the rate of spread to 0.78 of the quasi-steady state ROS
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North west bushfire attack
This modelled fire burns up-slope through Eucalyptus spp open forest to attack from the west.
• Fuel loads used are the “woodland” fuel loads of Australian Standard AS3959 of 15t/ha
surface fuel & 25t/ha total fuel.
• Separation of the house from the hazard: 12m
• Slope in the vegetation = 0.40.
• Site slope = 0.40
• Fire run = 280m reducing the rate of spread to 0.69 of the quasi-steady state ROS
• Flame width = 30m
7.6 Results of bushfire attack modelling
Table 9: Assessed Bushfire Attack Levels (BAL) for the modelled bushfire attacks
Modelled bushfire attack
Modelled bushfire parameter North bushfire attack East bushfire attack North west bushfire attack
Separation from the hazard 16 m 14 m 12 m
Vegetation Woodland Woodland Woodland
Modelled surface fuel load 15 t/ha 15 t/ha 15 t/ha
Modelled total fuel load 25 t/ha 25 t/ha 25 t/ha
Vegetation slope (degrees) 5.3 0 0.4 0 0.4 0
Site slope (degrees) 5.3 0 0.4 0 0.4 0
Fire run 350 m run 500 m run 280 m run
Proportion of QSS ROS 0.73 0.78 0.69
QSS ROS 1.30 km/h 0.93 km/h 0.93 km/h
Modelled rate of spread 0.95 km/h 0.73 km/h 0.64 km/h
Flame width 100 m 100 m 30 m
Radiant heat 17.86 kW/m2 17.79 kW/m2 17.56 kW/m2
Modelled fire line intensity 12271 kW/m 9429 kW/m 8267 kW/m
Modelled flame length 9.18 m 7.74 m 7.16 m
Level of construction (BAL) BAL 19 BAL 19 BAL 19
The worst case scenario from Table 9 above is BAL 19
See Appendix 2 for a detailed software reports from which the above table was generated.
7.7 Bushfire Attack Levels for each building elevation
Australian Standard AS3959 specifies that the Bushfire Attack Level for the building elevation
opposite the bushfire attack direction can be reduced one level due to radiant heat shielding by the
house but as more than one bushfire attack id assessed as BAL 19, no shielding is available so the
Bushfire Attack Levels for each of the building elevations is:
• BAL 19 for all building elevations
7.8 Assessment of vegetation clearing
• The only clearing of vegetation is within the approved building envelope which achieves the
bushfire risk mitigation recommendations in this bushfire management plan.
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8 Bushfire risk management solutions and recommendations
8.1 Bushfire hazard mitigation recommendations: Considerations
As shown above, the house:
• Is mapped as High Potential bushfire Intensity on the Noosa Plan Bushfire Overlay,
• Is mapped as Very High Potential bushfire Intensity on Queensland Potential Bushfire
Intensity mapping,
• Is assessed as Potential bushfire Impact Buffer Class when assessed using a site-specific
assessment,
• Is confirmed to be in a Bushfire Prone Area and
• Is assessed as BAL 19 for all building elevations.
Accordingly, this section will recommend bushfire mitigation requirements for the above assessments.
Legislative requirements are detailed in:
• The Noosa Plan: Bushfire Overlay Code
• Australian Standard AS 3959 Construction of Buildings in Bushfire Prone Areas and
• Queensland State Planning Policy 2017
8.2 Recommendations for the Noosa Plan Bushfire Overlay Code
It is recommended that:
• The level of bushfire hazard shown on a Bushfire Hazard Overlay Map is confirmed through
the preparation of a site-specific bushfire hazard assessment prepared in accordance with
the Planning scheme policy for the bushfire hazard overlay code and
• The house is sited in a low hazard area in the lot and achieves a setback of 10 metres from
any retained vegetation.
As the house location is in a Potential bushfire Impact Buffer Class area and a reliable reticulated
water supply is not available,
It is recommended that:
• The premises to have a minimum water supply capacity of 5,000 litres dedicated for fire
fighting purposes which is sourced from a separate tank, a reserve section in the bottom
part of the main water supply tank, a swimming pool installed immediately upon
construction or a permanent dam.
8.3 Recommendations for Australian Standard AS3959
Australian Standard AS3959 specifies Deemed to Satisfy bushfire resistant methodologies based
on the assessed Bushfire Attack Level.
Accordingly, it is recommended that :
• The building solution for all building elevations of the house address the Deemed to Satisfy
requirements of Sections 3 and 6 of Australian Standard AS3959 -2018: Construction of
buildings in bushfire-prone areas.
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8.4 Recommendations for siting and landscaping
It is recommended that:the building to have a bushfire fuel free zone around the house. 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 building 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.
8.5 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 12: Methodologies for minimising the lodgement points for burning debris around the house
(Source: Queensland Government, 2000)
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8.6 Recommendations for on-going management of 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
8.7 Recommendations for Part E: Natural hazards of Qld State Planning Policy 2017
It is recommended that, notwithstanding that the proposed building location is in an area of
Potential Bushfire Hazard:
• The building siting, construction, firefighting resources etc outlined in Section 7.2
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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9 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, (2015). Regional ecosystem descriptions.
Available from http://www.ehp.qld.gov.au/ecosystems/biodiversity/regional-ecosystems, accessed
April 2021.
Department of Environment and Heritage Protection, (2015). Regional Ecosystem Maps /
Regrowth Vegetation Maps and PMAVs, Available from
<http://www.ehp.qld.gov.au/ecosystems/biodiversity/regional-ecosystems/maps/index.php>,
accessed April 2021.
Noosa Council, (2018). The Noosa Plan 2020, Noosa Council, Tewantin, Queensland.
McCarthy, GJ, Tolhurst, KG & Chatto K, (1999), Overall Fuel Hazard Guide: 3rd Edition,
Department of Natural Resources and Environment, Victoria
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
Standards Australia, (2018). Australian Standard™ AS 3959-2018: 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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10 Appendices
10.1 Appendix 1: Regional Ecosystem full descriptions
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10.2 Appendix 2: Software report from the bushfire behaviour modelling
Note that this report has a reduced rate of spread due to a reduced fire run. See Sections 7.2 & 7.5
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Note that this report has a reduced rate of spread due to a reduced fire run. See Sections 7.2 & 7.5
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Note that this report has a reduced rate of spread due to a reduced fire run. See Sections 7.2 & 7.5
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10.3 Appendix 3: 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:
• 12 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)
• 13 years experience in bush fire fighting
• 13 years experience in bushfire ecology (includes fire and biodiversity monitoring and
ecological fire management plans for conservation areas)
• 14 years experience in natural resource management, ecology, ecological restoration,
environmental management planning etc
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10.4 Appendix 4: 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,
New York.
Cheney, N.P. and Bary, G.A.V. 1969. The propagation of mass conflagration in a
standing eucalyptus forest by the spotting process. Paper 46. In collected papers,
Mass Fire Symposium Volume 1. Commonwealth of Australia, Defence Standard
Laboratories Maribyrnong Victoria, 19pp.
Cheney, N.P. and Gould, J.S. 1997. Letter to the editor – fire growth and acceleration.
International Journal Wildland Fire 7(1):1-5.
Luke, R.H. and McArthur, A.G. 1978. Bushfires in Australia. Australian Government
Publishing Services, Canberra, 359pp.
McArthur, A.G. 1966. Weather and grassland fire behaviour. Forest Research Institute,
Forest and Timber Bureau of Australia, Leaflet no. 100, Canberra.
McArthur, A.G. 1967. Fire behaviour in eucalypt forest. Forest Research Institute,
Forest and Timber Bureau of Australia, Leaflet no. 107, Canberra.
McArthur, A.G. 1973. Forest fire danger meter, Mark V. Forest Research Institute,
Forest and Timber Bureau of Australia, Canberra.
McAlpine, R.S. and Wakimoto, R.H. 1991. Acceleration of a fire from point source to
equilibrium spread. Forest Science 37:1314 -1337.
Noble, I.R., Bary, G.A.V, & Gill, A.M. 1980. McArthur fire danger meters expressed as
equations. Aust. J. Ecol. 5, 201-203.
Zhenxiang Tan, Stuart Midgley and Grahame Douglas 2005. A Computerized Model for
Bushfire Attack Assessment. Paper presented at the Symposium of Planning for
Natural Hazards, Wollongong.