Fire Behavior Considerations for safe and productive fire suppression

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Fire Behavior Considerations for safe and
productive fire suppression

• Marty Alexander, PhD, RPFSenior Researcher
• Wildland Fire Operations Research Group FERIC -
  Western Division

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... further major advances in combating wildfire
are unlikely to be achieved simply by continued
application of the traditional methods. What is
required is a more fundamental approach which can
be applied at the design stage Such an
approach requires a detailed understanding of
fire behaviour ... Drysdale (1985) Introduction
to Fire Dynamics
Publication on Workshop CD
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• Outline of Presentation
• Fire Behavior Fundamentals
• Fire Suppression Principles
• Fireline Breaching
• Fire Suppression Requirements
• vs. Fire Behavior
• Fire Growth and Production Rates
• Safety Issues
• Thoughts on Future Directions

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FIRE BEHAVIOR 101 The Fundamentals
Fire behavior is defined as the manner in which
fuel ignites, flame develops, fire spreads and
exhibits other related phenomena as determined by
the interaction of fuels, weather, and topography.
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Point Source Fire Growth
Most wildland fires start from a single ignition
point (e.g., escaped campfire, lightning strike).
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A fire originating from a single point source
gradually increases its rate of forward progress
until a rate of spread approaching a "quasi"
steady rate state is reached.
Equilibrium or Steady-State Forward Rate of Fire
Spread
ROSeq
ROSt ROSeq (1 - e-at)
Forward Rate of Spread -- ROSt
Elapsed Time Since Ignition (t)
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Typical Rates of Spread for Wildland Fires
Ground or Subsurface Fires lt 0.01 m/min Surface
Backfires in Forests 0.1 1.0 m/min Surface
Head Fires in Forests 1 - 10 m/min Crown
Fires in Forests 15 - 200 m/min Grass Fires
up to 250 - 350 m/min
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Elliptical Fire Shape
A fire starting from a single point source
ignition will maintain a roughly elliptical
shape provided the fuel, weather and topography
remain relatively uniform.
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Area ? a b Perimeter 2 ?
?(a2b2)
2 a - long semiaxis b - short semiaxis
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Rough Rules of Thumb Rate of Perimeter Increase
Head Fire Rate of Spread x 2.5 Fire Perimeter
Length Forward Spread Distance x 2.5
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Area Burned in Relation to Rate of Spread The
area burned for a given period of time increases
in direct proportion to the rate of spread as
follows 2n were n is the proportional increase
in rate of spread.
For example, if a fire were to double its rate of
spread from 5 m/min to 10 m/min then the area
burned after one hour would 19.5 ha rather than
4.9 ha.
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What else does a wildland fire do following
ignition besides increase its spread rate and
size?
It consumes or eats fuel and
a visible flaming combustion reaction.
it produces heat energy and light in
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Fire Intensity
I H x W x
R
Fire Intensity (kW/m)
Fuel Consumed (kg/m2)
Rate of Fire Spread (m/sec)
Heat of Combustion (18 000 kJ/kg)
Fire Intensity Spectrum 10 kW/m Lower limit of
surface fire spread 1000 kW/m Limit of
suppression capability by
hand crews 10 000 kW/m Active crown fires have
developed 100 000 kW/m Major conflagrations
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Fire intensity is related to size of flames
For crown fires, flame height approximately2X
stand height
Simple Formula for Field Use (For surface fires
intermittent crown fires) I 300 x (L)2 L
Flame Length (metres)
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Elliptical Fire Intensity The head and rear or
back of the fire are fastest and slowest moving
parts of fire, respectively, while the flanks
are intermediate. This differences are reflected
in the flame lengths experienced about the fire
perimeter.
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Q 60(1 - exp- I / 3000 D) Q Radiation
Intensity (kW/m2) I Fire intensity (kW/m) D
Distance from Flame Front (m)
Fire
Intensity
Distance From Flame Front (m)
(kW/m)
1
5
10
20
30
40
50
60
70
80
2
Radiation Intensity (kW/m
)
500
9.2
2.0
1.0
0.5
0.3
0.2
0.2
0.2
0.1
0.1
1000
17.0
3.9
2.0
1.0
0.7
0.5
0.4
0.3
0.3
0.2
2000
29.2
7.5
3.9
2.0
1.3
1.0
0.8
0.7
0.6
0.5
3000
37.9
10.9
5.7
2.9
2.0
1.5
1.2
1.0
0.9
0.7
4000
44.2
14.0
7.5
3.9
2.6
2.0
1.6
1.3
1.1
1.0

• 1.0 kW/m2 firefighters can withstand indefinite
  
• skin exposure
• 7.0 kW/m2 maximum exposure for a firefighter
• with PPE for 90 sec
• 52.0 kW/m2 fibreboard will spontaneously ignite

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FIRE SUPPRESSION 101 Basic Principles
Fire suppression is defined as all the activities
concerned with controlling and extinguishing a
fire following detection.
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To stop a fire one can (1) Remove the fuels
ahead of the spreading combustion zone
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To stop a fire one can (2) Reduce the
temperature of the burning fuels
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To stop a fire one can (3) Exclude oxygen from
reaching the combustion zone
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In practical terms this generally means creating
a physical barrier in front of the advancing fire
edge by removing the fuels or cooling/smothering
the flames with water, covering them with mineral
soil, suppressants (e.g., foam), or chemical fire
retardants by various means from either the
ground or the air.
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The use of fire in the suppression operations can
take many forms (e.g., to reduce intensity of,
slow, or steer a wildfire to remove potentially
dangerous fuel concentrations to widen and
strengthen control lines to expedite mop-up).
Regardless of the specific use in the
application of fighting fire with fire, the
main objective is to speed up and/or strengthen
control actions on free-burning wildfires.
It is the most economical, fastest, safest, and
least damaging means of widening control lines.
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FIRELINE BREACHING The breaching or crossing a
barrier to fire spread can occur by one, all or
any combination of the following mechanisms

• Spotting (via wind borne firebrands
• Thermal radiation, either
• by pilot or spontaneous
• ignition
• Direct flame contact by the fires leading edge
• Fire whirls

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Minimum Firebreak or Fireguard Width? Perry
(1990, p. 273) A field rule of thumb says,
Construct line 1.5 times the height of the
fuel.
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Byrams (1959) Rough Rule of Thumb (in the
absence of severe spotting) Minimum Firebreak
or Fireguard Width Flame Length X 1.5
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Probability of Grass Fire Breaching Mineralized
Firebreak vs. Fire Intensity Firebreak Width
Model from Experimental Fires, Northern
Territory, Australia
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Probability of grass fire breaching a mineralized
firebreak for trees absent (A) or present (B)
within 20 m of the upwind side of the firebreak
based on Wilsons (1988) model
Good agreement between Byrams (1959) rough rule
of thumb and Wilsons (1988) A relation at the
90 level.
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Fire Suppression Requirements vs. Fire Behavior
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Fire Intensity Minimum Control Requirements
lt 500 kW/m hand tools 500-2000 kW/m water
under pressure and/or heavy machinery
2000-4000 kW/m helitankers airtankers using
chemical retardants 4000 kW/m very difficult
to control
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The U.S. Hauling Chart (Anderson Rothermel
1982)
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Fire Growth and Production Rates
One of the major characteristics that
distinguishes wildland fires from structural or
urban fires is their horizontal spread or growth
potential.
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Free-burning fire growth with the passage of time
since ignition before containment action begins
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Schematic diagram illustrating the various
activities involved in controlling and
extinguishing a wildfire as a function of
cumulative area burned and elapsed time since
ignition
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In order to achieve successful fire containment
the fireline production rate of the appropriate
suppression resource must exceed the rate of
perimeter increase
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Effect of Fire Containment on Fire Growth
1966 Monbulla Fire, South Australia (Douglas 1966)
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An example of fireline production rates by fuel
type for three commonly used fire suppression
resources (after Schmidt and Reinhart 1982)
Generalized fuel type Short grass Tall
brush Conifer stand Logging slash
Crew with hand tools (m/person-hour) 80 13 40 20
5-person pumper crew (m/hour) 805 402 463 402
Medium-sized bulldozer (m/hour) 1509 734 694 785
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Hot-Spotting Suppression Capabilities or
Production Rate
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Fireline Production Rates of Bulldozers
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http//www.fs.fed.us/t-d/programs/fire/production/
index.htm
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U.S. Fuel Type Classification
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U.S. Dozer Fireline Construction Rates (Single
Pass) in Chains Per Hour by Dozer Type, Fuel
Type, Slope Steepness Direction of Movement
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• Factors Affecting Fireline Production Rates of
  Heavy Equipment
• Operator skill and experience
• Age and condition of equipment
• Soil and rock conditions
• Time of day
• Fuel characteristics
• Terrain conditions
• (e.g., slope steepness)

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Fire Safety
there is one overriding challenge to fire
management that of maintaining full respect for
the power of fire and the effects of this power
on both wildland environments and the people who
live and work in these environments.
J.S. Barrows (1974)
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Thoughts on Future Directions
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A More Systematic Approach is Needed With
Respect to Studying and Modelling Fireline
Production Rates Than Has Been the Case to Date
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Tie Fireline Production Rates to Canadian FBP
System Fuel Types
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FBP System Fuel Types have been linked to Aerial
Application of Foam Requirements
Recommended Foam Consistencies for Aerial Attack
on Wildfires in Canadian Forest Fire Behavior
Prediction (FBP) System Fuel Types (adapted from
Ogilviie et al. 1989)
Ground Support (within 30 minutes)
No Ground Support
FOREST FLOOR
FOREST FLOOR
Shallow
Deep
Deep
Shallow
TREE CANOPY
TREE CANOPY
WET
DRIPPING
DRY
DRIPPING
FBP System fuel types C-1, C-7, S-1, O-1)
FBP System fuel types C-1, C-7, S-1, O-1)
FBP System fuel types C-2, S-2, S-3)
FBP System fuel types C-2, S-2, S-3)
Open
Open
WET FOLLOWED BY DRY-optional
WET FOLLOWED BY DRY
WET FOLLOWED BY DRIPPING
DRIPPING
Closed
Closed
FBP System fuel types C-4, C-5, C-6, D-1)
FBP System fuel types C-3, M-1, M-2 M-3, M-4)
FBP System fuel types C-4, C-5, C-6, D-1)
FBP System fuel types C-3, M-1, M-2 M-3, M-4)
i.e., two loads.
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More Realistic Field Studies Hard Data
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Live Fire Training
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THE END
ANY QUESTIONS?
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APPENDIX U.S. Dozer Fireline Construction Rates
(Single Pass) in Chains Per Hour for Type II and
Type I Dozers by Fuel Type, Slope Steepness
Direction of Movement and U.S. Tractor-Plow
(Drag or Mounted) Production Rates in Chains Per
Hour on Level to Rolling Terrain
From http//www.fs.fed.us/td/programs/fire/prod
uction/index.htm
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