Fire and Fuels
8/31/2010
THE FIRE TRIANGLE
HEAT
OXYGEN
FIRE
FUEL
The Wildland Fire Environment
(influences fire behavior)
Weather
Topography
FIRE
Fuels
What are Fuels?
Any organic material that is living or
dead that can ignite and burn.
Fuels can be found in the ground (ground
fuels), on the ground (surface fuels), or in
the air (aerial fuels).
Ground fuels
•below surface (“duff”)
•usually compact --> smoldering, creeping
•deep roots, duff, buried logs
•important in terms of line construction and mop-u
Surface fuels
•on surface
•litter, grasses, shrubs (to 6 ft.)
•important in terms of line construction
and mop-up
• Aerial fuels
•crown or canopy
•open vs. closed canopy
•important in terms of fire spread and fire
behavior due to torching, crowning, and spotting
Principle Fuel Characteristics
1. Fuel Loading
2. Size and Shape
3. Compactness
4. Horizontal Continuity
5. Vertical Arrangement
6. Chemical Content
7. Moisture Content
1. Fuel Loading
The oven dry weight of fuels in a given
area, usually expressed in tons/acre or
lbs/acre
2. Size and Shape
Surface-area-to-volume ratio (SA / V)
– ratio of fuel’s surface area to total volume
Example: Grass vs. Logs – which has a higher SA/V ratio?
Grass = high SA/V; Log = low SA/V
* The higher the ratio, the finer the fuel
How does SA/V affect
fuel moisture?
Fuels with high surface area
to volume ratios (fine fuels)
lose moisture more quickly
than larger fuels, therefore
they ignite more quickly
3. Compactness
= The spacing between fuel particles.
Closely Compacted
– less surface exposed (remember SA/V ratio?)
– restricts oxygen and inhibits convection and radiation
Loosely Compacted
– larger amount of surface exposed
– allows for oxygen circulation and heat movement
How does compactness
affect ignition and
combustion?
5. Horizontal Continuity
The horizontal distribution of fuels at various levels or planes.
How does horizontal continuity affect fire behavior?
Influences direction of fire spread, rate of spread,
and level of fire travel (surface vs. aerial)
Ladder Fuels
Vertical Arrangement
The relative heights of fuels above the
ground as well as their vertical continuity.
6. Chemical Content
Certain types of fuels contain volatile chemicals
which increase their flammability. Chemicals
include, pitch, tar, resins, oils, wax,etc.
Examples include- chaparral, palmetto,
fountaingrass, pine, juniper
7. Fuel Moisture Content
The amount of water in a fuel expressed as
a percentage of the oven-dry weight of that
fuel.
= (amount moisture/dry weight) * 100
Definition of Equilibrium Moisture
Content:
No net gain or loss of moisture between
fuels and the surrounding air.
Does equilibrium moisture content occur
for all fuels?
* Occurs in small, fine fuels.
* Never occurs in larger fuels.
Dead Fuel Time-Lag
Timelag- the rate at which a fuel
gains/loses moisture
relates to relative humidity,
precipitation, size of fuels…
* Will small pieces of fuel gain/lose water faster or
slower than large pieces of fuel?
Time-Lag Categories
1-hour - 0 to 1/4” in. diameter
10-hour - 1/4” to 1” in. diameter
100-hour - 1” to 3” in. diameter
1000-hour - 3” to 8” in. diameter
Which category gains/loses moisture
fastest? Why?
Definition of Moisture of
Extinction:
Moisture of Extinction is the fuel moisture
content at which a fire will not spread, or
spreads only sporadically and in a nonpredictable manner.
- usually around 25%
- lower for grasses; higher for logs
- also depends on tissue chemical content
Moisture content of live
vs. dead fuels:
Are they different?
Why or why not?
Living fuels: hold between 30 % to over 300 % water
*fluctuate slowly
Dead fuels: hold between 2 % to 30 % water
*fluctuate rapidly
Live-to-Dead Ratio
Amount of live fuel to dead fuel.
-- How does live-to-dead ratio affect fire behavior?
Higher amounts of dead to live fuels =
higher flammability (less moisture to drive out).
Examples of low live-to-dead ratios:
Over mature fuel complexes, fire damaged,
insect damaged, diseased
**Factors Influencing Fuel
Moisture Content
- Weather (ppt, dew, ground moisture,
wind, RH-evaporation)
- Biological/physiological processes in
plants
- Disturbance
Stages of Vegetative Development
Stage
Moisture content %
1.
Fresh young foliage
300
2.
Maturing foliage.
200
3.
Mature foliage
100
4.
5.
Entering dormancy, coloration starting,
some leaves dropping.
Completely cured (= dead fuel).
50
< 30
Disturbance Factors
Contributing to Live Fuel
Moisture Loss – may cause
Abnormal Fire Seasons:
–
–
–
–
–
Long drought periods.
Natural disease and insects.
Early frost.
Harvesting of timber.
Blowdown and ice storms.
NOT ALL FUEL WILL BURN!
Total fuel
– all the plant material
Potential fuel
– the material on a site that might burn
in an intense fire (less than the total fuel)
Available fuel
– the fuel that is available for combustion
in a given fire.
* varies by site and environmental
conditions
Available Fuel
Fuels that will ignite and support combustion at the flaming
front under specific burning conditions.
What factors influence fuel availability?
Total fuel, fuel type, weather, chemical content, fuel
moisture, season, live:dead, …
What will ignite more easily:
Solid wood, decayed wood, or litter?
And why?
• Thermal conductivity
The quantity of heat transferred per unit of area per unit time per
degree of temperature gradient.
- Increases as the density and size of the fuel increases.
• Heat capacity
The amount of heat required to raise the temperature.
- Increases with density.
- High density-fuels require more heat for ignition than
low-density fuels.
Homework - Select a recently or
currently burning fire…
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•
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Name & location
How did it start?
Fuel type?
Topography?
Fire behavior?
Weather conditions (RH/TEMP – see outcome
sheet)