Transcript Fuels
Fire and Fuels 8/31/2010
THE FIRE TRIANGLE OXYGEN FIRE FUEL HEAT
The Wildland Fire Environment (influences fire behavior)
Weather
FIRE
Topography 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-up
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 non predictable 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 1.
Fresh young foliage 2.
Maturing foliage.
3.
Mature foliage Moisture content % 300 200 100 4.
Entering dormancy, coloration starting, some leaves dropping
.
5.
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…
• Name & location • How did it start?
• Fuel type?
• Topography?
• Fire behavior?
• Weather conditions (RH/TEMP – see outcome sheet)