Fuels and Fire Behavior

David Veale

District Leader

Pleasanton

• Any combustible material• Live or dead plant material• Houses, sheds, etc., can also be

fuels

Fuels

Recognize and Identify Fuel Characteristics that Influence the Behavior of

the Fire.

• Fuel type

• Fuel loading

• Fuel availability

Fuels and Fuel Characteristics

Grass

FUEL TYPES

Grass - Shrub

Shrub

FUEL TYPES

Timber - Understory

Timber - Litter

FUEL TYPES

Slash - Blowdown

FUEL LOADING• Quantity of fuels in an area• Expressed in Tons/Acre

Fuel Arrangement:

• The manner in which fuels are spread over a certain area.

• Horizontal continuity- Affects fire’s rate of spread.

Uniform fuels

• Include all fuels distributed continuously over the area.

• Relates to fire spread.

Patchy Fuels

Include all fuels distributed unevenly over the area, or areas of fuel with definite

breaks or barriers present.

Aerial Fuels

Surface Fuels

Ground Fuels

Ladder Fuels

VERTICAL ARRANGEMENT OF FUELS

Ground FuelsAll combustible materials lying beneath the surface including:• deep duff• roots • rotten buried logs• peat • other organic

material

Important in terms of line construction and mop-up, not as much for fire behavior.

Surface FuelsAll materials lying on or immediately above the ground including: • needles • leaves• grass • downed logs • stumps • large limbs • low shrubs

Important in terms of line construction and mop-up, most important regarding fire spread and fire behavior.

Ladder Fuels

Any fuels that provide a

connection between the surface and

upper canopy.

Important in terms of fire spread and fire behavior by providing a path for the fire to travel to the

canopy. It can be linked to torching or crowning.

Aerial Fuels

All green and dead materials located in the upper forest canopy including:

• tree branches • crowns • snags • moss• high shrubs

Important in terms of fire spread and fire behavior due to torching, crowning, and spotting.

Fuel moisture content is the amount of water in a fuel expressed as a percent

of the oven dry weight of that fuel.

Fuel Moisture

• Determines how well a fuel will ignite and burn.

• Dry fuels will ignite and burn much more easily

than the same fuels when they are wet.

• Before a wet fuel can burn, the moisture it

contains must evaporate.

Fuel Moisture

Fuel Moisture

Timelag: • The rate at which a fuel gains/loses

moisture, relates to:- relative humidity - size of fuels - precipitation

• Fuel size plays important role

FUEL SIZE CLASSES AND SHAPE

• Physical characteristics of

fuels

• Divided into four categories on the basis of their size

Dead Fuel TimelagCategories

• 1-hour - 0 to 1/4” diameter• 10-hour - 1/4” to 1” diameter• 100-hour - 1” to 3” diameter• 1000-hour - 3” to 8” diameter

1 Hour Fuels 10 Hour Fuels

100 Hour Fuels 1000 Hour Fuels

Surface-Area-to-Volume Ratio:

Fuels with high surface area to volume rations (fine fuels) lose moisture more quickly than larger

fuels, therefore they ignite more quickly.

Higher surface area relative to the volume:• fuel moistures will change rapidly

• fuels will ignite and burn out quickly

Size and ShapeSurface area to volume ratio

03-24-S290-EP

OneCubicfoot

Surface Area6 square feet

1’

1’

1’

1’

Surface Area18 square feet

12”

3” 3” 3” 3”

3”

3”

3”

3”

Fine Dead Fuel Moisture• Dead vegetative material 0 – ¼”

diameter• When combined with high winds, most

responsible for rapid spread of flames• Calculated by relative humidity and

temperature• Changes hourly• Calculated on-site

The fire environment has been described with a triangle showing:

• Weather

• Fuels

• Topography (Terrain)

Look Up, Down, and Around

There are seven Fire Environment Factors that fireline personnel must monitor:

1. Fuel Characteristics2. Fuel Moisture3. Fuel Temperature4. Terrain5. Wind6. Stability

7. Fire Behavior

Look Up, Down, and Around

Fuel CharacteristicsIndicators: Continuous fine fuels

Fuel CharacteristicsIndicators: Heavy loading of dead and down

Fuel CharacteristicsIndicators: Ladder fuels

Fuel CharacteristicsIndicators: Tight crown spacing

Fuel CharacteristicsSpecial Conditions: Firebrand sources

Fuel CharacteristicsSpecial Conditions: Numerous snags

Fuel CharacteristicsSpecial Conditions: Frost and bug kill

Special Conditions: Preheated canopy

Fuel Characteristics

Fuel CharacteristicsSpecial Conditions: Unusual fine fuels

Fuel CharacteristicsSpecial Conditions: High dead to live ratio

Fire BehaviorIndicators: Trees torching

Fire BehaviorIndicators: Smoldering fires picking up

Fire BehaviorIndicators: Small firewhirls beginning

Fire BehaviorIndicators: Frequent spot fires

Reviewing the 7 Factors

1. Fuel Characteristics2. Fuel Moisture3. Fuel Temperature4. Terrain5. Wind6. Stability7. Fire Behavior

Bastrop FireSept. 2011

Spot fires

Wind Direction

Grasses are not being preheated by the fire. Ignition is much less intense than a head fire and fire movement is slow. Flame Front

Backing Fire

Previously burned Area

Wind Direction

Wind Direction Grasses are being preheated by the fire. Ignition is rapid and more intense.

Flame Front

Head Fire

Sparks are blown ahead of fire

Wind Direction

Two flanking fires used to burn in a fire lane using a wet line technique

Wet LineWet Line

8-10 feet

Rising heated air between two flame fronts will cause flames to move toward each other

Burning in Fire Lanes

HOW TO BURN

Pre-established fire line

Back Fire

Headfire

Strip Headfire

Wind Direction

Headfire

Creeping Backfire Across Firebreak

Pre-established fire line

Back Fire

Headfire

Strip Headfire

Pre-established fire line

Strip Headfire

Back Fire

Headfire

Pre-established fire line

Strip Headfire

Back Fire

Headfire

Wind Direction

Wind Direction

Wind Direction

Slash burns with slash stacked around the base can top kill trees.

Slash Burn

20-25% humidity

5 mph wind

Surface Wind Direction

Upslope burn with wind

Upslope burn

Hill

Surface winds blowing across hill can create a backdraft on the opposite side of the hill

Effects of wind and slope on fire behavior

QUESTIONS?