Wildland Fire Behavior

Component 1

Fuel-grass, shrub, timber litter, logging slash

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LIVE FUEL MOISTURE

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STAGE OF VEGETATIVE DEVELOPMENT________________________________________

Fresh foliage, annuals developing early in growing cycle.

Maturing foliage, still developing with full turgor.

Mature foliage, new growth complete and comparable to older perennial foliage.

Entering dormancy, coloration starting, some leaves may have dropped from stem.

Completely cured.

MOISTURE CONTENT

Percent300

200

100

50

Less than 30, treat as a dead fuel.

PROBABILITY OF IGNITION

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A rating of the probability that a glowing firebrand will cause a fire.

Wind

10 mi/h

20 mi/h

3 mi/h

20 mi/h

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Fuel exposure to windFuel exposure to wind

Partly shelteredPartly sheltered(patchy timber)(patchy timber)

WindWind

Unsheltered (no foliage, Unsheltered (no foliage, near clearings)near clearings)

Fully shelteredFully sheltered(under timber,(under timber,flat or gentle flat or gentle slope, near baseslope, near baseof steep mtn.)of steep mtn.)

Partly shelteredPartly sheltered(under timber(under timbermidslope, windmidslope, windon slope)on slope)

UnshelteredUnsheltered(ridgetops)(ridgetops)

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PERCENT SLOPE

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% SLOPE = RISE IN FEETRUN IN FEET

X 100 %

RISE

RUN

A MATHEMATICAL MODEL IS A SET OF

EQUATIONS

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ASSUMPTIONS OF THE FIRE SPREAD MODEL

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1. Fire is spreading at the flame front.

2. Fire is free burning.

3. Fine fuels control rate of spread.

4. Uniform and continuous fuels.

5. Surface fire.

6. Uniform weather and topography.

RATE OF SPREADDISTANCE/TIME

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STARTING TIME

TIME

ENDING TIME

DISTANCE

FLAME LENGTHFEET

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POINT SOURCE

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Point source model is designated for firesburning on flat ground or where the wind is blowing in the direction of the slope plus

or minus 30°.

upslope

max

slop

e

± 30º

SPREAD CALCULATION

SD=PT X ROS

SD = SPREAD DISTANCEPT= PROJECTION TIMEROS = RATE OF SPREAD

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SPREAD DIRECTION

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WIN

D

NO

WIN

D

WIN

DUP

SLOPEUP

SLOPEUP

SLOPE

WIN

D

DOWN SLOPE

DOWN SLOPE

DOWN SLOPE

POINT SOURCE PREDICTION LIMITATIONS

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Calculations with windspeeds less than 2.5 miles per hour are generally too low.

Better predictions result withwindspeeds greater than 2.5 milesper hour.

FIRE PERIMETER AND SHAPES

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Fire perimeter and shapes are based on smooth ellipses - actual perimeter of the fire edge would likely be greater length and follow topographic relief.

APPROXIMATE FIRE SHAPES ASSOCIATED WITH MIDFLAME

WINDSPEEDS OF…..

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Wind Direction

Fire Start

2.5 mi/h

5 mi/h

7.5 mi/h

15 mi/h

10 mi/h

AREAS OF USE SUPPRESSION

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Determination of locations to place crews,equipment, helicopters and fuel breaks.

Development of the wildland fire situation analysis.

PRESCRIBED BURNING

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Distance between spot fires to accomplishan objective.

Calculating timing of ignition to take advantage of diurnal weather patterns.

Managing wilderness fires.

Development of escaped fire contingencyplanning.

EFFECTIVE WINDSPEED

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The midflame adjusted for the effectof slope on uphill fire spread.

EXAMPLES OF EFFECTIVE WINDSPEED

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EFFECTIVE WINDSPEEDIS 5 mi/h

3 mi/h

60%5 mi/h

MAXIMUM SPOTTING DISTANCE

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When torching trees, piles or wind-driven surface fires loft firebrands,which are then carried by the prevailing wind.

SOURCES OF FIREBRAND

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•Torching trees

•Burning pile

•Spreading surface fire

FACTORS RELATING TO THE SPOTTING PROBLEM

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•Probability of production of firebrands.

•Windspeed.

•Fire intensity.

•Number of firebrands

Dispatching Priorities

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FUELSWEATHER

TOPOGRAPHY

Predicting “Real Time” Fire Behavior

Running Surface Fire

Wildland Fire Situation Analysis(WFSA)

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Prescribed Burning

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•Estimate the behavior of escapes or spots.

•Assess fuel and weather conditions at burn time.

•Develop burn prescriptions.

•Develop containment and control plans.

FIRE PLANNING

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•Preattack

•Describing consequences

•Environmental documents

Rate of Spread =

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Spread DistanceElapsed Time

FLAME LENGTH*

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Indicator of intensity

Observable

*Remember flame length is not equal to flame height.