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Basic Fire Behavior

Basic Fire Behavior. Walt Thomson Assistant Director of Field Conservation Services The Nature Conservancy Florida Chapter. Objectives: * Identify Fire Behavior Terms * Explain Fire Triangle * Phases of Combustion * 3 Elements of Heat Transfer *Discuss ROS and Flame Length

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Basic Fire Behavior

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  1. Basic Fire Behavior Walt Thomson Assistant Director of Field Conservation Services The Nature Conservancy Florida Chapter

  2. Objectives: * Identify Fire Behavior Terms * Explain Fire Triangle * Phases of Combustion * 3 Elements of Heat Transfer *Discuss ROS and Flame Length * Identify the Parts of a Fire * Describe Fire Environment Elements (3) * Basic Fuel Description - 3D * Identify Basic Fuel Types * Intensity vs Severity

  3. Fire Triangle OXYGEN HEAT FUEL

  4. Heat Transfer RADIATION CONVECTION CONDUCTION

  5. Branches preheated by convection and radiation Brush preheated by radiation Tree trunk preheated by radiation Log preheated by Conduction and radiation Litter or duff preheated by Conduction and radiation

  6. Flaming Glowing Smoldering Pre-Ignition Phases of Combustion

  7. Rate of Spread • Forward Rate of Spread (*Measured in Chains per Hour) • Head Fire: Burning with the wind or up-slope • Flank Fire: 90 deg. to the wind or slope • Backing Fire: Against the wind or down-slope

  8. Types of Fire in 3D space • Surface Fires: • Ground Fires: • Crown Fires: • Passive: • Active: • Independent:

  9. Types of Crown Fires

  10. Intensityvs. Severity • What is the Difference???

  11. Fire Intensity is... • Measures the dimensions of the fire itself. • Examples: • Flame Length • Peak Temperature • HUA • Reaction Intensity • FLI • Residence Time

  12. Fire Severity is... • A measure of the effect of the fire intensity on parts of the ecosystem • Examples: • Percent of fuel cover consumed. • Leaf or needle Scorch • Char Height • Bark char depth

  13. Intensity/Severity - Relationships • Low Intensity - Low Severity • Low Intensity - High Severity • High Intensity - Low Severity • High Intensity - High Severity

  14. Three Principal Environmental Elements Affecting Wildland Fire Behavior WEATHER FUELS TOPOGRAPHY

  15. FUEL Surface fuels: Ground fuels: small conifers mineral soil downed woody litter litter duff shrubs grass forbs.

  16. FUELS • Fuel Type • Fuel Moisture • Size and Shape • Fuel Loading • Horizontal Continuity • Vertical Arrangement

  17. Fuel Particle Properties Affecting Fuel Moisture Content • Fuel Moisture • Composition • Surface Coating • Living or dead • Amount of Decomposition

  18. Live Fuel Moisture“Live Fuels are Different” WHY???? Live Fuels Can Either Be a Heat Source of Sink: WHY???

  19. Categories of Fuels Light fuels: grass, leaves, shrubs Heavy fuels: Limbs, logs, stumps

  20. Fuel Model Categories • Grass Dominated • Primary carrier of fire is grass • Shrub Dominated • Primary carrier is brush or litter • Timber Litter Dominated • Primary carrier is litter beneath the stand • Logging Slash • Primary carrier is logging slash

  21. WEATHER Temperature • Measure of warmth or coldness of a substance • Fuel and ground temperature due to direct radiation from the sun • Higher ground and fuel temperature enables fuel to ignite/burn easier • Fuel temperatures can vary as much as 50o due to shading

  22. WEATHER Relative Humidity • Ratio of amount of moisture in the air to amount of moisture it could hold at a certain temperature and pressure, expressed in % • As RH increases, fuel moisture increases • Grasses and other light fuels respond quickly to RH • Logs and other heavy fuels respond slowly to RH

  23. WEATHER Precipitation • Increases fuel moisture • Amount • Duration • Light fuels affected quickly • Large fuels affected more by duration than by amount – time to absorb moisture before it runs off

  24. WEATHER Wind - Increases supply of oxygen • Drives convective heat into adjacent fuels • Influences spread direction and spotting • Carries moist air away replacing it with drier air • Dries Fuels • Raises fuel moisture if the air contains moisture

  25. Topography • Aspect- Direction a slope faces • Slope- Steepness • Position of Fire- Top, middle, or bottom of slope • Shape of Country- Narrow canyons & box canyons • Elevation - Relates to curing of fuels, precipitation, length of fire season, etc.

  26. Mountains Cause Channeling of Wind Saddle

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