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Fuels and Fire Behavior

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Fuels and Fire Behavior Fuels and Fire Behavior David Veale District Leader Pleasanton Fuels • Any combustible material • Live or dead plant material • Houses, sheds, etc., can also be fuels Recognize and Identify Fuel Characteristics that Influence the Behavior of the Fire. Fuels and Fuel Characteristics • Fuel type • Fuel loading • Fuel availability FUEL TYPES Grass Grass - Shrub FUEL TYPES Shrub Timber - Understory FUEL TYPES Timber - Litter 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. VERTICAL ARRANGEMENT OF FUELS Aerial Fuels Ladder Fuels Surface Fuels Ground Fuels Ground Fuels All 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 Fuels All 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 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 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 Timelag Categories • 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 Shape Surface area to volume ratio 1’ 1’ 3” One 3” 1’ Cubic foot 3” 3” 12” 1’ 3” 3” 3” 3” Surface Area Surface Area 6 square feet 18 square feet 03-24-S290-EP 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 Look Up, Down, and Around 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 Characteristics 2. Fuel Moisture 3. Fuel Temperature 4. Terrain 5. Wind 6. Stability 7. Fire Behavior Fuel Characteristics Indicators: Continuous fine fuels Fuel Characteristics Indicators: Heavy loading of dead and down Fuel Characteristics Indicators: Ladder fuels Fuel Characteristics Indicators: Tight crown spacing Fuel Characteristics Special Conditions: Firebrand sources Fuel Characteristics Special Conditions: Numerous snags Fuel Characteristics Special Conditions: Frost and bug kill Fuel Characteristics Special Conditions: Preheated canopy Fuel Characteristics Special Conditions: Unusual fine fuels Fuel Characteristics Special Conditions: High dead to live ratio Fire Behavior Indicators: Trees torching Fire Behavior Indicators: Smoldering fires picking up Fire Behavior Indicators: Small firewhirls beginning Fire Behavior Indicators: Frequent spot fires Reviewing the 7 Factors 1. Fuel Characteristics 2. Fuel Moisture 3. Fuel Temperature 4. Terrain 5. Wind 6. Stability 7. Fire Behavior Bastrop Fire Sept. 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 Previously burned Area Backing Fire Wind Direction Wind Direction Grasses are being preheated by the fire. Ignition is rapid and more intense. Flame Front Sparks are blown ahead of fire Head Fire Wind Direction Rising heated air between two flame fronts will cause flames to move toward each other Wet Line Wet Line 8-10 feet Two flanking fires used to burn in a fire lane using a wet line technique Burning in Fire Lanes HOW TO BURN Pre-established fire line Back Fire Strip Headfire Headfire Wind Direction Headfire Creeping Backfire Across Firebreak Pre-established fire line Back Fire Strip Headfire Headfire Pre-established fire line Back Fire Strip Headfire Headfire Pre-established fire line Back Fire Strip Headfire 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 Effects of wind and slope on fire behavior 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 QUESTIONS?.
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