Phases of wildfires
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Transcript of Phases of wildfires
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Phases of wildfires
• Preignition (energy absorbing)– Preheating – drying out– Pyrolysis – chemical degradation
• Combustion (energy liberating)– Fire triangle in place
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Fire triangle
All components must be present for fire to occur
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Why wood burns
• Wet wood – high heat capacity of water absorbs heat making the wood difficult to ignite
• Once dry it ignites at 572°F, its flash point• Flammable gases given off
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Conditions for wildfire
• Plant material + oxygen + heat
• Combustion equation
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How to fight fire
• Remove or reduce one component
• Water – reduces heat• Slurry – blocks oxygen• Remove fuel
– Cut or clear area– Light backfire
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• Ladder fuel effect
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What spreads fire?
• Fuel types
• Weather and wind
• Topography
• Its own behavior
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Types of fuel and results
• Grass, trees, shrubs, slash piles, homes• Rapid advance
– Grass fires: about 4 mph with 6-8 ft flames– Shrub fires: oily material 8 mph, 50 ft flames
• Understory fuel source important– No litter little damage, fast moving– Lots of litter hot, damaging fire
• Ladder fuels crown fires
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Flat topography; no wind
Flat topography; wind
Hillslope and wind
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Smoke plume – these can produce the fire’s own weather
What is the primary heat moving process here?
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Types of fires
• Ground fire– Creep along, mainly
smolders, few flames
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• Surface fires– Variable intensity– Burns low vegetation
and lower parts of trees
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• Crown fires– Burns upper parts of
trees– Can produce
firestorm– Move rapidly– Impossible to stop
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Results of crown fireCoconino National Forest
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Causes of wildfires
• Human caused (85%)– Arson – 26%– Equipment – 10%– Juveniles - 4%– Campfires – 3%– Railroads – 3%– Other/unknown – 50%
• Naturally caused (15%)
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One day – two BIG firesOct 8, 1871
• Extremely windy conditions affect both areas• Peshtigo, WI
– 15 mile wide fire front– Traveled 40 miles northward– 1,152 died
• Chicago, IL– O’Leary barn caught fire– 300 died– 3.3 sq mi destroyed
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Weather that produced winds
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Extent of Peshtigo fires
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Central Chicago following fire
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California
• Chaparral – shrubland plant community; contains a lot of oily, dried vegetation
• Fires occur every year• Major fires in 1991
– Oakland and Berkeley Hills– 25 died, 2,449 homes destroyed; 437 apts– “only” 1,600 acres but $1.5 billion damage– Caused by cooking fires in a camp set up by
homeless people
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Oakland fires in 1991
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California fires
• Santa Ana winds– Common October to
March– High pressure over
Nevada– Cool, dry air
descends over mtns– Air heats up,
generating winds
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Major fires due to Santa Ana winds
• October/November 1993– Santa Ana winds– 15 major fires in southern California– 3 dead, 1,150 homes, $1 billion damage– 215,000 acres [300 sq mi]
• October/November 2007– Mexico border to Santa Barbara– 350,000 homes evacuated– >500,000 acres burned– > 2,100 homes destroyed
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• October/November 2008– Santa Ana winds– Fires in Montecito, Sylmar
• Montecito fire due to bonfire– Interstates closed– More than 1,200 homes destroyed– Almost 50,000 acres
• August 2009– Station Fire, 161,000 acres NE of Pasadena– Arson caused
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Fire suppression• 20th century approach – put fires out!!• Trees per acre increased dramatically• In 1970s decision was to let fires started naturally
burn; human-caused extinguished• Prescribed burns
– Formerly “controlled burns”– Los Alamos, NM, burned in May 2000; 280 homes
destroyed
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Yellowstone National Park
• Burn areas shaded
Dry winter in 1987-88Low moisture content and many beetle ridden treesFires began in June and July 1988By mid-August very dry and fire lasted until Nov1.4 million acres burned, about half of the parkNatural-burn policy in place since 1976
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Rodeo-Chediski Fire of June 2002
Two separate fires grew together;5,000 firefighters
More than 500 homes burned;732 sq mi destroyed
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Wallow FireMay and June 2011
• Largest fire in AZ history burned 538,000 acres, including 15,000 acres in New Mexico
• Caused by campfire lit by two cousins
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Aftermath of fires• Rejuvenation of land
– naturally due to released nutrients and opening of seeds– replanting
• Erosion and landslides– Formation of hydrophobic layer caused by oils and organic
compounds vaporizing and recondensing in cooler layers under the surface
• Expense to fight fires– Fiscal cost– Loss of life
• Reduced air quality
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Schultz Fire north of Flagstaff June 20, 2010 burned 15,000 acres
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Extremely high winds rapidly spread fire caused by a campfire
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Area north of Flagstaff following 1996 fire
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Summary
• Good:– Increase in soil nutrients and regeneration of
vegetation (aspen, conifers)– Reduction of potentially larger fires
• Bad:– More erosion, runoff, mass wasting, loss of life
(human and animal)
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What is there to learn?
• Restrict development in heavily forested areas• Decrease fuel sources with prescribed burns• Undertake preventative measures through
education
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A healthy forest