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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. Fire triangle. All components must be present for fire to occur. Why wood burns. - PowerPoint PPT Presentation

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Phases of wildfires

• Preignition (energy absorbing)– Preheating – drying out– Pyrolysis – chemical degradation

• Combustion (energy liberating)– Fire triangle in place

Fire triangle

All components must be present for fire to occur

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

Conditions for wildfire

• Plant material + oxygen + heat

• Combustion equation

How to fight fire

• Remove or reduce one component

• Water – reduces heat• Slurry – blocks oxygen• Remove fuel

– Cut or clear area– Light backfire

• Ladder fuel effect

What spreads fire?

• Fuel types

• Weather and wind

• Topography

• Its own behavior

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

Flat topography; no wind

Flat topography; wind

Hillslope and wind

Smoke plume – these can produce the fire’s own weather

What is the primary heat moving process here?

Types of fires

• Ground fire– Creep along, mainly

smolders, few flames

• Surface fires– Variable intensity– Burns low vegetation

and lower parts of trees

• Crown fires– Burns upper parts of

trees– Can produce

firestorm– Move rapidly– Impossible to stop

Results of crown fireCoconino National Forest

Causes of wildfires

• Human caused (85%)– Arson – 26%– Equipment – 10%– Juveniles - 4%– Campfires – 3%– Railroads – 3%– Other/unknown – 50%

• Naturally caused (15%)

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

Weather that produced winds

Extent of Peshtigo fires

Central Chicago following fire

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

Oakland fires in 1991

California fires

• Santa Ana winds– Common October to

March– High pressure over

Nevada– Cool, dry air

descends over mtns– Air heats up,

generating winds

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

• 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

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

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

Rodeo-Chediski Fire of June 2002

Two separate fires grew together;5,000 firefighters

More than 500 homes burned;732 sq mi destroyed

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

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

Schultz Fire north of Flagstaff June 20, 2010 burned 15,000 acres

Extremely high winds rapidly spread fire caused by a campfire

Area north of Flagstaff following 1996 fire

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)

What is there to learn?

• Restrict development in heavily forested areas• Decrease fuel sources with prescribed burns• Undertake preventative measures through

education

A healthy forest