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Natural Hazards
Wildfires
Chapter 12
Learning Objectives
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Understand wildfire as a natural process that becomes a
hazard when people live in or near wildlands
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Understand the effects of fires
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Know how wildfires are linked to other natural hazards
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Know potential benefits provided by wildfires
Learning Objectives, cont.
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Know the methods employed to minimize the fire hazard
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Know the potential adjustments to the wildfire hazard
Introduction to Wildfire
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Nature’s oldest phenomena.
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Before humans, fires would burn until they ran out of
fuel naturally.
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Initiates plant re-growth, when the cycle restarts.
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Natural fires allowed humans to harness fires for their
uses.
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Heat, light, cooking, hunting, etc.
Wildfire as a Process
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Self-sustaining, rapid,
high temperature
biochemical reaction
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Requires
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Fuel
Oxygen
Heat
Three Wildfire Phases:
Preignition
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Fuel achieves temperature and humidity favorable to
ignition.
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Preheating
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Pyrolysis
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Fuel loses water and other chemical compounds
Processes that chemically degrade fuel
Products include volatile gases, mineral ash, tars, etc.
These processes produce the fuel gases.
Three Wildfire Phases:
Combustion
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Begins with ignition.
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External reactions liberate heat and light.
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Lightning, volcanic activity, and human action.
Ignition doesn’t always lead to wildfires.
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Preignition absorbs energy, combustion releases energy.
Sufficient fuel must be present.
Ignition is not a single process but occurs repeatedly as
wildfire moves.
Three Wildfire Phases:
Combustion, cont.
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Flaming combustion
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Dominates early fire
Rapid high temperature conversion of fuel into heat
Characterized by flames and large amount of unburned material
Smoldering combustion
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Takes place at lower temperatures
Does not require pyrolysis for growth
How does this
process compare to
an ordinary
thunderstorm?
Convection in Wildfires
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Transfer occurs as convection and radiation.
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Radiation heat increases surface temperature of fuel.
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Gases become less dense and rise.
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Rising gases remove heat and combustion products from
zone of flaming.
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Pulls in fresh air to sustain combustion.
Three Wildfire Processes:
Extinction
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Point at which combustion ceases.
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There is no longer heat and fuel to sustain fire.
Fire Environment: Fuel
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Leaves, twigs, decaying material, grass, shrubs, etc.
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Peat–Unconsolidated deposit of partially decayed wood,
leaves, or moss.
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Size affects ignition and movement.
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Landslides, hurricanes, and tornadoes can arrange
debris to facilitate fires.
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Organic materials can dry out during droughts to become
fuel.
Fire Environment:
Topography
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Fuel moisture content is affected by location.
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Drier fuels are found.
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On south-facing slopes in Northern Hemisphere
Slopes exposed to prevailing winds
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Mountainous areas circulate winds up canyons during
daytime.
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Wildfires preheat fuels upslope, making it easier to
spread.
Wild Fire Environment: Weather
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Wild fires are common following droughts.
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Can bring “dry thunderstorms” with lightning to start fires, but rain
evaporates and can’t extinguish them
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Fires burn more when humidity is lowest.
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What happens to fire conditions:
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At night?
During the day?
<Humidity goes UP at night
when air temperature is lower>
Salt Lake billboards say that
burning embers can travel
up to a mile.
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Wind direction and strength help preheat
unburned materials.
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Winds carry embers to ignite spot fires
ahead of front.
Types of Fires: Ground Fires
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Creep along under ground surface
Little flaming, more smoldering
Types of Fires: Surface Fires
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Move along surface
Vary in intensity
Types of Fires: Crown Fires
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Flaming is carried via tree canopies.
Driven by strong winds and steep slopes.
Can destroy the entire forest
Crown Fires can be compared to
other ‘conflagrations’ and ‘fire
storms’
Wind speeds can rival aerial
bombing and even atomic
weapons.
<Dresden, Tokyo, Hiroshima,
Liverpool>
Geographic Regions at Risk
from Wildfires
Is there a
correlation
between wildfires
and dry climates?
Dry country may
grow less ‘fuel’
but the fuel
ignites more
easily.
What are the
implications for
global warming?
Effects of Wildfires on
Geologic Environment
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Soil changes
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Water-repellent hydrophobic layer
Increases runoff and erosion and flood events
Soil erosion and landslides
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Removal of anchoring vegetation on steep slopes
Precipitation often exaggerates the effect of fires on landslides
Effects of Wildfires on
Atmospheric Environment
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Create their own clouds <evaporated water condenses
back into clouds and precipitation>
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Release smoke, soot, and gases contributing to pollution
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Contribute to smog formation
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Formation of ground-level ozone
Linkages of Wildfires with
Climate Change
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Climate change increases intensity and frequency of
wildfires.
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Caused by changes in temperature, precipitation, and the
frequency and intensity of severe storms.
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Increases in temperature, decreases in humidity.
Grasslands replacing forests creating more fuel.
Lightning strikes increase ignitions.
Insect infestations make trees more vulnerable to fire.
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North America is infested with ‘pine beetles’ that
persist more easily with warming atmosphere.
Effects of Wildfires on
Biological Environment
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Vegetation
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Animals
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Fire can destroy some vegetation.
Weakens others.
Some plants use fire to propagate.
Most animals may flee unharmed.
Habitats are altered.
Humans
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Water quality is affected.
Smoke and haze produce eye, respiratory, and skin problems.
Destroys personal property.
Natural Service Function of
Wildfires
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Benefits to soil
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Increases nutrient content
Reduce populations of microorganisms
Benefits to plants and animals
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Reduces the number of species of plants
May trigger a release of seeds in some species
Removes surface litter for grasses
Recycles nutrients in system
Animals benefit from increased plant life
Minimizing the Wildfire
Hazard: Fire Management
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Task is decide when fires should be allowed and when
suppressed
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Science
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Fire regime for site
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Types of fuel available
Fire behavior
Fire history
Education
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Educating people to reduce their risk
Minimizing the Wildfire Hazard:
Fire Management, cont.
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Data collection
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Mapping vegetation and potential fuel
Moisture content
FPI (Fire Potential Index) maps
Prescribed burns
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Controlled burns to manage forests
Reduces fuel for more catastrophic fires
Necessary to predict the behavior of the fire and control it
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People do not adequately perceive risk of wildfires.
People make demands on government for suppression.
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Development on brush-covered slopes
Insurance may give people false sense of security
Millions of buildings now exist in areas that are prone to wildfires.
Do fire prevention efforts just lead to bigger fires?
What are air quality factors when buildings burn?
Do government agencies make money on leasing land for buildings?
Might they need that money for fire supression?