Transcript Natural Hazards

Natural Hazards
Wildfires
Chapter 13
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 fire hazard
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Know potential adjustments to wildfire hazard
Introduction to Wildfire
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Before humans, fires would burn until they ran out of
fuel naturally.
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Fire initiates new plant growth and clears out dead
matter.
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Humans introduced “slash and burn” technology for
farming, which made weak soils even worse.
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Fire is still used in many places, including the United
States, to prepare soil for farming.
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
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Inserting water into this model
helps stop or prevent fire by
blocking fuel from “oxidizing”.
water
Three Wildfire Phases:
Preignition
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Fuel achieves temperature and low humidity favorable
to ignition.
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Preheating
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Fuel loses water and other chemical compounds
Pyrolysis
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Processes that chemically convert hydrocarbons into fuel
Products include volatile gases, mineral ash, tars, etc.
Three Wildfire Phases:
Combustion
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Begins with ignition – self-sustaining combustion.
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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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Pre-ignition absorbs energy, combustion releases energy.
Sufficient fuel must be present, dry enough as well.
Ignition is not a single process but occurs repeatedly as wildfire
moves: drying  igniting  combusting  more drying.
Wildfire Phases:
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Flaming combustion
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Dominates early fire
Rapid high temperature conversion of fuel into heat  heat is
needed to dry and ignite more fuel.
Characterized by flames and large amount of unburned material
Smoldering combustion
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Takes place at lower temperatures
Large amounts of fuel not fully combusted.
The key is conversion of hydro-carbon (organic) material into
fuel  combustion -- the result is conversion of organics into
in-organic minerals.
Exhaustion of fuel results in decline of fire.
How does this
process compare to
an ordinary
thunderstorm?
Heat, buoyancy,
rising air, low
atmospheric
pressure, water
vapor released from
organic material.
Convective
rising of humid,
hot air
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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Hot gases are less dense, especially with liquid water
converted into vapor - - rising air.
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Rising vapor can convert back to liquid with cooling
altitude.
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Rising air pulls in fresh air to sustain combustion.
Heat transfer by
radiation can be
enough to start the
combustion
process.
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 organic material, grass, shrubs,
etc.
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Peat–Unconsolidated deposit of partially decayed wood,
leaves, or moss.
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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 – or be dried-out by heat-related pests (pine beetle).
Fire Environment
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Fuel moisture content is affected by weather and slope
exposure.
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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 pre-heat 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
and pre-dry 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
Surface Fires
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Move along surface
Clears-out dead wood, duff, debris
May preserve trees and improve habitat and watershed.
Types of Fires: Crown Fires
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Flame 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>
Crown Fires (and conflagrations, infernos, fire storms)
How is this any different
from a tornado or
hurricane?
The internal convective
process creates an everlarger fire until fuel is
exhausted.
This is a “positive” cycle
in which more leads to
more until fuel runs out.
<both accelerate with
additional “fuel”>
Severe weather relies on
energy in humidity
Severe fires require
hydrocarbon fuel.
In fact, whirlwinds or vortices of
spiral-rising air are sometimes seen
in the most severe wildfires.
As flames heat the air, evaporation
occurs, drawing lighter, more
buoyant air aloft, encouraging yet
more in-flow of surface oxygen for
combustion.
Western states experience
more wild fires
There is a
correlation
between wildfires
and dry climates?
Dry country may
grow less ‘fuel’
but the fuel
ignites more
easily.
How is this
hazard related to
climate change?
Western USA
Effects of Wildfires on
Geologic Environment
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Soil changes
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Water-repellent hydrophobic layer – water “beads off” instead of
being absorbed.
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Increases runoff and erosion and flood events
Soil erosion and landslides
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Removal of anchoring vegetation on steep slopes
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Precipitation often exaggerates the effect of fires on landslides
Wildfires cause air pollution
and rain
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Fire heat evaporates water in plants and soil. Hot,
humid air rises  decompresses  cools  condenses
and then precipitates that water back to earth.
The 2007 Utah Neola fire exhibited daily cumulo-nimbus
clouds overhead as evaporated water condensed and
rained back to earth each afternoon.
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Smoke, soot, and gases contribute to pollution.
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Ground-level ozone may result from oxide gases
combining with solar rays.
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.
 Increases in temperature, decreases in humidity. <drying>
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Lightning strikes cause ignitions.
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North America is infested with ‘pine beetles’ that
persist more easily with warming atmosphere.
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Once again, the 2007 Utah Neola fire is a good
example – firefighters finally gave up
– “Kill ‘em all” <they said>
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, often for the better in the long run.
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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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 – mineral ashes become plant
nutrients
Animals move more easily
More precipitation is absorbed by the soil rather than evaporated
Minimizing the Wildfire Hazard
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Science – learning more about the role of fire in a
healthy world ecosystem
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Education and regulation – are people the real problem?
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Policy and Practice – Is “Smokey the Bear” right – or
wrong?
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Does it really depend on the situation?
Meanwhile,
is there a better way to keep state and federal agencies
funded without having to earn revenue by filling the wild
with buildings and people who need rescue?
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People do not adequately perceive risk of wildfires.
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People make demands on government for suppression.
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Insurance may give people a false sense of security.
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Millions of buildings now exist in fire-prone wild land.
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Burning buildings create very toxic fumes from synthetic
materials, such as plastics that contain chlorine,
cadmium, etc.
Minimizing the Wildfire Hazard
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Mapping and data collection
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Prescribed burns
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Controlled burns to manage forests
Reduces fuel to prevent more catastrophic fires
Utah’s Governor became quite angry in 2012 when a
prescribed burn got out of control and consumed
several expensive homes in Southern Utah.
Fourteen Utah homes were burned in 2013 due to
wildfire.
Mr. Allred managed a State of Utah research project into
machinery for conversion of cellulosic forest waste into
methanol vehicle fuel.
Using that methanol for vehicles that collect and process
more forest waste is a self-sustaining process that
produces little pollution.
Grooming forests to prevent periodic fires is better than
allowing fires that may get out of control, create air
pollution and endanger people and property.
Unfortunately, turning forest waste into methanol fuel is
more difficult than making ethanol fuel with corn.