Natural Hazards Wildfires Chapter 12. Learning Objectives Understand wildfire as a natural process...
-
Upload
percival-donald-jordan -
Category
Documents
-
view
217 -
download
2
Transcript of Natural Hazards Wildfires Chapter 12. Learning Objectives Understand wildfire as a natural process...
Natural Hazards
WildfiresChapter 12
Learning Objectives Understand wildfire as a natural process that
becomes a hazard when people live in or near wildlands
Understand the effects of fires
Know how wildfires are linked to other natural hazards
Know potential benefits provided by wildfires
Learning Objectives, cont. Know the methods employed to minimize the fire
hazard
Know the potential adjustments to the wildfire hazard
Introduction to Wildfire Nature’s oldest phenomena.
Before humans, fires would burn until they ran out of fuel naturally.
Initiates plant re-growth, when the cycle restarts.
Natural fires allowed humans to harness fires for their uses. Heat, light, cooking, hunting, etc.
Wildfire as a Process Self-sustaining, rapid,
high temperature biochemical reaction
Requires Fuel Oxygen Heat
Three Wildfire Phases: Preignition Fuel achieves temperature and humidity
favorable to ignition.
Preheating Fuel loses water and other chemical compounds
Pyrolysis Processes that chemically degrade fuel Products include volatile gases, mineral ash, tars, etc.
These processes produce the fuel gases.
Three Wildfire Phases: Combustion Begins with ignition.
Preignition absorbs energy, combustion releases energy.
External reactions liberate heat and light. Lightning, volcanic activity, and human action.
Ignition doesn’t always lead to wildfires. Sufficient fuel must be present.
Ignition is not a single process but occurs repeatedly as wildfire moves.
Three Wildfire Phases: Combustion, cont. Flaming combustion
Dominates early fire Rapid high temperature conversion of fuel into heat Characterized by flames and large amount of unburned
material
Smoldering combustion Takes place at lower temperatures Does not require pyrolysis for growth
How does this process compare to
an ordinary thunderstorm?
Convection in Wildfires Transfer occurs as convection and radiation.
Radiation heat increases surface temperature of fuel.
Gases become less dense and rise.
Rising gases remove heat and combustion products from zone of flaming.
Pulls in fresh air to sustain combustion.
Three Wildfire Processes: Extinction
Point at which combustion ceases.
There is no longer heat and fuel to sustain fire.
Fire Environment: Fuel Leaves, twigs, decaying material, grass, shrubs,
etc.
Peat–Unconsolidated deposit of partially decayed wood, leaves, or moss.
Size affects ignition and movement.
Landslides, hurricanes, and tornadoes can arrange debris to facilitate fires.
Organic materials can dry out during droughts to become fuel.
Fire Environment: Topography Fuel moisture content is affected by location.
Drier fuels are found. On south-facing slopes in Northern Hemisphere Slopes exposed to prevailing winds
Mountainous areas circulate winds up canyons during daytime.
Wildfires preheat fuels upslope, making it easier to spread.
Wild Fire Environment: Weather
Wild fires are common following droughts. Can bring “dry thunderstorms” with lightning to start fires, but
rain evaporates and can’t extinguish them
Fires burn more when humidity is lowest.
What happens to fire conditions: 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.
Wind direction and strength help preheat unburned materials.
Winds carry embers to ignite spot fires ahead of front.
Types of Fires: Ground Fires
Creep along under ground surface Little flaming, more smoldering
Types of Fires: Surface Fires Move along surface Vary in intensity
Types of Fires: Crown Fires 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 Soil changes
Water-repellent hydrophobic layer Increases runoff and erosion and flood events
Soil erosion and landslides Removal of anchoring vegetation on steep slopes Precipitation often exaggerates the effect of fires on
landslides
Effects of Wildfires on Atmospheric Environment Create their own clouds <evaporated water
condenses back into clouds and precipitation>
Release smoke, soot, and gases contributing to pollution
Contribute to smog formation Formation of ground-level ozone
Linkages of Wildfires with Climate Change Climate change increases intensity and
frequency of wildfires.
Caused by changes in temperature, precipitation, and the frequency and intensity of severe storms. Increases in temperature, decreases in humidity. Grasslands replacing forests creating more fuel. Lightning strikes increase ignitions. Insect infestations make trees more vulnerable to fire.
North America is infested with ‘pine beetles’ that persist more easily with warming atmosphere.
Effects of Wildfires on Biological Environment Vegetation
Fire can destroy some vegetation. Weakens others. Some plants use fire to propagate.
Animals Most animals may flee unharmed. Habitats are altered.
Humans Water quality is affected. Smoke and haze produce eye, respiratory, and skin problems. Destroys personal property.
Natural Service Function of Wildfires Benefits to soil
Increases nutrient content Reduce populations of microorganisms
Benefits to plants and animals 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 Task is decide when fires should be allowed and
when suppressed
Science Fire regime for site
Types of fuel available Fire behavior Fire history
Education Educating people to reduce their risk
Minimizing the Wildfire Hazard: Fire Management, cont.
Data collection Mapping vegetation and potential fuel Moisture content FPI (Fire Potential Index) maps
Prescribed burns Controlled burns to manage forests Reduces fuel for more catastrophic fires Necessary to predict the behavior of the fire and control
it
People do not adequately perceive risk of wildfires. People make demands on government for
suppression.
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?