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?