Natural Hazards

WildfiresChapter 13

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 fire

hazard

Know potential adjustments to wildfire hazard

Introduction to Wildfire

Before humans, fires would burn until they ran out of fuel naturally.

Fire initiates new plant growth and clears out dead matter.

Humans introduced “slash and burn” technology for farming, which made weak soils even worse.

Fire is still used in many places, including the United States, to prepare soil for farming.

Wildfire as a Process Self-sustaining, rapid,

high temperature biochemical reaction

Requires Fuel Oxygen Heat

Inserting water into this model helps stop or prevent fire by blocking fuel from “oxidizing”.

water

Three Wildfire Phases: Preignition Fuel achieves temperature and low humidity

favorable to ignition.

Preheating Fuel loses water and other chemical compounds

Pyrolysis Processes that chemically convert hydrocarbons into

fuel Products include volatile gases, mineral ash, tars, etc.

Three Wildfire Phases: Combustion

Begins with ignition – self-sustaining combustion. Pre-ignition 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, drydry enough as well.

Ignition is not a single process but occurs repeatedly as wildfire moves: dryingdrying igniting combusting more dryingdrying.

Wildfire Phases: Flaming combustion

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 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

Transfer occurs as convection and radiation.

Radiation heat increases surface temperature of fuel.

Hot gases are less dense, especially with liquid water converted into vapor - - rising air.

Rising vapor can convert back to liquid with cooling altitude.

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

Point at which combustion ceases.

There is no longer heat and fuel to sustain fire.

Fire Environment: Fuel Leaves, twigs, decaying organic material, grass,

shrubs, etc.

Peat–Unconsolidated deposit of partially decayed wood, leaves, or moss.

Landslides, hurricanes, and tornadoes can arrange debris to facilitate fires.

Organic materials can dry out during droughts to become fuel – or be dried-out by heat-related or be dried-out by heat-related pestspests (pine beetle).

Fire Environment

Fuel moisture content is affected by weather and slope exposure.

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 pre-heat 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 and pre-dry 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

Surface Fires Move along surface Clears-out dead wood, duff, debris May preserve trees and improve habitat and

watershed.

Types of Fires: Crown Fires 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)

The internal convective process creates an ever-larger fire until fuel is exhausted.

This is a “positive” cycle in which more leads to more until fuel runs out.

How is this any different from a tornado or hurricane?

<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 Soil changes

Water-repellent hydrophobic layer – water “beads off” instead of being absorbed.

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

Wildfires cause air pollution and rain

Fire heat evaporates water in plants and soil. Hot, humid air rises decompresses cools condenses and then precipitates that water back to earth.

Smoke, soot, and gases contribute to pollution.

Ground-level ozone may result from oxide gases combining with solar rays.

The 2007 Utah Neola fire exhibited daily cumulo-nimbus clouds overhead as evaporated water condensed and

rained back to earth each afternoon.

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. <drying> Lightning strikes cause ignitions.

North America is infested with ‘pine beetles’ that persist more easily with warming atmosphere.

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 Vegetation

Fire can destroy some vegetation. Weakens others. Some plants use fire to propagate.

Animals Most animals may flee unharmed. Habitats are altered, often for the better in the long run.

Humans Water quality is affected. Smoke and haze produce eye, respiratory, and skin problems. Destroys personal property.

Natural Service Function of Wildfires

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 – mineral ashes become

plant nutrients Animals move more easily More precipitation is absorbed by the soil rather than

evaporated

Minimizing the Wildfire Hazard

Science – learning more about the role of fire in a healthy world ecosystem

Education and regulation – are people the real problem?

Policy and Practice – Is “Smokey the Bear” right – or wrong?

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?

People do not adequately perceive risk of wildfires.

People make demands on government for suppression.

Insurance may give people a false sense of security.

Millions of buildings now exist in fire-prone wild land.

Burning buildings create very toxic fumes from synthetic materials, such as plastics that contain chlorine, cadmium, etc.

Minimizing the Wildfire Hazard

Mapping and data collection

Prescribed burns 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.