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Chapter 13 Wildfires
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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
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Learning Objectives, cont.
Know the methods employed to minimize the fire hazard Know the potential adjustments to the wildfire hazard
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Introduction to Wildfire
Nature’s oldest phenomena, fueled by trees and then by grasses Before humans, fires would burn until they ran out of fuel naturally Initiates plant regrowth, when the cycle restarts Natural fires allowed humans to harness fires for their uses Heat, light, cooking, hunting, etc.
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Wildfire as a Process Self-sustaining, rapid, high-temperature biochemical oxidation reaction Requires: Fuel, oxygen, and heat Essentially, wildfires are a way to remove vegetation Fires reverse the process of photosynthesis Water vapor and carbon dioxide are the most abundant gases released in fire
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Figure 13.2
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Three Wildfire Phases: Preignition
Fuel achieves temperature and water content favorable to ignition Preheating Fuel loses water and other volatile compounds Compounds that are easily vaporized Pyrolysis Processes that chemically degrade fuel Products include volatile gases, mineral ash, tars, etc. Heat, radiating from flames, causes both preheating and pyrolysis in advance of the fire These processes produce the fuel gasses
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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 single process, but occurs repeatedly as wildfire moves
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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 Hindered by blanket of noncombustible material that forms as volatile gases are removed from the fuel Smoldering combustion Takes place at lower temperatures Does not require pyrolysis for growth
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Figure 13.3
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Heat Transfer in Wildfires
Heat 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
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Figure 13.4a, b
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Three Wildfire Processes: Extinction
Point at which combustion ceases There is no longer heat and fuel to sustain fire
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Fire Environment: Fuel
Leaves, twigs, decaying material, grass, shrubs, etc Peat – unconsolidated deposit of partially decayed wood, leaves, or moss Fuel 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
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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
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Figure 13.4c
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Fire Environment: Weather
Fires 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 Wind direction and strength help preheat unburned materials Winds carry embers to ignite spot fires ahead of front
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Types of Fires: Ground Fires
Creep along under ground surface Little flaming, more smoldering Burn in Duff, decaying organic matter in the soil Drained or temporarily dry swamps and marshes Thicker peat deposits below the soil
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Figure 13.6
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Types of Fires: Surface Fires
Move along surface and vary in intensity Burn slowly with smoldering, limited flaming burn grass, shrubs, dead and downed limbs, leaf litter, and other debris Figure 13.7
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Types of Fires: Crown Fires
Flaming is carried via tree canopies Driven by strong winds and steep slopes Figure 13.8
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Geographic Regions at Risk from Wildfires
Areas that are near grasslands, woodlands, shrublands, or tundra are at risk of wildfires during drought Even deserts, rainforests, marsh and swamplands can experience wildfires Large wildfires most common Alaska and the western contiguous states in the United States In the Canadian Rockies Belt that extends from the Yukon Territory southeast to Lake Superior then east to Labrador The wildfire risk area tends to shift from year to year
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Figure 13.9
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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
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Figure 13.10
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Effects of Wildfires on Atmospheric Environment
Wildfires create their own clouds Release smoke, soot, and gases contributing to pollution Contribute to smog formation Formation of ground-level ozone
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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
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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
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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
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Yellowstone Fires of 1988 1976 policy for Yellowstone National Park of allowing natural burns in all wilderness areas Summer 1988 lightning strikes ignited 50 fires in the park In July, fire crews were deployed, but fires were beyond control September rains slow fires and November snow extinguish them Cost $120M to fight fires Yellowstone still has a natural burn policy Fires were beneficial to environment around Yellowstone
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Minimizing the Wildfire Hazard: Fire Management
Task is to decide when fires should be allowed and when suppressed Scientific understanding is critical to fire management Fire regime for an ecosystem Types of fuel available Fire behavior Fire history Education Educating people to reduce their risk
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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
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Perception of the Wildfire Hazard
People do not adequately perceive risk of wildfires Example: California Development on brush-covered hillslopes Demand for hill property increases Insurance may give people false sense of security
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Adjustments to the Wildfire Hazard
Fire Danger Alerts and Warnings Red flag warnings Fire Education Codes and Regulations Fire Insurance Evacuation
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Table 13.1
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End Wildfires Chapter 13
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