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Earth Science - Lecture 36 Notes.docx

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McMaster University
Earth Sciences
Sergei Basik

November 27, 2013 WILDFIRES Fire Process and Behaviour  Wildfires are natural part of forest evolution  Approximately 8,000 wildfires occur each year in Canada.  The average area burned in Canada is 2.5 million ha/year  Benefit ecosystems by: o Thinning forests o Reducing understory fuel o Permitting growth of different species and age groups of trees  Factors that affect wildfire behavior are: o Fuel o Weather o Topography Fuel  Fuel requires three components: o Fuel o Oxygen o Heat  Types of fuel available, its distribution and moisture content determine how quickly fire ignites and spreads  Fuel loading: amount of burnable material  Primary sources are trees and dry vegetation  Burn at high temperature by reaction with oxygen in air  Cellulose: o Compound of carbon, hydrogen and oxygen o Breaks down to carbon dioxide, water and heat  Natural oils and saps add to combustibles  Large relative surface area accelerates ignition and burning  Heavy fuels with small surface area ignite with difficulty and burn slowly  Ladder fuels: low brush and branches that ignite first and allow fire to climb into higher treetops  Crown fire: burns through treetops with incredible speed and heat Ignition and Spreading  Fires can be: o Naturally started by lightning strikes (13%) o Intentionally set for beneficial purposes (24%) o Set accidentally or maliciously (26%)  Fires ignite and progress by: o Radiation o Convection o Firebrands or burning embers
(firebrands carried by wind ignite spot fires) November 27, 2013 Topography  Local topography can: o Funnel air o Accelerate fire o Cause more rapid spreading o Chimney-like funneling effect in canyons and on hillsides  Fires generally move rapidly upslope and slowly downslope Weather Conditions  Fires start more easily and spread rapidly during dry weather with low fuel moisture  Few years of less than normal moisture: dehydrates soil, lowers water table, providing less 
moisture for vegetation  Winds accelerate fires by: o Directing flames at new fuels o Bringing in new oxygen  High winds or extreme amounts of dry fuel can initiate firestorms that generate their own winds 
as convective updraft  Climate change: more frequent and larger wildfires will emit greater amount of carbon 
monoxide and carbon dioxide into atmosphere Secondary Effects of Wildfires  Major fire can lead to other hazards sometimes more disastrous than fire itself (ex. floods and 
landslides)  Organic material burns into hydrocarbon residue that soaks into ground  Water can no longer infiltrate surface soils  Unprotected soils are easily gullied  Flow can turn into flash flood or debris flow  Flood risk remains significantly higher until vegetation is restored (up to 5 years after a 
wildfire) Air Pollution  Major wildfires can spread smoke, soot, ash for 1,000 km or sometimes around the planet  Pollutants include carbon monoxide, hydrocarbons, nitrogen oxides Forest Management Policy  Princip
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