WildfiresDavid AlexanderUniversity College London
Forest and rangeland fires (wildfires)are a source of importantecological and economic damage andrisk to human lives and settlements.A wildfire burns out of control andthreatens people, buildings or resources.
Synonyms:-• bushfire (Australia)• forest fire, range fire (North America)• environmental fire• wildfire.
Phases of wildfire:-• confined - limited to apredetermined area usingnatural and man-made barriers• contained - surrounded by a controlline so that it cannot spread further• under control - completelyencircled by the control lineand burning in predictable ways.
The causes of wildfires:-• lightning strikes (a naturalcause of varied importance)• human negligence and vandalism(greatest at theurban-rural interface)• vegetative fermentation leading tospontaneous combustion are possiblebut not probable causes of wildfires.
Dendrochronology: the frequency ofwildfires in the past is visible in damageto the xylem of the tree, which can bedated by counting the annual rings.
Model of the acceleration of a wildfirein an open environmentTime (minutes)Proportionofareaburnt
The dimensions, duration and intensityof wildfires varies according to:-• the type, inflammability and watercontent of wood: the loading factorfor combustible material• meteorological conditions(temperature, humidity)• the speed, direction andvariability of winds• topography and groundslope in the local area.
The essential ingredientsof a wildfire are:-• combustible material• heat• oxygen.
Classification of wildfires:-• ground fire: combustion without flamesin the roots and undergrowth• surface fire: in rangelands,undergrowth, etc.• spot fire: localised, concentrated• dependent crown fire: the crowns oftrees catch fire but the fire ispropagated in the undergrowth.
(a) Ground fire (c) Dependent crown fire(d) Running crown fire(b) Surface fire
More elements of the classification:-• running crown fire: the fire ispropagated through the tree crowns• a “backfire” can occur if thewind changes direction and thecombustible material has notbeen completely consumedA wildfire can climb a slopepropelled by wind convectionand by heat radiation.
Convective wildfire which climbsa slope propelled by wind turbulence
Wildfire fuel can be classified as:-• aerial material: branches, leaves, moss• surface material:low-level vegetation, wood,leaves, grass, detached branches• buried and composting materials• mineral soil and roots.
Some environmental effectsof wildfires:-.... on vegetation:• can stimulate the growthof rangelands and meadows• can encourage the germination of seeds• can stimulate floweringof bushes and plants• but can restrict thegrowth of woody plants.
.... on soil:• they can purify soil by destroyingtoxins, fungi and parasites• but can accelerate erosion of the soil.... on birds:• can destroy nests and causefragmentation of habitatsEcological environments createdor maintained by wildfire showthe greatest diversity of species.Environmental effects of wildfires:-
Australia has an extremely high wildfirerisk: 40% of the nation contains adequatecombustible material, the climate is dry,temperatures are high and damagingfires are common.
Mediterranean maquis (chaparral inCalifornia) is highly inflammableMediterranean vegetation gives rise tovery hot fires (540-1100 degrees C),fed by dry bushes that arefull of highly inflammable oils.
The elimination of Mediterraneanvegetation by wildfire can:-• impermeabilise surfaces by depositingnatural polymers (hydrophobic residues)• cause surface slides of dry ravel• cause mudflows and debris flows• contribute to the formation of gullies.
Mitigation of wildfire:-• controlled fire - can reduce fuelloading levels, but it remainscontroversial and can cause damage• total elimination of wildfires allowsmassive accumulations of inflammablematerial and therefore leadsto much bigger and hotterfires that spread more rapidly.
Monitoring of wildfire risk:-• lookout towers and field patrols• aerial surveillance• infrared monitoring from theground, the air and satellites.
INDEX OF FUEL AVAILABILITYALGORITHM USED TOCALCULATE AN INDEXOF FUEL AVAILABILITY
Wildfire fighting:-• one needs to reduce the supply ofheat, oxygen or fuel to the fireThe attack:• train personnel• use parachute brigades in areasthat are difficult to reach• construct corridors of low fuel loading.
The attack:• dump water or flame retardantchemicals from aircraft• Canadair CL-215 "Super Scooper"aircraft are able to take up6,000 litres of water in 12 seconds• drop water on flames from helicopters.
C-130 aeroplanes can carry 11,000 litresof flame-retardant chemicals
California Firescope, a centralisedsystem of fire fighting:-• an Incident Command System (ICS)• a coordination system for variousagencies that work in the field(Multi-Agency Coordinating System,MACS) -- for 28 agencies• an Operations CoordinationCenter (OCC) for field attacks.
Average annual number of forest firesin 12 U.S. statesTotal numberCaused by lightning
California Firescope includes an informationsystem and data bank, which covers:-• weather forecasting• infrared surveillance of areasat greatest risk of wildfires• immediate reports on the stateof fires and suppression activities• advice on the best tacticsto adopt for fighting fires.[X]