natural hazards wildfires
TRANSCRIPT
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Wildfires
David Alexander University College London
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Forest and rangeland fires (wildfires) are a source of important
ecological and economic damage and risk to human lives and settlements.
A wildfire burns out of control and threatens people, buildings or resources.
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Synonyms:-
• bushfire (Australia)
• forest fire, range fire (North America)
• environmental fire
• wildfire.
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Phases of wildfire:-
• confined - limited to a predetermined area using natural and man-made barriers
• contained - surrounded by a control line so that it cannot spread further
• under control - completely encircled by the control line and burning in predictable ways.
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The causes of wildfires:-
• lightning strikes (a natural cause of varied importance)
• human negligence and vandalism (greatest at the urban-rural interface)
• vegetative fermentation leading to spontaneous combustion are possible but not probable causes of wildfires.
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Dendrochronology: the frequency of wildfires in the past is visible in damage to the xylem of the tree, which can be
dated by counting the annual rings.
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Model of the acceleration of a wildfire in an open environment
Time (minutes)
Prop
ortion
of
are
a b
urnt
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Map of the evolution of a wildfire in the USA
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Are
a inv
olve
d
Maximum spread of fire
Peak of fire
Dying out
Dying out
Extinct
Flash over
Latent
Start of fire
Accele- rated
diffusion
Main fire
phase
Time
Emission of smoke and hot gases
Beginning
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The dimensions, duration and intensity of wildfires varies according to:-
• the type, inflammability and water content of wood: the loading factor for combustible material
• meteorological conditions (temperature, humidity)
• the speed, direction and variability of winds
• topography and ground slope in the local area.
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The essential ingredients of a wildfire are:-
• combustible material
• heat
• oxygen.
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Classification of wildfires:-
• ground fire: combustion without flames in the roots and undergrowth
• surface fire: in rangelands, undergrowth, etc.
• spot fire: localised, concentrated
• dependent crown fire: the crowns of trees catch fire but the fire is propagated in the undergrowth.
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(a) Ground fire (c) Dependent crown fire
(d) Running crown fire
(b) Surface fire
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More elements of the classification:-
• running crown fire: the fire is propagated through the tree crowns
• a “backfire” can occur if the wind changes direction and the combustible material has not been completely consumed
A wildfire can climb a slope propelled by wind convection
and by heat radiation.
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Convective wildfire which climbs a slope propelled by wind turbulence
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Convective wildfire that climbs a slope
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Ground fire
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Running crown fire
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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.
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Some environmental effects of wildfires:-
.... on vegetation:
• can stimulate the growth of rangelands and meadows
• can encourage the germination of seeds
• can stimulate flowering of bushes and plants
• but can restrict the growth of woody plants.
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.... on soil:
• they can purify soil by destroying toxins, fungi and parasites
• but can accelerate erosion of the soil
.... on birds:
• can destroy nests and cause fragmentation of habitats
Ecological environments created or maintained by wildfire show
the greatest diversity of species.
Environmental effects of wildfires:-
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Australia has an extremely high wildfire risk: 40% of the nation contains adequate combustible material, the climate is dry,
temperatures are high and damaging fires are common.
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Mediterranean maquis (chaparral in California) is highly inflammable
Mediterranean vegetation gives rise to very hot fires (540-1100 degrees C),
fed by dry bushes that are full of highly inflammable oils.
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The elimination of Mediterranean vegetation by wildfire can:-
• impermeabilise surfaces by depositing natural polymers (hydrophobic residues)
• cause surface slides of dry ravel
• cause mudflows and debris flows
• contribute to the formation of gullies.
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Mitigation of wildfire:-
• controlled fire - can reduce fuel loading levels, but it remains controversial and can cause damage
• total elimination of wildfires allows massive accumulations of inflammable material and therefore leads to much bigger and hotter fires that spread more rapidly.
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Monitoring of wildfire risk:-
• lookout towers and field patrols
• aerial surveillance
• infrared monitoring from the ground, the air and satellites.
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INDEX OF FUEL AVAILABILITY
ALGORITHM USED TO CALCULATE AN INDEX OF FUEL AVAILABILITY
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Wildfire fighting:-
• one needs to reduce the supply of heat, oxygen or fuel to the fire
The attack:
• train personnel
• use parachute brigades in areas that are difficult to reach
• construct corridors of low fuel loading.
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The attack:
• dump water or flame retardant chemicals from aircraft
• Canadair CL-215 "Super Scooper" aircraft are able to take up 6,000 litres of water in 12 seconds
• drop water on flames from helicopters.
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Bell-412 helicopters
can transport 1,400 litres
of water.
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C-130 aeroplanes can carry 11,000 litres of flame-retardant chemicals
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California Firescope, a centralised system of fire fighting:-
• an Incident Command System (ICS)
• a coordination system for various agencies that work in the field (Multi-Agency Coordinating System, MACS) -- for 28 agencies
• an Operations Coordination Center (OCC) for field attacks.
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Average annual number of forest fires in 12 U.S. states
Total number Caused by lightning
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California Firescope includes an information system and data bank, which covers:-
• weather forecasting
• infrared surveillance of areas at greatest risk of wildfires
• immediate reports on the state of fires and suppression activities
• advice on the best tactics to adopt for fighting fires.
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