natural hazards wildfires

Wildfires

David Alexander University College London

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.

Synonyms:-

• bushfire (Australia)

• forest fire, range fire (North America)

• environmental fire

• wildfire.

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.

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.

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.

Model of the acceleration of a wildfire in an open environment

Time (minutes)

Prop

ortion

of

are

a b

urnt

Map of the evolution of a wildfire in the USA

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

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.

The essential ingredients of a wildfire are:-

• combustible material

• heat

• oxygen.

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.

(a) Ground fire (c) Dependent crown fire

(d) Running crown fire

(b) Surface fire

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.

Convective wildfire which climbs a slope propelled by wind turbulence

Convective wildfire that climbs a slope

Ground fire

Running crown fire

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 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.

.... 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:-

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.

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.

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.

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.

Monitoring of wildfire risk:-

• lookout towers and field patrols

• aerial surveillance

• infrared monitoring from the ground, the air and satellites.

INDEX OF FUEL AVAILABILITY

ALGORITHM USED TO CALCULATE AN INDEX OF FUEL AVAILABILITY

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.

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.

Bell-412 helicopters

can transport 1,400 litres

of water.

C-130 aeroplanes can carry 11,000 litres of flame-retardant chemicals

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.

Average annual number of forest fires in 12 U.S. states

Total number Caused by lightning

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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