Module 3 - Fire Behavior

L. C. P. B. - Module 3

Fire Behavior

The Fire Triangle

FUEL

HE

AT A

IR

These 3 Components All Must Be Present to Have Fire:

Fire Behaviour

Creeping

Is Described in the Following Terms:

Fire Behaviour

Smouldering

Is Described in the Following Terms:

Fire Behaviour

Running

Is Described in the Following Terms (cont’d):

Fire Behaviour

Torching

Is Described in the Following Terms (cont’d):

Fire Behaviour

Is Described in the Following Terms (cont’d):

Crowning

Fire Behaviour

Is Described in the Following Terms (cont’d):

Spotting

Combustion

Gaseous VapoursReleased

Raised to ignition Temperature

Preheating

Flaming

‘The conversion of living & dead fuels intocarbon dioxide, water vapour & heat energy (flame)’

Three Phases:

Gaseous VapoursIgnited

Combustion

Flaming

Glowing

Three Phases:

Gaseous VapoursIgnited

Charcoal or solidphase

Heat Transfer‘Heat energy is transmitted from burning to

unburned fuels’

Convection

Movement ofmasses ofhot air (can

cause spot fires)

Heat Transfer

‘Heat energy is transmitted from burning to unburned fuels’

Conduction

Radiation

Throughsolid matter

In straightlines from

warm surfacesto coolersurfaces

Fuels

Quantity Amount of fuel available

Type Association of fuels that are distinctive (species, form, size, arrangement and continuity).

Size Fine, medium, coarse

Arrangement Horizontal and vertical distribution of all combustible materials within a particular fuel type

‘These fuel characteristics can effect fire behaviour’…..

Fuels

Distribution Continuity of fuels over an area

Fuel Moisture Content The amount of fuel available for combustion. The lower the moisture content, the greater the amount of fuel available.

‘These fuel characteristics can effect fire behaviour’…..

Crown – ‘standing & supported forest combustibles not in direct contact with surface fuels’

Crown

Surface

Sub-Surface Duff layer

LadderFuel

Fuel Quantity/Type

Surface - ‘all combustible material lying above the duff layer between the ground and ladder fuels’

Crown

Surface

Sub-Surface Duff layer

LadderFuel

Fuel Quantity/Type

Subsurface - ‘all combustible material below the litter (duff) layer of the forest floor (e.g., roots, punky wood and peat)’

Crown

Surface

Sub-Surface Duff layer

LadderFuel

Fuel Quantity/Type

Fuel Type

Distinctive species Ex: pure stand of jack pine

Form

Size Ex: immature, mature

Arrangement Ex: horizontal or vertical

Distribution (continuity) Ex: continuous or broken (by barriers)

The association of all the elements of a particular fuel:

Fuel Size

Ignite readily

Consumed rapidly

Cured grasses, fallen needles, leaves and small twigs

Fine Fuels

Fuel Size

Too large to be ignited until after the leading edge of the fire front passes

Small enough to be completely consumed

Medium Fuels

Fuel Size

Large diameter woody or deep organic materials

Difficult to ignite

Burn more slowly than fine or medium fuels

Coarse Fuels

Fuel Arrangement‘Horizontal & vertical distribution within aparticular fuel type’

Laddered FuelsVertical Continuity betweensurface fuels and crown fuels

SlashDebris left as a result of forestry practices

Fuel Arrangement

‘When considering fuel arrangement, there are 2 situations to be aware of’ ….

SlashDebris left as a result of forestry practices

Intensity!

Laddered FuelsVertical Continuity betweensurface fuels and crown fuels

Spread!

Fuel Distribution

‘Continuity of fuels over an area’

Continuity can be broken by a natural or constructed barrier or a different fuel type

3 Types of Fires

Crown

Surface

Sub-Surface Duff layer

CombustionFlames:

FlameLength

FlameHeight

Flame Depth

‘The visible bi-products of combustion’

Duff Layer

Mineral Soil

Depth of Burn

Direction of spread

Fire Intensity

FlameLength

• Flame length is the main visual indicator of fire intensity

• 1.4 metres is approx. the upper limit where firefighters can work directly at head or flanks • The longer the flame length, the greater the intensity

‘The amount of heat energy released from the fire’

Parts of a Fire

Spot

Bay

Head

Back

Flank

Flank

Finger

Fire Perimeter

Effects of Weather

• Relative Humidity

• Precipitation

• Temperature

• Wind (direction & speed)

Factors to consider:

Effects of Weather (cont’d)

• Defined as the amount of moisture in the air at the prevailing temperature

• The amount of moisture directly affects the moisture content of the forest fuels

• All other factors being equal, a RH of 40% or less usually means fires will burn quite rapidly

• When the RH is greater than 65%, it usually means fires will burn slowly, if at all

Relative Humidity

Effects of Weather (cont’d)

• Defined as the presence of rain, hail or dew

• Influences the moisture content of forest fuels and impacts the RH in a given area

• Accumulated rain is measured in millimetres (25 mm = 1 inch)

Precipitation

Effects of Weather (cont’d)

Defined as the degree of hotness or coldness of a substance

High temperature contributes to the drying of forest fuels

When temperature is increased, less heat from the fire is required during pre-heating (pyrolisis) stage

Temperature

Effects of Weather (cont’d)

Defined as the natural movement of air parallel to the earth’s surface

Can influence fire behaviour by:

– moving moist air away from or over fuels

– blowing burning embers outside the fire perimeter

– carrying burning embers that have been lifted above the ground by convection (may start new fires some distance away)

– bending convection columns closer to unburned fuels which will pre-heat them

– bringing a continuous flow of oxygen to the fire

Wind

Effects of Topography

SOUTH NORTH

SLOPE – Upward or downward slant of the earth’s surface

ASPECT – Direction the slope is facing

This slope has a ‘Southern Aspect’

‘Topography is a description of the physical features of the earth’s surface’

Effects of Topography (cont’d)

Convection Columnis far from unburnedfuels. Convection Column

touches unburneduphill fuels. Radiantheat from flame frontpreheats uphill fuels.

Wind

Effects of Topography (cont’d)Physical Barriers - ridges, escarpments,hills, mountains, etc.

Can have a profound effect on local wind speed and direction

Firefighters must deal with local winds

The larger the barrier, the more significant the impact

Effects of physical barriers on local winds are described as: – barrier effects – channeling – funneling

Can result in upslope winds or downslope winds (dependent on time of day)

Effects of Topography (cont’d)

Barrier Effects

• Air can be deflected either along or over barrier

• On windward side, effect can result in change of direction with flow more parallel to barrier

• On leeward side, air that spills over top is more turbulent, gusty, potential for upslope winds

Leeward side

Windward side

Effects of Topography (cont’d)

Channeling

• Air motion that enters a valley will often change direction

• Will flow along the contour of a valley

• Can occur along a river (especially if banks are steep).

Effects of Topography (cont’d)Funneling

• Air that encounters a ridge with a saddleback or mountain pass may funnel through the pass or pass over top of the ridge

• Can lead to locally high wind speeds in the pass or gusty winds on leeward side

Effects of Topography (cont’d)

Upslope WindCOOL AIR

WARM AIR

• During the day, a layer of air in contact with a slope becomes warmer and lighter than the surrounding air at the same level

• Creates rising air on the face of the slope

• Air accelerates as it moves upward

• Can increase fire behaviour into the extreme category (especially at the top)