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RX100 Module 3 - Fire Behaviour
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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)