What is fire intensity | Char height | Ignition
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Intense fires behave quite differently from less
intense "cool" fires
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Fire behaviour describes the physical attributes of individual
fires — the height and depth of the flames, the speed with
which the fire moves, the size and shape of the various fronts, and
the intensity of the fire. Many of these attributes can be inferred
from post-fire features of the burnt landscape, such as the height
of blackened or scorched leaves, the size of standing twigs
consumed by the flames, and the degree of fuel consumption, i.e.
fire patchiness. Knowledge about fire behaviour is important to
understand the likely extent and effectiveness of fires — be
they wanted or unwanted — that may affect our patch of land,
and so that we can understand the effects of fires on the landscape
and atmosphere.
One measure of fire behaviour is 'fire intensity'. This
represents the rate at which energy is released, and is measured as
kilowatts (a unit of work) per metre of fire front. It is a
function of the heat yield of the fuel (heat per unit mass burnt as
kilojoules/kg), the amount of fuel per unit area (kilogram per
metre squared: kg/m2) and the rate of forward spread of
the fire front (m/second).
These three components of fire intensity can be measured or
estimated.
Fuel loads can be determined by direct harvest, by using
indirect pasture-estimation techniques such as BOTANAL or by using
calibrated, photo-graphic fuel standards. It is important to
estimate the degree of fuel consumption, i.e. the proportion of the
available fuel that will be burnt. Heat yield is often assumed to
be 20,000 kJ/kg for mixed fuel types. However, the heat content of
various fuel components varies between species.
A low intensity fire mostly less than
1000kW/m
A higher intensity fire
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Rate of spread is the hardest component to measure, but it can
be estimated if the time to arrival of the flame front at three or
more points can be determined accurately (within one second). The
most important determinants of rate of spread are wind speed,
relative humidity and fuel moisture. These factors vary throughout
the day and seasonally, from early dry season to late dry season.
Rate of spread—and hence intensity—can therefore be
manipulated by careful consideration of ignition time—both
during the day (or night) and from month to month.
Savanna fires generally move at speeds of 0.1–2 metres per
second (m/s). Fuels loads are generally in the range of 2–8
tonnes per hectare (t/ha), with fine fuel consumption rates of
50–100%. Fire intensities in general range from 500 to 10,000
kiloWatts per metre (W/m), and rarely exceed 20,000 kW/m. In
southern Australian eucalypt forests where fuel has accumulated to
near maximum levels (in excess of 30 t/ha), fire intensities can be
as high as 50,000– 100,000 kW/m.
Over a five-year period at Kapalga in Kakadu National Park,
early dry season fires (lit in early June) averaged about 2000 kW/m
whereas late dry season fires (lit in late September) averaged
about 8000 kW/m. Flames from 500–1000 kW/m fires are less
than 1 m high, but can reach 2–4 m if the intensity is above
5000 kW/m.
An indication of flame height is given by char height —
the height above ground of blackened leaves that are still attached
to trees or shrubs. Char heights increase by about a metre for
every 2500 kW/m (shown right).
Leaf scorch height—the height above ground to which the
leaves in the tree canopy are killed, and thus 'browned' —
also varies with intensity. Scorch height increases by about 3 m
for every 1000 kW/m of fire intensity up to about 8000 kW/m (shown
below). At higher intensities the tops of the canopies of even the
tallest trees are scorched.
Fires greater than 2000 kW/m tend to burn all the available fuel
whereas less intense fires create a mosaic of burnt and unburnt
patches across the landscape. These rules of thumb concerning crown
and ground scorch can be used to gauge the intensity of a fire
after it has passed.
Ignition type is an important factor affecting fire behaviour.
Point sources of ignition lead to elliptical — shaped
fires as shown at right. Only the country at the head of the
ellipse is burnt with maximum intensity; the flanking country on
the sides and back end of the ellipse is burnt at much lower
intensities. Thus there is considerable variation in fire intensity
across the landscape.
Line ignitions, such as those along roadsides, lead to fires
burning on a broad front; such fires accelerate to maximum rates of
spread very quickly — in a matter of minutes. Thus, compared
with a point-source ignition, fire intensity is more uniform across
the front, more of the country is affected by the heading fire, and
there is less variation in intensity across the landscape.
Perimeter fires — lighting up lines on more than one front
— can also create broad, fast-moving, more intense fires.
These variations in behaviour, as a consequence of ignition type,
have implications on how we may use fire and why.
