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EnviroNorth > All Regions > Fire > Fire Behaviour

Fire behaviour

What is fire intensity | Char height | Ignition type

Intense fires behave differently to less intense, cool fires

Intense fires behave quite differently from less intense "cool" fires

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.


What is fire intensity?

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.

Low-intensity-fire

A low intensity fire mostly less than 1000kW/m


Intense-header-fire

A higher intensity fire

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.

charscorchSlide1


charscorchSlide2


Char height 

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

elliptical

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.