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The recent tragic events in Victoria have focused our attention fire and the vunerability of our homes, farms and forests. Whilst we will learn more in the coming months about how fires behave on days of extreme weather there is much we already know about how 'normal' fires behave and the impact that farm trees might have.
The unfortunate fact is that trees and forests provide the fuel that feeds a wildfire. However, farm plantings, plantations and native forests can be designed and managed so as to reduce the risk to life and property. Trees can even save them.
On this page we provide some basic background on fire behaviour and offer some links to research groups and useful publications.
Fire Behaviour
Bushfires are usually described by the characteristics of the vegetation they burn. Whilst grass fires are the most common in farming areas, bushfires are often the most threatening to life and property. Tree growing on farms changes the characteristics of the vegetation and therefore the way fires behave, their intensity and how they can be controlled. Fire behaviour is influenced by three factors:
• weather
• fuel type and arrangement
• topography.
Fire hazardous weather
Wind speed, relative humidity and temperatures are the weather factors that have the most influence on fire behaviour. In south-eastern Australia, high fire danger conditions develop when high-pressure systems develop in the Tasman Sea in summer, causing hot, dry winds to be blown from the interior of the continent towards the coast. If a frontal depression pushes up against the high-pressure system, wind speed can become very strong with gusts up to 100 kilometres per hour ahead of the change.
Whilst we usually think of the danger coming from the northwest, most lives and property are lost when the wind changes to the south west. If there is no rain with the change the dry winds blow back turning the edge of a narrow finger of fire into a wide burning front.
The rate at which a fire spreads increases dramatically with increasing wind speed. Research suggests that if the wind speed doubles, the rate at which a grassfire spreads might increase three-fold. A fire's intensity is dependent on the amount of fuel burnt and the time taken for it to burn. So, an increase in the rate at which the fire spreads directly increases its intensity. The hotter the fire, the greater the damage to vegetation and the more difficult it is to fight.
Farmers cannot change Australia's climate. But they can anticipate a fire's likely behaviour and plan accordingly. For example, they can design and manage shelterbelts and plantations so that they reduce windspeeds and slow the rate at which a grassfire could spread.
Fuel type and arrangement
Fuel is any material that can be burnt by fire. Its size, moisture content, arrangement and other characteristics influence the way the fire burns and the risk of spot fires. Fine fuels—including grass, pine needles, leaves and bark—dry out as the fire approaches. They then burn quickly. Coarser fuels, such as twigs and branches, burn for longer after the fire front has passed.
The arrangement of fine fuels is critical. Pastures trampled by stock will burn more slowly than a standing pasture. The more continuous the fuel, the easier it is for the fire to run, pre-drying any fine fuels as it goes. Reducing, compacting or separating the fine fuel component of a farm forest dramatically reduces a fire's intensity.
Some types of vegetation contain oils or other compounds that increase their flammability and might allow them to say alight for longer. Eucalyptus leaves, for example, contain oils that vaporise as they are heated and burst into flame when ignited. Many tree and shrub species have high moisture content in their leaves during the fire season and are less likely to burn. When selecting tree and shrub species farmers should consider their:
• bark type;
• leaf shape, moisture content and oil content;
• propensity to retain dry leaves or branches during the dry season; and
• effect on the vigour and density of understorey plants and groundcovers.
The flammability of tree foliage can be tested by holding a green branch over a hot fire. Let the leaves heat up before ignition. You'll soon identify the better species to grow near your house.
Topography
Ground slope directly affects the rate at which a fire spreads. Fires travel much faster uphill than downhill and every 10 degree increase in slope doubles the speed at which a fire spreads. Aspect is also important because it affects the quantity of fuel and its moisture content. North-westerly aspects are generally the driest but may not carry a lot of fuel.
A note from Kevin Tolhurst (Melbourne University Fire Researcher)
The diagrams from McArthur (1967) in Fig.1, show the importance of two weather factors: wind speed and relative humidity. In particular, the diagram showing the relationship between fuel moisture content and rate of spread rises very steeply once the fuel moisture content drops below 10%. This is a very critical range and is associated with relative humidities of less than 20% - in the range very difficult for meteorologists to predict even on a daily basis.
Similarly, the second diagram shows that the rate of spread of a fire increases proportionally greater at higher wind speeds than at lower wind speeds. Both wind speed and fuel moisture content are important in the efficiency of heat transfer and the spotting process.
Fine fuel moisture content is affected by the amount of exposure to wind, solar radiation, recent rainfall and atmospheric relative humidity. However, the McArthur fire danger rating system only incorporates relative humidity along with air temperature, and recent rainfall. What is even more troubling, is that these factors affecting fine fuel moisture content, vary rapidly over the course of the day and even a few hours of extreme weather is sufficient for fires to burn very large areas.
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