Bushfire basics: what you need to know

By Bianca Nogrady

17 December 2015

6 minute read

A picture of a bushfire.

Inferno: a typical Australian bushfire scene.

If you cast your mind back to high school science, you might recall something called the ‘fire triangle’. The three sides of the triangle – oxygen, heat and fuel – represent the three essential ingredients for a fire. Take any one of them away, and a fire won’t happen.

Bushfires have a triangle all of their own. The ‘fire behaviour triangle’ – topography, fuel and weather – represents the three key factors that influence how a bushfire behaves. Weaken any one of these and a bushfire becomes more manageable. If all three of these elements favour the bushfire, it’s time to seriously batten down the hatches. Unfortunately, we only have a say in one of those three elements.

Topography

Topography is well and truly beyond our control, and it has a major impact on the speed with which a fire will spread. Put simply, a fire will move faster uphill than across flat ground or downhill.

“The rule of thumb is that the rate of forward spread of a fire on a slope will double the equivalent rate of spread on flat ground for every ten degrees of slope,” says Dr Andrew Sullivan, bushfire behavior expert at CSIRO Land and Water.

But there is an upper limit; when the ground becomes too steep, like on a cliff face, vegetation becomes too sporadic and can actually impede a fire. Fires can also ‘spot’ from the top of hills, and thereby overcome the potential handicap of a downward slope or a break in the fuel.

Weather

A picture of bushfire.

Bushfire in the Top End of Australia.

Weather is also beyond our control. In a country prone to extreme droughts and hot, windy days, the climate and weather are perhaps the greater contributors to bushfire risk and bushfire severity in Australia. As anyone who has tried to light a campfire in a downpour will know, wet fuel doesn’t burn very well. It needs to dry out first. And so it is with the Australian bush. The more humid and damp the conditions, the less combustible the fuel is. Long periods of hot dry weather lead to desiccated, highly flammable fuel loads.

Unfortunately, climate change forecasts suggest that we are likely to experience more of the kind of extreme fire weather that is associated with more devastating bushfire events. To make matters worse, the bushfire season is predicted to extend.

“Climate change forecasts are suggesting that spring isn’t going to change for the southern part of Australia but autumn is generally going to be drier and hotter,” says Sullivan. “The bushfire season is going to get longer, firefighters are going to get more tired, resources going to get more stretched.”

This could mean that the end of the southern hemisphere bushfire season will overlap the start of the northern hemisphere season. Given our current reliance on the northern hemisphere’s large fire-fighting airtankers and helitankers such as the famed Elvis Aircrane, it puts our access to those valuable fire-fighting resources at risk.

Weather also includes wind. The relationship between wind speed and bushfire spread is essentially linear; the faster the wind speed, the faster the fire will spread. While climate change will undoubtedly affect wind, we don’t have nearly as much confidence in our forecasts of how.

Fuel

Now we come to fuel; the only element of the bushfire triangle that we can influence. Fuel – its availability, arrangement, size, amount and moisture content – decides the speed and intensity of a bushfire.

“Fires will spread faster in finer and more open fuel such as grasslands than they will in forests, where the fuel that actually powers the fire is much coarser and slower to combust,” Sullivan says.

The type of vegetation decides the flammability of the fuel: the finer (and drier) the fuel, the more easily it will burn.

“When we talk about fuel loads, we’re talking about fine fuels less than 6 mm in diameter, so leaves, twigs and bark on the floor are the primary component of fuel,” Sullivan says.

“Large materials such as fallen branches and trees themselves can burn and often do burn, but they burn well behind the front of the fire.”

In extreme conditions, fires can burn through the canopy of trees, called ‘crowning’ but even in this situation, the fire must be supported by the fuel in the understorey of the forest. While the Australian landscape is home to a host of different fuel types, the current fire danger rating system is predicated on just two: grassland and forest.

“There are whole bunches of vegetation that aren’t either grass or forest, such as in the Hawkesbury sandstone region which has more of a temperate shrub which behaves somewhere in between forest and grass,” Sullivan says.

Add in the fact that these areas often feature quite complex topography, and this can make it tricky to predict fire behaviour in these areas. While these three elements – topography, weather and fuel – shape the intensity of a bushfire, a different set of calculations are made to determine the risk of a bushfire starting in the first place.

A picture of a fire.

Vegetation feeding a bushfire.

Rating fire danger

All Australians are familiar with the Fire Danger Index, which is actually subdivided into the Forest Fire Danger Index (FFDI) and Grass Fire Danger Index (GFDI). This number decides the fire danger level and whether a total fire ban needs to be called. The lowest rating – low-moderate – represents a score of 0-11 on the Forest Fire Danger Index, and total fire bans kick in at 50. The highest rating – ‘catastrophic’ in all parts of Australia except for Victoria where it is called ‘Code Red’ – is a score above 100 in forests and 150 in grasslands.

“These are primarily weather indices, so the Bureau of Meteorology calculates them for the fire agencies and the fire agencies use those values to determine their level of preparedness and the potential for calling a total fire ban,” Sullivan says.

The FFDI and rating system is now facing another overhaul that is designed to take into account levels of vulnerability, exposure and preparedness.

Managing the risk

Of all the factors that influence the risk and severity of a bushfire, the only one we have any degree of control over is fuel. We can modify fuel structure and reduce fuel load in our immediate vicinity by clearing around houses and further afield, fuel reduction burns can make a big difference ahead of fire season.

“The key thing that we’ve found is that there is a strong link between initial attack success, so when firefighters first turn out to a fire, and the level of hazard represented by the fuel,” Sullivan says.

If we can reduce the hazard through prescribed burning, then there’s a greater chance that fires can be controlled at an early stage, even under bad fire weather conditions. It’s our only weapon and we better use it. Here’s an experiment we conducted with the NSW Rural Fire Service  to look at how fire behaves under different conditions in a grassland setting:

Top ten things you need to know about bushfire behaviour:

  1. The steeper the land, the faster the bushfire will spread up it; for every 10 degrees in uphill slope, the speed of a fire will double.
  2. Fires can overcome downward slopes or breaks in fuel by ‘spotting’; throwing burning embers well ahead of the fire front that can start new fires.
  3. The lower the humidity and higher the temperatures, the drier the fuel and the easier it is to ignite and the more intense the fire will be.
  4. The bulk of a bushfire’s energy actually from the fine fuel on the ground; stuff less than 6mm in size such as twigs, leaves and bark. This is what the fire feeds most on, rather than the heavier fuel such as branches which tends to burn after the fire front has passed through.
  5. In extreme conditions, fires can ‘crown’, which is when they burn through the canopies of the trees rather than burning through the undergrowth.
  6. Wind speed has a linear relationship with the speed of the bushfire; the faster the wind, the faster the bushfire will travel.
  7. The Fire Danger Index is an open-ended score of fire risk; in the Forest Fire Danger Rating System, ‘severe’ is above 50 points, ‘extreme’ is above 75, and ‘catastrophic/Code Red’ is above 100.
  8. Once the fire danger rating gets above ‘extreme’, there is little that can be done to control bushfires.
  9. Reducing fuel hazard through clearing and hazard reduction burns is one of the few things we can do before bushfire season to reduce the risk of bushfires starting and their intensity when they do, and improve the chances of firefighters getting them under control.
  10. Climate change is forecast to increase the number of extreme bushfire weather events but also to extend the bushfire season well into autumn.

Fire away

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