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Fire danger: topography, weather and fuel

By Bianca Nogrady

December 17, 2015

Bushland on fire

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, weather and fuel – 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

A slope with burnt trees
Kinglake, Victoria, following the Black Saturday bushfires in 2009. Image: Nick Pitsas

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 behaviour 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

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.

Rating fire danger

A sign with fire danger ratings
Fire danger rating signs are a familiar sight in Australia. Image: J C Merriman/Flickr

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.

Originally, the Fire Danger Index topped out at 100, which was intended to represent the worst possible combination of conditions, but since they were developed, those conditions have been exceeded a number of times.

The Fire Danger Index is now an open-ended scale – because there’s always the possibility that conditions can get worse – but Sullivan says that once you get above an FDI of 50, it’s essentially splitting hairs.

“The truth of the matter is that there’s nothing that can be done once you get above 50 – that’s why it was called ‘extreme,” he says. Conditions around the Victorian Black Saturday bushfires earned an FDI well above 100, but Sullivan says the introduction of the ‘catastrophic/Code Red’ rating was more about messaging.

“When things got really, really bad, above 100 – and it doesn’t take much to start kicking that number up really high – they wanted to have another layer that conveyed the seriousness of the situation,” he says.

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

“The level of potential impact in central Sydney is dramatically different than what you would get in the Blue Mountains, so the new system is intended to take into account those aspects which the current system was never designed to do but is being used for.”

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.


Read more: Bushfire resilience: preparing yourself and your property

One comment on “Fire danger: topography, weather and fuel

  1. Dr Robert Dumsday says:

    As a young man I was caught in a fire that burnt a large part of South Gippsland (mid 60s?). My cousins were caught in an associated grass fire but survived under wet hessian bags as the fire went over them. I was caught in a forest (in a CFA truck) where the fire came back on us due to a wind change – it crowned over us but no-one was injured. More recently I witnessed Black Saturday at our small farm at Flowerdale. Due to mis-guided policies on forest management it was worse than it should have been and I don’t blame SP Ausnet for its severity – the fire could have been started by any number of causes. As you conclude, reducing fuel loads is critical.

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