The Layman's Guide to Fire Suppression
Fire comprises three components, oxygen, heat and fuel. Often referred to as the "Fire Triangle".
If one of these components is removed, the fire will die.
When fire fighters are at work, they are trying to remove at least one of these components. When tackling a wildfire, there are several tools at their disposal. Most of these are basic tools that have been around for over a hundred years. An axe, a shovel or maybe a “Pulaski” which looks like a cross between a hoe and an axe and is a specialist tool invented specifically for the purpose of fighting wildfires. (1) All these tools are designed to remove one of the three components.
Taking fuel first – you might have seen fire fighters walking along with a "drip-torch" to set fire to vegetation in order to remove fuel from an advancing fire in a carefully controlled way. And when you hear about creating a ‘fire-break,’ the purpose is the same, removing the fuel in order to kill the fire.
In terms of the heat component, it’s a different approach again. When water is sprayed onto a fire, it is an attempt to suppress the fire by removing heat (the energy). When water is applied to a fire, it is taking energy away from it. Why water? It is practically the most effective substance you could use. (2) In some cases, it can be water that’s had chemicals added to it – but it’s still water that’s removing the heat. The purpose of other chemicals is to spread the water across the fuel or to cause it to remain longer on the fuel.(3) Water is dropped directly on the fire, rather than ahead of the fire.
Retardants, unlike suppressants, are generally dropped ahead of the fire. Retardants help to remove two fire components, heat and fuel. There are different types of retardants, with new ones are continually being trialed. In the early days, the focus was on chemicals that decompose when heated by the fire and in so doing absorb heat energy. (4) This chemical reaction is called endothermic. Nowadays, the retardants used on wildfires are basically fertilizers. (5) Their principle purpose is to deny the fire access to the fuel. The way they do this is to form a char that coats the fuel and makes it less combustible. Retardants are dropped ahead of the fire in order that the fuel is coated with the retardant that then reacts and forms the char as the fire approaches. It is important that the retardant remains on the fuel, for it to be effective. (6)
The skill, expertise and experience of the fire fighters is crucial to determining the tactical approach to dealing with the wildfire. The decision to deploy fire fighters and the strategy adopted is based upon many interacting factors. The behavior of wildfires is difficult to predict, as it will be a function of different things including topography, weather, fuel and other factors.
The issue of fire spread is very important. Not only because it is what the fire fighters will be so concerned about but also because the ultimate damage caused by the wildfire will result from spread. The science behind fire spread is being enhanced constantly as research improves our understanding. (7) How it manifests on the ground so that wildfire behavior can be predicted more accurately is vital research that is still continuing.
There are many issues facing a fire chief when the call comes in. Apart from getting confirmation about the nature and extent of the fire, he/she will want to know about the surrounding environment, the weather condition and the landscape and fuel. This is why local knowledge is so crucially important. The majority of wildfires are dealt with in the first attack. But for some, it’s not possible to contain them early, so the focus then turns to managing the direction of spread. If it can be shunted away from populated areas towards a large expanse of water, that is ideal. If not, is there a logical place such as a highway where a team could be deployed to prepare the ground by back burning any residual fuel and dig in there. The emphasis is on managing fire spread through attacking the fire on the flanks rather than a direct assault.
Scale and Speed
The unpredictable nature of wildfires makes fighting them difficult and dangerous. Add to that, two other factors, speed and scale. The speed of movement of the wildfire front can be alarming. It may be running at five to ten miles per hour, but it could be higher than that. In 2009, the disastrous fire in Victoria, Australia was recorded on one day running at over 30 MPH. (8) At that speed, there is minimal time to think let alone plan. It comes down to trying to react correctly to a rapidly changing scenario.
Then there’s scale. It is hard to comprehend the scale of a mega fire. The Fort McMurray fire is very large, but it is not the biggest we have seen, at least not yet. Scale adds a complexity all of its own. The line may be spread over several miles, over different topographies and with different fuel beds. Add to that the fact that a mega fire creates its own weather system. Most people are aware of the devastation a tornado can cause. A mega fire can create a ‘firewhirl,’ which is similar but far more deadly. The upwards vortex is a swirling mass of flame that carries fire brands high into the sky to frequently fall ahead of the fire line and cause new fires where they land. (9) In these circumstances, the fire chief has to manage the situation, minimize loss through communication with other emergency services and protect his team.
This guide is designed to help understand the basics of fighting wildfires and cannot do justice to the years of experience and the depth of expertise that the fire fighting service depend upon to make the right calls when confronting mega fires. Hopefully this will give you a basic understanding of the complexity of the job.
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(1) The Pulaski was invented by Ed Pulaski, a USFS Ranger in 1911. It’s a cross between an axe and a hoe/adze.
(2) Water has a very high specific heat capacity, which means it can take more energy out of a fire than almost any other substance.
(3) These chemicals are called surfactants and include things like detergents and foaming agents. Their purpose is to lower the surface tension between the liquid and the fuel.
(4) An early one used was branded Huntite and was a compound of Magnesium and Calcium Carbonate.
(5) There are different products or brands available, and new ones will be tested for compliance by the USFS before being used. As an example, diammoniumphosphate is a fertilizer but is used by fire fighters to combat wildfires. They will also have colouring added to help pilots make effective aerial drops
(6) In 2010, the USDA Wildland Fire Chemical Systems in Missoula carried out experiments to determine what proportion of the retardant remains on the fuel and what proportion falls to the ground. Whilst there are a number of different factors involved in this, it is reasonable to say that around half remains on the fuel.
(7) See “Role of buoyant flame dynamics in wildfire spread” published August 11th 2015, written by Mark A Finney et al.
(8) Black Saturday Bush fires in February 2009 were the worst wildfires Australia had ever experienced. They left 173 people dead and destroyed 3500 buildings and 1.1M acres.
(9) This is the process called spotting. Fire fighters have to be assigned to this task as otherwise the best efforts to hold a line will be jeopardised by spotting.