Trainer’s Corner: Wildland fire behaviour for structural fire fighters (1st of a series)
Wildland fire behaviour for structural fire fighters (1st of a series)
December 10, 2007 By Ed Brouwer
Although we have made great advances in understanding fire behaviour, every situation is different. Fire is dynamic. Winds can gust and shift direction suddenly. Flames can reach a different fuel type, causing them to increase the rate of spread and/or change direction without warning. Nowhere is this more of a concern than in a wildland/urban interface situation.
Wildland/urban interface fires, by their size, location and risks, frequently require the co-ordinated efforts of fire fighting agencies with differing missions, training and equipment.
Wildland fire fighting: Wildland fire fighters generally respond to forest fires with a mission to protect valuable natural resources that are usually found in remote areas where no piped water supplies are available. There may be a delay in discovering and reporting fires in remote areas; consequently, these fires tend to be larger when fire fighters arrive. Wildland fire fighters are trained and equipped to work from the perimeter around the fire in an attempt to contain the fire. A large fire’s perimeter could extend for kilometres.
Structural fire fighting: Structural fire fighters are trained to attack fires in individual buildings. These fire fighters usually rely on water systems providing ready access to piped water for direct fire suppression. Fire crews are equipped and located for a response time to each emergency incident of mere minutes, so that a fire might even be contained in one room, and the rest of the structure and contents saved.
This separation of missions has worked well enough, but more and more we are seeing a mixing of forests and homes (the wildland/urban interface). It is paramount to fire fighter safety that structural fire fighters learn as much as possible about wildland fire behaviour.
Awareness of the methods of heat transfer may help to better predict where the fire could go and how fast it will get there.
Heat transfer is the method of moving heat from one source to another object. For the purposes of this article, we are talking about combustion and the transfer of enough heat from a burning object to ignite a second object. When discussing the following three methods of heat transfer, remember that they can and often do work together.
Radiation: Heat is transferred from one object to another object through the air without direct contact (sun warming your skin).
Convection: Heat transfers by convection when it moves in a current from a hotter area to a cooler area.
Conduction: When heat energy, such as a flame, comes in direct touching contact with a second object.
There are three key factors that influence fire behaviour. They are: fuel, weather and topography.
Fuel can be grouped into two general categories: light and heavy.
Light fuels include any fuel with a diameter of less than a 1/2-inch (grasses, dead leaves, pine needles). They are generally easy to ignite and burn rapidly. They can serve as kindling for heavy fuels. Fires in tall grasses result in intense burning and, as in the tragic case of volunteer fire fighter Kelly York who was severely burned over 35 per cent of her body, injury to fire fighters. “Many fire fighters are surprised to learn that tragedy and near-miss incidents occur in fairly light fuels, on small fires, or on isolated sectors of large fires, and that fire behaviour is relatively quiet just before the incident. Most of us believe that the high-intensity crown fire in timber or heavy brush is what traps and kills forest fire fighters. Yet, with rare exceptions … most fires are innocent appearing just before the accidents” (Wilson and Sorenson, 1978).
Heavy fuels include tree trunks, growing limbs, stumps, logs and branches that have fallen to the surface or been cut.
The thicker diameters of this fuel can absorb more heat before ignition, so they are slow to ignite. Heavy fuels are subject to intense, long-lasting burning.
Fuel loading refers to the quantity of fuels in a given area available for combustion. The arrangement of heavy fuels over a certain area affects their rate of ignition and spread.
• Horizontal continuity describes the arrangement of fuels across the surface in horizontal terms.
• Uniform fuels describes a thick arrangement where the fuels are in contact with each other and provide a continuous path for firespread.
• Vertical arrangement refers to the distribution of fuels in a vertical dimension. Any fuel situated above a burning fuel is subject to strong convection heating as well as radiant and conduction heating.
The progression of vertical fuel arrangement can be described in three categories: ground fuels, surface fuels, and aerial fuels.
Ground fuels include all combustible materials found beneath the surface (deep duff, roots, rotten buried logs, and other organic material). Although slow to ignite, ground fuels can hide below the surface and are difficult to extinguish completely.
Surface fuels include all materials resting on the surface or immediately above the ground. Light fuels on the surface can easily ignite, but the limited volume of fuel available right at the surface can limit the firespread.
Aerial fuels include all of the green and dead vegetation that is situated in the upper forest canopy. The vertical dimension allows ready access to sufficient oxygen to support combustion. Crown fires in aerial fuels can spread very rapidly and dramatically.
Ladder fuel is a term that describes material on or near the ground that will carry fire to the crown of the tree.
Weather has a strong effect on fire behaviour. The categories of effects that the fire fighter needs to study for basic awareness are temperature, relative humidity and wind.
At the individual fire fighter level, heat makes any physical operations more difficult. Especially if you are still trying to fight wildland fires while wearing bunker gear. Thankfully, most departments have purchased protective coveralls. As well, fire fighters should drink plenty of water to keep hydrated. Apparatus should carry boxes of bottled drinking water. Wildland fire fighters commonly lose one to two litres of sweat per hour.
Increasing temperature also increases the effects of convection currents. But the most important consideration is that heat contributes to the drying of fuels, and dry fuels are easier to ignite than moist fuels.
Relative humidity (RH) is the ratio of the amount of moisture in the air (water vapour) compared to the amount that the air could hold at the same temperature and pressure if it were saturated. Low humidity in the air takes moisture out of fuels, high humidity allows more moisture in the air to be absorbed into fuels. When a fuel has more moisture, it is harder to ignite and burn.
30/30 cross describes the condition where the RH drops below 30 per cent and temperature rises above 30 degrees C. If this occurs on the fireline you should expect extreme fire behaviour. Plan accordingly.
Wind is one of the most important influences on fire behaviour.
The stronger the wind, the faster the spread of fire (see box).
Fires create some of their own wind. Convection currents of heated air rise above a major fire, causing a pressure differential at ground level that results in fresh air rushing in, which continues to feed the fire as long as fuels are available to burn.
Wind patterns determined by the time of day always need to be considered when planning operations for fires, especially around slopes and canyons. Daytime winds tend to move up the faces of slopes, while night-time winds tend to move down-slope.
Topography refers to the surface features of land. Features of topography that are important for size-up considerations of fire fighters include aspect, slope and terrain.
Aspect describes the direction in which a slope faces in relation to the sun. More direct sunlight generally falls on the south and southwest slopes, with resulting higher temperatures, lower humidity, lower fuel moisture and sparser and lighter fuels. These areas are critical in terms of wildland fire starts and spread. North aspects of slopes are more shaded and have more fuels that are heavier. This shady side has lower temperatures, higher humidity and higher fuel moistures.
Slope is the angle of incline on a hillside. The importance of this to fire fighters is that the steeper the slope, the faster a fire burns. Higher on the slope ahead of the fire, the fuels become heated by radiant and convection heat currents. Burning material on slopes can also roll downhill to start other ignitions.
A slope that rises at a 45-degree angle is said to have a 100 per cent grade. That means the land rises 100 feet in elevation as you move 100 feet in a straight, level direction. Here is the significance of slope: on a 30 per cent slope, fire will burn twice as fast as it will on level ground. Take a long wooden match, light it. Holding it at 90 degrees it burns slowly, but if you hold it at a 45-degree angle it burns more rapidly.
Terrain includes land variations such as canyons and ridges. A narrow canyon can cause radiant heat to raise the fuel temperature on the opposite slope, drastically increasing the chance of multiple ignitions.
Ridges experience more wind primarily because they are elevated above the surrounding land. When a fire moves up a slope towards a ridge, it gathers speed and intensity. Fires burning at the ridge of a canyon or hill can exhibit unusual fire behaviour.
Winds can increase when blowing through saddles due to the funnelling effect of the constricted pass. On the other side, winds spread out again and can result in eddy action.
When sizing up a fire include the presence of any barriers (natural or man-made). Rivers, lakes, wet swampy areas, rock outcroppings and bare ground are examples of natural barriers. Old burns, roads, highways and reservoirs are examples of man-made barriers.
Rate of spread (ROS) is expressed in both metres per minute or kilometres per minute. A moderately slow firespread would be 2.1 m/min, where as a very fast ROS would be 65 m/min, (crown fire). Wind has the most effect on ROS. Every 13-km increase doubles the ROS. Slope has the next greatest impact. Every 25 per cent increase in slope doubles the ROS. If the IC is unaware of the importance of reading the ROS he or she can end up a hose length short of the fire. Or worse yet, place crews in harm’s way.
Multiple ignitions: When conditions are such that firebrands are lofted into the air, the probability for multiple subsequent ignitions over a wide area increases. These multiple ignitions can overwhelm any fire fighting force. Fire entrapment is a very real and present danger.
Note: Fire spotting is one of the major ways that fires spread and homes are lost in wildland/urban interface fires. Firebrands can come down on combustible roofs, on combustible items stored adjacent to homes, or on other nearby combustible fuels and ignite. The resulting spot fires may go unnoticed and unanswered when an area has been evacuated of residents, when fire fighters are spread too thin or when the spot fires are too numerous.
Problem fire behaviour describes fire activity that in some way presents a potential hazard to fire fighters. To reduce the hazard, adjust the tactics being used. Being able to size-up a fire scene and predict or anticipate fire behaviour is the best way to avoid problem fire behaviour.
In Part 2 we will address suppression and safety tactics. In the mean time, stay safe out there … and please remember to train as if their lives depend on it – because they do. By the way, I am researching information for an article on RIT for volunteer fire departments. I would appreciate hearing from anyone who operates with rapid intervention teams.
Ed Brouwer is the Fire Chief/Training Officer for Canwest Fire and a member of the Osoyoos (B.C.) Fire Dept. The 17-year veteran of the fire service is also a Fire Warden with Ministry of Forests, a First Responder III instructor/evaluator, Local Assistant to the Fire Commissioner and a fire service motivational speaker and chaplain. E-mail firstname.lastname@example.org .
Print this page