Brush and Wildland Fires
Wildland fires can be devastating to the environment and cause harm to plants, animals, and humans alike. California has been known to experience widespread wildland fires due to a combination of human and climate activity and recently, Canada has also experienced widespread wildfires that have carried dangerous, thick smoke into the United States causing low air quality and colder temperatures.
Firefighters and fire emergency response teams are essential to controlling and eliminating unchecked wildfires. Part of the battle of fighting wildland fires is understanding how they are started and how they spread so quickly. In this post, we’ll break down the causes of brush and wildland fires and how they are fueled.
What Causes Wildfires?
Wildland fires, or wildfires, can be caused by a variety of actions and variables. Some of the most common causes of wildfires are unattended campfires, uncontained burning debris, downed electrical equipment like power lines, intentional arson, and dropped cigarettes. The National Parks Service states that about 85% of wildland fires in the U.S. are caused by humans and according to the National Interagency Fire Center, California has the highest rate of wildfires and number of acres burned in the U.S.
Extreme heat temperatures, combined with low amounts of annual rainfall, cause flora and soil to weaken and dry up. Wildfires are also more prone to occur in areas of prolonged drought, making grasses, fallen leaves, foliage, and trees prime tinder for fires caused by lightning strikes.
What Is A Fire Triangle?
A fire triangle, also known as a combustion triangle, is a model that illustrates the three essential elements to ignite and sustain a fire. The three elements needed to start a fire are heat, fuel, and oxygen. If one of those factors is removed, the combustion stops and the fire will cease.
Heat can be caused by ignition or combustion reactions that in turn, create more heat as they burn, further fueling the fire. Fuel can be any material that is flammable including wood, fabric, paper, or dried foliage. Different materials and elements burn at different rates and temperatures and can often be the most difficult element of a fire to control. Lastly, oxygen, or another type of oxidizing agent, is required to maintain the fire. The earth’s atmosphere is 21% oxygen, so heavy fire blankets, fire extinguishers, or fire retardants are used to eliminate the fire’s access to oxygen.
Depending on the type of fire, heat can be transferred in three main ways:
- Radiation: heat generated from a burning object affecting an adjacent object
- Convection: heat transferred through the movement of liquid or gas
- Conduction: heat transferred from one object to another by direct contact
To properly battle and extinguish a wildfire, it is important to understand what is fueling the spread, how it is moving, and what type of fire it is.
Types of Wildland Fire Fuel
As mentioned above, fuel is any type of flammable material that can burn to continue to feed burn. Wildland fires have three main fuel types including:
- Surface Fuel
- Subsurface Fuel
- Aerial Fuel
Wildland surface fuels burn on the forest floor and can include grasses, leaves, vegetation, and other ground debris. The spread rate of forest and wildland surface fires often depends on the density, size, slope, terrain, and weather conditions of the area. Oftentimes, the most effective way to manage wildland surface fires is directly with water and the removal of surface fuels.
Wildland subsurface fuels consist of duff (including topsoil, decayed leaves, peat, and tree needles) and decayed or dead woody material. These materials can be found at a depth anywhere from three to four inches in dry areas and three feet in wet areas. These materials are compact, have limited access to oxygen, and are protected from wind, allowing for a persistent slow burning fire that can be more difficult to suppress.
Wildland aerial fuels consist of upper foliage and the crowns of standing trees. In some cases, a crown fire, a fire that spreads in the treetops, may develop and move quickly. A crown fire spreads ahead of the surface fire and will generally travel at the same speed of the wind moving it.
Weather and Topography Impact on Wildfires
Although fuel has a major impact on the severity of a brush or wildland fire, weather and topography also play a major role in how fast and far a fire spreads.
Weather conditions like wind can greatly increase a fire’s access to oxygen, causing it to burn stronger. Wind can also drive convection heat into other fuel sources, removing moist air and replacing it with dry air, drying fuels which allows them to burn easier. Wind has an influence on the fire spread direction as well.
Relative humidity (RH) and precipitation increase fuel moisture, which can make them difficult to burn and increase the potential for greater smoke production during a fire. Temperature also impacts the environment by making the area more susceptible to burn easier or not and also impacts a firefighter’s ability to withstand the environment and eliminate the fire.
Topography is also a major factor in how a wildfire burns and spreads. The steepness and slope of the land will affect how quickly a fire spreads and in what direction it moves it. The steeper the slope, the quicker a fire will move.
Based on the way a wildfire moves, its parts can be broken up into a few main categories including the heel (or backend of the fire), the head (the front of the fire in the direction it is burning), the left and right flank (the farthest reaching fire spread on each side of the main burn), and the fingers (extended fire paths from the center).
Brush and wildland fires are a major environmental, safety, and health concern. In order to properly combat the spread of wildfires, it is essential to have a plan and a deep knowledge of how they spread to mitigate damage and injury whenever possible.
In addition to his leadership role at Ward Fire Equipment,
John May has more than 30 years of experience in the Fire Service. John is a Level I & II Fire instructor in the State of NY.
