How do you stop a desert wildfire?

Answered by Kelsey Griffee, Fire Information & Prevention, BLM-Idaho Falls District Office

Wildfire is a mesmerizing natural occurrence created by the chemical reaction of oxygen, heat and fuels. Oxygen is supplied from the air all-around us and spurred by wind. Heat is sparked by lightning or human folly. Fuels come in the form of grass, brush and trees. This chemical reaction is deemed the fire triangle and is also the basis for how firefighters suppress a wildfire.

By removing one side of the triangle a fire can be stopped. Oxygen can be removed by smothering the fire with dirt. Called “dry mopping,” firefighters shovel dirt onto the fire, stirring it over and over till the area is cold to the touch. Heat is removed through water, which is sprayed from hose lines or dropped from the sky by helicopters or planes. Finally, removing fuel from the equation is often the most successful way to stop a wildfire. Firefighters dig line, which looks like a trail around the fire’s edge. They remove vegetation till they hit mineral soil, ceasing the fire’s progress. Handcrews of 20 people may dig miles of line or in more accessible terrain bulldozers will build blade wide fireline. Then controlled burns may be set to consume the fuel ahead of the fire and snuff it out. Even fire retardant works to remove fuel by fully coating vegetation in a soapy mixture of fertilizer and water. Retardant incapacitates the fire’s ability to ignite the covered vegetation.

2017 fire in east Idaho desert. Bulldozer is punching line along the fire’s edge. Photo by BLM-Idaho Falls District

East Idaho desert fires are driven by wind, which propels fire through the fine, flashy fuels of the sagebrush steppe ecosystem. To stop these fast-moving fires, firefighters must use all available tactics. Retardant lines are dropped out in front of the fire’s head and backed up with flanking dozer lines. Then firefighters with engines secure these lines with water until the fire is pinched out. Weather and difficult topography can turn this all to naught by creating extreme fire behavior, which forces retreat to more advantageous locations. Mother Nature is a worthy adversary, but we must remember not all fire is bad.

Helicopter and bulldozer work together to slow the 2018 Grassy Ridge Fire. Sagebrush is coated in fire retardant from earlier. Photo by Kelsey Griffee, BLM-Idaho Falls District

Fire Triangle

Let’s conduct three experiments to limit one of each side of the fire triangle – oxygen, fuel, and heat – to understand the needs of fire.
Safety Precautions: Hair pulled back and sleeves rolled up, safety glasses on, and an adult helper.

BLM Idaho Falls District engine battles the 2018 Grassy Ridge Fire near Dubois. Photo taken by Kelsey Griffee, BLM- Idaho Falls District

Make sure your experiment is conducted on a fire retardant surface.

Materials: long-reach lighter, pillar candle, metal pie pan, water, glass jar, aluminum foil, scissors, screwdriver

Limiting Oxygen

  1. Place the candle on the metal pan.
  2. Light the candle.
  3. Place the glass jar over the lit candle until the rim of the glass is 1 inch from the pan, leaving a gap between the glass and the table. Observe.
    When a glass is lowered over a candle flame and a gap is left, the hot gases in the glass expand and create a higher pressure than the atmosphere. The high pressure will not let the oxygen in even though there is a gap at the bottom of the glass.

Limiting Fuel

  1. Cut a piece of foil 1-inch x 1-inch. Cut a small slit in the middle of one side to the center.
  2. Light the candle.
  3. Carefully slide the lit wick into the slit so that the foil separates the wax from the flame. Observe.
    The candle flame’s fuel is melted wax. The aluminum foil slid between the wick and the melted wax cuts off the amount of fuel the fire needs to keep burning.

Limiting Heat

  1. Light the candle.
  2. Place the metal end of the screwdriver next to the lit wick of the candle. Observe.
    The candle’s heat is provided by the burning wick. The metal on a screwdriver absorbs heat. When we hold the screwdriver’s metal end to the flame, touching the flame without touching the wick, the metal absorbs the heat taking the heat out of the candle wick.

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