How firefighters contain a forest fire.
We keep hearing the term “containment” in news reports of wildfires: The burn is 5% contained; 17% contained; 22% contained.
We had a burning question: what does “contained” mean, exactly?
“A fire can be 100% contained and can still be burning,” says Jim Gersbach, public information officer at the Oregon Department of Forestry. “That’s what throws people. You’ve still got smoke and flames.”
Fires need three inputs to burn: heat, oxygen, and fuel.
Since you can’t throw buckets of water or blankets on a raging forest fire to remove heat and oxygen, authorities focus on starving the blaze of fuel.
Forest firefighters expend countless hours of labor constructing firelines, typically by digging shallow trenches 10 to 12 feet wide — crude barriers meant to scrape away any potential fuel and stop the fire from gaining ground.
The containment number refers to how much of the fire’s perimeter has been surrounded by firelines, lakes, or rivers.
So if the fire stretches for 10 miles around, and fire fighters have built a trench along three of these miles, and a lake borders another two miles, the fire is 50% contained.
Here’s a real life example: As of September 20, according to InciWeb, the Forest Service’s incident tracking webpage, the Eagle Creek fire is 48,668 acres and is 46% contained. That means almost half of the fire’s edge has been surrounded by fire lines, trenches that clear the area of fuel sources.
There’s a big difference, however, between containment and control.
Even a wildfire that has been completely surrounded by fire lines — 100% contained — is still very wild.
Stray embers can fly across the line and spark another conflagration on the other side. Strong winds can also push the fire across.
“They may not feel that the line is wide enough or that they got all the spot fires that can jump that line,” Gersbach says.
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Achieving control requires erecting barriers, removing fuel, and cooling especially active areas of the fire, called “hot spots.”
In the 16,436-acre Horse Prairie fire that scorched Douglas County, fire fighters laid a hose around the perimeter to reinforce the trench.
To sum up, in Gersbach’s words, “containment means the fire fighters are reasonably sure they can stop the spread of the fire.”