08/12/2002

Associated Press

Researchers investigating the aftermath of major wildfires now think most houses are destroyed by errant embers, burning after the main fire has swept through.

The discovery could change the way firefighters combat wildfires, said Jack Cohen, a research scientist at the U.S. Forest Service Fire Sciences Laboratory in Missoula, Mont.

The wildfire in Murdock, Wash. destroyed four homes Saturday night. (KGW Photo)

"We get the sense that it all happens at once, that the whole neighborhood explodes into flame, and that's not necessarily the case," he said.

The discovery means if firefighters can safely wait out the speeding front of a wildfire -- the so-called "crown fire" -- they can snuff small sparks that pose the larger threat to homes afterward.

"You don't need to just throw up your hands and say, 'A crown fire goes through and all the houses go up,' " said Cohen, who studies fire behavior. "This provides an additional opportunity for protection."

A research team deployed fireproof instruments to measure the heat and speed of one Oregon wildfire last month in hopes of helping fire crews seize on the opportunity.

"We're trying to ask, 'How can firefighters make the best use of this new knowledge we have to protect life and property?' " said Bret Butler, a research engineer with the Forest Service's Fire Behavior Project in Missoula.

Firefighters know wildfires turn most terrifying and erratic when low flames jump into dense treetops. Feeding on crispy pine needles, those fires rush through forests at speeds of a few miles per hour.

Usually houses within 50 feet of crown fires will burn -- but Cohen found recently that fire never came that close to destroyed buildings in Arizona and elsewhere.

At the Rodeo-Chediski fire, which blackened 470,000 acres and incinerated more than 400 buildings in Arizona in June, roughly four of every five burned homes were surrounded by unburned trees or lawns.

In some cases, clues left around homes showed the fire never came closer than 100 feet from the house.

Experiments by Cohen and others in northern Canada bolstered his suspicions.

Researchers set up samples of wooden home siding and lit crown fires in adjacent trees. Although the walls of flames loomed up to 65 feet and briefly leaped much higher, they did not generate enough lasting heat to ignite the siding -- as long as the siding was about 100 or more feet away.

As long as vegetation is sparse, a crown fire won't carry flames to a house's front door.

"Because the flame front is moving so fast, it's just not there long enough to ignite a house," Butler said.

Instead, about 80 percent of homes destroyed in wildfires begin blazing when flaming embers fly as far as a half-mile to land on or near them.

By the time these houses go up in flames -- sometimes four to six hours later -- fire crews have moved on to the next big crown fire.

"Someone with a backpack pump could have saved it," Ronald Wakimoto, a professor of forest fire sciences at the University of Montana.

And Cohen suspects flaming homes are more likely than flaming trees to set other homes on fire.

"In higher-density areas, if you don't operate as a community, you will certainly burn as one," he said.

Cohen said these lessons will help firefighters better protect homes in future fire seasons.

"If crews don't have to withdraw so far ... they can get back in to pick up the small ignitions," Cohen said. "If no one's around to deal with it, all we see is the ashes."

(Copyright 2002 by The Associated Press. All Rights Reserved.)