Forest Service says fires are linked to landslides

1997 Lower Banks Slide

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by Stephanie Zepelin

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KTVB.COM

Posted on August 10, 2012 at 11:58 PM

Updated Thursday, Dec 5 at 12:54 PM

BOISE COUNTY -- Highway 55 was closed earlier in the week for a rock slide onto the road. Concerns about falling rocks have continued. Rockfalls and landslides are often seen in fall, winter, and spring, when there's rain or snow. But fires can help cause landslides to happen.

"There was a major fire event in the mid 90s and subsequently there was a major landslide event that occurred that winter," said David Olson of the Boise National Forest.

Olson said when trees and plants burn in a fire, the soil is less stable.

"You can see a debris torrent or landslide from that root structure being lost like what occurred in the banks area in the mid-90s," he said.

About ten years before the landslide in Banks in 1997, the Forest Service said there was a large fire. That fire helped cause the landslide that wiped out Lower Banks.

"Well after a fire occurs, obviously there is very little vegetation," said Olson. "So if you have a thunder cell that comes through, if you have a heavy rainstorm that might occur, there's nothing to really hold that water in place on the steep slope."

Olson said that can cause a flash flood or debris flow, about three or four years after a fire.

"In the eight to 10 year range, as the roots on the trees begin to die if the trees have died, then you start losing your subsoil structure."

Combine those conditions with heavy snow or rain, and you can have a huge landslide.

"Burned areas will be something we have to watch out for because we know that if some kind of a weather event occurs, whether it's a heavy rainstorm or later in years a rain on snow event, we could definitely see landslide potential," Olson said.

Olson can't say if the Springs Fire or other fire will cause a landslide. But he said it will be interesting to see what happens in the area.

Forest Service research said the possibilities of a landslide occurring can be 80-90 percent greater in a burned area versus a non-burned area.

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