Rain brings new concern for Pierce County wildfire-scorched hillsides: Landslides

VIDEO: Concern for landslides as rain hits Western Washington

PIERCE COUNTY, Wash. — During the Sumner Grade fire, which burned thousands of green wooded acres, a good soaking rain would have been regarded as a blessing.

Two weeks later, the first rains of fall have state geologists scrambling to assess a new and different danger -- landslides.

When the hills were raging in wind-driven wildfires, trees and plants on the steep hillsides were left in ashes. Rain falling on acres of blackened hillsides is creating a slick, oily surface, where water rolls off, like rain off a freshly-waxed car.

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It’s all very unstable, and now state Department of Natural Resources geologists are very concerned about the hillsides above state Highway 410.

“It is an area with a public safety concern,” said Casey Hanell, the state geologist with DNR.

Hanell said when wildfires scorch and cook the soil, the earth becomes water-resistant. Rainwater will roll right off of it.

A United States Geological Survey video illustrates how a small mudslide stream down a wildfire-ravaged hill can quickly magnify, becoming an extremely powerful force within seconds.

The current can bring down boulders and trees, and anything below the flow could be in serious danger. In some cases, homes can be pushed off their foundations.

In the Sumner Grade fire, KIRO 7 saw a lot of homes right on the edge of steep burning slopes in nearby Bonney Lake.

“That’s a watch-out situation that you’ll want to be paying attention to,” he said.

That’s why geologists are doing an analysis everywhere wildfires have burned. They hope to warn people about the potential dangers of post-fire mudslides before they happen in areas where until now, fall rains were always absorbed in soil--and by thirsty plants.

“You’re losing some of that capacity for the landscape to soak up water or trap water in tree canopies before it gets to the ground,” Hanell said. “You just get more water getting to the ground and less able to soak into the ground."