Does satellite photo foreshadow slide trouble?

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by CHRIS INGALLS / KING 5 News

Bio | Email | Follow: @cjingalls

KING5.com

Posted on March 27, 2014 at 10:29 PM

Updated Thursday, Mar 27 at 10:41 PM

SEATTLE -- Dave Montgomery is one of Washington’s foremost landslide experts.

But he’s not above using Google for his research.  And what he sees in an undated Google Earth satellite image, taken sometime before the slide, has intrigued him.

“You can clearly see the river cutting into the toe of the slope,” said Montgomery, as he pointed at the computer screen in his office at the University of Washington.

Montgomery is fixed on the “revetment,” a wall built along the Stillaguamish River right across from Steelhead Drive.  It was constructed after a landslide there in 2006 to reinforce the riverbank and prevent sediment from building up in the river.

But at one point at the northeast end of the wall, the logs appear to have broken apart and there are signs that the river is starting to undermine the vast slope above it.

“You can clearly see the river cutting into the toe of the slope,” said Montgomery.

Montgomery says he can’t say for certain that this was early evidence that erosion was increasing the landslide hazard.

And he says it’s way too early to point fingers at whether the wall could have had any impact that contributed to the slide.

But he says it’s worth investigating to see if it was a contributing factor.

“It’s one of those questions that should be looked into as we search for answers on this slide,” said Montgomery.

The Stillaguamish Tribe, which oversaw construction of the $1 million retain wall, says it has no reason to believe it had anything to do with the slide.

Environmental manager Pat Stevenson says the “erosion is not in a significant place” and that the condition of the wall “had very little to do with what happened on Saturday.”

Stevenson says the wall was built to hold back sediment that was ruining the fish run.  It wasn’t designed to hold up the entire slope.

Officials are also looking at whether logging above the site of Saturday’s slide could have been a factor.

Many experts agree that logging or toe erosion would only have been contributing factors to the slide.

They say the primary reasons that triggered the slide were the heavy winter rains and the loose soil beneath the slope.  Those factors have long been responsible for a long history of slides in the area.

The retaining wall was constructed through grants to the Stillaguamish Tribe, which wanted to fortify the slope to keep sentiments from harming salmon runs.
 

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