The Science And Social Challenges Behind Landslide Prevention

David Montgomery, a geologist at the University of Washington, Seattle, and author of The Rocks Don't Lie, explains the mechanics of a mudslide.

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AUDIE CORNISH, HOST:

For more on what may have triggered the landslide, we called David Montgomery. He's a geologist with the University of Washington, Seattle. He says while it's not clear what exactly set it off, there is one obvious contributing factor.

DAVID MONTGOMERY: We had a very wet March. So rainfall is one of the things that drive landslides. But there's other things that may contribute as well. In this particular case, there was a history of previous sliding at the site. And once a hill slope has slid, it's vulnerable to further sliding, because the material that makes up the slope is weakened. And why had this slope failed in the past? Well, it's fairly weak glacial material to begin with.

So think of it as a, you know, as a wall of sand and silt that's hundreds of feet tall. It's not like an outcrop of really hard rock.

CORNISH: What tools are there for identifying slopes that are vulnerable to landslides?

MONTGOMERY: Well, this particular site had been mapped out the landslide by the USGS and by state geologists. So one tool is to basically good old-fashioned geologic mapping. You can run models to try and look at the role of topography on controlling instability, sort of where the most unstable places ought to be. And you can run models in terms of keeping track of the rainfall that a region has happened, and seeing if we're getting close to the point where you'd expect to see landslides become a possibility.

The really, really hard thing to do, though, is to actually predict a particular slope will fail in a particular storm, on a particular day. And yet that's where a lot of hazardous assessment boils down: Do you want to know that? And it's very difficult to actually do that.

CORNISH: Now, short of moving, I mean, are there any preventative measures that people can take?

MONTGOMERY: Well, again, it depends on the nature of the landslide hazard. For this particular one, it would have taken a pretty massive engineering approach to actually stabilize that slope that have failed many times in the past, and then did once again. I don't think anyone had talked about actually doing that. And then, of course, you have the problem that the slope is probably owned by people who are different from the people living across the river, who are actually impacted by it.

This particular community that was impacted was on the floodplain of the river - it was flat ground. I mean, if you look just at that piece of property, you would have thought that the hazard it would've been from flooding, not from the hillside across the river coming down.

So we have this problem of how you connect the geological knowledge of the hazard to where it might go, but hasn't gone yet, and get that kind of information to the people who might be able to use it in their decision-making.

CORNISH: So what, if any, lessons can scientists, people take away from a disaster like this?

MONTGOMERY: Boy, well, one clear lesson is that slopes that have failed in the past may cut loose again without warning. And that geology can give you insight, but it won't necessarily give you the answers of, you know, should someone live there, should we allow people to live there, how might we regulate development in high hazard zones. These are questions that are - you know, transcend the scientific and get into the social.

And what's just absolutely tragic is that sometimes it takes a situation like this to get that conversation going.

CORNISH: David Montgomery, a geologist at the University of Washington, Seattle. He's also author of "The Rocks Don't Lie: A Geologist Investigates Noah's Flood."

Thanks so much for speaking with us.

MONTGOMERY: Thank you.

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