Landslides don't bring David O'Longaigh down.

For 12 years, O'Longaigh has been the supervising engineer for bridges and structures at the Portland Bureau of Transportation. That's a job that usually involves managing the design of bridges, as well as keeping an eye on the 157 bridges, 566 retaining walls and 188 stairways that PBOT currently owns.

But this past week has been different. Since Dec. 6, Portland has been deluged by more than 8 inches of rain. The result: flooding, power outages and 20 landslides within city limits—all of them occurring in the first 72 hours of heavy rain between Dec. 6 and 9. Most of these landslides occurred in the posh West Hills (see map at right).

O'Longaigh manages the team of transportation engineers who orchestrate the city's response when a landslide smashes into a road. Last week, PBOT says, crews removed at least 900 cubic yards of debris from Portland roads. That's enough dirt and rocks to fill 75 dump trucks.

O'Longaigh took a brief break from wading through mud to tell WW how he can tell if a landslide is still dangerous, what falling rocks cost the city and what it would take to remove the threat completely.

WW: What does the city's response to a landslide look like?

David O'Longaigh: Our first thing when we get there is to assess the slide to see if there's any energy left so that it's still going to move.

We've had a number of moderate slides along the banks of creeks where the roadway is above a creek and there's been a fall towards the creek. If we don't get there immediately, the road could fail. Our crews [install] a small rock buttress—angled rock that knits together and creates a really stable slope. It takes maybe three to four hours, and you've saved literally thousands of dollars of repair on the street.

Portland seems to have a consistent, repeated pattern because the West Hills generally feel the same everywhere: low-height hills, similar yards, similar soil types. Over time you get a feel for a slide, you almost have seen it before. And we do rely on rapid assessment, because when it rains, it pours. We don't just have one, sometimes we have many that day that we've got to get to.

How do you know when you can leave a slide alone?

Soil has natural angles of repose, and silty soil has a natural angle of between 20 to 30 degrees, depending on how stiff it may be. You gauge that angle in the slope as you're observing it. You can see if there's any, for example, trees remaining that look like they're unstable, ready to topple, and if there are, we try to take those down before they do fall.

How much do landslides cost to fix?

The small ones, the ones that we call "pop-outs," where the earth pops out and we pick it up and walk away, the cost is very minimal. It's a crew arriving with a truck, loading material and then driving away. So the cost of that is probably a lot less than a thousand dollars per incident.
But the moderate ones are between $50,000 and $100,000, and the really significant ones that occur maybe once every five years can be a half a million to $1 million.

Are the numbers of landslides increasing much?

We have good winters and we have bad winters. On an average good winter, you have maybe 12 incidents. But in a severe winter, five to 10 dozen. We have more this week than we've had the last two winters combined. It's too early to say whether this is going to shape up to be a bad winter, overall, although it has been one hell of a week.

What was the most memorable landslide you saw last week?

What was dramatic is when [West] Burnside [Street] became a river on Monday [Dec. 7]. That blew out the ditches, because there was so much water coming down the hill, and Burnside Street was filled with debris and water, almost like a riverbed, and walking across Burnside you're literally walking through six inches of water. My foot was underwater. It was a bizarre feeling to see Burnside like that, almost like a return to the wild.

Are there problem areas on the city's radar? Any way to proactively 
fix them?

That's an interesting question. The West Hills is defined by how it looks, like natural slopes, vegetation, trees, ditches. To engineer ourselves out of landslides, it would completely alter the West Hills and its appearance. Instead of West Hills, it'd be West Walls. You'd be building retaining walls everywhere, and I'm pretty sure it wouldn't be the idyllic stone wall like you find in Italy. They'd be ugly concrete block walls. Engineering is a great tool, but it doesn't solve every problem.

MORE: Find the landslide nearest you with this interactive map.