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Landslide risk highest where disaster struck before

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FILE PHOTO -  In Nov. 2014, a landslide closed a section of Highway 224 near milepost 36 near Estacada. The highway had already been closed between mileposts 31 and 38 since the 36 Pit Fire because of increased risk of slides exacerbated by recent heavy rains and the undergrowth having been burned away from the steep slopes near the highway.

If a massive earthquake triggered landslides across Oregon, what would be the state’s pricetag for repairs?

Oregon’s geology department says $1 billion.

The risk posed by landslides — even during a much less severe scenario — is real.

More than 7,000 residents and 3,000 buildings are located on large, deep-seated landslides in Clackamas County, and the total value of that land is estimated at $832 million.

The historical record shows that roughly 7 percent of county soil has shifted over the years, though some of these slides are decades or centuries old.

But the state’s Department of Geology and Mineral Industries (DOGAMI) believes it doesn’t matter much how old the evidence of a past landslide is, especially when it comes to deep-seated slides.

“These landslides should be considered just barely stable and in most cases would require only a small change in stability to reactivate,” a recent study of the region states.

So what parts of Estacada are the most dangerous for these sudden cascades of rock and dirt, and how does the state study landslide risk?

“We go into deep geologic time,” explained Bill Burns, one of the department’s top rock researchers.

“Some (landslides) could be from 10, 20 or 30 years ago, some are thousands of years old, some are 10,000,” he said. “How old they are is less important than where they are.”

With 7,000 confirmed landslides in the Portland metro area’s geologic record, the risk is real.

In Clackamas County alone, DOGAMI researchers have found evidence of 370 landslides over the last 45 years, which they estimated caused roughly $137.5 million in damages.

Landslides are known to have external triggers like heavy rains and flooding. It’s no coincidence that some of the worst landslides in the area happened during the February floods of 1996.

Destruction costs from landslides that year was pegged at $75 million, according to a state estimate.

“Landslides can happen very quickly, with no warning,” said Ali Ryan Hansen, an agency spokeswoman. “There’s potential for damage to property, but also loss of life.”

Burns, the geologist, just finished publishing a paper on how to map the areas of Oregon deemed high risk for landslide disasters. The information is vital for city and regional planners, plus ordinary homeowners looking out for their own safety.

“Having a published methodology makes sure we’re consistent no matter where in the state we’re mapping,” Hansen noted.

Burns has previously worked on landslide-threat maps for the Bull Run Watershed (where a quarter of Oregonians get their drinking water) and Clackamas County. A risk map for Multnomah County is expected to land sometime next year.

Whatever you do, don’t picture Burns next to a tripod, holding a level and protractor. These risk maps compile reams of geologic data, historical accounts and the latest topographic imaging. Geologists now frequently use a system of plane-mounted laser and radar to plot the Earth’s contours, not old-fashioned land surveying equipment.

It’s called LIDAR, and the technology can “see through” vegetation to the underlying soil in ways that humans can’t.

All this work sounds highly technical, and it is. But at its core, Burns is simply trying to predict Mother Nature’s next calamity before it happens.

“What we see over and over is that (for pre-existing landslides), either a portion of it reactivates, or the whole thing reactivates,” Burns said. “So the absolute most important thing is figuring out where those existing landslides are.”

So where are these existing slides and their corresponding hot spots for danger?

The good news is that 90 percent of Estacada is considered a low hazard zone for serious landslides. That number drops to 66 percent when looking at shallow landslides, which tend to move faster, but affect smaller areas.

However, the high elevation land directly to the east of Estacada High School is some of the highest rated for landslide activity. Milo McIver State Park is another trouble spot.

Springwater Road is buffered on both sides by landslide risk zones. There are reports of landslides on Southwest Highway 224 dating back to the 1996 floods.

“Once you get into the Gorge, into Troutdale and east of there, there are a lot of landslides,” Burns recounted.

Burns believes this data will help homebuilders site new developments and stay away from obvious trouble spots. For instance, construction crews would want to avoid channeling stormwater onto a steep slope with a high hazard level.

“People figuring out whether or not they’re in an existing hazard area is the key first step,” he said.