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Oaks Bottom trails reopened; history of slides cited by geologists

by: Merry MacKinnon According to a letter by BES Chief Engineer Bill Ryan to Environmental Services' Bureau Director Dean Marriott – referring to this partcular landslide last March above the Oaks Bottom Wildlife Refuge connector trail – Oaks Bottom bluff, like other bluffs along the banks of the Willamette River, is “prone to instability”. In his letter, a copy of which was obtained by <i>THE BEE</i>, Ryan wrote, “This particular bluff has a recent history of slumps and slides from minor to major in impact.”

About the time THE BEE was going to press for May, the Portland Bureau of Parks and Recreation finally elected to reopen all but one of the Oaks Bottom trails it had closed because of ongoing landslide hazards.

However, the number of landslides in last March's heavy rain, along the bluff rimming Oaks Bottom Wildlife Refuge in Sellwood and Westmoreland, was disturbing - especially for homeowners living in houses at the edge of the slope.

But to geologists who study landslides, what happened there, while unfortunate, was not all that surprising.

'What we're finding is that landslides are a widespread and chronic hazard in Oregon,' observed Bill Burns, engineering geologist with Oregon Department of Geology and Mineral Industries.

Burns and other geologists have compiled data on past landslide activity in the Portland metropolitan area. Their subsequent landslide inventory maps for the 'Portland quadrangle - Multnomah and Washington Counties' depict where past landslides have occurred, including some along Oaks Bottom bluff. The maps help identify susceptible areas.

'Where landslides occurred in the past is key to where they'd occur in the future,' Burns explained.

One factor that makes landslides so common is Oregon's history of massive floods and major volcanic eruptions. And, with landslide hazards, it's not just what lies deep underneath the soil that counts, but what's close to the surface as well.

'If you look at our maps showing the west side of the Willamette River, there are landslides all over,' Burns said - including the only-recently-discovered ancient slide which continues to compress the west end ramp of the Sellwood Bridge.

The Westside slide history is because of Portland's West Hills geology, which features basalt covered with a thin layer of wind-blown silt, he said.

But on the Eastside's Oaks Bottom bluff, the basalt is buried under thick layers of sand and silt deposited by the many successive ancient catastrophic 'Missoula Floods'.

'And then, over time, the Oaks Bottom slope has been eroded by the Willamette River,' Burns added.

Consequently, with enough rain, the water-saturated slopes, especially steep ones, can suddenly turn into debris flows - some small, like those in Oaks Bottom, but others large, like one years ago in the Columbia River gorge that consumed a house.

An additional worry is the potential for earthquakes to trigger landslides.

'One of our bigger fears is the secondary effects - such as landslides and tsunamis - of earthquakes,' Burns revealed. 'Anywhere there's been landsliding in the past - if there were an earthquake, there could be a landslide again, especially in winter when the soil is wet.'

Burns' advice for those living in houses at the edge of the bluff above Oaks Bottom, and who are concerned about slope stability, is to hire a geo-technical engineer.

'They can calculate the stability of a slope,' Burns said.

The Public Affairs Manager for the Portland Bureau of Environmental Services, Megan Callahan, echoed Burns advice: 'We would encourage the property owner to engage a geo-technical engineer,' she said.

A professional geo-technical engineer should examine the site for drainage, the degree of the slope, water, and the location of past landslides.

'You want to find out the geology of the slope," Burns recommended.

Oregon Department of Geology landslide maps can be viewed online, at: www.oregongeology.org/pubs/ims/p-ims-033.htm . You can also obtain a copy of the 'Homeowner's Guide to Landslides' online by downloading: www.oregongeology.org/landslide/homeowners-landslide-guide.pdf .