Corps orders safety evaluation of Fern Ridge Dam.

Byline: SCOTT MABEN The Register-Guard

Federal officials say the potential for Fern Ridge Dam to collapse in a large earthquake warrants an in-depth study of its structural integrity and the condition of the sandy base on which it sits.

A recent, routine seismic evaluation of the wide earthen dam on the Long Tom River, 12 miles west of Eugene, indicates that a large-magnitude quake could cause liquefaction in the base of the dam, the U.S. Army Corps of Engineers said Tuesday.

The phenomenon occurs when loose, saturated sands are subjected to seismic shaking, in effect causing earth to move as if liquid.

Liquefaction at Fern Ridge could cause "significant loss of foundation strength and possible damage to the dam itself," said Rich Hannan, chief geological engineer for the corps' Portland district.

The design of the dam and its location both factor into the potential risk, he said.

Built in 1941, Fern Ridge is an earth-filled dam 6,330 feet long and only 44 feet tall. The corps will try to determine the extent of the sand zone beneath the dam as part of its investigation.

The liquefaction threat emerged from relatively new information that suggests the potential for a large-magnitude earthquake off the Oregon Coast along the Cascadia Subduction Zone.

That's not to say people living below the dam should be worried, Hannan said. The preliminary study was based on the worst-case scenario, and the probability of a large subduction zone quake in the next 50 years is relatively low, he said.

But from a design standpoint, the probability is considered high enough to be a concern, he said.

"No one should worry specifically about the effects of damage to Fern Ridge Dam," Hannan said. "If the unthinkable were to happen - the failure of Fern Ridge Dam - calculations show there would only be minor flooding east of the community of Monroe."

Those floodwaters would not reach Monroe until 8 1/2 hours after the dam failed, giving residents plenty of time to evacuate, he said.

When Fern Ridge Lake is full, it contains 9,000 acre feet of water. An acre foot is enough water to cover an acre of ground to a depth of one foot.

There are no obvious structural deficiencies at Fern Ridge Dam, which has performed without problems for more than 60 years, corps officials said.

In addition to studying the sand zone, the agency will evaluate the amount of seismic shaking the dam can withstand and estimate the anticipated damages to the dam should a large quake hit. If the expected damages are found to be significant, the corps would pursue design changes to strengthen the foundation, Hannan said.

The study will begin as soon as practical, he said. Cornforth Consultants Inc., a Portland geotechnical firm specializing in dams, seismic studies and landslides, did the preliminary investigation on the dam and has been hired to further evaluate the dam's seismic stability.

Cornforth will discuss its preliminary findings today at the Oregon Water Resources Department's annual dam safety workshop in Wilsonville.

The corps performs safety inspections on each of its dams at least every five years. In addition, the agency has ongoing studies focusing on seismic safety in the Willamette Valley since the mid-1980s.

The corps maintains 13 multiple-purpose dams in the valley.