Report finds multiple issues with aging dam
Dec 15, 2013
Failure is unlikely, but continued deterioration inevitable
By Scott Hammers
An engineering report by PacifiCorp suggests the Newport Avenue Dam is unlikely to experience a catastrophic failure, but without repairs it will continue deteriorating until it can no longer maintain normal water levels.
The 21-page report, published Dec. 10, summarizes the findings of a dam inspection conducted Oct. 31 and Nov. 1 that led PacifiCorp to conclude the dam is not worth repairing. The inspection was scheduled after a new leak was discovered in the dam earlier in October, causing water levels in Mirror Pond to drop approximately 2 feet.
The author of the engineering report, PacifiCorp’s chief dam safety engineer Roger Raeburn, said Friday the new hole is big enough for a basketball to pass through it. Even if the hole was patched, Raeburn said multiple elements of the century-old dam are deteriorating, with each failure putting excess stress on the other elements and leading to further failures.
“You could almost consider it a natural progression, it’s the aging of the dam,” Raeburn said.
The dam includes 21 separate “bays,” each of which consists of a log cabin-style box built of large timbers, weighted down with loose rocks, ranging in size from, in Raeburn’s words, “a Nerf football to a five-gallon bucket.”
Seven of those bays were constructed as “needle bays,” in which the rock is suspended on a platform above the river bottom, creating a void at the base of the dam. The “needles” — no longer in use — were vertical planks which could be moved up or down, allowing the dam operator to control how much water flowed under the dam.
On the upstream side of the dam, horizontal wooden planks are affixed to the timber bays. Raeburn said due to gaps between planks and the porousness of the rock fill, none of the bays are completely watertight — even those in good condition allow 20 to 30 gallons a minute to flow through — the needle bays are much leakier than the rest of the dam.
“We have had the greatest amount of deterioration in those needle bays,” he said. “For whatever reason, the episodes we’ve had, like we’re having right now, have happened in the needle bays.”
Raeburn said there are multiple elements to the needle bays that contribute to their deterioration. When the needles were decommissioned, the planks added to the upstream side of the needle bays were smaller than those used in other parts of the dam, he said, creating more gaps for water to flow through the dam.
Several upstream planks on the needle bays have broken or fallen off, Raeburn said, allowing excess water to flow into the rock and timber portions of the dam. In at least five of the 14 needle bays, the platform that held the fill rock in place has collapsed. Raeburn was unable to inspect the condition of the rock fill or its supporting platform in bay 11, site of the newest leak, due to the amount of water flowing through that section of the dam.
Once the rocks have shifted or fallen, some of the smaller rocks are washed downstream, Raeburn said. Without the weight of the rock to support the wooden components of the dam, the load on the upstream planks is increased, he said, leading to the failure of more planks and the loss of more rock fill.
One bright spot in the inspection is that given their age, the timbers and many other wooden components are in good condition.
Holes drilled into the timbers found no soft or rotting wood, Raeburn said, and the nails holding wooden parts together appeared to be firmly in place.
“That’s one of the beauties of the rock-filled dam, if you keep wood wet, it’ll last a long time,” Raeburn said. “If you get it wet, and dry it, get it wet, and dry it, it won’t.”
Raeburn’s report does not include any cost estimates of possible repairs.
PacifiCorp spokesman Bob Gravely said the dam is not so far gone that it could not be repaired to again generate electricity — the generators have been off since the discovery of the leak in order to keep the pond level up — but that needed repairs aren’t cost effective given the facility’s meager generation capacity.
“That’s where the economics come in, do you do that, at what cost, and to what end?” Gravely said. “And for us, the question is do you do what you could to prolong the life to generate power, and does that make sense economically?”
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