Dam inspections indicate Foster, Green Peter in good shape

Sean C. Morgan

After conducting its five-year inspections on Foster and Green Peter dams last week, The U.S. Army Corps of Engineers found nothing unexpected or out of the ordinary.

Every five years, the Corps conducts a multi-disciplinary inspection of all of its dams, said team leader Matt Chase, a structural engineer who has worked for the agency for 17 years. The inspection team includes personnel from several engineering disciplines, including structural, mechanical, electrical and hydraulic. It also includes geologists.

Chase said his team spent Wednesday and Thursday of last week at Green Peter Dam and Monday at Foster Dam.

“Typically, we’ll walk the crest,” Chase said, and they’ll inspect the “galleries” inside the dam, along with the gates and the powerhouse. “We want to maintain the safe operation of the project. We’re looking for signs of distress. We need to get in front of it.”

Last week, the inspectors found nothing major, he said. Upon completion of the inspection, the team completes a report with a list of recommendations for maintenance and for repairs.

The scrutiny is among several regular inspections conducted by the Corps, he said. All of the dams have less intensive annual inspections, as well as quarterly inspections by the project staff and monthly inspections based on instrument monitoring.

“Our dam safety program encompasses a lot of different kinds of activities,” said Ross Hiner, Dam Safety Program manager. The inspectors look at the structure and performance. They track water pressure, water levels and water flows through the dams.

They monitor data over time and try to identify the shifting of the dam structure in the fractions of an inch, Hiner said. Embankment dams, like Foster, will settle and move fractions of an inch over decades, but even concrete dams, like Green Peter, can flex.

Hiner said crews conduct a climbing inspection every five years as well.

“A lot of things you can’t see 20 to 30 feet away,” he said.

It was through the five-year climbing inspection program that the Corps noticed the deformation of the spillway gate trunnion arms at Foster Lake in 2008. The agency repaired and strengthened the gates and has since gone on to repair several other spillways in the Willamette Basin, including Green Peter. Three other dams in the Willamette Basin are scheduled for the same repair. Detroit dam’s gate repair project is under construction this year.

The gates themselves are key to dam operations, Hiner said. “Those gates get tested very frequently – on a daily basis.”

This year’s inspections around Sweet Home went well this year, he said.

“What I’ve heard from the team lead, we didn’t find anything out of the ordinary that we weren’t expecting,” Hiner said. That’s the result of annual and daily operations keeping eyes on the projects. With those eyes constantly on the project, the Corps shouldn’t find any surprises during its larger inspections.

He addressed the two or three leaks in the face of Green Peter Dam. Like The New Era, the Corps of Engineers frequently hears questions from the public about them.

The Corps is aware of the leaks, he said, and they are tied to the technology used when the dams were constructed.

At the joints of the concrete blocks that make up Green Peter Dam, the Corps used copper water stops that run along the joints to keep water from moving through the dam. The blocks are designed to allow the dam to expand and contract with the temperature without cracking the concrete.

Today, the Corps would use a synthetic polymer as a water stop, Hiner said. The copper stops corrode and wear over time, allowing water to enter the dam from the lake side.

That water travels between the blocks until it reaches the downstream side where it plummets down the face of the dam into the river or bank below.

“It’s something we’ll put into future budget requests,” Hiner said, adding that the repair is difficult and requires drilling down 200 to 300 feet into the dam, but it’s not going to erode the dam’s joints.

In the meantime, the Corps would like to begin collecting and monitoring the flow to determine how much is coming through and also how changes in the pool and weather affect that flow and the leaks, Hiner said.

The inspection program is just a part of the Corps safety program, Hiner said. The Corps is just wrapping up a periodic risk assessment of Green Peter Dam, and the results will be available later this year. Foster Dam is undergoing a risk assessment.

During risk assessments, the Corps dives into a lot of detail to look at possible future outcomes of various hazards that might affect the dams, particularly earthquakes, he said. The risk assessments primarily consider the potential consequences of a hazard, the loading on the dam and the performance of the dam. At the center of it is the size of the dam, the volume of water stored behind it and downstream population centers, which adds to potentially high consequences for a dam failure and a higher risk level.

He noted that concrete dams, like Green Peter, usually perform “very well” in earthquakes, while mostly earthen dams, like Foster, have a few more variables to consider.

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