Sean C. Morgan
Green Peter Dam will remain closed through Dec. 1 while the U.S. Army Corps of Engineers repairs and upgrades one of the spillway gates.
The Corps started repairing gates throughout the Willamette Valley Project after a 2008 inspection revealed buckling in the strut arms of the spillway gates in Foster Dam. The Corps repaired the first of those four gates in 2009. Since then, the Corps has completed repairs at two other dams in the Willamette Basin.
“We were seeing a lot of friction in the trunnion pins,” said Corps Spokesman Scott Clemans. The trunnion pin serves as the pivot point for the gates, with the struts stretching from the pins to the gates. The friction can cause the struts to twist and deform.
The Corps is replacing the pins with new greaseless trunnion bearings, replacing the strut arm structures supporting the gates, replacing motors and electrical systems and strengthening the gates, which are 48 feet tall and 45 feet wide. The structures are original. Green Peter Dam was completed in 1967.
“The standards for Tainter gate design have evolved since these gates were put in place,” Clemans said.
The Corps has found twisting and buckling in spillway gates throughout the Willamette Basin, he said, although the Corps has not observed any deformation yet in the Green Peter gates.
“What we’re concerned about is these gates won’t perform like we want them to, particularly when there is water pressure on them,” he said, adding that the gates may not open properly or, in the worst case, may actually break.
“We don’t use them all that much,” Clemans said. They are used much more often at other projects for downstream temperature control.
Generally, the gates are the last resort in flood control, he said. Most of the time, water is sent through regulating outlets and through the power house.
After discovering problems at Foster Dam, the Corps began looking at other dams, finding more problems and prompting the Corps to adopt “interim risk reduction measures,” Clemans said. Under the measures, the Corps does not let the water on the gate reach above the top strut.
“So we’ve lost some storage in the reservoirs, in Green Peter and many others,” he said. As a result, downstream, following a series of large storms, property owners will see higher water for longer periods.
Right now, the water level at Green Peter is 20 feet below the spillway crest, lower than normal low-pool levels, Clemans said.
The Corps has finished work at Foster, Big Cliff and Dexter re-regulation dams, Clemans said. “Now we’re starting to work on storage projects.”
In addition to Green Peter, the Corps is working on a gate at Fall Creek Dam.
The project costs $3.2 million, Clemans said. The second gate will cost $3 million. Completion of the second gate should be in spring 2016. The second gate should take less time because much of the preliminary work has been completed already.
Knight Construction of Deer Park, Wash., is the contractor on this job for the Corps.
Knight also completed the project at Foster.
The first spillway gate is scheduled for completion in February, Clemans said. So far, it is on schedule and on budget.
In preparation for the project, the Corps coordinated with nearby landowners, including Cascade Timber Consulting, Seneca Jones Timber Company and Giustina Resources, and with emergency services such as Sweet Home Fire and Ambulance District and Oregon Department of Forestry.
The Corps started work two months ago, Clemans said, with a total closure of the dam road from Oct. 1 to Dec. 1. The idea was to begin work as fire season wound down but before the flood season began.
“We have a big honkin’ crane parked up there,” Clemans said. “In the middle of a critical lift, that crane is not going anywhere.”