Fish biologists, power marketers and hydro operators at the Skagit performed a carefully orchestrated dance between July 15 and 17 to prepare the Gorge Dam spill gates for an inspection, seal rehabilitation, and a new coat of paint.
It’s the kind of work that is fundamental for Seattle City Light’s mission of providing reliable, low-cost, environmentally responsible power to our customers.
The two counter spill gates, weighing a total of 1,120 tons, were immobilized and wrapped in a containment envelope to allow the work. But to access them, operators first needed to draw the Gorge reservoir down 50 feet. This would be tricky in any hydro project, but more so in a salmon bearing stream in the middle of a national park.
The operation was coordinated with Power Marketing, Water Resource Planning, Skagit Operations and the System Control Center, to ensure that power was safely generated during the process, and that downstream flows were not excessive for fish and recreational uses.
The drawdown took 60 hours to complete, under careful monitoring by North Cascades National Park and City Light biologists, who relocated fish such as bull trout that got trapped in drying ponds in the main river channel.
This project has been on hold the last two years due to higher than normal snowmelt flows that did not provide the window of time between fish activities and the fall storms. Prior to turning over the job to the contractor, a special condition assessment was done by Skagit crane, paint and safety crews, along with engineers from the Seattle Power Production group.
The work will be completed by Long Painting, Knight Construction and Magnum Construction Services, with contract management by City Light Construction Management and Team Skagit. It will continue through Oct. 1
The existing structure is the third dam for Gorge Powerhouse. The original dam was a wood structure that provided a small impoundment and diversion to the power tunnel. In the 1930s, a slightly higher concrete masonry dam was built, and finally in the late 1950s the existing “high” dam was constructed.