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Gorge 2nd Tunnel


This project has its roots in 1949 when Seattle City Light was authorized to install a fourth generator (Unit 24) at Gorge Powerhouse. This increased Gorge's output, but it also increased the volume and speed of the water moving through the power tunnel. Increased water speed is undesirable for power production because higher velocity creates more drag, which, in turn, reduces the amount of electrical energy produced by the generators. The Gorge 2nd Tunnel (G2T) project proposes to build a 22 foot diameter, 11,000 foot long tunnel, running in parallel with the existing power tunnel, to reduce that drag. This efficiency improvement will turn the same amount of water into more energy production. This $70 million project is expected to capture nearly 56,000 Megawatt–hours of energy a year, with no change in flow through the powerhouse – enough to power 5,000 homes. City Light will be able to credit this project as new renewable energy under Initiative 937. The renewable energy captured by the project translates to a reduction in greenhouse gas emissions equivalent to keeping 6,000 cars off the road.

The second tunnel will provide additional environmental benefits. The rock tailings from the tunnel boring machines will be used as fill in a former quarry site at Bacon Creek. After the site has been re–contoured, it will be restored as wildlife habitat. The project will require an amendment to the FERC licensing agreement which will include formalization of the voluntary operational flow practices that City Light has adopted to help keep the Skagit River a healthy place for fish. One of the things that is remarkable about this project is that all these benefits are coming to us with only minimal, temporary impacts.

Project Milestones
  • Submit FERC Amendment Application – June 30, 2011
  • Design Complete – Mar 2012
  • FERC Amendment Approval – Jul 2012
  • Construction Dates to Be Determined