Gorge 2nd Hydropower Tunnel

Jacobs Associates is leading the preliminary design effort of the Gorge 2nd hydropower tunnel through approximately 11,000 feet (3,353 m) of solid rock between Gorge Powerhouse and the intake at Gorge Dam on the Skagit River in Washington State. The proposed Gorge 2nd Tunnel will be a largely unlined, 14 to 18 foot (4.3 m to 55.5 m) diameter tunnel bored between the Gorge Powerhouse and the intake at Gorge Dam. It will parallel the existing tunnel, increasing generating efficiency by reducing head loss. This will allow Seattle City Light to better optimize the powerhouse without altering flows to the Skagit River. This is important for maintaining Seattle’s long standing stewardship of the Skagit River fishery.

The preliminary design phase includes hard rock Tunnel Boring Machine (TBM) boreability and tunnel lining studies, tunnel diameter optimization and layout studies, hydraulic transient analysis, site layout evaluations, water quality studies, a water management plan, and cost estimates of the recommended tunnel and connection configurations.

In conjunction with this work, Jacobs Associates is leading preliminary geotechnical investigations, which will produce a geotechnical data report and geotechnical baseline report strategy memorandum. If the findings from the preliminary design phase prove technically and economically favorable, work will proceed onto final design and construction.