Gorge 2nd Hydropower Tunnel

Jacobs Associates is leading the final 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, 22-foot (6.7 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 longstanding stewardship of the Skagit River fishery.

The preliminary design phase included 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 led geotechnical investigations, producing a geotechnical data report and geotechnical baseline report strategy memorandum. The firm also assisted with FERC relicensing documentation and tunnel condition assessments. The findings from the preliminary design phase proved technically and economically favorable, allowing the project to proceed into final design in 2010, with construction anticipated to begin in 2013.