Dam expansion in Alaska

Located approximately eight kilometers east of the city of Sitka in Alaska, U.S.A., the Blue Lake Expansion hydroelectric project will help supply 100% of the city’s power needs.  Hatch provided planning, design, and engineering support during construction for the project expansion that included the unprecedented height extension of an existing thin concrete arch dam, refurbishment of both underground and surface water conveyances for higher pressure rating, construction of new intake tunnel and surge chamber, and  installation of new generating units in a new powerhouse.

The City of Sitka was looking for ways to better meet the increasing demand for energy without reliance on expensive diesel generation.  At the time, two hydroelectric facilities, Blue Lake and Green Lake with diesel generator back-up, were providing electricity for the city.  Due to its remote location on Baranof Island in Alaska, Sitka is unable to connect to the mainland power grid and is accessible only by air and sea. It was determined that one of the existing facilities would be upgraded.

The Blue Lake facility was chosen for expansion based on favorable geological conditions at the existing dam site and reservoir rim that would allow a significant increase in reservoir head. The existing Green Lake hydroelectric facility together with the upgraded Blue Lake facility will provide energy independence for Sitka. Diesel generation will provide back-up power in the event that either hydro plant is out of service or lake storage is depleted.

Based on Hatch’s structural and geotechnical analysis and design efforts, the existing 160 foot high concrete arch dam was raised by 83 feet. Technical challenges included updating the probable maximum flood (PMF) studies, analysis of the potential for plunge pool scour at the base of the dam, non-linear structural analysis for the dam raise, and stability analysis of rock slope stability with the increased dam height increased arch thrust loads.

POWER Magazine featured the Blue Lake Expansion project in its December 2015 edition. Read the article here.