Energy independence for Sitka
Remote dam expansion awarded by Engineering News Record
The Blue Lake facility, located approximately eight kilometers east of the city of Sitka, 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.
For the expansion, the existing 145-ft-tall concrete arch dam was raised by 83 ft to its maximum geotechnical height. Getting water to the turbines also required building a new intake tunnel and concrete intake structure, involving 120 vertical ft of rock excavation on the shores of Blue Lake.
Hatch provided planning, design, and engineering support during construction for the project expansion that included the unprecedented height extension of the 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.
There were several challenging aspects to the project, including getting all of the key materials and concrete via weekly barge shipments to site. The dam’s location in a canyon allowed vehicle access from only one side. The team relied on three cranes to get the work done.