Barnard added 83 ft. onto the 145-ft.-tall, 1950s-era concrete arch dam built into a narrow slot canyon on Baranof Island to bring the dam to its maximum geotechnical height.
In an effort to eliminate their dependence on diesel fuel for additional power, the City and Borough of Sitka set out to expand its hydropower generation by raising the Blue Lake Dam and constructing a new powerhouse. The dam impounds the 3.25-mile-long Blue Lake Reservoir from which the area has been drawing most of its power for more than 50 years. The project included construction of a new ogee spillway with wing walls atop the dam extension and a new 15 MW hydroelectric powerhouse equipped with three 5.3 MW horizontal Francis turbine generators. Getting the water to the turbines required a new intake tunnel and concrete intake structure, which required significant vertical rock excavation on the shores of Blue Lake.
This project required in-water work with strict attention to the environment, given that Blue Lake contains some of the cleanest water in the U.S. and acts as the primary water resource for Sitka and the community at large. Water quality was never an issue throughout the duration of the project. The project included challenges inherent to construction in a remote location and throughout the continued operation of Sitka’s existing hydroelectric operations. Additionally, work was completed through a 65-day generation outage to commission the new intake, gatehouse, intake tunnel, penstock, and new powerhouse, which was completed 14 days ahead of schedule.
Location: Sitka, Alaska
Owner: City and Borough of Sitka, Alaska
Construction Manager: McMillen, LLC
Contract Type: Fixed Lump Sum & Fixed Unit Price
83-ft. dam raise
500 LF of 8-ft.-dia. penstock
360 VF 11-ft.-dia. surge shaft