US$10 million reservoir expansion under way at 25-MW Swan Lake hydroelectric facility in Alaska

Swan Lake Dam

A US$10 million modernization project is under way to increase the Swan Lake Reservoir storage capacity at the 25-MW Swan Lake hydroelectric project in Alaska, from about 86,000 acre-feet to 111,800 acre-feet.

Located on Revillagigedo Island at the head of Carroll Inlet, about 22 mi northeast of Ketchikan, construction work began this week despite unexpectedly high water levels at the scheme, which is owned and operated by the Southeast Alaska Power Agency (SEAPA).

The work is expected to be completed by the end of this year or early 2017.

Currently, the Swan Lake facility includes:

  • Swan Lake Reservoir with a surface of 1,500 acres at normal maximum elevation and useable storage capacity of 86,000 acre-feet between elevations 330 ft and 271.5 ft;
  • A concrete thin arch dam, 174-ft in height;
  • A 100-ft-long uncontrolled ogee spillway section with a crest elevation of 330 ft;
  • A 2,217-ft-long by 11-ft-diameter water conveyance tunnel;A powerhouse that contains two Francis turbine generating units with a total installed capacity of 25 MW; and
  • A 115 kV transmission line extending 30.5 mi to the existing S.W. Bailey Substation.

Construction crews encountered difficulties installing safety walkways, form anchors and other equipment when beginning the project due to higher than normal water levels in the reservoir. The higher levels resulted from snowmelt during last year’s warm winter and high rainfall amounts during this spring, according to published reports.

Project managers say they are working to protect crews working on the parapet -- the low protective wall running along the top edge -- at Swan Lake Dam by further reducing reservoir levels in anticipation of relatively high rainfall amounts this summer.

Highlights of the expansion include:

  • Increasing the reservoir full-pool elevation 15 ft, providing about 25 to 30% more storage;
  • Flashboards and a gate will be installed in the existing fixed spillway slot;
  • Parapet wall reinforcement; and 
  • An increase in the height of water intake equipment.

Annually, according to SEAPA, the expansion will provide up to 12,000 MWh of additional energy. In terms of offsetting diesel generation, increased hydropower output will displace up to 800,000 gallons of expensive diesel fuel consumed by its member utilities. The utility also said results from the modernization project would eliminate about 18 million pounds of CO2 from being emitted into the atmosphere.

Eric Wolfe, SEAPA's director of special projects, told the agency's board the week of June 12 that the work would result in increased water storage at Swan Lake and also increase flexibility for usage of energy generated at SEAPA's other plant, the 25-MW Tyee Lake project. The project is located at the head of Bradfield Canal, about 40 mi southeast of Wrangell, 60 air-miles northeast of Ketchikan and 70 air-miles southeast of Petersburg.

Near the end of project construction, the state of Alaska, under the Alaska Energy Authority, assumed ownership of the project from the city of Ketchikan and began commercial operation in June 1984. Ownership transferred from the state to the Four Dam Pool Power Agency (FDPPA) in 2002, and SEAPA assumed ownership when the FDPPA was restructured in 2009.

The ownership exchange provided financing to construct a power line connecting Ketchikan to Tyee Lake via Swan Lake. The new power line allowed for better utilization of hydropower to displace Ketchikan diesel generation with the dispatch of SEAPA assets Swan Lake and Tyee Lake, governed by a new, long-term power sales agreement.

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