Grand Coulee Dam - Expansion - Third Powerplant

Third Powerplant

After World War II, the growing demand for electricity sparked interest in constructing another power plant supported by the Grand Coulee Dam. One obstacle to an additional power plant was the great seasonality of the Columbia River's streamflow. Today the flow is closely managed—there is almost no seasonality. Historically, about 75% of the river's annual flow occurred between April and September. During low flow periods, the river's discharge was between 50,000 cu ft/s (1,400 m3/s) and 80,000 cu ft/s (2,300 m3/s) while maximum spring runoff flows were around 500,000 cu ft/s (14,000 m3/s). Only nine out of the dam's eighteen generators could run year-round. The remaining nine operated for less than six months a year. In 1952, Congress authorized $125,000 for Reclamation to conduct a feasibility study on the Third Powerplant which was completed in 1953 and recommended two locations. Nine identical 108 MW generators were recommended, but as matters stood, they would be able to operate only in periods of high water.

Further regulation of the Columbia's flows was necessary to make the new power plant feasible. Water storage and regulation projects in Canada would be needed, as well as a treaty resolving the many economic and political issues involved. The Bureau of Reclamation and Army Corps of Engineers explored alternatives that would not depend on a treaty with Canada, such as raising the level of Flathead Lake or Pend Oreille Lake, but both proposals faced strong local opposition. The Columbia River Treaty, which had been discussed between the U.S. and Canada since 1944, was seen as the answer. Efforts to build the Third Powerplant were also influenced by competition with the Soviet Union, which had constructed power plants on the Volga River that were larger than Grand Coulee. On September 16, 1964, the Columbia River Treaty was ratified and included an agreement by Canada to construct the Duncan, Keenleyside and Mica Dams upstream. Shortly afterward, Washington Senator Henry M. Jackson, who was influential in constructing the new power plant, announced that Reclamation would present the project to Congress for appropriation and funding. To keep up with Soviet competition and increase the generating capacity it was determined that the generators could be upgraded to much larger designs. With the possibility of international companies bidding on the project, the Soviets who had just installed a 500 MW hydroelectric generator on the Yenisei River indicated their interest. To avoid the potential embarrassment of an international rival building a domestic power plant, the Department of the Interior declined international bidding. The Third Powerplant was approved and its appropriation bill was signed by President Lyndon Johnson on June 14, 1966.

Between 1967 and 1974, the dam was expanded to add the Third Powerplant. Beginning in July 1967, this involved demolishing the northeast side of the dam and building a new fore-bay section. The excavation of 22,000,000 cu yd (16,820,207 m3) of dirt and rock had to be accomplished before the new 1,725 ft (526 m) long section of dam was built. The addition made the original 4,300 ft (1,300 m) dam almost a mile long. Original designs for the powerhouse had twelve smaller units but were altered to incorporate six of the largest generators available. To supply them with water, six 40 ft (12 m) diameter penstocks were installed. Of the new turbines and generators, three 600 MW units were built by Westinghouse and three 700 MW units by General Electric. The first new generator was commissioned in 1975 and the final one in 1980. The three 700 MW units were later upgraded to 805 MW by Siemens.

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