The 6809MW Grand Coulee dam on the Columbia river in the US is the country’s largest hydro power producer. It also ranks as the fourth largest in the world behind China’s Three Gorges, Guri dam in Venezuela and Itaipu on the Brazilian/Paraguay borders.
Grand Coulee dam is a key feature of the US Bureau of Reclamation’s (USBR) Columbia basin project in Washington state: a multi-purpose scheme which provides flood control, irrigation, hydro power production, recreation, and benefits for fish and wildlife.
Standing at 167m high and 1592m long, Grand Coulee was described as a ‘monster dam’ during construction in the 1930s. Although the dam is today a hydro power ‘giant’, the initial requirement for such a structure was increasing demands for irrigation water to support agricultural development within the Columbia river basin area. This can be traced back to the origins of ‘dry farming’ in the early 1900s, when large numbers of settlers undertook the practice of dry land farming on rangeland which had previously been used for grazing.
As the average annual rainfall over the entire area is less than 25cm, this could never have been a long term agricultural development. Above average rainfall over successive years after the grasslands were first ploughed made the practice seem more successful and appealing, but the reality was that dry farming was doomed to failure – except on land with deep soil and high water-holding capacities.
In 1902 the Reclamation Service (now the USBR) was established, and became increasingly aware of the difficulties associated with dry farming. Interest therefore was initiated in the possibility of using water from the Columbia river for irrigation. Two years later the agency undertook studies into the idea but the problem of lifting water from the river’s deep canyon, onto the plateau surface for farming, was more than such a newly established organisation could handle.
By 1918 there was growing local interest in proposals for an irrigation project in the area which could water farm land over 80km away. One idea was to build a high dam on the Columbia river at the head of the upper Grand Coulee (Grand Coulee is a unique geological feature in the ancient river bed which was formed when the original channel was blocked by glaciers during one of the Ice Ages.) A second proposal was to look at constructing a canal to convey water from the Pend Oreille river in Idaho across the plateau surface.
However in 1932 the US Corps of Engineers recommended that the Grand Coulee dam should be constructed, and water pumped out of the reservoir into the irrigation system using power generated at the dam. Excavation at the base of the dam got under way in December 1933, and transport connections to the dam site were also started. By August 1934 the first of two major contracts for construction of the dam and power plant was awarded to a consortium of three construction firms (Silas Mason Company, Walsh Construction Company and Atkinson-Kier Company) known as MWAK Company.
The plan for building Grand Coulee dam had originally focused on a two-staged approach. A low dam (107m high) was to be built first with a foundation designed so that a higher dam could be superimposed on it at a later date. A pumping plant and other components of the irrigation system would also be added at the second stage. The thinking behind this was an economic motivation – the government had baulked at the cost of building a high dam during the great economic depression of the 1930s. The plan to start with a lower dam was a blow for supporters of the irrigation scheme as it would not be able to pump water for irrigation, although it could be used to generate power.
However in June 1935 the Secretary of the Interior signed an order to change construction to the high dam. This wasn’t a surprise as there had been concerns about successfully placing newer concrete to old. Plus at the time there wasn’t much need for extra power while the low dam option would still not have provided any relief for irrigators.
In February 1938 the contract for completion of the dam and appurtenant works was awarded to Consolidated Builders. This was a merger of MWAC and Six Companies (the company which had built the Hoover dam). The two giants had decided to merge when bidding for the contract, as by doing so they could cut costs by 20%, stay within USBR’s estimates and still pay minimum wages as stipulated by the government.
From 1933-41 construction of the dam proceeded well. However, the onset of World War ll initiated changes and construction centred entirely on the installation of units in the left power plant. As the war progressed and the war industries demanded more power, work was brought forward. An order was issued to construct the right power plant in December 1941.
The last of the 18x108MW units was placed into commercial operation in September 1951. Eighteen years after construction had begun, the power plant was able to deliver maximum power production to meet needs in the Pacific Northwest, as well as for pumping irrigation water across the Columbia basin farmland.
By 1954 USBR was looking into proposals for a third power plant at Grand Coulee. This had been prompted by the rapid expansion of the Pacific Northwest power market which had experienced power shortages during 1951-2. Work on the project didn’t begin until 1967 and was completed in May 1980 when the sixth unit swung into operation.
Grand Coulee is one of the largest concrete structures in the world, and the largest to be built in North America. The 167m high concrete gravity dam raises the water surface 116m above the old river bed, and contains over 9Mm3 of concrete.
The original dam was modified for the third power plant by the addition of a 356m long, 61m high forebay dam along the right abutment. The length of the main dam, forebay dam and wing dam is 1592m. The spillway is controlled by 11 drum gates, each 41m long, and is capable of spilling 28Mm3/sec when Lake Roosevelt reservoir behind the dam is at full pool.
The average discharge at Grand Coulee over a period of years is approximately 3000m3/sec. On 12 June 1948, during an historic Columbia river flood period, the maximum discharge (turbine and spill) was recorded at 18,000m3/sec. The April to July inflow accounts for 65-70% of the total annual inflow volume.
Grand Coulee power complex consists of:
• Left power plant (1155MW) – contains 9x125MW units and 3x10MW station service generators, located at the downstream face of the main dam.
• Right power plant (1125MW) – contains 9x125MW units and is located at the downstream face of the main dam on the east side of the spillway.
• Third power plant (4215MW) – contains 3x600MW and 3x805MW units. This is located on the downstream face of the forebay dam.
• Pump-generating plant – 314MW located at the wing dam on the left abutment.
After over 30 years of operation the six generating units in the third power house are now ageing and becoming problematic, which has resulted in increased power outages and reduced reliability. As a result USBR is undertaking a refurbishment project. Speaking about the project, US Interior Secretary Ken Salazar said that it will allow USBR to modernise and maintain Grand Coulee dam so it can still be a key component in the nation’s energy mix. ‘Grand Coulee dam is a vital part of our nation’s infrastructure,’ he added.
The main focus of the project is to refurbish turbines G19-24 in the third power plant. Work will be performed on the generators, turbines, shafts and auxiliary equipment with the overall goal of ensuring that they will provide another 30 years of service.
Units G-19 to G-21 are equipped with Francis turbines, with a runner throat diameter of 10.3m, designed by Dominion Engineering Works and manufactured by Willamette Iron and Steel Company. Units G-22 to G-24 are equipped with Francis turbines, with a runner throat diameter of 9.7m, designed and manufactured by Allis-Chalmers.
Generators G-22, G-23, and G-24 are rated to produce a total of 2415 MVA with an annual power production value of over US$100M. These units will be overhauled first and the work will include inspecting, refurbishing or replacing worn mechanical and electrical components, including bushings, seals, controls, and deteriorated wiring. Stator cores or windings for units G-22, G-23, and G-24 were replaced recently and should not need attention beyond inspection and cleaning. Rotors will be inspected and repaired as needed.
Units G-19, G-20, and G-21 will be uprated in overall unit capacity with new generator and turbine components.
TablesTable 1 Key facts