| Folsom Dam | |
|---|---|
Folsom Dam Release |
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| Location | Folsom, California |
| Construction began | 1948 |
| Opening date | 1956 |
| Construction cost | US$81.5 million |
| Dam and spillways | |
| Height | 340 ft (103 m) |
| Length | 1400 |
| Impounds | American River |
| Reservoir | |
| Creates | Folsom Lake |
| Capacity | 977,000 acre·ft (1.205×109 m3) |
| Surface area | 10,000 acres (4,000 ha) |
| U.S. Geological Survey Geographic Names Information System: Folsom Dam | |
Folsom Dam is a concrete gravity dam on the American River in Northern California, about 25 miles (40 km) northeast of Sacramento. Folsom Dam is 340 feet (100 m) high concrete and 1,400 feet (430 m) long, flanked by earthen wing dams. It was completed in 1955, officially opening the following year.
The dam, at the juncture of the north and south forks of the American River, was built by the United States Army Corps of Engineers, and upon its completion, was transferred to the United States Bureau of Reclamation. The dam and lake are part of the Central Valley Project, a multipurpose project that provides flood control, hydroelectricity, drinking water and water for irrigation. Upgrades to provide increased flood protection for Sacramento are scheduled to be completed in 2015.
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Folsom Power Plant is located at the base of the dam. It has three Francis turbines with a combined capacity of 199 megawatts. The power plant's electricity production is intermediate, between peaking and base load. It operates during the day, when the demand and price for electricity is the highest.
The current Folsom Dam, built in 1955, replaced an earlier, smaller dam that had been completed in 1893 by Horatio Gates Livermore. The earlier dam had fed the Folsom Powerhouse, generating electricity that was transmitted to Sacramento over a 22 miles (35 km)-long distribution line, the longest electrical distribution system in the world at the time.[1] The remains of the earlier dam can be seen downstream from the Folsom Lake Crossing.
The current dam was originally authorized by Congress in 1944 as a 355,000 acre·ft (438,000,000 m3) flood control unit. Folsom Dam was reauthorized in 1949 as a 1,000,000 acre·ft (1.2 km3) multiple-purpose facility. Construction of the dam began in October 1948 and was completed in May 1956.[2]
Some old settlements and towns were flooded when the lake was filled, including Mormon Island and Salmon Falls.
On July 17, 1995, a spillway gate failed, and nearly 40 percent of Folsom Lake drained before it could be repaired. Nearly 40,000 cubic feet (1,100 m3) flowed through the broken gate. The freshwater reaching San Francisco Bay confused salmon and striped bass, whose instincts told them that fall rains had arrived, and they began their annual fall migrations months ahead of schedule.[3]
After a year-long investigation, the United States Bureau of Reclamation attributed the failure to a design flaw. The gate, which rolls up and down like a giant garage door, rotates on giant brass and steel pivot pins. Corrosion on the bearing surfaces, caused by an inadequate and not sufficiently waterproof lubricant, greatly increased friction in the gate's pivot bearings, which seized up as the gate was being lifted. The gates are supported by a system of radial beams that transmit the live load of the water pressing against the gate to the concrete dam structure through the main pivot bearings (one on each side of each gate). The gates are raised with an electric motor which engages a chain drive attached to the gate, through a very high-ratio reduction gear set. The torque multiplication provided by the reduction gear allowed the drive motor to continue lifting the gate despite the seized pivot bearing, placing a lateral bending stress on the gate support structure that it was not designed for (these radial beams are intended to take only strait-line compressive loads). Eventually, the lateral loading stressed the support beams to their yield point, and the structure buckled. [3][4]
After the 2001 terrorist attacks, the Bureau of Reclamation analyzed potential targets for vulnerability and measures that could be taken to eliminate or reduce possible threats. With 500,000 residents in the vicinity of the Folsom Dam, the possibility of an attack on the dam was great enough concern for Bureau officials to close Folsom Dam Road. The road over the dam had been a major artery for the city of Folsom. With its closure, traffic became severely congested during rush hour. The impact was so great that residents and city officials petitioned the federal government to reconsider the road closure, which the government initially considered. Continued security concerns prevented them from re-opening the road and a new bridge, named Folsom Lake Crossing, was constructed and opened on March 28, 2009.[5]
During a severe storm in December 1964, the inflow into Folsom Lake reached a record high of 280,000 cubic feet (7,900 m3) per second, with a river release of 115,000 cubic feet (3,300 m3) per second. This exceeded the downriver levee capacity by 15,000 cubic feet (420 m3) per second.[6] In 1986, nearly 500,000 people faced the possibility of flooding when engineers at Folsom Dam were forced to open the spillway gates after heavy rains.[3]
The Bureau of Reclamation's Safety of Dams Program determined the risk of flooding in the Sacramento area made it one of the most at-risk communities in the United States.[7]
Two projects to increase flood protection are currently underway. The first will raise the surrounding dikes by 7 feet (2.1 m) to increase flood protection. The second, a new spillway, is designed to handle the runoff from large storms that otherwise might flood the region.[8][9]
In a satirical comedy video, The Onion offered a fake reportage about a memorial pre-emptively created to the victims of a disaster that will die once a future, totally avoidable disaster occurs and the dam collapses.[10]
11. http://www.usbr.gov/mp/jfp/news-info/Final%20JFP%201-page%20Flyer%204-2-07.pdf
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