Design and Operations
Seven Oaks Dam is a 2,980-foot (910 m) long arched embankment structure standing 550 feet (170 m) high above the Santa Ana River and 650 feet (200 m) above its deepest foundations. The dam is 40 feet (12 m) wide at the crest and over 2,200 feet (670 m) wide at the base. Situated close to the San Andreas Fault, Seven Oaks is designed to withstand an 8.0 ML earthquake. The reservoir has a gross storage capacity of 145,600 acre feet (0.1796 km3), with 113,600 acre feet (0.1401 km3) reserved for flood control and the remainder for sediment accumulation. At full pool, the reservoir lies at an elevation of 2,604.4 feet (793.8 m) and has an area of 780 acres (320 ha). Water releases are controlled by a 1,623-foot (495 m) long tunnel outlet that runs through the base of the structure, and a 500-foot (150 m) wide ungated overflow spillway located just southeast of the dam. The dam controls runoff from an area of 176 square miles (460 km2) or 209 square miles (540 km2) including the drainage area of Baldwin Lake, which overflows into the Santa Ana River system during periods of heavy runoff.
In tandem with Prado Dam, Seven Oaks prevents about $140 million of flood–related damages each year in Orange, Riverside and San Bernardino Counties and provides flood protection to more than two million people. During flood events, Seven Oaks Dam will store runoff as long as the reservoir of Prado Dam is rising, then release the captured water as quickly as downstream channel conditions allow. Seven Oaks is designed to completely contain a "Reservoir Design Flood" of 85,000 cubic feet per second (2,400 m3/s), corresponding to a 350-year flood event, reducing it to a peak outflow of 7,000 cubic feet per second (200 m3/s). The dam is capable of withstanding an inflow of up to 180,000 cubic feet per second (5,100 m3/s), or the "Probable Maximum Flood". In addition, controlled releases from the dam allow about 10,000 acre feet (0.012 km3) of additional groundwater recharge in the upper Santa Ana River basin each year. Captured floodwater from the Seven Oaks Dam benefits local water districts, as river water costs only a fraction of water imported through the State Water Project, which supplies most agricultural and municipal needs in the area.
Since its construction, Seven Oaks Dam has not yet been filled to capacity. The highest lake level on record was about 43,000 acre feet (0.053 km3), or less than a third of capacity, on March 8, 2005. The high water presented an opportunity to test large water releases through the dam's outlet works, but the trial had to be cancelled because the high flows destroyed sections of the concrete tunnel lining. The tunnel was repaired and later tested successfully in 2011.
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