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Brantley Dam is on the Pecos River at mile 478.5, about 13 miles upstream from the city of Carlsbad, New Mexico. It is about 10 miles upstream from the Avalon Dam in the Carlsbad Project and 25 miles downstream from Artesia.
The Brantley Project area extends about 16.5 miles above the damsite. The Pecos River watershed is in a semi-arid region. Average annual precipitation in the Pecos River Basin varies from about 10 inches near Pecos, Texas, to greater than 33 inches in the higher mountain elevations of northern New Mexico.
The project's main purpose is to replace McMillan Dam, which was declared unsafe. Additional benefits include irrigation, flood control, fish and wildlife enhancement, and recreation.
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The east wing dam is an earth and rockfill section 730 feet long with a maximum height of 150 feet where it croses the Pecos river. The crest width is 24 feet, and the crest elevation is 3.308 feet above mean sea level.
The west wing dam is an earth and rockfill section 8,020 feet long with a maximum height of 120 feet where it ties to the main dam.The crest width is 24 feet, and the crest elevation is 3.308 feet above mean sea level.
The outlet works consist of two four-foot-square conduits controlled by tandem hydraulic slide gates. The invert elevation is 3,210.7 feet mean sea level with a design capacity of 1,450 cubic feet per second at reservoir elevation 3,259.5 feet mean sea level and a maximum capcity of 1,800 cubic feet per second at water surface elevation 3,283 (the top of the flood control pool).
The spillway is part of the central section of the main dam. It has six gated bays, each 50 feet wide. Six radial arm gates (tainter gates) control spillway discharge. Each radial arm gate is 50 feet wide by 25.24 feet high. Crest elevation is 3,259.5 feet mean sea level. At the maximum water surface elevation of 3,303.5, the spillway capacity is about 357,000 cubic feet per second.
A low flow outlet works is to the left of the spillway on the downstream side of the dam. This structure ensures a minimum flow of 20 cubic feet per second in the portion of the river between the dam and the junction with the spoillway channel. The structure consists of a 36-inch diameter concrete pipe between the stilling basin and the old Pecos River channel.
Carlsbad Irrigation District operates and maintains Brantley Dam for irrigation releases. There are two levels of flood control operations:
Dam safety evaluations in 1964 of McMillan and Avalon Dams showed that a large flood could exceed the existing spillway capacity of McMillan Dam and overtop the structure. Brantley was designed and construction relocations started in 1983.
Brantley Dam and Reservoir of the Brantley Project were authorized on October 20, 1972, by Public Law 92-514, for irrigation, flood control, fish and wildlife, recreation benefits, and to replace McMillan Dam, which was determined to be unsafe.
As the dam was being designed, Reclamation determined that the damsite should be relocated 2,100 feet downstream to a site with a more favorable foundation. This relocation resulted in higher construction costs, which exceeded the indexed cost ceiling. Public Law 96-375, October 3, 1980, raised the cost ceiling.
Construction relocated Highway 285, the Atchison Topeka and Sante Fe Railway, utility lines, a county road, and a cemetary. Excavation for the main dam began in May 1985, and work on the dam was completed in August 1988.
Brantley Dam has a 335,054 acre-feet capacity assigned to flood control functions.