By Dave Russell
The roar of heavy equipment, ping of boring rigs and thunder of blasting deep in Appalachian granite continue around Cedar Cliff Dam, and will at least until about July 2024 when new flood control measures are completed.
Preliminary work began in 2019 to allow Duke Energy and contractors to widen and upgrade one of the dam’s two spillways, channels designed to handle surplus water.
“The primary spillway, in conjunction with the power unit, is used to control the lake level under normal conditions,” Duke’s Chief Dam Safety Engineer Brad Keaton said. “If the primary gets overwhelmed, the auxiliary is the backup.”
So far, the primary spillway has handled every flood event thrown at it. The auxiliary spillway is seeing upgrades to control the lake in case of a massive flood, he said. The spillway’s width will increase from 95 feet to 145 feet, and the channel bottom lowered by 15 feet on average.
The widening of the second spillway involves digging into a hillside, creating about a 200-foot high cliff on the east side of the dam. Stone debris from the construction is loaded onto a barge and dumped into the lake about 1/4-mile upstream from the dam. The placement of excavated materials in the lake is in compliance with authorizations and permits issued by the Federal Energy Regulatory Commission, the U.S. Army Corps of Engineers and the state.
“We’re blasting about 250,000 cubic yards and about 2/3 of that will go into the lake,” Duke Energy’s Ken Karably said. “All that material after they blast it is run through a screen so they can separate the fines and put the larger rocks in the lake.”
Yellow floating booms drape turbidity curtains across the lake to prevent particulate matter from heading down the river and eventually into the Gulf of Mexico.
“Once this is widened and deepened, instead of these erodible fuseplugs, it’s going to be this new technology, fusegates,” Keaton said.
Currently two fuseplugs, earthen barriers designed to collapse in high water, stretch across the top of the auxiliary spillway. Fusegates are large concrete cubes designed to topple forward and allow floodwater to escape.
“The fusegates are a good solution for controlling an auxiliary spillway because they are passive,” Keaton said. “They act as a weir, and the water goes over the top of them without them rolling out of the way. They are designed to progressively start flipping as it exceeds a certain amount and will go, and if it exceeds a higher amount, another one would go, opening another section for flow. They would not all go at the same time.”
Six fusegates are planned for the spillway.
The impetus for the upgrades came from FERC, even though the fuseplugs have yet to be overtopped, Keaton said.
“FERC is requiring us to be able to handle a larger flood, thus the deepening and widening of the spillway,” he said.
A tunnel in the center of the lake in front of the dam supplies water – about 560 cubic feet per second – to two turbines about 1,100 feet west of the center of the lake. One powerhouse was built at the time of construction,and another was added in 2012, he said.
Plans call for the crest of the dam being rebuilt with a higher wall on it.
Cedar Cliff is one of about 50 dams operated by Duke Energy, Keaton said.
Construction of the 173-foot tall dam ended in 1952. The earthen structure measures 590 feet across with the primary spillway on the west side and an auxiliary spillway on the east side.
A lake drawdown plan and environmental report, including a water quality monitoring plan, was prepared in 2018, reviewed by state and federal resource agencies and authorized by FERC.
Site development work got underway in late 2019. This included constructing a new bridge across the primary spillway channel, refurbishing the primary spillway gate and its hoist, establishing laydown areas, tree clearing, improving access roads and performing work at the boat ramp.