Raccoon Mountain Gets a Health Check

A mountaintop reservoir disappears as engineers inspect TVA’s signature pumped-storage plant, affording us a rare view of the inner workings of this marvel of engineering.

On any ordinary Saturday afternoon, Chattanoogans flock to Raccoon Mountain to enjoy leisurely hiking and biking and enjoy the world-class view across the blue waters of the reservoir. This month, however, they’d find their access blocked and the reservoir completely drained—a gray empty pit.

Where did all the water go? The answer is simple: The 107+ billion gallons of water that normally fill the reservoir were released into the Tennessee River on Oct. 17 so that TVA could perform maintenance on the pumped-storage plant that lies deep in the mountain beneath it and inspect the 8,500-foot-long rock-filled dam at the top of the mountain.  

Engineers quite literally pulled the plug on the reservoir, or plugs, that is—four giant ball valves that serve the same purpose as the stopper in your sink or bathtub, only on a grand scale. The purpose? So that routine maintenance could be performed on them. “Each spherical valve has a 10-foot diameter and weighs 183,000 pounds,” says Thomas Gamble, Raccoon Mountain plant manager. “The only way we ever get to inspect the inside the of equipment  is to drain the lake entirely, which we do every five years—think of it as a doctor’s visit but for machinery.”

The approach to the health check is holistic. Above ground, trucks and crews are removing debris and foliage that could possibly interfere with the smooth operation of the pumped storage facility, performing large-scale reservoir cleanup. Inside the facility, engineers are using LIDAR—a remote sensing technology similar to RADAR that senses surface integrity using lasers rather than radio waves—to check every inch of Raccoon Mountain’s 1,080-foot shaft for cracks, imperfections or areas of excessive wear that might need to be repaired. The powerhouse and switchyard, too, are being checked and—where needed—repaired or upgraded.

Meanwhile, crews are also attending to the four giant Allis-Chalmers pump turbines that will return the water to the top of the reservoir when the health check is done, paying special attention to the gaskets that maintain the water seal just like, again, your kitchen faucet, only on a massive scale.  

This go-round, two of the four spherical valves will be replaced; each was special ordered from Voith Hydro out of York, Pennsylvania, and each will cost approximately $750,000, according to Gamble.

The team on site right now is appropriately mighty: whereas Raccoon Mountain normally has a staff of 35, during this health check hundreds of workers are working around the clock. That’s because this valuable asset is a secret weapon in TVA’s power portfolio—and it deserves nothing but the best in care.

When not being serviced, Raccoon Mountain works like a battery, storing potential energy in the water at the top of the lake. When TVA is facing a time of peak demand, it can “pull the plugs” to release water through all of the four shafts and be generating 1,600 megawatts of electricity within a matter of minutes—and keep generating for a full 24 hours. That’s an amount of electricity on par with a nuclear plant—enough to power 935,000 homes. Then the plant can pump the water back to the top, and get it ready to do it again. (Read more about the engineering behind this Mountaintop Marvel.)

How does TVA deal with this critical asset being offline? “We always schedule the maintenance for autumn, when demand peak is lower overall and winter rains haven’t picked up in full,” Gamble explains. “The rest of the system can compensate—all the conventional hydro plants are carrying more load right now, there may be more starting and stopping of gas plants. It’s all a big balancing act, but TVA’s system can handle it.” 

The work will be done and lake refilled by Nov. 21 in time for the annual Pangorge, the Tennessee River gorge adventure race, in which participants snake around Raccoon Mountain’s shores, oblivious to the potential energy that lies beneath them.