Dam's rating goes from worst to best

PIEDMONT, Mo. -- Clearwater Dam has become the first project in the nation to go from the highest risk of failure to the lowest held by American dams.

The Piedmont structure now is a model for future dam repairs, after 12 years and more than $210 million of work, according to the U.S. Army Corps of Engineers.

The dam was re-evaluated in the spring, losing its DSAC 1 classification for a DSAC 4 ranking, said Fred Esser, interim operations manager.

The DSAC 1 ranking in the Dam Safety Action Classification System means high urgency -- a dam will almost certainly fail under normal operations -- and, at DSAC 4, low urgency.

"The structure itself is very sound," Esser said. "It was a big relief to know the efforts they put into the dam have paid off. ... It's something we can be proud of."

About 450,000 people visit Clearwater Lake annually, and the dam is credited by the corps with preventing more than $278 million in flood damage since it was completed in 1948. Original dam construction cost less than $10 million.

About a dozen dams remain at high urgency. and these projects are adapting the work done at Clearwater for their repairs, said David Howell, a resident engineer during much of the repairs.

An underground wall built on the lake side of the earthen dam extends into bedrock and runs the length of the dam.

The wall's construction required enough concrete to pour 17 miles of two-lane highway, or nearly 81,000 cubic yards. The dam, by comparison, is built from 5.5 million cubic yards of earth.

The cutoff wall was needed to resolve sinkhole problems discovered on the lake side of the dam in early 2003, Howell said. Less-extensive repairs also had been tried in the 1940s and 1980s, according to Howell.

While caves and sinkholes are common in southern Missouri, this had the potential to undermine the Clearwater structure, Esser said. The newest sinkhole meant water had found a way to move past the original repair, he said.

Work at Clearwater used an improved wall design that required 277 interlocked concrete panels up to 34 inches thick. These extended from 100 to 195 feet below ground, into bedrock. It used rectangular panels that were larger than a previous design made of circular panels, Howell said.

"The bigger panels meant less joints, which meant less risk of seepage, which is a huge benefit," he said.

Each side of the wall is lined with a thick grout curtain. A pressurized mix of cement and water was injected into the earth to act as a glue, filling small fractures and holding rock together, Howell said. The grout extends down to 275 feet in places.

The cutoff wall and grout represented the most intensive repair work and was completed in 2012.

Most recently, Esser said, a line of riprap was laid along the platform that runs the dam's length. This large stone is a kind of armor plate, Esser said, and the third layer of fail-safes in place.

Blankets of clay and sand sit below the riprap.

"If there should be an anomaly that would open, the sand is going into that hole and plug it, and then the clay would fall in," Esser said.

To earn the DSAC 4 rating, Clearwater had to pass inspections of the dam structure of the earth and concrete, Esser said. Also considered was the risk to downstream communities if the dam failed.

"We've really made a lot of strides. We are very much at the forefront as far as being a safe dam," Esser said. "They learned through our process a lot of things that went well."

There is a fifth safety class -- normal -- but none of the nearly 700 Corps-owned dams has reached that rank, Esser said, because there always is some risk.

"There is no perfect dam," he said.

Clearwater has no more extensive work planned, Esser said. A fish-cleaning station is planned for River Road, but otherwise, the dam is at the end of its fiscal year and will not begin other work.

The Clearwater project manages 18,606 total acres, according to the corps. It includes 1,630 acres of water, with more than 10,350 acres of water at the top flood pool.