Key phase of Clearwater Dam project finished

CLEARWATER LAKE, Mo. -- With the completion last week of a major portion of a nearly $248 million rehabilitation, Clearwater Dam is on schedule to be the first dam project completed by the U.S. Army Corps of Engineers in the nation to receive a top safety rating.

The corps' nearly 300 flood damage reduction dams have safety ratings between 1 and 5, with the lowest referring to structures that would almost certainly fail under normal operations. Only 10, or 3 percent, have a Dam Safety Action Classification (DSAC) 1 rating, which the Clearwater Dam now enjoys.

Risk reduction measures, including more aggressive management of lake levels, began at Clearwater after a sinkhole caused by long-term seepage was discovered almost nine years ago. It was located on the upstream face of the earthen portion of the dam.

Seepage at the structure was recorded as early as 1950, two years after construction was completed.

The rehabilitation project is expected to finish by 2014, however Tuesday marked the completion of its primary element.

The final 160 cubic yards of concrete needed to complete a 4,100-foot cutoff wall were poured into an underground panel dug to a depth of between 120 and 200 feet. There are 277 such panels in the wall, which will resolve chronic seepage problems, according to the corps.

Some 75,000 cubic feet of concrete, requiring 7,500 truck loads, was used to construct the wall. This would equal 25 miles of a 1-foot deep, four-lane highway.

"This is this district's largest project now, in scope and dollar wise," explained Col. Glen Masset, commander of the U.S. Army Corps of Engineers Little Rock District. "This project is well ahead of schedule."

Masset was on the construction platform as the cutoff wall was completed, finishing Phase II of the rehabilitation project.

The effectiveness of the cutoff wall has already been demonstrated, Masset said during the work.

"We saw the benefit during the last high water event, in April in May. Historic seepage is gone," he said. "The cutoff wall was only 75 percent done then."

Lake levels reached within one-eighth of an inch of its historic crest, 566.6 feet, set in 2002.

Exploratory drilling and grouting occurred during Phase I.

Phase III will see the construction of a hard-packed clay wall from the top of the roadway to the construction platform. During this time, three feet of rock will be removed and replaced with three feet, or 46,000 cubic yards, of sand. Workers will then pack 100,000 cubic yards of clay on top of the sand.

Risk reduction measures will continue for a time after completion of the project, said Laurie Driver, corps public affairs specialist.

"Those will stay in place while we monitor to make sure everything is performing the way it should," Driver said.

They include reducing lake levels to conservation levels as quickly as possible without adding to downstream flooding.

Masset praised the efforts of construction workers and contractor Bencor for completing the project safely and well. Construction has continued year-round, with the exception of extremely high lake levels in April and May.

Pertinent address:

Clearwater Lake, MO