PIERRE, S.D. - The U.S. Army Corps of Engineers conducted inspections of all six tunnels at the Oahe Dam Stilling Basin north of Pierre, S.D., Aug. 29.

Inspections are typically conducted annually during the early summer season at the stilling basin, said Oahe Dam Operations Manager Eric Stasch. Due to record releases this summer, this year's inspection was pushed back until recently.

Unlike typical annual inspections where only half of the tunnels are checked, Corps personnel inspected all six tunnels.

"The entire tunnel is about one mile in length," said Stasch. "The control structure is right in the center line of the dam."

Here, the tunnel is divided into the downstream (river) and upstream (reservoir) sections.

Project personnel drove boats 1/2 mile into the 19-foot diameter tunnels to visually inspect the tunnel walls for any cracks, spalls or distress in the concrete. At the outlet tunnel gate, directly under the control structure, they inspect it for any signs of concern. A sheet of steel also lines the tunnel immediately downstream of each gate covering several feet of concrete.

The inspections Aug. 29 were precipitated by issues with closing the outlet tunnel gate on tunnel two last week, according to Stasch.

During inspection of tunnel two, inspectors found that the lower roller chain on the left side of the outlet tunnel gate was missing, said Stasch.

Each outlet tunnel gate has four roller chains, two on each side, which aid in raising and lowering the gate when under pressure. The chains resemble bicycle chains and can weigh anywhere from 1,000 to approximately 2,000 pounds.

The loss of the lower left chain caused issues with closing the outlet tunnel gate last week, said Stasch, and closing the gate completely was halted due to binding of the gate caused by the pressure from water releases. To alleviate the pressure, the emergency gate was lowered, which removed the pressure on the gate and allowed the gate to be pulled up to the control structure where the chain can be replaced and allow continued use of the outlet tunnel gate.

As part of the inspection, all six outlet tunnels were temporarily closed for four hours causing releases out of Oahe to drop from 80,000 cubic feet per second (cfs) to 57,000 cfs. The 57,000 cfs was released exclusively from the power plant. This caused an approximate 1 -- 1 1/2 foot drop in water elevation in the Missouri River near the Pierre / Fort Pierre area.

Besides the loss of the lower roller chain, there were no issues with the other rarely used outlet tunnels, even with record water releases. The lower roller chain was replaced and a more permanent repair will be made at a later date.

"We've only used those tunnels three times since the project was completed," said Stasch. "The typical release from those tunnels is zero."