YANKTON, S.D. — Gavins Point Dam popped a few buttons in the beating it took during the flood of 2011.
Among the estimated $10.5 million in repairs planned this year at Gavins Point is replacing broken or missing cast-iron spillway grates. Twenty are known to be broken or missing. More than 200 others are under water and will be inspected this month.
"That is a big unknown for us,'' said David Becker, Gavins Point operations manager.
The grates cover deep cylindrical holes drilled to bedrock under each of the spillway's 250 concrete slabs. The holes dissipate hydraulic pressure created during high water releases through the dam's spillway gates.
Becker used a rusty, broken grate as an illustration during a briefing Friday for Brig. Gen. John McMahon, commander of the U.S. Army Corps of Engineers' Northwest District in Portland, Ore.
McMahon led a delegation of Omaha- and Portland-based engineers and others on a tour this week of the corps' six Missouri River dams, from Montana to Nebraska. The purpose of the tour was to check the status of post-flood damage assessments and repairs.
Corps officials have identified 122 repairs to be done this year. The cost is estimated at more than $186.3 million — plus approximately $54.5 million the following two years — at Fort Peck, Garrison, Oahe, Big Bend, Fort Randall and Gavins Point Dams.
Gavins Point is at the lowest point on the river of the six dams. It is about 200 miles upstream from Omaha and impounds Lewis and Clark Lake.
Becker said his primary concerns at Gavins Point include the condition of the spillway's concrete slabs, bank degradation below the dam and restricted flows through the power plant.
Sonar inspections of the spillway have shown no problems under the concrete, he said.
"We have no major structural issues. The (spillway) gates work. They just need paint, and they'll get new cables,'' he said.
Peak releases at Gavins Point during the summerlong flood reached 160,000 cubic feet per second — more than twice the previous record, in 1997.
Becker said the high flows scoured away 12 feet of river bottom along a 1,200-yard stretch of the north bank downstream from the dam. That piece of shoreline functions as a dike separating the dam's outflows from Lake Yankton.
The lost rock and soil is significant because it could allow seepage from the lake into the river and threaten the dike's integrity.
Lake Yankton — which is about 15 feet higher than the nearby river channel — abuts the back side of the dam. Pressure that the lake exerts helps prevent seepage from the dam.
Becker said the river side of the dike will be armored with rock this summer to prevent future erosion and degradation.
"We need to treat this like a dam,'' he said.
Trees and marsh debris coming down Lewis and Clark Lake will plague the Gavins Point power plant for two or three more years, Becker said.
During the flood, debris jammed and damaged the trash racks that protect the power plant's water intakes. The problem continues. As a result, Gavins Point's turbines have lost about 10 percent of their generating capacity, Becker said.
Cleanup of the debris is scheduled to begin next week.
"We're looking forward to a nice, normal year,'' Becker said.
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