Cracked Columbia River dam not anchored in bedrock

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Updated: May 1, 2014, 10:18 AM

 

BEVERLY — Repairing Wanapum Dam will likely require anchoring the entire spillway to bedrock, not just one section where a crack was discovered, a utility official said.

The $61 million repair estimate includes the cost of installing anchors on each of the spillway's 12 monoliths or massive concrete sections, Grant County Public Utility District chief financial officer Kevin Nordt said Tuesday.

Engineers have said when the Columbia River dam was built in the early 1960s, designers believed its mass alone would be enough to hold it in place, The Wenatchee World reported.

The reason for the 65-foot crack has not been officially determined, but officials have suggested it could have been caused by water pressure. The reservoir was lowered after the crack was discovered Feb. 27 at the dam, which is just south of the Interstate 90 bridge near Vantage.

The utility is working with federal dam authorities, state agencies, tribes and a panel of independent experts to study the damage and plan repairs.

Until a repair plan is approved, it's uncertain how many of the anchors would be installed, Nordt said.

The anchors would run from the top of the spillway through the concrete structure and anchor in bedrock using grout, utility spokesman Chuck Allen said Tuesday. The anchors can be cinched down to hold the spillway in place.

Some drilling is already underway to gauge the size of the crack and to create holes for the anchors, Allen said.

Utility customers will pay about a third of the repair costs and the district can pay for the rest from reserves and financing without an additional rate increase, officials said.

The 26-foot drawdown of the pool behind the dam reduced its capacity to generate electricity. It also affected operation of the Chelan County Public Utility District's Rock Island Dam, 36 miles upriver. The Rock Island pool was drawn down a few feet to protect generating equipment. Both utilities have modified their fish ladders to help migrating spring chinook and steelhead get past the dams.