Drawdown under way after crack discovered in Wanapum Dam spillway

A drawdown of water behind Washington’s Wanapum Dam continued today as crews work on stabilizing a crack discovered on one of the dam’s spillway sections, the Grant County Public Utilities District said.

A 65-foot-long by 2-inch-wide horizontal crack was found by divers February 26 after a worker noticed a slight bend in a conduit that runs the length of the dam, indicating a potential shift in Wanapum‘s structure.

The resulting discovery has led Grant PUD to coordinate with the Federal Energy Regulatory Commission (FERC) and upstream dam operators, dropping the Columbia River forebay levels to the lowest they have been since the reservoir was filled in 1964.

The affected spillway is one of 12, which, combined, are capable of 80,000 cubic feet of water per second based on current river conditions.

Even under a worst case scenario, however, failure of one spillway would not cause immediate danger.

“If one of the spillway sections failed, the remainder of the spillways and the main dam structure would remain intact,” the utility said in a release. “Under current conditions, the amount of water that would flow through this section of the dam would be within the range of normal river conditions.”

The dam is also home to a 1,038-MW Wanapum hydroelectric plant. Grant PUD said it will continue generating power despite the drawdown.

The facility is a sister plant to the Priest Rapids project. Combined, the two have an output capacity of nearly 2,000 MW.

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Michael Harris formerly was Editor for HydroWorld.com.

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