A mathematical error made during the pre-construction design of Wanapum Dam was the primary contributing factor in creating a spillway fracture discovered earlier this year, Grant County Public Utilities District said.
The mistake, detailed in a preliminary report released by Grant PUD and its consultants after an 11-week investigation, was discovered by engineers while examining the original design calculations of the Wanapum Dam spillway.
"When engineers recalculated the original design formulas, they found that additional concrete and/or reinforced steel should have been included in the construction of the monolith," a Grand PUD release said.
According to the utility, the miscalculation created a weak point on Wanapum Dam's spillway pier No. 4, which finally fractured after water pushing against it for decades. Exactly when the fracture formed, however, is still a mystery.
"The investigation indicates that the fracture may have originated a number of years ago and spread gradually over time, ultimately allowing enough water into the fracture to push the upper portion of the structure visibly out of place," the company said. "Grant PUD's monitoring programs never identified this section of the dam as being susceptible to this type of potential failure and did not identify the issues at the time the fracture began."
Repairs to the pier will require the installation of additional steel reinforcements through the concrete structure into bedrock. Wanapum Dam's remaining 12 spillway pier monoliths also have designs similar to No. 4, meaning additional steel reinforcements will likely be necessary as well. All repairs must be approved by the Federal Energy Regulatory Commission (FERC) before being conducted.
"Moving forward, this section of the dam will be monitored and evaluated throughout the repair phase and as the dam resumes normal operations," Grant PUD said.
Work on the spillway is expected to take place throughout the summer, potentially allowing the reservoir behind Wanapum Dam to be raised in the fourth quarter of 2014 by an additional 19 feet.
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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