Cause of Wanapum Dam damage determined

BEVERLY - A thorough investigation all of the way back to the 1950s has determined that a mistake made decades ago may be the reason for the spillway crack discovered in February at Wanapum Dam.

It's a costly mistake too. The repairs that must be made will cost more than half of the dam's original price.

An extensive and exhaustive review of all records led Grant PUD and its consulting engineers to believe the primary contributing factor to a fracture within a spillway was a mathematical error during the design phase of the dam.

The dam was designed before 1959, the year construction started. Construction was completed in 1963. The total project cost was $93 million. The initial estimate for the coming repairs is $61 million.

The fracture forming on spillway monolith No. 4 may have been exacerbated by a weak construction joint, Public Information Officer Thomas Stredwick said. The results of the investigation will be submitted to Grant PUD's Board of Consultants for final review.

According to Stredwick, a team of engineers discovered the mathematical error while examining the original design calculations of the 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 spillway pier monolith is the structure that supports the spillway gates. There are 13 spillway pier monoliths at Wanapum Dam. The fracture discovered in February formed on spillway pier number 4.

The original designers of the dam miscalculated that the weight of the spillway's concrete provided enough strength to resist the force of the water pushing against it, Stredwick said. This weak point on spillway pier No. 4 succumbed to the force of the water pushing against it after approximately 50 years of operation.

"Had Grant PUD engineers known of the design miscalculation earlier, the spillway could have been reinforced prior to a fracture forming," Stredwick said.

The fracture may have originated a number of years ago, ultimately allowing enough water into the fracture to push the upper portion of the structure visibly out of place. 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 issue at the time the fracture began.

This section of the dam will be monitored and evaluated throughout the repair phase and as the dam resumes normal operations, Stredwick said. Grant PUD is committed to following all FERC-approved dam safety and monitoring programs.

To repair spillway pier No. 4, additional steel reinforcement installed through the concrete structure into bedrock is anticipated. Steel reinforcement will likely be necessary to secure all of the spillway pier monoliths into bedrock.

The independent board of consultants will review and FERC must approve any repairs prior to implementation by Grant PUD.

Repairs are likely to occur throughout the summer, allowing the utility to potentially raise the reservoir behind Wanapum Dam in the fourth quarter of 2014 to an operating elevation of 560-562 feet above sea level.

Shortly after this intermediate river level is reached, limited public access to the Wanapum shoreline and reservoir will likely be restored, Stredwick said.