Grant PUD considering Wanapum Dam embankment analysis

EPHRATA — Grant County PUD commissioners will consider a proposal to hire the engineering firm Gannett Fleming to analyze the right embankment at Wanapum Dam to determine if it is at risk for erosion.

If approved, the 10-year contract would cost a maximum of $15 million.

“Our embankments were built well, and with the best practices of the time they were built,” said Rebecca Simpson, PUD engineering manager.

Utility district engineer Logan Castle said during a presentation at the commission meeting Tuesday that the analysis would focus the embankment on the Grant County side, and would look at the potential for what Castle called internal erosion.

Christine Pratt, PUD public information officer, said there is no risk to the public, but that utility district officials want to ensure the embankment remains stable.

However, the construction method meant that some of the work was done in the water, what Simpson called,” in the wet,” which limited the options. Some sections of the embankment are at higher risk than others, she said.

Portions of the embankment have settled at a rate about what engineers would expect, she said. Others have settled more than engineering projections, and engineers want to take a closer look, she said.

“We wanted to make sure we thought well ahead, and we prepared and we planned for the possibility there will be mitigation,” Simpson said.

The only existing core samples from the Wanapum Dam site were taken in the late 1950s, prior to the dam’s construction, she said.

Simpson said engineers analyze the dam and its structure about every five years to look for problems that might need to be addressed. The last analysis was in 2021, and that identified issues that hadn’t shown up in 2016. All of those issues are potential indicators of internal erosion.

In answer to a question from Commissioner Judy Wilson, Simpson said the opposite bank, on the Yakima County side, also is being analyzed, along with the embankment on the Grant County side at Priest Rapids Dam. The information has been submitted to a federally-mandated Board of Consultants, which analyzes the data and makes recommendations, she said. Those conclusions are then submitted to the Federal Energy Regulatory Commission.

Construction crews are currently working on the Yakima County embankment at Priest Rapids Dam to ensure it’s better able to withstand a major earthquake.