Rehabilitation and Repair

Priest Rapids Dam can now handle a 7.1-magnitude earthquake

Priest Rapids Dam, which impounds water for a 955.6 MW hydroelectric powerhouse, can now withstand a 7.1-magnitude earthquake.
Priest Rapids Dam can now handle a 7.1-magnitude earthquake
(Courtesy: Stantec)

Contractor Stantec has announced the completion of the Priest Rapids Right Embankment Improvement Project, enhancing the safety and resilience of the Priest Rapids Dam in Mattawa, Wash., along the Columbia River.

Priest Rapids Dam plays a crucial role in impounding water for a 955.6 MW capacity hydroelectric powerhouse. Thanks to this critical infrastructure project, undertaken in collaboration with owner Grant County Public Utility District No. 2 (Grant PUD) and IMCO General Construction, the dam is now able to withstand a 7.1-magnitude earthquake.

“We are thrilled to have successfully completed the Priest Rapids Right Embankment Improvement Project ahead of schedule,” said Mario Finis, executive vice president of energy and resources at Stantec. “Our collaboration with Grant PUD exemplifies our commitment to dam safety and community resilience. The project’s success was the result of a strong partnership spanning years of planning and dedication.”

To address growing concerns over the seismic resistance of the dam’s right embankment, Grant PUD commissioned the Priest Rapids Right Embankment Improvement Project in September 2021. Stantec’s team designed and constructed a downstream replacement dam, with the new structure mitigating potential liquefaction risks associated with the existing dam’s foundation.

The Priest Rapids Right Embankment Improvement project replaced the existing earthen dam and added reinforcement to the right bank of the river to address the stability of the embankment near the Priest Rapids Dam facility, ensuring the safety of the neighboring Wanapum Indian tribal village and improving earthquake safety.

A new roller-compacted-concrete (RCC) dam with a short connecting embankment and secant pile cut-off wall was tied into the existing embankment. The RCC dam was constructed on basalt bedrock downstream from the existing embankment, according to IMCO. RCC is an alternate construction method that offers many advantages, according to Stantec, which has prepared designs for more than two dozen RCC dams and rehabilitation structures, including gravity, thick-arch, and buttress dams. Stantec previously helped transform an existing concrete gravity dam into an arch dam using RCC for the Gross Reservoir Expansion Project in Boulder, Colo. The firm also used RCC to raise San Vicente Dam in Southern California.

“Completion of the Priest Rapids Right Embankment Improvement Project enhances the protection and preservation of the Wanapum people’s culture who live immediately downstream of the dam,” said Dale Campbell, senior manager of power production at Grant PUD. “This investment in Priest Rapids Dam is strategically critical for our customers as it reduces Grant PUDs public safety and financial risk for the foreseeable future.”

Late last year, Grant PUD project managers reported that the rehabilitation of the fifth turbine-generator at its Priest Rapids project was completed 38 days ahead of schedule. Rehab of the fifth of the 10 generating units at Priest Rapids required 88,000 work hours to complete, compared to 95,000 hours for the previous turbine-generating unit, according to Project Manager Eric Hull. Rehabilitation work at Priest Rapids began in August 2016. The final five units are scheduled to be rehabbed by late October 2029.