The Wanapum Spillway Incident: Response, Remediation and Lessons Learned

The Wanapum Hydroelectric Development is located on the Columbia River in central Washington. The Project, owned and operated by Public Utility District No. 2 of Grant County, Washington (GCPUD), is licensed by the Federal Energy Regulatory Commission (FERC) and has operated without incident since first filling in 1963 until early 2014.

In February 2014, GCPUD staff observed misalignment of the bridge railing at spillway pier 4 and uneven leakage at the adjacent radial gates. Investigations were started immediately. Surveys indicated that the top of Pier 4 had moved downstream about 2.5 inches. Diver inspection of the upstream face revealed a 2-inch open crack spanning the full length of monolith 4. GCPUD initiated a drawdown of the reservoir, implemented the Emergency Action Plan and quickly engaged its engineering consultants. Detailed investigations were initiated including a potential failure modes analysis (PFMA) and a comprehensive root cause analysis. Both concluded that this type of failure could occur at the other spillway monoliths.

GCPUD developed a multi-task strategy to analyze, design and implement remedial measures while restoring the reservoir in stages. Remedial measures included the installation of post-tensioned anchors with localized concrete repairs. The reservoir will be restored to the normal maximum pool level after completion of all remedial measures. This presentation addresses:
• Description of the damage observed,
• Findings of the root cause analysis and PFMA,
• Expedited design, regulatory approval and construction,
• Enhanced surveillance and monitoring,
• Impacts to fisheries, recreation and cultural resources, and
• Lessons learned.

The final design included the installation of 61-strand post-tensioned anchors in each pier and post-tensioned bar anchors in the spillway ogee. Lift joint drains were drilled from the drainage gallery in each monolith to relieve internal hydrostatic pressures. The Project required close cooperation between GCPUD, its consultants, construction contractors and the FERC which allowed design and construction to progress at an accelerated pace.