Hyrum Dam’s Seismic Issue Evaluation

In 2009, a possible dam deficiency was identified. Hyrum Dam was built over a relatively thin unconsolidated alluvial deposit that could be at risk for strength loss during a large earthquake. If large deformations were to occur because of the strength loss, the crest height might be reduced or cracking of the embankment may occur, and could potentially lead to failure. In order to assess the situation, a field investigation was completed to measure the in situ properties of the dam embankment and the alluvium; and a seismic hazard analysis was performed. A comprehensive analysis was then undertaken to estimate potential deformation and overtopping. Using the data from updated seismic loading, field investigation, and analysis, a risk analysis was performed for potential failure modes. The annualized failure probability was estimated, using an event tree approach. The risk analysis revealed that the likelihood of dam failure during a seismic event was below the Bureau of Reclamation’s (Reclamation) standards.