Oregon’s Semi-Quantitative Risk Assessments for Dams with Limited Design and Analysis Information

With risk assessment now an important dam safety function, Oregon dam safety staff have developed an assessment protocol for use on State regulated dams to support FEMA grant processes. Fifteen high hazard dams with known or suspected major safety deficiencies have been assessed, including quantitative evaluation of events associated with failures, loadings in those events, and loss of life from catastrophic failure. Events assessed in this project include flood, high water earthquake, and landslides above the reservoir or dam. The project evaluated seven general failure modes based on statistical frequency, including modes that have caused most historical U.S. incidents plus the seismic and landslide risks. Dam specific failure modes are also considered. The methods were influenced by a similar risk evaluation project for rapidly moving landslides and loss of life in Oregon. The use of historical failure frequencies is a more established means of determining risks.

Existing file information was comprehensively reviewed with inspection elements based in information and gaps in the dam safety program files for those dams. The product is a consistent procedure and includes a rating of confidence in the information. Results are calculated in terms of expected loss of life on an annualized basis and shown in this paper on a figure developed following the federal guidelines for dam safety template. Parts of the risk assessment process are applicable to all dam safety regulators. This process would benefit from more detailed failure investigations added to dam failure statistics, and from additional input from experts.