CS16B - Concurrent Session 16B: 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 use on 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, loading 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 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 inspections modified based on those reviews. Information is summarized in data spreadsheets, and corrected by lead technical staff and moved into analysis spreadsheets. The product is a consistent procedure, with guidance if consulting engineers were to use this procedure and includes a rating of confidence in the information. The results in tabular form, and in order of risk in terms. The risk analysis has provided useful lessons, a determination of the dams posing the highest risk to people, and prioritizing needs for analysis and remediation or dam removal, and using contract and grant funds for the highest risk dams. Parts of the process are applicable to all dam safety regulators, with a need for more consistent evaluation of floods, modification of inspection protocols to support risk, and to have additional staffing to complete a full risk evaluation on a large number of dams.