Dam safety decisions for bradbury dam incorporating risk analysis and risk assessment

With new seismic loading and geotechnical data on the foundation obtained in 1995, decisions were required on the need for safety modifications for Bradbury Dam, California. Dam safety, the concerns of downstream populations, the effective utilization of water, the validity of technical data and information and many other factors were taken into consideration in the decision process. Risk assessment was also incorporated into the dam safety decision process at two separate stages with a detailed risk analysis performed in the second stage. A preliminary risk assessment was made after the results of the initial field investigations and initial analyses of the dam and appurtenant structures indicated that during an earthquake, dam failure could occur. Due to the potential for foundation liquefaction and embankment deformation, it was viewed that the risk to the downstream public was unacceptable. The preliminary risk assessment was used as input to the decision that interim (short term) actions were required to reduce this risk. A risk analysis, described in this paper, was carried out later in the analysis phase when more detailed data with respect to recurrence intervals for various load conditions and results from more detailed geotechnical and structural analyses were available. The results from the risk analysis was input to the risk assessment/decision process that was used to decide how to establish modification requirements and prioritize modification design schedules and funding.