Estimating Overall Risk of Dam Failure: Practical Considerations in Combining Failure Probabilities ANCOLD

With the move to a risk based approach to dam safety there has been a concomitant focus on estimating the probability of failure of dams. The majority of risk guidelines relate to the total probability of failure and therefore the individual probabilities estimated for different components and loading conditions need to be combined. In most cases the failure modes of various components of a dam are not mutually exclusive and therefore the failure probabilities are not simply additive. This paper discusses the requirements for estimating probabilities in dam safety risk analysis both for assessing the risks associated with an existing dam and the justification for upgrades. The approach taken to calculating probabilities of failure in the risk analysis model can have a significant influence on the magnitudes of key inputs to decisions about dam safety upgrades. Ignoring adjustment for common cause failure modes can lead to overestimation of the estimated total probability of failure and the annualised incremental loss of life, and underestimation of the cost per statistical life saved. A worked example is used to illustrate the importance of appropriate combination of probabilities in quantitative risk analysis.