2024 – Combining Risk Estimates Over Multiple Failure Modes

David Margo, C. Haden Smith

The U.S. Army Corps of Engineers has developed and implemented two practical models for dam and levee risk analysis. The joint failure modes model allows one or more failure modes to occur
during the same hazard event, while the weak link competing failure modes model identifies the failure mode with the lowest capacity as the first, and only, failure. These two risk models address many of the limitations and criticisms associated with current methods used in the dam and levee industry. They provide point estimates for the probability of system failure and system risk, thereby avoiding challenges associated with bounded estimates. Dependency in failure mode capacity and dependency in failure events are explicitly modelled, eliminating the need for ad-hoc adjustment procedures. The models offer a more defensible framework for assessing failure event dependency compared to normalization procedures like the common cause adjustment. Risk analysts can choose between joint failure modes and weak link competing failure modes depending on whether multiple failure modes could occur during the same hazard event or whether the first failure could prevent additional failure modes from developing. Considerations for model selection and best practices for portraying risk estimates are proposed. The joint failure modes and weak link competing failure modes models provide a computationally efficient, practical, and credible risk estimate, thereby improving risk-informed decisions.