Delivering a Risk-informed Approach to Dam Safety 

Established in 1961, the Irvine Ranch Water District serves more than 600,000 people across Orange County, California, with high-quality drinking water and recycled water for irrigation and landscaping. IRWD operates five reservoirs that are instrumental in balancing water demands, holding winter precipitation and recycling water for summer use when supply is limited. 

The California Department of Water Resources Division of Safety of Dams classifies these structures as “extreme high hazard,” which, if they fail, could cause mass evacuations, result in loss of life or critical infrastructure, and drastically disrupt water storage and delivery in the area. 

We partnered with IRWD to develop and implement a state-of-the-art, risk-informed dam safety program that provides a rigorous, systematic and thorough framework to improve public safety and trust, decision-making quality and support. The framework also proactively provides IRWD with information needed to understand each dam’s condition, maintain their safety and prioritize actions. 

As a first step in implementing the risk-informed dam safety program, HDR and IRWD performed a risk analysis on four of IRWD’s five dams using screening and semi-quantitative risk analysis methods. The fifth dam, Syphon Dam, is integrating risk-informed design into its dam improvement project. The program helped identify critical potential failures and verified that the design meets state safety requirements and IRWD’s guidelines. 

Through risk analysis workshops, the team identified and described each dam’s potential failures, estimated their likelihood and determined downstream consequences. Each dam’s cumulative risks were presented on a semi-quantitative risk analysis matrix, which provided an understanding of the dam’s risks, identified the most crucial potential failure modes and prioritized actions to reduce uncertainty or identify interim measures to reduce risk. 

The risk-based approach provided a framework for IRWD’s transition to a risk-informed dam safety program. Many of the dam safety recommendations included studies and further analyses, which IRWD is currently implementing at Rattlesnake and Santiago Creek Dams. Current and future planned dam safety activities are occurring within a robust risk-informed decision-making process that challenges conventual understanding of dam performance.