Risk-Informed Decision Making – FERC Perspective

1. Risk-Informed Decision Making – FERC Perspective Natural Hazards: Seismicity Events David Lord, P.E., D2SI Dam Safety Risk team – Portland, Or

2. What is Risk-Informed Decision Making (RIDM) • Decision-making, which has as an input the results of a risk assessment. • Risk information will play a key role in decisions related to dam safety but will not be the only information to influence the final decisions. • RIDM involves a balancing of benefits and risks while reducing risks as much as is practicable.

3. Risk • Risk = Probability of Failure x Consequences • Probability of Failure = Probability of Load x Adverse Structural Response Given Load (PF = PL x PAd ) • Adverse Response includes all steps of event tree leading to failure.

4. From Deterministic to Probabilistic • Current standards-based program relied on conservative deterministic analyses. • Probable Maximum Flood (PMF) and Maximum Credible Earthquake (MCE) estimates • Structural analyses relied on these point estimates • Probabilistic Analyses give a curve of frequency estimates for the loading, not one particular point.

5. Probabilistic Loadings • Probabilistic loading has the benefit that different issues can be considered including: • Probable level of reservoir during earthquake • Range of inputs from far distance Cascadia Subduction Zone source to nearby infrequent local sources • Current standards-based current regulatory framework does not address how to use this information for decision making.

6. Evaluating Frequency Loading Estimates • Some form of fragility analyses are needed to evaluate how the dam performs during a range of frequencies. • If structure ok for a 1/5000 annual exceedance probability (AEP) event, but not ok for 1/10000 AEP, is this acceptable? • Reclamation and US Corps of Engineers have developed techniques for determining acceptability

7. Consequence Risk (Potential Life Loss) • The current hazard classification system does not distinguish between one only potential life loss (PLL) and 10000. • Risk consequences evaluate a range of PLL from 0, to 1, to many thousand.

8. Risk-Informed Decision Making (RIDM) • Decisions are made by using a chart that compares F (the likelihood of failure) to N (the PLL) • The fN (or FN) charts have a one to one relationship between frequency and PLL for societal risk

9. USACE Societal Tolerable Risk Chart

10. Why Risk at the FERC?

11. Why Risk at the FERC? • Enables a better informed decision. • Enables a better understanding of the dam • Provides a better justification for decision-making • Makes dams safer.

12. Mid-Columbia Prototype Risk Analyses • Six prototype sections • Workshop No. 1 held on May 22 and 23, 2012 • Grant Co PUD: Wanapum Gate System • Developed a system model of gate system under seismic loading

13. Prototype Risk Analyses • Workshop No. 2 held on November 6 through 9, 2012 • Chelan Co PUD: Chelan Surge Tank • Chelan: Rocky Reach Spillway Gate System • Grant: Priest Embankment (Liquefaction)

14. Prototype Risk Analyses • Workshop No. 3 to be held in June 2013 • Chelan: Rock Island North Abutment Wall • Grant: Wanapum Future Unit System - Anchors

15. Mid-Columbia Risk Analyses • The FERC likes the prototype studies for many reasons • RIDM can provide more understanding of particular issues as follows: • Combining the frequency of loading with consequences can provide a better perspective on the risk of a particular loading • Frequency aspects of loading event can be evaluated using risk, e.g., more likely loading versus more unlikely.

16. Mid-Columbia Risk Analyses • RIDM provides more understanding (cont.) • Many MCE values are typically comparable to 2% in 50 year ground motions, an approximately 1/2500 AEP loading. (Note that not all MCEs are comparable to a 2% in 50 year ground motion.) • This calls into question whether an MCE is actually the maximum load that needs to be considered for dam evaluation. • Note that Wanapum Dam deterministic loading are well above the 1/10000 AEP loading.

17. Wanapum Deterministic vs Probabilistic Loading

18. Wanapum Seismic Loading • Wanapum probabilistic results may be adequately conservative at a higher frequency earthquake, e.g., 1/5000 AEP, than the equivalent deterministic result. • Using the probabilistic loading would then result in less potential for remedial work.

19. Risk Scalability • The prototype risk analysis technique is simplified and scalable. • Simplifying assumptions are made to determine if the dam system meets the selected tolerable risk guidance. • Process ends at any of four steps after acceptable performance at the lower step based on PLL. • This scaling technique means that only necessary detailed analyses are completed, and unnecessary steps are not completed.

20. Completeness • Simplifying assumptions reduce the cost without sacrificing completeness. • A full risk quantitative risk analysis (QRA) of spillway gate systems can be quite costly. • The prototype captures the full system model without analyzing every detail reducing the cost. • If it is clear that the dam’s fragility is acceptable under the loading used, no further work is needed. • A full QRA could be completed if needed.

21. Systems Model • Analysis of selected dam sections as systems rather than just individual components. • The full system model is described • The models develop for the Wanapum and Rocky Reach gate systems revealed very useful information about the various aspects of the spillway gate system • This information allows the dam owner to fix issues that had not been previously appreciated. • For example, anchorage of backup power systems and safety of dam operators and their continued ability to operate the spillway gates.

22. Usefulness of New Risk Analysis Tool • Tool is a useful way to develop a result during the FERC’s integration of risk into its dam safety program. • Any necessary modifications to the tool can be made at a later date building upon the previous effort. • Refining the tool is part of the on-going work of the prototype development.

23. Potential Issues • Tool assumes seismic hazard uses half the risk space. • Future work includes the need to understand where the risk is in relation to the tolerable risk guideline including analyzing all of the PFMs. • We recognized that the FERC does not currently have a tolerable risk guideline which may vary from current guidance.

24. Tolerable Risk • Risks are tolerable if they are adequately managed and driven “as low as reasonably practicable” (ALARP). • An ALARP evaluation will be needed in the future to fully establish tolerability.

25. Conclusions • Prototype Risk Tool is a potentially useful tool • Risk-informed decision-making can provide more understanding of dam safety issues because it includes likelihood of failure and consequence. • Prototype Risk Tool is simplified and scalable.

26. Conclusions • Reasonably complete analysis of complete dam systems can be done at relatively low cost. • Developing a system model is a very productive effort in understanding risks. • This technique is a useful tool. • Any potential changes and needed modifications can be made at a later date.

27. QUESTIONS?