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Step 3: Develop Failure Event Trees and Probability Estimates All plausible PFMs assigned to Categories I or II (Table 3) were then carried forward for risk analysis. Probability estimates were assigned to each step involved in the initiation and progression to failure. Probabilities for initiation and each step towards progression were estimated using as guidance empirical tabulations for similar failure mechanisms that have been developed by the U.S. Bureau of Reclamation, U.S. Army Corps of Engineers and others, as documented in the recently updated manual of Best Practices in Dam and Levee Risk Analysis (USBR, 2012). The process described as follows results in a calculation of annualized failure probability. The annualized failure probability (1) is defined as the product of the load probability (PL) times the probability of failure under the load (PF), as follows: f= PLXPF 1. The annual load probability (PL) is typically included at the beginning of the event tree. For purposes of flood loading, the PMF is assumed have a recurrence interval of 100,000 years. This corresponds to an annual load probability of 1 X 10 -5. 2. The probability for each subsequent event on an event tree is then estimated. The value Of PF will be computed as the product of probabilities assigned for each node on event trees leading to dam failure. To simplify the process, for some nodes the estimated probability was assigned based on subjective descriptors as summarized on Table 4. Table 4 Descriptive Probabilities Descriptor Probability Virtually certain 0.999 Very likely 0.99 Likely 0.9 Neutral 0.5 Unlikely 0.1 Very unlikely 0.01 Virtually Impossible 0.001 Table 4 is derived from USBR (2012) Chanter 13 — Subjective Probability Table 8 -2 available at httn: / /www. usbr. gov/ssle/damsaLetvI Risk/ BestPractices/ 13- SubjectiveProbabilitv20121116 pdf San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 38 February 2014