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a dam safety risk. If future monitoring indicates an unexplained reduction in the drain flows or <br />steady rise in the pore pressures within the downstream portion of the dam, this PFM should be <br />re- evaluated and investigations initiated to understand the cause of the changed seepage and <br />phreatic conditions. <br />5.5 SUMMARY OF RISK ESTIMATES <br />The results of the PFMA for probability of failure (f) and consequence categories of failure are <br />graphed on the risk matrix shown on Figure 19. The results also are summarized on Table 8. <br />Shown for comparison to Figure 19 Figure are the color -coded regions used for the descriptive <br />consequence categories (0 through 4) as defined on Table 5. <br />PFM - 1 graphs over a range of risk as indicated by the oval shape encompassing the PMF and <br />100 -year events. For major floods approaching a PMF, the range of risk extends slightly above <br />the tolerable risk guidelines, in the yellow zone. This justifies taking action to reduce risk, but is <br />not highlighted as needing to take expedited action. Additional studies are recommended in <br />Section 6.3 to evaluate potential actions that could be taken to reduce risks for extreme flood <br />events. PFM -2 graphs in the green zone and therefore falls well within acceptable risk ranges <br />without any further actions. <br />Table 8 Summary of Risk Estimates and Justification for Action <br />Potential <br />Failure Mode <br />Description <br />Probability of <br />Failure <br />Justification <br />to Take <br />Action <br />PFM -1 <br />Overtopping of Drop Structure at Left Dam Abutment <br />Yes - for <br />during Flood Event leading to Scour Erosion and Breach <br />5 x 10-6 (PMF) <br />extreme flood <br />of Dam <br />5 x 10-6 ( >100 -yr) <br />event risk <br />reduction <br />PFM -2 <br />Internal Erosion or Piping of Embankment Materials <br />10- <br />2 x 1010 (flood) <br />No - monitor <br />Exiting on Downstream Face of the Dam near the Under- <br />3 x (normal) <br />piezometers <br />drain Pip Outlets <br />and drains <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 51 February 2014 <br />