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associated with a major PMF -scale event to 1 for flood events greater than but much closer to a <br />100 -year than a PMF event. The assigned consequence category of 4 for PMF -scale flood events <br />is considered conservative, because even during a PMF event, release of impounded flood water <br />from the facility is not a certainty, as discussed under Positive Factor 5. Also, the tailing are <br />drained and consolidated and therefore will have limited mobility as discussed under Positive <br />Factor 7. The principal reason for assigning a category of 4 to the extreme (PMF) event is due to <br />the proximity of the facility to the town of San Luis and potentially limited warning time to the <br />population. <br />The results are graphed on Figure 19 at the end of this section. These results indicate that risks <br />are below established risk guidelines except for extremely remote flood events, which graph in <br />the yellow zone. Potential risk reduction measures to address this potential failure mode for <br />extreme floods may be warranted. Recommendations for additional analyses to develop <br />potential risk reduction measures for extreme flood events are described in Section 6.3. <br />Table 6 PFM -1: Overtopping of Drop Structure during Flood Event <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 44 February 2014 <br />Probability Range <br />Node Description <br />Comments <br />Probability Descriptor <br />PMF <br />>100 -yr <br />Loading Event <br />Storm larger than a 100 -year <br />Event must be larger than <br />event occurs and generates <br />100 -year event to initiate <br />10.5 <br /><10_2 <br />flows in the South Diversion <br />sustained overtopping; <br />Ditch <br />maximum event is the PMF <br />Initiation — Erosion <br />Flows overtop drop structure at <br />Starts <br />sufficient depth and for sufficient <br />Very Likely (PMF event) to <br />0.99 <br />0.1 <br />duration to initiate erosion on <br />Unlikely (100 -year flood) <br />the slope <br />Progression —head <br />Concentrated erosion on the <br />Very Likely (PMF event) to <br />cut(s) form <br />slope causes formation of head <br />Unlikely (100 -year flood) <br />0.99 <br />0.1 <br />cuts <br />Progression —head <br />Head cuts reach top of slope <br />Very Likely (PMF event) to <br />cut(s) advance upstream <br />and connect to flowing ditch <br />Unlikely (100 -year flood) <br />0.99 <br />0.1 <br />bottom <br />Progression — breach <br />Breach expands laterally and <br />opening enlarges <br />begins to erode adjacent dam <br />Very Likely (PMF event) to <br />0.99 <br />0.1 <br />embankment and tailing <br />Unlikely (100 -year flood) <br />materials <br />Intervention fails <br />Site is accessible and <br />inspections and monitoring <br />Neutral <br />occurs regularly; could get <br />0.5 <br />0.5 <br />materials to site in a timely <br />manner <br />Dam Breaches <br />Tailing material is released with <br />Very Likely (PMF event) to <br />0.99 <br />0.1 <br />storm water <br />Unlikely 100 ear flood <br />Estimated Probability of Failure (0 <br />5 x 10-6 <br />5 x 10-8 <br />Consequences Category <br />4 <br />1 <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 44 February 2014 <br />