Laserfiche WebLink
• At peak flow, 68% of flows will enter impoundment area; peak inflow into <br />impoundment: 1930 cfs <br />• Volume of flood bypassed via South and North Ditches (assuming overtopping but no <br />erosion of ditch embankments): 245 acre -feet; volume entering impoundment: <br />199 acre -feet <br />• Approximate pool elevation with additional 199 acre -feet added to the existing <br />free -water pool volume: 8613.2; dam freeboard at maximum PMF pool: 8.6 feet <br />• Overtopping at South Diversion Ditch drop structure in left dam abutment: 3.1 feet <br />maximum, 1336 -1412 cfs peak, approximately 5 hours duration. <br />5.1.3 Seismic Data <br />Design Earthquake peak ground acceleration (PGA) = 0.6 g <br />Design Earthquake magnitude = 7+ <br />Design Earthquake estimated recurrence interval = 16,200 years (probability = 6.2 X 10-5) <br />5.2 PFMA PROCESS SUMMARY <br />A simplified potential failure modes analysis (PFMA) and semi - quantitative risk estimate <br />process was used to evaluate the current condition of San Luis Tailing Dam. The process <br />adopted a streamlined methodology based on accepted, systematic PFMA procedures that are in <br />general accordance with the Bureau of Reclamation's "Dam Safety Risk Analysis Best <br />Practices" (USBR, 2012). The PFMA focused on a reasonable range of credible potential failure <br />modes (PFMs) associated with static, earthquake and flood loading conditions. Downstream <br />flood impacts due to dam failure and population at risk estimates are not available for the San <br />Luis Tailing Facility. Therefore the risk analysis is semi - quantitative in that the probability <br />(likelihood of occurrence) of a potential failure mode is quantified using statistical data, but the <br />consequences are defined by descriptive categories only. <br />The PFMA process involved the following steps: <br />Step 1: Identify Potential Failure Modes <br />This step involved identifying, fully describing and evaluating potential failure modes by listing <br />adverse and favorable factors. For each candidate potential failure mode (PFM), the physical <br />conditions specifically related to the mode of failure were reviewed. The initiating event and <br />step -by -step sequence to failure were then developed. Factors that were considered to make the <br />failure mode more likely and conceivable (adverse factors), or less likely and remote (positive <br />factors) are listed. <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 36 February 2014 <br />