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to be subject to erosion; erosion gullies from surface runoff erosion have been observed <br />on and near the dam embankment and adjacent abutment slopes. <br />2. During a PMF -scale extreme flood event, hydraulic models predict the drop structure will <br />be overtopped by more than 3 feet and for approximately 5 hours; this would most <br />certainly cause severe erosion and head - cutting of the slope in the left dam abutment in <br />the vicinity of the structure. <br />3. A substantial volume of the PMF -scale storm water is predicted to overtop the South <br />Diversion Channel berm and will enter the impoundment area; the resulting flood pool in <br />the impoundment area will be larger and closer to the dam than the flood pool from a <br />much smaller event (i.e., event larger than a 100 -year flood but much smaller than the <br />PMF). For major flood events, there is a higher risk that an erosion gully breach that is <br />initiated at the left abutment will head cut sufficiently far back into the tailing deposits to <br />connect with the flood pool and release the impounded water downstream. <br />4. The drop structure currently does not have a trash rack. During a major storm event, <br />large vegetation debris could be transported down the South Diversion Ditch and could <br />block the intake to the drop structure. This would increase the likelihood of <br />overwhelming the hydraulic capacity of the structure and overtopping of the drop <br />structure during higher frequency flood events. <br />Based on the positive and adverse factors listed, PFM -1 was assigned to a preliminary screening <br />Category of I -III. Category I is appropriate for larger flood loading events approaching the <br />PMF event that would have more significant consequences and higher likelihood of progressing <br />to failure. <br />PFM -1 was further evaluated by semi - quantitative risk analysis. Table 6 summarizes the nodes <br />on the event tree leading to failure, and the estimated range of probabilities associated with each <br />node. The probability range was bounded by the worst -case loading condition (PMF) and the <br />minimum triggering event to cause overtopping of the drop structure, which is assumed as the <br />100 -year flood. The probabilities for these triggering events are the inverse of the recurrence <br />intervals, which are: 1 /100 = 0.01 for the 100 -year flood, and 1 /100,000 = 0.00001 for the PMF <br />event. Subsequent nodes on the event tree were assigned a descriptive probability and value as <br />summarized on Table 4. <br />The consequences of this potential failure mode are highly dependent on the scale of the <br />triggering event (magnitude and duration of the flooding) and, especially the ratio of the amount <br />of floodwater that is bypassed around the facility to that which would be routed into the facility <br />during the flood. If the breach occurs when the impoundment is storing a significant amount of <br />flood water and the stored water is released through the breach in combination with the water <br />that is being routed via the diversion ditch system, downstream consequences will be more <br />severe than if the breach occurs at the abutment but does not release any impounded water. <br />Consequence categories (defined on Table 5) were assigned as a range from 4 for breaches <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 43 February 2014 <br />