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risk guidelines for potential failure modes ranging from slope instability, cracking and <br />deformation under seismic events and internal erosion under seepage forces. This conclusion is <br />based on normal conditions under which the facility is currently operated: with a minimal -size <br />(approximately 20 to 80 acre -feet, 16 to 32 acre) sustained free -water pool located on the <br />topographic low spot on the surface of the impounded tailing, several hundred feet upstream <br />from the dam, which ensures that the tailing deposits near the dam are drained and unsaturated. <br />The principal dam safety concern identified by this evaluation is related to uncertainties about <br />the performance of the facility's storm water diversion system under rare major to extreme flood <br />events, in excess of a 100 -year recurrence interval event. With or without recommended minor <br />improvements to the South Diversion Ditch that are described in the report's recommendations <br />(Section 6.2.1), this evaluation did not identify a significant risk to the safety of the dam for <br />floods up to a 100 -year event. <br />In the event of major to extreme floods approaching a very rare probable maximum flood (PMF), <br />analyses presented in this report indicate that there is some risk of damage, uncontrolled erosion <br />and scour along the South Diversion Ditch system, specifically due to predicted overtopping at <br />the Drop Structure. The risk posed to the dam during extreme flood events is associated with the <br />potential for deep scour erosion of the left dam abutment at the Drop Structure, which could <br />potentially result in loss of the Drop Structure, head - cutting and erosion of the channel alongside <br />the dam embankment. This specific condition was evaluated as a potential failure mode <br />(PFM -1) and is described in Section 5 of the report. The estimated range of probabilities and <br />risk associated with this potential failure mode extends slightly above tolerable risk guidelines, <br />but only for extreme flood events on the order of an extremely rare PMF. For lesser (more <br />frequent) flood events, risks are below tolerable risk guidelines. All other potential failure <br />modes that were identified through this study also were determined to be ruled out or well within <br />tolerable risk guidelines; and no further action to address those potential failure modes other than <br />continued diligence and monitoring is recommended. <br />Recommendations are provided in Section 6 of the report for actions to address noted minor dam <br />safety conditions, and for additional analysis and possible modifications to the facility to <br />improve the hydraulic performance during major and extreme flood events. The report <br />recommendations include the following: <br />Recommendations for Maintenance and UpQr ades to Address Noted Conditions <br />• Construct minor improvements to the South Diversion Ditch system to ensure that it <br />performs as envisioned by design for the 100 -year event. Recommended improvements <br />include: raise the berm by about 1 foot in some areas to prevent all flows predicted in the <br />South Diversion Ditch during the 100 -year event from entering into the impoundment <br />area, clear a constriction in the channel, and provide additional erosion protection <br />adjacent to the Drop Structure where some minor overtopping is anticipated to occur <br />during a 100 -year event. <br />ES -2 <br />