2014-03-07_REVISION - M1988112 (3)

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this review. Analyses presented in Section 4 of this report indicate that the maximum overflow depths at the drop structure calculated for the 100 -year design event would be a few inches and would occur for a short duration of approximately 20 minutes. The location of anticipated overtopping would likely occur at a swale on the south side of the Drop Structure. Overtopping at this location, and of this small depth and short duration, for a 100 -year event is very unlikely to result in severe erosion that could threaten the dam embankment. Recommendations are provided in Section 6.2.1 for minor modifications to the South Diversion Ditch to ensure desired performance of that system up to a 100 -year flood event. In the unlikely event of major to extreme floods approaching a IMF, the calculated depths and duration of flows over the left dam abutment at the Drop Structure could cause severe erosion in the vicinity of that structure. Maximum flow depths calculated for an extreme IMF event were over 3 feet and overflow durations may last for several hours. Overflows of this scale would most certainly result in severe erosion and potential breaching at the Drop Structure unless mitigative action is taken. The only dam safety risk identified by this evaluation that was above tolerable risk guidelines is the potential for severe erosion at the Drop Structure during an extreme flood event greater than a 100 -year event. This could result in loss of that structure due to undermining, head - cutting and erosion of the channel alongside the dam abutment, coupled with increased water level within the tailing impoundment during the flood event. This scenario was evaluated as a potential failure mode (PFM -1) described in Section 5. PFM -1 has an estimated range of risk, which extends slightly above tolerable risk guidelines for extreme flood events only. For lesser flood events (on the order of the 100 -year event) risks are well below tolerable risk guidelines for the potential failure mode identified. Recommendations are provided in Section 6.3 for additional studies that could be done to evaluate feasible mitigative actions to reduce risk under extreme flood loading conditions. 6.2 RECOMMENDED MAINTENANCE, REPAIRS, AND UPGRADES TO ADDRESS CURRENT CONDITIONS The recommendations for maintenance, repairs and upgrades to address current conditions are presented below and are summarized on Figure 20. 6.2.1 Maintenance and Improvements to South Diversion Ditch As discussed in previous sections, the South Diversion Ditch was originally designed and approved to completely bypass the 100 -year flood around the impoundment and to overtop to the impoundment and contain floods up to the PMF event. The current analyses shows partial overtopping of the drop structure will occur during the 100 -yr event. A minor amount of San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 54 February 2014