Laserfiche WebLink
6.2.3 Erosion Protection of Right Groin Area <br />The repaired right groin area should be seeded and erosion protected to prevent a repeat of the <br />erosion that occurred previously. Seeding of the area can proceed as soon as conditions allow. <br />Channel erosion protection placement will require engineered design of appropriately sized <br />riprap or other protection to suitably route and channel the flow. <br />6.3 RECOMMENDATIONS FOR ADDITIONAL STUDIES TO UPGRADE FACILITY <br />PERFORMANCE FOR EXTREME FLOOD EVENTS <br />As discussed in previous sections, the South Diversion Ditch was originally designed and <br />approved to completely bypass the 100 -year flood around the impoundment and to overtop to the <br />impoundment and contain floods up to the PMF event. With minor improvements as described <br />in Section 6.2, the South Diversion Ditch system should perform as envisioned by the original <br />design to fully divert floods up to a 100 -year event. With or without these minor improvements <br />the analyses presented in this evaluation did not identify a significant risk to the safety of the <br />dam for floods up to a 100 -year event. <br />If the objective is that the facility is to perform within tolerable risk guidelines under a PMF or <br />other approved Inflow Design Flood (IDF) that is substantially larger than a 100 -year event, <br />MGC advises that additional engineering studies will be needed. These studies should proceed <br />with the objectives being to identify and recommend appropriate structural upgrades to the <br />facility that will reduce risk of severe erosion at the Drop Structure and South Diversion Ditch <br />during a PMF (or other approved IDF), which could potentially result in an unacceptable release <br />of flood water or tailing material stored in the impoundment. The following sections outline the <br />recommendations for the additional engineering studies that would be required. <br />6.3.1 Design of an Auxiliary Overflow Structure at the Drop Structure <br />As described in the previous sections, with the existing configuration of the South Diversion <br />Ditch and Drop Structure a significant depth and duration of flow is expected to overtop the <br />Drop Structure during an extreme (e.g., PMF- scale) flood event. Flow will overtop at the drop <br />structure, and run down the slope at the left dam abutment. As described in Section 5.3.1 <br />(Potential Failure Mode -1), risk analysis indicates that action is justified to protect against scour <br />erosion at the Drop Structure for a PMF -scale event. Further analysis is necessary to determine <br />the specific structural measures that would be needed to attenuate and convey the estimated <br />excess flows around the Drop Structure and minimize the risk of scour erosion at the left dam <br />abutment. <br />San Luis Project Miller Geoteclmical Consultants <br />Tailing Dam Data Report 58 February 2014 <br />