2014-03-07_REVISION - M1988112 (3)

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6.0 CONCLUSIONS AND RECOMMENDATIONS 6.1 OVERALL EVALUATION Based on this review and evaluation, the San Luis Tailing Facility dam is in good condition and the embankment is determined to be structurally stable in accordance with accepted engineering standards (factors of safety) under static and earthquake loading, and for failure modes ranging from slope instability and deformation under seismic events to internal erosion under seepage. This conclusion is based on conditions under which the facility is currently operated: with a minimal -size (approximately 20 to 80 acre -feet, 16 to 32 acre) sustained free -water pool located on the topographic low spot on the surface of the impounded tailing, several hundred feet upstream from the dam. The principal dam safety concern identified by this evaluation is related to uncertainties about the facility's performance under major to extreme flood events, in excess of a 100 -year event'. The original design envisioned that the South Diversion Ditch would route all flows entering the ditch up to a 100 -year event around the impoundment, and the ditch would overtop into the tailing impoundment during floods in excess of a 100 -year event. The original design documents indicate that floods greater than a 100 -year event up to and including a PMF event would be accommodated within the impoundment area without overtopping the dam. As part of this evaluation, HEGRAS models were developed to determine the approximate locations, extent and duration of overtopping of the South Diversion Ditch system during the flood events analyzed (100 -year and PMF). Analysis provided in Section 4 shows that with minor modifications described in the following sections, there would be no overtopping of the South Diversion Ditch berm into the impoundment area during a 100 -year event, but there would be some overtopping of the ditch near the Drop Structure. For a PMF event, although a portion of the flow would overtop the berm (as indicated in original design documents) and enter the impoundment without overtopping the dam embankment, a large portion also would overflow the end of the channel at the Drop Structure. Whether or not erosion of an earthen embankment during overflow will ultimately lead to a full breach of an embankment relates to factors such as location of the overflow relative to the source of water, depth of flow, embankment geometry, erodibility of materials within the overflow section, and duration of overflow. These factors were taken into consideration for purposes of ' An IDF for this facility has not been established. This evaluation therefore considered a range of flood events consistent with the original facility design documentation. The original hydraulic design of the facility was based on diversion systems (run on controls) sized for a 100 -year event. The impoundment storage capacity was sized to fully contain the volume of a probable maximum flood (PMF) with sufficient freeboard to prevent overtopping the dam. This study evaluated dam safety based on these two events. San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 53 February 2014