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5.3 POTENTIAL FAILURE MODES <br />For each of the potential failure modes identified below, a failure mode description is provided <br />and the factors that make the failure mode more likely (adverse factors) or less likely (positive <br />factors) to occur are listed (Step 1). Each potential failure mode was then assigned to an initial <br />screening category (Step 2, Table 3). All Category I and II PFMs were carried through event - <br />tree risk analysis and estimated probabilities were assigned to initiation and all nodes on the <br />event trees (Step 3). The computed annual probability (f) was then graphed on the risk matrix <br />along the appropriate Consequence Level (Steps 4 and 5, Table 5, and Figure 18). <br />Two potential failure modes were identified for San Luis Tailing Dam and carried forward for <br />detailed evaluation, as follows. <br />5.3.1 PFM -1 Overtopping of Drop Structure at Left Dam Abutment during Extreme <br />Flood Event leading to Scour Erosion and Breach of Dam <br />Failure Mode Description: <br />This potential failure mode is triggered by hydrologic (storm) events substantially larger than a <br />100 -year precipitation event up to a probable maximum flood (PMF) event. This potential <br />failure mode involves overtopping of the concrete intake and 48 -inch diameter CMP drop <br />structure at the left dam abutment during a flood event larger than a 100 -year flood event. The <br />triggering hydrologic event must be larger than a 100 -year recurrence interval event to generate <br />sufficient flows in the South Diversion Ditch that will exceed the hydraulic conveyance capacity <br />of the drop structure and overtop the drop structure and ground adjacent. In order for the failure <br />mode to initiate and progress along an event tree, the overtopping flows must bypass over the <br />drop structure and down the slope at the left dam abutment at a sufficient depth and for a <br />sufficient duration to initiate erosion on the slope. Concentrated erosion would then cause <br />deepening of erosion channels until one or more of the erosion channels forms head cuts on the <br />slope. The head cut, or multiple head cuts would then advance upstream on the slope and the <br />erosion would progress laterally into the dam embankment and tailing deposits adjacent to the <br />ditch and structure. The erosion breach at the upper left dam abutment exposes tailing deposits <br />to the channelized flows in the ditch and erosion gully. Tailing materials are exposed and may <br />be transported downstream with the flood flows. <br />The following positive factors (making progression to failure less likely) and adverse factors <br />(making progression to failure more likely) were identified and considered in the process of <br />assigning node probabilities in the risk analysis. <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 41 February 2014 <br />