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4.0 HYDROLOGY/HYDRAULICS <br />4.1 SUMMARY OF EXISTING DATA AND ANALYSIS <br />The existing San Luis Tailing dam does not have a spillway. The facility was designed to <br />intercept and divert around the impoundment storm -water runoff up to a 100 -year, 24 -hour <br />precipitation event on the contributing watershed area. To handle larger floods, the dam crest <br />was maintained with sufficient freeboard above the level of tailing to fully accommodate runoff <br />volumes from the contributing watershed up to a probable maximum flood (PMF) event, without <br />overtopping the dam. Available design documentation for the existing storm -water diversion <br />system is included in Section D.6.5.6 of the 112 Permit Application Amendment Permit <br />No. M -88 -112 for the San Luis Project, dated September 1989 (BMRI, 1989). Available <br />calculations for the hydraulic design are provided in Appendix H of that report, while <br />calculations for the PMF design storm used in that analysis are provided in Appendix I. Drainage <br />basins associated with the design are shown in Figure D.6 -1 of the 1989 report. <br />As -Built Drawings (SRK, 1991), depicting the drop structure at the outlet of the South Diversion <br />Ditch are identified as Drawings 14711 -111 A, 14711 -111, and 14711 -112; the orientation of the <br />Diversion Ditch and Drop Structure can be seen on Drawing 14711 -103. <br />The information contained in the above historic documents was used to support the dam safety <br />hydraulic analysis described in this report, and is reproduced in the accompanying Data Report <br />(MGC, 2014). <br />4.1.1 Impoundment Elevation- Area - Volume <br />Ground surveys were requested as part of this dam safety evaluation to determine the existing <br />elevation- area - volume data for the facility. Surveys of the impoundment area were completed in <br />May 2013. The resulting elevation - area - volume data are summarized on Table 1 and graphed on <br />Figure 14. The maximum above - tailing, water - holding capacity of the impoundment area at dam <br />crest (elevation 8621.8) is 1,105 acre -feet. <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 22 February 2014 <br />