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• Spillway: None; impoundment has sufficient flood storage capacity to fully contain <br />runoff from the contributing watershed in excess of a probable maximum precipitation <br />event without overtopping the dam. <br />• Storm Water Management: storm water run -on is intercepted and conveyed around the <br />impoundment via perimeter berms and ditches, sized for a 100 -year event; a concrete and <br />CMP drop structure located at the left (south) dam abutment routes around the dam storm <br />water that is collected by the South Diversion Ditch from the largest contributing <br />drainages on the south side of the impoundment; storm water runoff from other, smaller <br />contributing drainage areas on the southeast and north sides of the impoundment are <br />diverted off site towards the northeast, or flow into the tailing impoundment area, <br />respectively. <br />• Outlet Works: None; water entering the impoundment area behind the dam infiltrates <br />the impounded tailing, is collected on top of an impervious geomembrane liner, and is <br />conveyed via a granular blanket drain and drain pipe network under the dam to a lined <br />outfall at the downstream toe of the dam. The collected drainage water is delivered to a <br />geomembrane -lined collection pond downstream from the dam and is then pumped back <br />up to the impoundment area for evaporation and re- infiltration. <br />• Freeboard: 13 to 14 feet vertical and approximately 600 feet horizontal on the current <br />ponded water surface elevation of approximately 8607 to 8608 feet; 6 to 8 feet above <br />tailing level at the upstream dam face <br />• Impoundment Surface Area: 136.7 acres at dam crest <br />• Flood Water Storage Capacity (above tailing level): 1105 acre -feet at dam crest <br />5.1.2 Hydrology Data <br />• Probable Maximum Precipitation = 14 inches (8 hour) <br />• 100 -year, 24 -hour Precipitation= 2.9 inches (basis of design for storm -water diversion <br />system) <br />• 100 -year, 24 -hour peak flows (assume modifications to South Diversion Ditch berm and <br />drop structure overflow erosion protection are implemented as described in Section 6.2): <br />o South Diversion Ditch: 292 cfs; 100% of flow will be contained in the ditch and none <br />will overtop and enter impoundment area <br />o North Diversion Ditch: 80 cfs diverted away from impoundment area towards <br />northeast <br />o Overtopping at South Diversion Ditch drop structure: < 1 foot maximum, 112 cfs <br />peak, approximately 20 minutes duration. <br />• PMF peak flows and volumes: <br />o Peak inflows from watersheds into South and North Diversion Ditches: 4315 cfs <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 35 February 2014 <br />