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4.1.2 Drainage Area and Runoff Conditions <br />The watershed area on the south and east side of the disposal area is approximately 0.98 square <br />miles or about 626 acres. This area is divided into four ephemeral sub - catchments which are <br />delineated as catchment areas A, B, C and D on Figure D.6 -1 in the 112 Permit Application <br />Amendment Report (BMRI, 1989). Elevations within the catchment area vary from <br />approximately 8,600 ft at the tailing facility to about 9582 ft at the southerly limit of the <br />watershed boundary. The watershed slopes in a northerly direction towards the facility at an <br />average gradient of approximately 8.5 percent. <br />Runoff from catchment areas A, B, and C, a combined drainage area of approximately 537 acres, <br />is intercepted by a perimeter diversion ditch along the south and east side of the tailing disposal <br />area, referred to herein as the South Diversion Ditch. Flows conveyed in the South Diversion <br />Ditch are diverted around the left (south) abutment of the dam via a structure that comprises a <br />concrete inlet, a sloping 48 -inch diameter corrugated metal pipe (CMP) and a concrete energy <br />dissipation structure at the pipe outlet. <br />Runoff from catchment area D, approximately 89 acres, is intercepted along the southeastern <br />perimeter of the tailing disposal area and is reportedly diverted away from the facility towards <br />the northeast into a topographic saddle that drains into the Rito Seco. <br />Precipitation that falls directly onto the tailing facility and runoff originating from the small <br />contributing watershed on the north side (a combined area of about 204 acres, referred to as <br />catchment area E) is not diverted, and is contained within the impoundment area behind the dam. <br />4.1.3 Existing Hydrologic/Hydraulic Design Data <br />Based on the available design documentation, the storm -water interception and diversion <br />facilities were designed to have a hydraulic conveyance capacity to handle a 100 -year, 24 -hour <br />design storm event, defined as 2.9 inches of precipitation (BMRI, 1989). During a 100 -year <br />event, the diversion system was to intercept storm runoff flowing from catchment areas A, B and <br />C and divert these flows towards the west, around the south side of the tailing impoundment. <br />Calculated runoff intercepted from these three drainages during a 100 -year, 24 -hour design storm <br />varies along the South Diversion Ditch and ranges from approximately 224 cubic feet per second <br />(cfs) at the upper end of the ditch to a maximum of 292 cfs at the bottom end near the drop <br />structure. These flows are conveyed over the left side of the dam via a 48 -inch conduit at the <br />drop structure and into an energy dissipation structure below. 100 -year storm runoff from <br />catchment area D, having a peak discharge of approximately 80 cfs, is diverted around the east <br />end of the impoundment and directed into a topographic saddle that drains into the Rito Seco <br />northeast of the tailing facility. Runoff from the north side is not diverted and flows directly into <br />the tailing impoundment area behind the dam. <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 24 February 2014 <br />