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• <br />2.Surface drainage from the Whirlwind waste rock pile and all other surface facilities will be diverted <br />through ditches and culverts to the mine sediment pond that, in turn, discharges to an ephemeral <br />drainage in Lumsden Canyon. These collection ditches, which will be reclaimed at the conclusion of <br />mining, have been designed for the 10-year, 24-hour storm event. They have maximum sideslopes of <br />2H:1 V and freeboard levels of 0.3 feet except next to topsoil stockpiles where the freeboard depth is <br />1.0 feet. The sediment pond will fully contain the 10-year, 24-hour event and will safely pass the 100- <br />year 24-hour event. A principal spillway pipe is used to drain the pond for storm events up to the 100- <br />year event. Larger flows will be handled though a trapezoidal emergency spillway that also leads to the <br />ephemeral drainage. The sediment pond is completely incised in the flat area at the low point of the <br />Whirlwind disturbed area and will have excavated slopes of 2H:1 V that will be topsoiled and <br />revegetated. It has a surface area of 0.46 acres and a volume of 4,608 cubic yards. <br />3.Surface drainage from the ore stockpile pad will be directed to a sump that will overflow into a <br />synthetically lined tank (Untreated Water Tank) that feeds the water treatment system. Water pumped <br />• from underground will also enter this system. The Untreated Water Tank is designed to utilize 40,000 <br />gallons of capacity for daily operation of the mine dewatering and treatment system. The tank will also <br />have 75,000 gallons of emergency capacity for continued mine pumping during planned or unplanned <br />plant maintenance activities, and 49,000 gallons for inflow from the ore pad and the tank surface for a <br />100-year storm event. Therefore, the total design volume of the tank is 164,000 gallons. The tank will <br />have a minimum freeboard of 1.5 feet. This tank is also described as part of the Water Treatment <br />System designed by Lyntek in Appendix H. <br />18.1 Whirlwind Surface Drainage <br />Surface drainage in the Whirlwind portal area will include installation of long-term drainage controls <br />in the facility areas (i.e., buildings and water treatment system) and phased drainage controls at the ore <br />pad and waste pile areas. The proposed drainage ditches and culverts are illustrated on Map C-6. <br />Collection ditches within the site are shown in light blue and diversion ditches around the site are <br />shown in purple. All collection ditches drain to the sediment pond while diversion ditches divert <br />• surface water runoff from undisturbed areas around the site. The sediment pond is sized to safely pass <br />Whirlwind Mine 07 (rev. April 08) D-29 <br />