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lower rainfall presented in TR-55, which is more widely used in Colorado for determining peak <br />• flows. All ditches have a maximum slope of 2H:1 V and 0.3 feet freeboazd. Any ditch with a design <br />flow greater than 5.0 feet per second is designated for arip-rap lining, which is also designed to <br />withstand the peak flow from the 100-yeaz 24-hour event. <br />2. Surface drainage from the Whirlwind waste rock pile and all other surface facilities will be <br />diverted through ditches and culverts to the mine sediment pond that, in turn, discharges to an <br />ephemeral drainage in Lumsden Canyon. The sediment pond will fully contain the 10-year, 24-hour <br />event and will safely pass the 100-yeaz. 24-hour event. A principal spillway pipe is used to drain the <br />pond for events up to the 100-yeaz event. Lazger flows will be handled though a trapezoidal <br />emergency spillway that also leads to the ephemeral drainage. The sediment pond is completely <br />incised in the flat azea at the low point of the Whirlwind disturbed azea and will have excavated <br />slopes of 2H:1 V that will be topsoiled and revegetated. It has a surface azea of 0.46 acres and a <br />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 pond (Lined Untreated Water Pond) that feeds the water treatment system. Water <br />pumped from underground will also enter this system. The Lined Untreated Water Pond is designed <br />to utilize 20,000 gallons of capacity for daily operation of the mine dewatering and treatment <br />system, although average pumping and treatment is expected to be much less. The pond will also <br />have 75,000 gallons of emergency capacity for continued mine pumping during planed and <br />unplanned plant maintenance activities, and 50,000 gallons for inflow from the ore pad and the <br />pond surface fora 100-year storm event. Therefore, the total design volume of the pond is 145,000 <br />gallons. The pond will have 2H:1 V sideslopes and a minimum freeboard of 1.5 feet. The pond depth <br />will be 10.0 feet. This pond is also described as part of the Water Treatment System designed by <br />Lyntek in Appendix H. <br />The volume of the Lined Untreated Water Pond, shown on Map C-2, was calculated using the <br />Survcadd program. The volume calculation, shown below, was made at an elevation 1.5 feet below <br />the top of pond berm, which is the same as the surrounding treatment azea ground surface (7050' <br />elevation). This volume is approximately 160,000 gallons, or about 15,000 gallons more than the <br />• 145,000 gallon design criteria. The 160,000 gallons is equivalent to 21,400 cubic feet or 790 cubic <br />Whirlwind Mine July 07 D-25 <br />