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handle the runoff from the 100-year, 24-hour storm event. As shown on Map C-6, the sump will be <br />• located at the bottom of the bin area. Surface water runoff will flow directly into the sump from the <br />lower pad. Water runoff from the upper bench will be transferred to the bin level sump by a 6-inch <br />diameter HDPE vertical pipe, which is shown on Map C-4. <br />Surface water runoff from above the Packrat will be diverted azound the site by diversion channels <br />located along the west perimeter of the pad. The channels will be duected into culverts where they <br />pass under portal azeas. All diversions, collection ditches, and culverts were designed using the <br />100-yeaz peak flows. <br />16.3 Design Method <br />The hydrology designs, including ditches, culverts, drainage basins, peak flows, sediment pond, <br />etc. aze shown on Map C-6 Mine Surface Hydrology Designs. All calculations for the designs aze <br />shown in Appendix B. <br />~, For both the Whirlwind and Packrat sites, the 100-full-yeaz type 2 storm event was chosen for <br />designs. Information for the 2.90 inch event was obtained from the NOAA Atlas 2, Volume III for <br />Colorado. <br />The hydrologic soils group for the Sili , Sedgran, and Bodot soils is "C", although for conservative <br />design purposes, a "D" hydrologic soil group was used. <br />The Curve Number Method was developed to allow calculation of the overall time of concentration <br />(Tc) under a wide range of conditions. The method is designed for azeas of 2,000 acres or less. <br />The calculations require a proper understanding of the input requirements: <br />The HYDROCAD computer modeling program was used which uses the SCS Unit Hydrograph <br />Method for watershed runoff modeling. <br />~~ <br />Whirlwind Mine June 07 D-29 <br />