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• <br />18.2 Packrat Surface Drainage <br />As the Packrat portal area is very small (i.e., about an acre), installation of an unlined collection sump <br />is considered sufficient to control runoff and sediment from the disturbed area. This sump will handle <br />the runoff from the 100-year, 24-hour storm event. As shown on Map C-6, the sump will be located at <br />the bottom of the bin area. Surface water runoff will flow directly into the sump from the lower pad. <br />Water runoff from the upper bench will be transferred to the bin level sump by a 6-inch diameter <br />HDPE vertical pipe, which is shown on Map C-4. <br />Surface water runoff from above the Packrat will be diverted around the site by diversion channels <br />located along the west perimeter of the pad. The channels will be directed into culverts where they pass <br />under portal areas. All diversions, collection ditches, and culverts were designed using the 100-year <br />peak flows. <br />18.3 Design Method <br />The hydrology designs, including ditches, culverts, drainage basins, peak flows, sediment pond, etc. <br />are shown on Map C-6 Mine Surface Hydrology Designs. All calculations for the designs are shown in <br />Appendix B. <br />For both the Whirlwind and Packrat sites, the 100-full-year type 2 storm event was chosen for designs. <br />Information for the 2.90 inch event was obtained from the NOAA Atlas 2, Volume III for 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 areas of 2,000 acres or less. The <br />calculations require a proper understanding of the input requirements: <br />• <br />Whirlwind Mine 07 (rev. April OS) D-32 <br />