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SMALL AREA EXEMPTION <br />TRAILER AND ADJACENT TOPSOIL STOCKPILE AREA <br /> <br />1.0 INTRODUCTION <br />Backfilling, grading, and topsoiling hu been completed at the Raton Creek Mine. All [he <br />topsoil hu been removed from the topsoil stockpile and the entire stockpile azea including the <br />trailer pad area hu been regraded, seeded, and mulched. In addition, Ditches TS-1 and TS-2 are <br />no longer required and have been removed. The area hu been gnded so that all runoff is <br />directed to the northwes[. The total regraded azea is approximately 1.4 acres. Given these <br />changes, EFCI hu revised the drainage control plan for this area. Runoff from the regraded site <br />will now be controlled by a sediment fence located in the northwest corner of the disturbed area <br />as shown on the Mine Site Water System and Refuse Map (Map 12). <br />2.0 HYDROLOGY DESIGNS <br />Runoff peak discharge calculations were performed utilizing the SEDCAD+ computer model, <br />consistent wi[h the methodology previously discussed in [his permit document. The sediment <br />fence is designed to safely pus the peak discharge from a 10-year, 24-hour storm event. <br />2.1 Hydrology inputs <br />All hydrology inputs are shown on the SEDCAD+ computer printou[s (see attached). <br />• The curve number (CN) used was obtained from Procedures for Determining Peak I:loun in <br />Colorado and Technical Release No.33 ((/SDA•SCS, 1980). Bued on soil type and vegetation <br />cover, the CN was determined using Table 2-2d, USDA-SCS TR-55, for the following <br />conditions: <br />Disturbed (Mulched) <br />Soil Type: C <br />Ground Cover: Poor <br />Cover Type: Herbaceous <br />CN: 87 <br />2.2 Results <br />Peak Discharge <br />10-year, 24-hour event: 2.7 cfs <br />2.3 Sediment Fence Requirements <br />Given the peak discharge and permittivity of sediment fence, the amount of sediment fence <br />surface area required co safely pus the peak discharge from a 10-year, 24-hour event was <br />determined u follows: <br />Sediment Fence Permittivity: 15 gal/min/f[r or 0.033 cfs/ftr (manufacturer's <br />• specificttion) <br />Required Surface Area: 2.7 cfs/0.033 cfs/ft' ~ 82 ft=~ <br />a:m x„o~'uneaic <br />= ,.;~~,,, Reeisee <br />