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_Y ~~ File SL-01, Permit No. C81-010 <br />' Trappzr Mining Inc. <br />I (/23/98 <br />Page 4 <br />The Division's review of Trapper's re-runs of SEDCAD+ has determined that the CN <br />conversion factors aze not as useful as previously thought. The conversion factors assume <br />that 5 days of rainfall on Trapper Mine totaling more than 2.1 inches would boost the runoff <br />potential of the ground surface from an original CN 42 to an effective CN of 62. This <br />assumption appears invalid, though, because much of that quantity of rainfall over 5 days on <br />Trapper would probably be lost to infiltration and evapotranspiration, and would not <br />significantly increase runoff potential. Consequently, the increase in CN would be neglible. <br />(Trapper's soils aze capable of absorbing any one-hour rainfall of a Type I[, 5-year storm <br />based on measured infiltration rates (Table 4.8-14 and Appendix G, part III of permit).) <br />5. Division's alternate method of comparing pre- and post-mining sediment yields. Given <br />the previously described limitations of the curve number method for determining runoff, the <br />Division has used an alternate method for checking Trapper's curve number-based sediment <br />yield prediction. The alternate method is a comparison of pre- and post-mining soil loss <br />rates. The Division calculated the pre- and post-mining soil loss rates using the Universal <br />Soil Loss Equation (USLE). <br />In addition, the Division checked its USLE-calculated soil loss rate for the post-mining <br />conditions. This check was accomplished by calculating the post-mining sediment yield <br />from bond release parcels using field estimates of the sediment volumes that have <br />accumulated in ponds. <br />a) Pre-mining USLE calculation of soil loss from bond release parcels. The Division's <br />USLE calculation of the annual pre-mining soil loss from the fully reclaimed drainages <br />that contain bond release blocks is: <br />A = RKLSCP <br />_(30)(0.25)(3.0)(0.047)(1.0) <br />= 1.06 tons per acre per year <br />The sources and rationale for selecting the pazameters in the equation aze shown in the <br />following table. <br />Parameter Value selected Source/rationale <br />R 30 SCS map of R factors for Colorado <br />K 0.25 Trapper permit <br />LS 3.0 SCS "LS" table: 1000-ft. 8% slope <br />C 0.047 SCS "C" factor table for permanent <br /> pasture and rangeland: Appreciable <br /> brush, 45% canopy rounded to 50% <br /> (acreage-weighted data from Table <br /> 2.3-13 of permit), grass type, 56% <br /> ground cover (p. 4-102 f of permit) <br />P 1.0 SCS "P" factor table: 8% slope <br />