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File SL-OI, Permit No. C81-010 <br />Trapper Mining Inc. <br />11/23/98 <br />Page 5 <br />b) Post-mining USLE calculation of soil loss from bond release parcels. The <br />Division calculates the USLE for the annual post-mining soil loss from the fully <br />reclaimed drainages that contain bond release blocks as: <br />A = RKLSCP <br />_ (30)(0.25)(3.0)(0.020)(1.0) <br />= 0.45 tons per acre per year <br />The sources and rationale for selecting the parameters in the equation are shown in <br />the 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-fr. 8% slope <br />C 0.020 SCS "C" factor table for permanent <br /> pasture and rangeland: No <br /> appreciable canopy, grass and litter <br /> surface cover, 75% ground cover <br /> (average allowable cover on pazcels, <br /> Table 2.2-1 of submittal) <br />P 1.0 SCS "P" factor table: 8% slope, "up <br /> & downhill" <br />c) Sediment delivery ratios for pre- and postmining conditions. USLE calculates <br />gross soil loss rather than net soil loss. Gross soil loss is all of the soil freed from a <br />parcel of land by erosive forces. The eroded soil may or may not be transported out <br />of the drainage basin over a period of several years. Some of the eroded soil is <br />deposited within the basin. Net soil loss (or sediment yield) is the fraction of eroded <br />soil that is transported out of the basin. The percentage of all eroded soil that is <br />transported out of the basin is the sediment delivery ratio, calculated as net soil loss <br />divided by gross soil loss. The Division estimates that the pre-mining sediment <br />delivery ratio was 10% less than the post-mining delivery ratio (see attached <br />Appendix A). <br />d) Conversion of USLE soil loss rate to sediment yield rate.. Reducing the <br />Division's USLE-calculated pre-mining soil loss rate of 1.06 tons/acre/year by 10% <br />gives apre-mining sediment yield rate of 0.95tons/acre/year. <br />e) Post-mining sediment yield rates indicated by pond sediment volumes. In order <br />to check our USLE-calculated post-mining sediment yield rates, the Division has <br />estimated the post-mining sediment yield from the four fully reclaimed drainages <br />