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The SEDCAD 4 models serve to demonstrate that for the selected design event, less sediment is <br />produced from the reclaimed lands than from the undisturbed lands. Given the nature of the vegetation <br />characteristics, and consequently of the runoff characteristics occurring under both pre-mining and post- <br />mining conditions, it is necessary to select an atypical design storm input in order to produce the <br />requisite. 0.5 inches of runoff necessary to maintain program accuracy. For the purposes of this <br />demonstration, an input design event of 2.75 inches per 24 hours was utilized. This design storm input <br />produces the desired minimum 0.5 inches of runoff from the pre-mining blocks and serves as the basis <br />of comparison for the pre-mining and post-mining conditions. <br />Curve Numbers. (CN) representative of the vegetation at Trapper for both the pre-mining and post-mining <br />conditions were selected from tables available within the SEDCAD 4 program. For the pre-mining areas, <br />a CN of 48 was selected from the CN table for Arid & Semi-arid Rangelands using the oak-aspen - <br />mountain brush mix cover type in "Fair" hydrologic condition with a hydrologic soil group classification <br />(HSG) of B. For the post-mining areas, a CN of 61 was selected from the CN table for Other Agricultural <br />Lands using the pasture, grassland, or range - continuous forage for grazing cover type in "Good" <br />hydrologic condition with an HSG classification of B. The different CN values serve to model the <br />differing runoff responses of the described cover types. More pronounced initial precipitation <br />abstractions are accounted for in the lower CN selected for the pre-mining areas. After the initial <br />abstractions are considered, erosion and sediment yield are evaluated considering vegetative cover <br />conditions at ground level below the mountain brush vegetative type canopy. The documented cover <br />percentage values of the vegetation occurring on the pre-mined and reclaimed lands were considered in <br />selecting the CN model input values and are reflected primarily in the hydrologic condition classifications <br />chosen. The CN values representative of the vegetation at Trapper for both the pre-mining and post- <br />mining conditions were modified to reflect Antecedent Moisture Condition III (AMC III) for the purposes of <br />this demonstration in order to produce comparable model outputs which show runoff and sediment <br />transport occurring. <br />Further distinctions between the reclaimed lands and pre-mining conditions are characterized using the <br />Control Practice (CP) factor model input variable. The C factors used in the models were obtained from <br />the SEDCAD 4 table for Permanent Pasture, Rangeland, Idle Land, and Grazed Woodland utilizing <br />cover and canopy data as appropriate. For pre-mining conditions, a canopy providing 50% cover and <br />consisting of appreciable brush or bushes (fall height of approximately 2 meters) was used in conjunction <br />with a range site area-weighted herbaceous cover measurement in accordance with the pre-mining cover <br />data presented on Table 4.4-2 of Permit No. C-81-010. For post-mining conditions, no appreciable <br />canopy was utilized along with the actual herbaceous cover values existing on the reclamation as <br />determined by direct transect measurements. These tabular C values were subsequently adjusted to <br />reflect the more roughened condition prevalent on the reclaimed lands as compared to undisturbed <br />areas. <br />14