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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 tt~e nature of the vegetation <br />characteristics, and consequently of the runoff characteristics occurring under both pre-mining and post- <br />mining wnditions, an atypical design storm input was selected in order to produce the requisite 0.5 inches <br />of runoff necessary to maintain program accuracy. For the purposes of this demonstration, the 100-yr <br />recurrence interval design event of 2.20 inches per 24 hours was utilized. This design storm input <br />produces the desired minimum 0.5-inches-of runoff-from the post-mining.blocMs 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 are selected from tables _available_within_the_SEDCAD_ 4_program. For the _pre-mining <br />evaluations, a CN of 75 was selected from the CN table for Cultivated and Agricultural Lands using the <br />small grain (straight-row and crop residue) cover type in 'Poor" hydrologic condition with a hydrologic soil <br />group Gassification (HSG) of B. For the post-mining areas, a CN of 61 was selrcted from the CN table for <br />Other Agricultural Lands using the pasture, grassland, or range -continuous forage for grazing cover type <br />in "~ "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 absVactions <br />are accounted for in the lower CN selected for the post-mining areas. After the initial absVactions are <br />• considered, erosion and sediment yield are evaluated considering vegetative cover and related soil <br />conditions at ground level. The CN values representative of the vegetation at Trapper for both the pre- <br />mining-and post-mining conditions are-modified-to reflect.Antecedent_Moisture.Condition III (AMC III) for <br />the purposes of this demonstration in order to produce comparable model outputs showing sufficient <br />- runoff to fall within the bounds of accuracy for the program. Trapper believes this modification provides a <br />valid basis foccomparison under the prescribed conditions. <br />Soil erodibility or "K" factor distinctions are modeled based upon knowledge of tlne pre-existing soils under <br />cultivation as compared to the mixture of salvaged soils typically used in reclamation. Specifically, two <br />dominant soil types, the Forelle and Savage, were present within the majority ol` the mined areas formerly <br />utilized as cropland. Both these soils are known to be erosive. A "K" factor of 0.34 is utilized to describe <br />the pre-mining scenario while a "K" factor of 0.25 is used to describe reclaimed conditions. <br />Representative slope lengths for sedimentology evaluations are differentiated under the pre-mining and <br />post-mining scenarios as well. The representative slope length is defined as the distance from the point of <br />origin of overland flow to either the point where slope decreases such thart deposition of entrained <br />sediment can occur or flows become concentrated in defined channels. For pre-mining conditions, a <br />relatively shorter_ representative slope-length_(50') is utilized to reflect the likelihood that flows became <br />. concenVated over much shorter distances due to the influence of surface tillage practices associated with <br />wheat farming operations. Overland flows under these conditions are assumed to have persisted over <br />shorter distances because of the characteristics of the regularly tilled and roughened cultivated soil <br />10 <br />