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~. <br />~~' <br />October l7, 1996 <br />Ms. E. Cosby <br />Page 3 <br />1.7 Muskingum Routing Parameters <br />The Muskingum routing parameters were also calculated using the SCS upland method. The hydratdic lengths, <br />drainage heights, and associated slopes were determined from scaled drawings. The values calculated between <br />each structttre and/or sub-watershed are shown in the SEDCAD+ printouts. <br />1.8 Curve Number <br />The run-off curve number is a factor relating the amount of rainfall to the amount of run-0ff for a given area. <br />Curve numbers were evaluated based on vegetation and soil type for the given watershed. The values used <br />were obtained from previously approved curve numbers presented in the Colowyo Permit Application. The <br />following curve numbers were used for the analyses (Soil Conservation Service, 1986): <br />TABLE 1 <br />CURVE NUMISERS <br />Area Type Hydrologic Soil Group Yegefatioa Curve Number <br />Disturbed C Nane 9l <br />Disturbed Spoils B None 86 <br />1 to 2 Ycar Reclamation B Herbaceous, 60% cover 61 <br />3+ Year Reclametion B Herbaceous, 70% cover 58 <br />Undisturbed C Sagebrush, Good <br />Condition 47 <br />2.0 SEDIMENTATION CALCULATIONS <br />The sediment calculations used to model the Colowyo mine site were performed utilizing the SEDCAD+ <br />computer modeling program developed by Civil Software Design. The SEDCAD+ computer model is based on <br />the Revised Universal Soil Loss Equation (RUBLE). The input parameters used in the sedimentation analyses <br />are presented in the following sections. <br />2.1 Rainfall Factor <br />A rainfall factor of 30 was used for the Colowyo mine site. This factor is consistent with other erosion analyses <br />performed for the Colowyo Coal Company. <br />2.2 Soil Erodibility Factor <br />The soil erodibility factor for the subject site was estimated based on soil survey information presented in the <br />Colowyo Permit Apphcation. An average value of the dominant soil types USC used for the Colowyo mine site. <br />