2015-09-09_PERMIT FILE - C1981019A (16)

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1.9 BASE FLOW In some cases a fixed base flow is include in the flow reporting to a sediment pond. Specifically, a base flow of 0.15 cfs was used for the Streeter and Prospect Pond designs. A base flow of 0.03 cfs was used for the West Pit Pond design. Base flows were not used in any other pond design calculations. Pit water will not be discharged through outfalls that have not been permitted for pit water discharge under an approved CDPS permit. Additionally, for those outfalls approved for pit water discharge but where this discharge was not included in the pond design as a base flow, pit pumpage will not occur during any storm runoff or snow melt less than or equal to the 25 -year, 24 -hour storm event. 2.0 SEDIMENTATION CALCULATIONS The sedimentation calculations for each pond on the Colowyo Mine are performed using the SEDCAD4 Computer Modeling Program developed by Civil Software Design. The SEDCAD4 Computer model is based on the Revised Universal Soil Loss Equation ( RUSLE). The input parameters used in the sedimentation analyses are presented in the following sections. 2.1 RAINFALL FACTOR, R A rainfall factor of 19 is used for the Colowyo mine site. This factor is consistent with other recent erosion analyses performed for the Colowyo Coal Company. 2.2 SOIL ERODIBILITY FACTOR, K The soil erodibility factors for the Colowyo Mine are estimated based on soil survey information presented in the Colowyo Permit Application. Note that for historic (existing) designs, an average of 0.28 was used for K. Future designs will be based on a K value of 0.30. TABLE 2 SOIL ERODIBILITY FACTOR DOMINANT SOIL TYPE SOIL TEXTURE SOIL ERODIBILITY FACTOR (K) Burnette Series Loam 0.28 Work Series Loam 0.31 Average 0.30 2.3 LENGTH -SLOPE FACTOR, LS Input parameters to determine the LS factor were obtained from scaled maps of the watershed areas. Values measured from the scaled maps are shown in the SEDCAD4 printouts. Within disturbed subwatersheds, the representative length was capped at 400 feet. Lengths of drainage paths needed for the "LS" factor determination in the RUSLE option in SEDCAD are determined by tracing the flow path back to the subwatershed headwaters. In most cases this distance exceeds the nominal limit of 400 feet described in accepted references such as "Applied Hydrology and Sedimentology for Disturbed Areas ", 1985, by Barfield, Warner and Haan, making reference to the original 1965 Wischmeier and Smith USDA publication. In such cases the length of the watershed has been capped at 400 feet. This practice has been recommended based on personal conversations with Dr. Richard Warner, one of the developers of the SEDCAD program. 2.4 CONTROL FACTOR, C The control factor is used to account for vegetative or mulch cover. The following values are utilized in Exhibit 7 Page 5 Revision Date: 3/18/13 Revision No.: TR -95