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Technical Revisia~ No. 79 (Second Response) May 5, /997 Page 2 15. MCC has recalculated rip rap sizing for the RPE sediment ditches (A, B, C, and D) using the SEDCAD' Version 3 program by Civil Software Design. The Pennsylvania Department of Environmental Resources (PADER) method of rip rap sizing based upon the permissible velocity concept, was utilized to calculate recommended rip rap dimensions. The calculations are attached and are summarized as follows: Ditch A: D,,,,x = 7.5 in, Dsu = 6.0 in, Dso = 2 in Ditch B: D,,,,x = 18 in, Dsu = 9.0 in, Dso = 3 in (see note below) Ditch C: Dm,x = 7.5 in, Dso = 6.0 in, Dso = 2 in Ditch D: D,,,,,~ = 7.5 in, Dso = 6.0 in, D~a = 2 in Because the computed velocity for Ditch B was only 2 fps, the erodible channel design option was used to confirm that native colloidal clay is sufficient and no rip rap is necessary. The erodible channel design routine computed a flow velocity of 2.79 fps using colloidal clay, which can withstand flow velocities up to 5 fps. MCC will use the above rip rap dimensions, with the exception that Ditch B will not be rip rapped, but instead have native colloidal clay or equivalent as its channel base. According to the SEDCAD user manual instructions the Simons/OSM method is not appropriate for channels with widths outside the range of 6 to 20 feet. Ditches A, B, and D have full flow (using 100-year, 24-hour storm event peak flow estimates) width dimensions generally less than 6 feet which makes the PADER method more appropriate. This is apparent as the computed flow velocity for Ditch A using the Simons/OSM method was nearly 74 fps (or 50 mph) and is not realistic. 24. Please note that placing liners in sedimentation ponds or under refuse piles is not required by the Regulations of the Colorado Mined Land Reclamation Board for Coal Mining. MCC is designing these structures to include liners, which is above the minimum regulatory requirements for both structures. As stated previously, MCC will utilize a 60-mil HDPE liner in the sedimentation pond and a 30-mil PVC liner under the RPE. 25. MCC reiterates our commitment to resize the drains as needed based on field conditions and to quality control during construction. 26. See the response to question 25. 28. See the response to question 2. 30. MCC assumes that the Division's use of the term "post-construction monitoring" refers to post-construction of the Phase 1 cut slopes. "Post-construction stability monitoring" was not proposed because the cuts during Phase 1 will remain open only a short time before being backfilled with refuse. Once backfilled, the refuse pile contours will be higher than the original contours, buttressing the undisturbed slopes, and no stability issues should exist.