2017-11-22_PERMIT FILE - C1981019

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EXHIBIT 7, ITEM 23, PART A CLEAN WATER DIVERSION STRUCTURES To reduce the volume of runoff that would otherwise report to the pit, storm flows and flows from snow melt from portions the undisturbed areas uphill and to the south of the pit will be diverted around the disturbed area by means of a series of clean water diversions. One set (CWD-1 and CWD-3) will divert clean water runoff around the west side of the pit and the other (CWD-2 and CWD-4) will divert these flows around the east side of the pit. Another clean water diversion structure (CWD-5) will divert runoff away from a large cut slope on the Collom Haul Road. The five clean water diversions are CWD-1, CWD-2, CWD-3, CWD-4, and CWD-5. Exh. 7-23, Figure Al shows the locations, lengths, slopes, and profiles of clean water diversions CWD-1 through CWD-4 on a single drawing, although it should be noted that all four diversions will generally not exist at the same time. Exh. 7-23, Figures A2, A3, A4 and A5 respectively show the four pit clean water diversion individually and in context of the pit progression uphill and to the south. These maps also present the drainage area boundaries and drainage areas. Exh. 7-23, Figure A6 shows the Collom Haul Road clean water diversion location, profile, and its associated watershed boundary. As a broad overview, diversions CWD-1 and CWD-2 will be installed initially upon commencement of disturbance lower in the valley. As can be observed on Exh. 7-23, Figure A-1, the lower end of the western diversion CWD-1 will be cut off by the highwall sometime before 2020, by which time it will be replaced by CWD-3 further upstream. Similarly on the east side, the lower end of CWD-2 when mined through, will similarly be replaced by CWD-4. As CWD-3 and CWD-4 are mined through as the pit advances uphill and to the south, this signals the end of the clean water diversion system for the Collom Pit. CWD-5 is intended to be in place for the life of the Collom Haul Road. The four Collom Pit clean water diversion ditches (CWD-1 through 4) will be constructed as simple triangular ditches with a minimum depth of 3.5 feet, crabbing downward along the contour at a uniform slope of 1.5%, as shown on the attached figures. The Collom Haul Road clean water diversion will be constructed as a triangular ditch with a minimum depth of 1.5 feet, and a uniform slope of 1.0%. Prior to excavation of each ditch, the topsoil within the ditch footprint will be pushed off and stored immediately adjacent to the ditch on the downhill side,to be conserved and used during reclamation. The ditches and associated disturbances will be seeded upon completion of construction. Hydraulic design of the ditches was performed using SEDCAD, as described in more detail in the Introductory Text of Volume 2D in the document entitled "Methodologies and Assumptions for Sedimentation Pond Design Evaluations". The Regulations of the Colorado Mined Land Reclamation Board for Coal Mining require in Rule 4.05.3(2) that temporary diversions of overland flow and ephemeral streams with a drainage area less than one square mile be designed to handle at least the 2-year storm, and Rule 4.05.4(2)(b) would require the ditch to be designed for the 10-year, 24-hour flow for watersheds greater than one square mile. While the largest of these temporary Collom Pit clean water diversions is well below the one square mile threshold (CWD-2 at 178 acres), CWD-1 through CWD-4 have all been designed to convey the 100-year, 24-hour storm, in part as a means to reduce pit inflow and pumping requirements. Also, for CWD-1 through CWD-4 a curve number of 70 was used for the ditch designs, notwithstanding that the Introductory Text of Volume 2D, would allow a CN as low as 63 for these undisturbed watersheds. Clean water diversion CWD-5 was modeled using the 10-year, 24-hour event with a curve number of 63. Exh. 7-23A-1 Revision Date: 8/23/17 Revision No.: TR-114