2016-08-08_PERMIT FILE - C1981019A (58)

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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. The four clean water diversions are CWD I, CWD2, CWD3 and CWD4. Exh. 7-23, Figure Al shows the locations, lengths, slopes, and profiles of all four clean water diversions on a single drawing, although it should be noted that all four diversions will generally not exist at the same time. Maps Exh. 7-23, Figures A2, A3, A4 and A5 respectively show the four ditches individually and in context of the pit progression uphill and to the south. These maps also present the drainage area boundaries and drainage areas. 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. The four clean water diversion ditches will typically 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. 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 clean water diversions is well below the one square mile threshold (CWD-2 at 178 acres), the ditches 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, 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 62 for these undisturbed watersheds. The hydraulic parameters and flow results are presented in the following tabular format. A separate SEDCAD program output is provided for each of the four ditches and attached. All of the ditches exceed the minimum freeboard requirement of 0.3 feet per Rule 4.05.3. In certain situations, it may be necessary to modify the final ditch horizontal and vertical alignments in the field should obstacles occur that would require minor plan modifications, but the robust designs presented herein are representative. In the upper portion of each ditch, it is expected that construction might present additional challenges as a result of the steepness of the surrounding terrain. In those cases, the use of a shallower V ditch with steeper sideslopes may be necessary. For example, above station Exh. 7-23A-1 Revision Date: 2/15/16 Revision No.: PR -04