REV88550 - Laserfiche WebLink

Home Browse Search

Mountain Coal Company West Elk Mine 6.0 SURFACE HYDROLOGY 6.1 Introduction Exhibit 51 Lower Refuse Pile A surface hydrology analysis was performed to size the sediment and runoff ditches located around the lower refuse pile for each of the four phases and determine if the existing sedimentation ponds were capable of handling the additional runoff associated with each phase. The peak discharges and maximum runoff used in the design size verification of all ditches and sedimentation ponds were based upon procedures developed by the Soil Conservation Service and the U.S. Department f Agriculture. (Urban Hydrology for Small Watersheds, Technical Release~No. 55, 1975 and from Procedures for Detetminine Peak Flows in Colorado.~oil Conservation Service and the U.S. Department of Agriculture, March 1980) All ditches and diversions were designed to accommodate the peak discha r the 100-year, 24-hour precipita- 2j tion event with an adequate freeboard o .5 f et. A triangular channel design was used for all ditches. The sedimentation pon was designed to store the 10-year, 24- hour storm event without discharge plus s invent storage for each phase. Also, the existing outlet structure for the sediment and was analyzed to insure that the system could discharge the peak flow from a -year, 24-hour precipitation event far each phase without overtopping. The llowing sections describe the design of the sediment control ditches, the clear water ditches, and the sedimentation basin for each phase of the lower refuse p' e. Positive drainages are current established and will be maintained along the south side of the lower refuse pile. Surface runoff from the refuse pile will be collected in the sediment ditches and c veyed to sediment pond MB-6. The drainage from the undisturbed area to the uth will be collected in the clear water ditches into the clear water discharge are Sylvester Gulch, or the North Fork of the Gunnison River. At no time will surface/runoff be allowed to collect at the head of the pile. The final configuration sediment ditch locations around the entire lower refuse pile are shown in Figure 2.0 in Appendix B. 6.2 Sediment/Control Ditches Sediment control ditches were constructed along the perimeter of each phase of the proposed lower refuse pile as shown on Figure LWP-002 thru LWP-004 and Figure 2.0. The/sediment control ditches were designed bas on a 100-year, 24-hour ` r itatton of 2.S~nches, a wa ile curve number f 75, - "f-concentratien- 8Z e~~~~u Hoff- _ - - _ .-The peak dts rge for each phase was 1, ~~~~ calcu acted based upon proce ures developed by the Soil Conservation Service 0~ me Toned in the introduction of this section. The diversion ditches were designed us' g a computer program called MANCRIT2, based upon Manning's equation for a riangular channel. The ditches were designed based upon a Manning's "n" value f 0.03, a required minimum freeboard of 0.3 feet, and ditch side slopes of 2:1.. 1Recommended ditch velocities for unlined channels were estimated to be from 3 to 23