Laserfiche WebLink
West Elk Mirse <br />• primary discharge structure elevations, can be visually determined. The lazger ponds usually <br />contain water and visual evaluations are less accurate. These ponds, MB-1 and MB-2R, will be <br />surveyed at least every three years to determine the actual sediment levels. <br />The OMLR requirements for the emergency spillway sizing are 25-year, 24-hour storm flows, <br />or 100-year, 24-hour storm flows, depending on the size of the structure. The spillways for MB- <br />3, MB-4 and MB-5 were sized based on the 25-year, 24-hour storm, while both MB-1 and MB- <br />2R were sized for the 100-year, 24-hour storm event in accordance with the requirements of the <br />State Engineer. Drainage basins for the spillway sizing are shown on Map 54, Sediment and <br />Drainage Control Facilities. Pond drainage hydraulics may be found in Exhibit 66. See also <br />Exhibit 47 for the design criteria for MB-5 and MB-6. <br />MB-1 and MB-2R have been classed in the United States Bureau of Reclamation (USBR) Hazard <br />Assumption "B" category. Accordingly, the probable maximum precipitation inflow design flood . <br />has been reduced to the "B" assumption. This assumption is justified in that the lazgest storm <br />drainage/sediment control pond has a storage volume of less than 15 (acre-feet). Furthermore, <br />the potential for loss of life is minimal as immediate downstream households are limited in <br />number and could be evacuated quickly. In addition, the impact on the Gunnison River is <br />minimal since the energy from such a small volume of water would be quickly dissipated. <br />The 10-year, 24 hour storm runoff design was adopted in accordance with CMLRB regulations. <br />• The Revised Universal Soil Loss Equation (RUBLE) was used to predict the sediment yield from <br />three years of runoff. MCC will maintain the ponds in compliance with the designs to effect <br />discharge in compliance with effluent limitations. In some cases, MCC has installed concrete <br />sediment traps above the pond inlets to facilitate sediment removal. These traps typically have <br />inside dimensions of 24' in length, 10' in width, and 5' in depth, and can contain 600 ft.' of <br />sediment. <br />A system of ditches and culverts has been designed to collect runoff from the 10-year, 24-hour <br />storm event and direct it to the runoff ponds. Although not required by CMLRB regulations for <br />coal mines, most of the haul and access roads located within the main mine facilities area are <br />drained to sedimentation ponds. Drainage basins tributary to these ditches are shown on Map 54. <br />Modifications to the original mine pond and ditch system can be found in Exhibit 48 and <br />Exhibit 66. <br />Combined ditch flows are summarized for the system as they progress toward the runoff ponds. <br />Ditch and culvert specifications are listed in Exhibit 66. A summary of inflows and volumes <br />for each pond is presented in Table 42. <br />The hydrologic parameters for watershed and subwatershed basins are summarized on Table 43 <br />in Exhibit 66. This Exhibit gives the area, runoff, volume, and peak flows, for each watershed <br />basin. Documentation of specific parameters such as curve number, rainfall and time of <br />concentration are also found in Exhibit 66. <br />• _ Rcsisrd Jan. 1993 TR64; Revised Aug. /993 7R6S; 1 /94 PROS; 1 /94 ]R67; 3/94 MR118; S/94 rR69, MR1I? & MRI?3; <br />2,05-2J 6/94 MRI?7; Rrvisrd Jw. 1994 MR128; Resiscd Srpt. /994 MRlJ8; Retiisrd Nov. J994 MRf 44; II/94 AfR145 <br />