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These ponds have been constructed to contain an estimated three yeazs of sediment storage volume <br />and a runoff storage volume resulting from the 10-year 24-hour rainfall event. Sediment will be <br />excavated, allowing for additional storage, when sediment has reached the maximum storage <br />capacity. Sediment volumes aze estimated by a quarterly visual evaluation. The smaller ponds <br />MB-3, MB-4, and MB-5, usually dry during a portion of the yeaz and sediment levels, relative to <br />the primary dischazge structure elevations, can be visually determined. The larger ponds usually <br />contain water and visual evaluations aze less accurate. These ponds, MB-1, MB-2R, and SG-1 will <br />be surveyed at least every three years to deternune the actual sediment levels. <br />The CMLRB requirements for the emergency spillway sizing aze 25-yeaz, 24-hour storm flows, or <br />100-yeaz, 24-hour storm flows, depending on the size of the structure. The spillways for MB-3, <br />MB-4 and MB-5 were sized based on the 25-yeaz, 24-hour storm, while MB-1, MB-2R, and SG-1 <br />were sized for the 100-yeaz, 24-hour storm event in accordance with the requirements of the State <br />Engineer. Drainage basins for the spillway sizing are shown on Map 54, Sediment and Drainage <br />Control Facilities. The RPE pond has been designed to completely contain the runoff from a 100- <br />year, 24-hour storm event. Designs aze included in Exhibit 70. <br />MB-1 and MB-2R have been classed in the United States Bureau of Reclamation (iTSBR) 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, the <br />potential for loss of life is minimal as immediate downstream households are limited in number <br />and could be evacuated quickly. In addition, the impact on the North Fork of the Gunnison River is <br />minimal since the energy from such a small volume of water would be quickly dissipated. <br />The 10-yeaz, 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 yeazs of runoff. MCC will maintain the ponds in compliance with the designs to effect <br />discharge in compliance with effluent limitations. hi 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 fk.3 of <br />sediment. <br />A system of ditches and culverts has been designed to collect runoff from the 10-yeaz, 24-hour or <br />lazger storm event and direct it to the runoff ponds. Although not required by CMLRB regulations, <br />most of the haul and access roads located within the main mine facilities area aze drained to <br />sedimentation ponds. An exception is the Sylvester Gulch haul/access road; however, best <br />management practices for stormwater treatment will be utilized. Modifications to the original mine <br />pond and ditch system can be found in Exhibit 48 and Exhibit 66. Ditches and culverts associated <br />with the RPE area are contained in Exhibit 70. Ditches and culverts are shown on Maps 54, 54A <br />and 54B. <br />Combined ditch flows are summarized for the system as they progress towazd the runoff ponds. <br />Ditch and culvert specifications aze listed on Table 44, Exhibit 66. A summary table of inflows and <br />volumes for each pond is presented in Table 42 and in Exhibit 66. <br />The hydrologic parameters for watershed and subwatershed basins aze summarized on Table 43 in <br />2.05-29 Revised Jan 1996 RN03; RevisedNav. 1996 PR07; Rew'sedJan 1997 rR79; Revised Jul. 19977R82; Revised/un. 1999 TR90 <br />