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allows for the storage of runoff from two 10-year, 24-hour storm events (1.95 acre-feet) as well as storage of 6.7 years volume of sediment accumulation (1.90 acre feet). During the review of the permit revision application, it was noted that the above capacities were determined using a CN of 80. This CN is inappropriate for this particular pond due to the high degree of waste material compaction required during construction of the waste embankment. This extreme compaction precludes Soil Group A as defined on page 3 of "Peak Flows in Colorado." This refuse should approximate, after compaction, a very poorly drained soil with a very slow rate of water transmission. The Division feels compacted refuse more appropriately fits in a CN of 85. The applicant, in response to this concern, subsequently revised the pond sizing calculations using a CN of 85. Waste Disposal Area Topsoil Stockpile Sediment Pond The proposed pond at the coal processing waste pile topsoil stockpile is designed to hold a volume of runoff and sediment of 0.31 acre-feet. This capacity allows for the storage of runoff from one and one-half 10-year events (0.18 acre-feet) as well as storage of 2.1 years' volume of sediment accumulation (0.18 acre-feet). The Division requires that a sediment clean-out marker be installed to indicate the level at which sufficient storage remains for both runoff and sediment generated by the 10-year, 24-hour storm event. The collector ditch into the pond will be stable under the flow from the 10-year, 24-hour storm event. The emergency spillway is designed to convey flow from the 25-year, 24-hour event without eroding. Due to the small size and incised nature of the pond, dewatering will be accomplished through the use of a pump once the water quality meets or exceeds effluent standards. Mine Bench Area Collection Ditches, Culverts and Diversions The bypass culvert conveying undisturbed flow beneath the mine bench fill is sufficient to pass the required 10-year, 24-hour peak flow. Velocities of water discharged from the culvert during the 10-year, 24-hour event will be moderate. Since the applicant proposes to install a dumped rock gutter at the discharge end of the culvert, the energy should be properly dissipated. The bypass culvert will discharge into a riprap-lined temporary diversion which will convey the flow into the natural drainage. Due to the high slope of the channel, flow velocities during the 10-year, 24-hour event could be as high as 16 feet per second. The applicant has committed to the installation of adequate channel lining to inhibit erosion of the diversion and appropriate measures to dissipate flow energy prior to discharge into the receiving drainage. The collection ditches and culverts conveying runoff from the disturbed area at the mine bench will adequately carry the flow resulting from the 10-year, 24-hour event. Ditches collecting disturbed flow and diverting -24-