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The collector ditch for the pond at the underground development waste disposal site will <br />be stable under flows from the 10-year, 24-hour event and the emergency spillway will be <br />stable during; flows resulting from the 2~-year, 24-hour event: As in the case for the pond <br />at the mine tench, the permit application has been amended in response to the original <br />Stipulation No. 2 to include provisions for installation of a dewatering device to dewater <br />the pond down to the maximum sediment storage level. <br />Central Facilities Area Sediment Pond- The proposed pond at the central facilities area is <br />designed to hold a volume of runoff and sediment of 5.53 acre-feet. This capacity allows <br />for the storal;e of runoff from two 10-yeaz, 24-hour storm events (3.5 acre-feet) as well as <br />storage of five years' volume of sediment accumulation (0.145 acre-feet). The Division <br />requires that a sediment clean-out mazker be installed in the pond to indicate the <br />maximum level at which sufficient storage remains for both runoff and sediment <br />generated by the 10-yeaz, 24-hour storm event. <br />Central Facilities Topsoil Stockpile Sediment Pond- The proposed pond at the central <br />facilities topsoil stockpile is designed to hold a volume of runoff and sediment of 0.3 <br />acre-feet. This capacity allows for the storage of runoff from one and one-half 10-yeaz, <br />24-hour storm events (0.20 acre-feet) as well as storage of three yeazs' volume of <br />sediment accumulation (0.10 acre-feet). The Division requires that a sediment clean-out <br />mazker be installed in the pond to indicate the level at which sufficient storage remains in <br />the pond for both runoff and sediment generated by the 10-yeaz, 24-hour storm event. <br />The collector ditch into the pond will be stable under the flow from the 10-yeaz, 24-hour <br />storm event. The emergency spillway is designed to convey flow from the 25-yeaz, <br />24-hour event without eroding. Due to the small size and incised nature of this pond, <br />dewatering will be accomplished through the use of a pump once the water quality meets <br />or exceeds effluent standards. <br />Coal Processing Waste Disposal Area Sediment Pond- The proposed pond at the coal <br />processing vvaste disposal azea is designed to hold a volume of runoff and sediment of <br />3.95 acre-feet. This capacity allows for the storage of runoff from two 10-year, 24-hour <br />storm events (1.95 acre-feet) as well as storage of 6.7 yeazs volume of sediment <br />accumulation (1.90 acre feet). During the review of the permit revision application, it <br />was noted that the above capacities were determined using a CN of 80. This CN is <br />inappropriate for this particulaz pond due to the high degree of waste material compaction <br />required during construction of the waste embankment. <br />This extreme compaction precludes Soil Group A as defined on page 3 of "Peak Flows in <br />Colorado." This refuse should approximate; after compaction, a very poorly drained soil <br />with a very :slow rate of water transmission. The Division feels compacted refuse more <br />appropriately fits in a CN of 85. The applicant, in response to this concern, subsequently <br />revised the pond sizing calculations using a CN of 8~. <br />29 <br />