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2-48 <br />Combined peak flow from drainage areas DI through D16 for a 25 year 24 <br />hour storm is approximately 38.10 cfs. An emergency spillway, designed to carry <br />this flow was constructed on the west side of the proposed sediment pond. The <br />location of this spillway is shown in Figure 2.2-3. See Appendix M, pages 12-73 <br />for calculation of flow for 24 hour storm recurrence intervals for 10, 25 and 100 <br />years. Baseline soil descriptions and vegetative cover percentages were used to <br />estimate runoff curve numbers (CN) values. As shown in Appendix M, pages 12- <br />14, Curve Numbers range from 73 to 89. <br />Hydrologic values for the individual sub-basins are shown in Appendix M, <br />pages 11-61. Peak discharge calculated with SedCad 4.0 for the drainage basins <br />shown on Figure 2.2-3 and were used to define the capacity of the culverts and <br />ditches. <br />The size of the sediment pond in conjunction with the precleaner sediment <br />trap (sump) along the haul road has made cleaning of sediments from the pond <br />an infrequent activity. Cleaning the pond is done as necessary, usually when <br />sediment reaches the 60 percent sediment level or when the pond is completely <br />" dry. The sediment pond must be dewatered if it contains water above the <br />maximum sediment level and discharge will be sampled each time it is released. <br />The pond will be manually dewatered through the primary spillway which consists <br />of a six inch diameter pipe. The pond. has a standard emergency spillway with <br />the inlet placed at the maximum water storage elevation. Dewatering events <br />should occur within 48 hours after the storm. <br />The material cleaned from the pond may be used as topsoil. If analysis of <br />the chemical and textural characteristics of the sediment indicate that the <br />sediment is suitable for topsoil. The operator submitted the analyses to CMLR in <br />1984 and received approval on November 8, 1984 (DOC. 5279) to use it as a <br />plant growth medium and to store it in the topsoil stockpile. Material cleaned from <br />the pond in 1992 has been stored in the drying area. Analysis of this material is <br />shown on Table 4.4-7. <br />TR-16 M`-Volume 1 (Rev 07/08) <br />