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2 - 48 <br />Combined peak flow from drainages DI, D2 and D3 fora 25 year/24 hour storm is <br />approximately 10.05 cfs. An emergency spillway, designed to carry this flow was constructed on the <br />notth side of the proposed sediment pond. The location and cross section of this spillway is shown <br />in Figure 2.2-3. The outslope of the embankment in this area is rip-rapped and vegetated for erosion <br />control. See Table 4.2-12 for calculation of flow for 24 hour storm recurrance intervals from 10 to <br />100 years. Baseline soil descriptions and vegetative cover percentages were used to estimate runoff <br />curve numbers (CN values) under antecedent moisture condition II. As shown on Table 4.2-10, CN <br />values ranged from 67 to 80. <br />Hydrologic values for the individual sub-basins are shown on Table 4.2-10. Peak discharge <br />values at the design poinu shown on Figure 2.2-3 were estimated to define the capacity of the <br />culverts and ditches on Table 4.2-13. <br />• The size of the sediment pond in conjunction with the precleaner sediment trap along the road <br />has made cleaning of sediments from the pond an infrequent activity. Cleaning the pond is done as <br />necessary, usually when sediment reaches the 60 percent sediment level or when the pond is <br />completely dry. The material is sometimes too wet to place directly on the topsoil stockpile; <br />therefore, it is first dewatered in the sediment drying area shown on Figure 2.2-2. <br />The material cleaned from the pond may be used as topsoil. If analysis of the chemical and <br />textural characteristics of the sediment indicate that the sediment is suitable for topsoil. Grand <br />Valley submitted the analyses to CMLR in 1984 and received approval on November 8, 1984 (DOC. <br />5279) to use it as a plant growth medium and to store it in the topsoil stockpile. Material cleaned <br />from the pond in 1992 has been stored in the drying area. Analysis of this material is shown on <br />Table 4.4-7. <br /> <br />M° Volume 1 10.12-97 <br />