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Sediment storage volume for 10 years of sediment of 0.21 acre feet <br />• is provided in the design. Modelling with SEDCAD+ utilized a <br />rainfall factor (R) of 26.5. This portion of Jackson County is <br />located in a rainfall zone of 20. There are 4.33 inches of <br />precipitation between December and March, resulting in a snowmelt- <br />corrected R of 26.5. <br />In the reclaimed area the sedimentology factor, 'K,' was derived <br />from the Phase II Bond Release documentation dated 8/16/90. A 100- <br />foot length was presumed to be the maximum distance water would <br />flow within the slopes of the watershed before initiating minor <br />rilling. The side slopes of the reclaim are 25~. The cover <br />practice (CP) of 0.017 is derived from a value for a canopy of tall <br />weeds or short brush (0.5 m) of 44~ vegetative canopy and 86.8$ <br />total cover. The 'K' for the pond itself is the same as for the <br />reclaim. A representative length of 275' was utilized, with a <br />slope of 2$. A cover practice (CP) of 1 reflected the presence of <br />fine silt from deposition. <br />Peak flows were estimated for the drainage area consisting of the <br />27.86 acres of reclaimed surface and 11.03 acres of undisturbed <br />area for a total of 35.38 acres. The pond covers an area of 3.51 <br />acres. The contributing watershed of reclaim and undisturbed land <br />was merged and run with a curve number of 84 per the Phase II Bond <br />• Release documentation dated 8/16/90. Three storms were run: the 5- <br />year 24-hour; the 10-year 24-hour and the 25-year 24-hour of 1.4", <br />1.6" and 2.0". <br />The principal spillway is a 24" perforated riser pipe with an <br />invert elevation of 8222.3'. There is a 24" barrel constructed of <br />CMP. There are four orifices, 2" in diameter at 8219' and 8222' <br />and the emergency spillway invert is at 8224.1'. There is 10.2 <br />acre feet of volume between the maximum sediment storage level and <br />the primary spillway invert at 8222.3'. The 5-year storm of 1.4" <br />generates 1.28 acre feet of runoff and the l0-year storm of 1.6" <br />generates 1.68 acre feet of runoff. Design flow through the primary <br />spillway can handle a l0-year 24-hour event without discharge over <br />the emergency spillway. <br />The 25-year storm of 2.0" generates 2.58 acre feet of runoff. <br />There is no discharge over the emergency spillway for the design <br />flow of a 25-year, 24-hour storm of 22.1 cfs. <br />• 4-2 <br />