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4.6.1.3 Moffat Area Sediment Pond Design <br />The sediment yields and peak runoff values were used to formulate a <br />sediment pond design that provide storage for the runoff from a 10-year, <br />24-hour and would provide a minimum of three years of sediment storage below <br />the primary spillway. The pond was designed as a dewatering pond utilizing a <br />pipe located above the three year sediment storage level. Sediment pond <br />values are listed in Table 4.6-8. Figure 9.6-1 shows the stage storage curve <br />for the pond <br />The pond is rectangular in shape with a dog-leg on the southern end. <br />This shape enables a maximum of storage while minimizing the amount of <br />excavation along the back slopes. The front slope of the pond consists of a <br />2.Sh:ly slope below grade with a 3h:ly slope above grade. The side slopes and <br />back slope are 2.Sh:ly with a terrace at the 7000' elevation along the <br /> backslope to facilitate sediment removal. Exhibit 4.6-3 shows a planar view <br /> of the pond and Exhibit 4.6-4 shows a cross section of the pond. <br /> The primary spillway is a 12-inch pipe, The pipe will be projecting on <br /> the inlet and outlet end. The crest of the primary spillway will be placed at <br /> the elevation of 6999.6' (Table 9.6-8). Exhibit 4.6-3 shows the location of <br /> the primary spillway. <br /> The combination of the principle and emergency spillway was designed to <br /> carry the runoff from a 25-year, 24-hour storm event for the pond. The same <br /> procedure that was used to determine the peak stage of a 10-year, 29-hour <br /> storm event was used to determine the peak stage of the 25-year, 24-hour storm <br /> event. Year 20 had the highest peak stage with a peak discharge flow of 30.19 <br /> cfs. The drainage patterns during this year differ significantly from the <br /> previous years in that all of the haulroads and associated culverts have been <br /> removed which shorten many of the flow paths from various portions of the <br /> watershed. Table 4.6-6 shows the subwatershed time of concentration and <br /> hydrograph routing parameters for each subwatershed. Table 4.6-7 shows the <br /> parameters for routing the subwatershed hydrographs between junctions and/or <br /> structures. Exhibit 4.6-2 indicates the watershed dissection used for the <br /> peak stage calculation and Appendix 9.6-C contains the computer model results. <br /> The emergency spillway for the pond was designed as a trapezoidal open <br />• channel with 3h:ly side slopes able to carry the peak flow at a non-erosive <br />4.6-29 <br />November 1,1989 <br />