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spillway up to the Proposed Emergency Spillway Elevation and is required to be at least as large as the total 10- <br />year, 24 hour runoff volume. Table 3 shows storage volume calculations for each pond. Figures 2, 3 and 4 <br />display the pond designs for NHN -001, NHN -002, and NHN -003 respectively. <br />The sediment storage is calculated assuming an annual R value of 20.0 for one (1) year. Thus each pond <br />requires maintenance and sediment removal at a minimum of once a year and after every major storm event. <br />Required sediment storage volume calculations are based on the Modified Universal Soil Loss Equation. <br />Sedimentation Pond Outlet Controls <br />There are three types of principal spillways commonly used: open channel, drop inlet and pipe culvert. <br />Considering the flexibility of control and modification, an 18 inch diameter pipe with a 6 inch diameter control <br />valve was chosen for all ponds, allowing the New Horizon North Mine to selectively drain each pond after <br />sediment has had adequate settling time. All three ponds are assumed to have 18 inch diameter outfall pipes, <br />though larger pipes are an option. The valve is located at the end of pipe inlet, controlled by a wheel on the pond <br />embankment as the example shown in Figure 5. The 6" valve shown in the picture is used to dewater pond once <br />water is clear enough to discharge and meet effluent levels. <br />The slope and length of pipes are listed in the Table 2. SEDCAD calculated these pipe sizes assuming gravity <br />flow, though pressure flow scenarios are likely with a manually operated drain valve on each pond. The outflow <br />pipe bottom should be positioned between 0.5 ft to 1.0 ft above the top of the sediment storage elevation, which is <br />5676.6, 5676.0 and 5686.0 feet for ponds NHN -001, NHN -002 and NHN -003 respectively, assuming the release <br />is slow as suggested by the SEDCAD manual. <br />Two proposed emergency spillway elevations are listed in Table 3 for each pond. The first listed elevation for <br />each pond in Table 3 is 18 inches below the top of embankment and the second elevation is two feet below the <br />top of embankment. For all ponds, it is recommended that an emergency spillway elevation of 18 inches below <br />the top of embankment be constructed to allow for extra pond capacity. In the case where no additional capacity <br />is necessary, spillway elevations of two feet below the top of embankment may also be constructed. These <br />designs are highlighted in bold in Table 3. Each pond includes 10% or more excess capacity with the proposed <br />emergency spillways and should capture any additional known flows, such as volume pumped into ponds from <br />the mining pit. The design alternative of an 18 inch emergency spillway depth is included in the case that <br />additional pond capacity is preferred. NHN -001, NHN -002 and NHN -003 include an emergency spillway design of <br />18 inches. <br />Each emergency spillway is designed with a 10 foot bottom width and 2:1 side - slopes on each side. A typical <br />emergency spillway cross - section is shown in Figure 6. The crest length will vary depending on the elevation of <br />the emergency spillway compared to the top of embankment. <br />Erosion control below spillways is necessary to provide protection downstream of spillways and nearby structures. <br />The appropriate anti - piping barriers will be used on the primary discharge pipes that extend through the <br />embankment per Rule 4.05.9(2). <br />5 <br />