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