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storm. Once no longer needed when the Section 36, Middle Pond, and Section 25 Pond are built, <br />the temporary sump will be removed. <br />Activities During_ Construction <br />Exhibit 7-23, Figure M2 shows the necessary activities that will occur as part of the Coal <br />Stockpile Pond construction. During the construction, it will be necessary to disturb <br />approximately 3.0 acres within the pond's physical construction limits. None of the other <br />upstream disturbances shown on Exh. 7-23 Figure M3 will be allowed until the pond is <br />completed. Following topsoil removal, excavation and the construction of the downstream <br />embankment portion will proceed. Ultimately, a storage capacity of about 8.44 acre feet is <br />anticipated. Excavated materials (not topsoil) will be stockpiled in the Initial Stockpile Area <br />shown on Exh. 7-23, Figure M2. Silt fence or s -fence will be installed down gradient of the <br />initial stockpile area (most will already be installed during construction of the temporary haulage <br />route) during construction of the West Collector Ditch and Coal Stockpile Pond. <br />Two ditches designated as the East Collector and West Collector Ditch will also be built during <br />this time period. Both of these are built along the contour to bring in drainage into the Coal <br />Stockpile Pond that would not otherwise be captured. Typical Collector Ditch dimensions are <br />shown on Exhibit 7-23, Figure M2. <br />Two SEDCAD models are provided to estimate the flows and runoff volumes for these two <br />collector ditches. Both of these collector ditches will be built as the excavation of the Coal <br />Stockpile Pond is underway. These two collector ditches are temporary in nature and are no <br />longer needed once other downstream sediment control structures are in place. The East <br />Collector Ditch is approximately 2,800 feet in length and a potential drainage area of about 139 <br />acres. The West Collector Ditch is about 3,657 feet in length and a drainage area of 14.1 acres. <br />Activities Following Construction <br />Figure 7-23, Figure J2 shows the planned location, dimensions and capacity of the Coal <br />Stockpile Pond. This structure is almost entirely subgrade, but it will have a small embankment <br />on the downstream side. To control runoff from this area, the Coal Stockpile Pond will be built <br />to completely contain the runoff from the 10 year, 24 hour storm event. The structure will be <br />evacuated by pumping when needed to maintain adequate storage. As shown below, the water <br />level will need to be maintained below elevation 7212.82. <br />The pond has been designed with 3H:1 V side slopes and a 12 foot wide embankment crest. <br />Total depth of the pond is less than 10 feet. Exhibit 7-23, Figure M1 shows the location of the <br />disturbances, curve numbers, drainage areas, watershed names, structure numbers, and other <br />information. An open channel emergency spillway 20 feet in width with 3H:1 V sideslopes will <br />be provided. No other outlet will be provided. <br />Two SEDCAD models are provided for this impoundment. The first provides sedimentology for <br />the resultant 10 year, 24 hour storm event, while the second model evaluates the effectiveness of <br />the pond's spillway system under the 25 year, 24 hour storm. All of the undisturbed areas are <br />modeled at a curve number of 63 while all disturbances are modeled at a curve number of 85, <br />based on Volume 2D, Exhibit 7, Table 1 values. <br />Exh. 7-23M-2 Revision Date: 5/17/17 <br />Revision No.: TR -111 <br />