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Portal Area. To control runoff from the reclaimed area, the pond would necessarily also control <br />runoff from the Scullion Gulch watershed above the reclaimed area (unless an elaborate diversion <br />system was constructed to bypass Scullion Gulch runoff). With a drainage area of approximately <br />5270 acres, the 10-year, 24-hour storm (1.6 inches) would yield about 152 acre-feet (assuming a <br />runoff curve number of 77). Thus, a sediment pond with a volume in excess of 160 acre-feet would <br />be required, resulting in a large amount of disturbance. This alternative is, therefore, not <br />considered feasible or desirable. <br />Disturbed area runoff will be controlled at the rail loop and slot storage areas during final <br />reclamation by the use of contour furrowing, mulching, jute netting and loose rock check dams. A <br />technical revision application will be submitted to DMG by March 15, 2000 to address the post-mine <br />reclamation of the East Portal and rail loop areas. <br />V.C Diversions <br />A channel diversion of Scullion Gulch was required at the "D" Portal Area to provide adequate area <br />for parking, mine entry, and surface facilities. This diversion consists of a large diameter <br />corrugated-metal pipe to bypass the main Scullion Gulch flow with smaller diameter lateral <br />corrugated-metal pipes to bypass tributary inflows in the portal area. Pipe dimensions and design <br />details are shown on Maps 29, 30, 31 and 32. Details of the pre-existing channel are shown on <br />• Maps 36, 37, 38 and 39. <br />Burying the Scullion Gulch drainage through the plant area has the added advantage of decreasing <br />the passibility of airborne coal dust or spills entering the drainage. In addition, this layout has <br />significantly reduced the number and size of required sedimentation ponds. <br />The Scullion Gulch bypass system has been designed to safely pass the peak flows resulting from <br />the 100-year, 24-hour rainfall event. Peak flows were determined using NRCS methods as <br />described in TP-149 (for watersheds with drainage areas of 2,000 acres or less) and Section 4 of <br />the National Engineering Handbook (for larger watersheds). Map 21 shows the drainage basins of <br />concern. Runoff calculations are included in Illustration 9. <br />The bypass system was sized using the conservation of energy principle (Bernoulli's equation) to <br />construct backwater curves along the culvert. Peak flows in the main and tributary channels were <br />assumed to reach the culvert simultaneously. Results of the backwater analysis are also included <br />in Illustration 9. <br />As noted on the design drawings of the Scullion Gulch bypass system, erosion protection is <br />provided at the inlets and outlets, as appropriate. Concrete hardwalls are provided at all inlets, <br />while inlet and outlet channels and banks are riprapped. Debris-control structures are provided at <br />• all inlets to prevent clogging. <br />Permit Renewal #3 (Rev. 1/00) V-17 <br />