1991-09-26_REVISION - C1981035 (3)

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Table 3. Mine Yard and adjoining areas drainage characteristics. Watershed Parameters for Mine Yard and Adjoining Areas Drainage Area = 29.8 acres Length to Divide = 1200 ft Curve Number = 74 Average Slope = 10% Lag Time = 7 minutes normal depth, flowing full hydraulic analysis. The pipe roughness used in the analysis is at the high end for standard corrugated metal pipe. Support computations are given in appendix 5. LOCATION EXST/NEW REQUIRED SLOPE DIAMETER DESIGN DISCHARGE (in) DISCHARGE reach 1 new 8 cfs 0.071 18 12.2 reach 2 existing 8 cfs 0.300 18 25.0 reach 3 new 8 cfs 0.060 18 11.2 reach 4 new 14 cfs 0.060 24 24.1 reach 5 existing 6 cfs 0.100 18 14.4 reach 6 new 6 cfs 0.038 18 8.9 Table 4. Design characteristics for each section of pipe in drainage plan. The lower eastside drainage channel from the end of pipe reach 4 to the discharge point on the boundary of the mine permit area will not change from that presented in TR 2. Sediment Ponds In order to achieve the required volume of 1.13 AF, the West Pond was extended in a westerly direction to its intersection with the natural slope. In addition, the embankments were redesigned to meet the MLRB 5:1 total slope regulation. As with the East Pond, the inslope is 2:1 and the outslope is 3:1 around the entire pond. Assuming a level top of embankment the inslope height is 6.5 ft and the outslope height varies around the pond. On the north side the outslope height is about 8-10 ft, on the east side it averages 3-5 ft. The natural ground will form the embankment on the west side. The topwidth is 8 ft. The volume of the proposed pond is 1.15 AF which exceeds the required volume of 1.13 AF. Volume computations for the West Pond are included in appendix 6. The elevation of the top of the West Pond embankment, based on an assumed data, is 108.0, 5