2008-01-25_REVISION - C1992080 (4)

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1„ Design Volume Calculations - Lower Sump Area Drainage Area (acres) 1.13 Design Event 100 yr. Storm Design Precipitation 3.2 Average CN 89 Calculated S 1.23595505618 Description Exposed gravel and rocky soil Calculated Runoff Volume (Q) 2.08 Calculated Runoff Volume (acre-ft.) 0.20 Ponding Percentage 5.00 [Actual Runoff Volume (acre/ft.) 0.19 As shown on the Hydrology Map, this sump will have the design capacity of .22 acre feet allowing for extra volume for sediment storage. Collection ditches will be installed that direct water from the disturbed areas into the sump. These ditches will be v-ditches of at least 1.0 feet deep and 2.OH:1.OV sideslopes. No lining will be employed since the grades are mild and they are leading to sediment sumps. With the demonstration of the adequacy of revegetation to control surface erosion at and below the North Pit reclamation (TR15), the Lower Sump will not be constructed. Alternative Sediment Control (SAE) Topsoil Stockpile # 2 Once topsoil is removed from this stockpile and while the area is final-graded, a berm of 24" height will be placed around the former pile area (2.69 acres) to prevent erosion from the area. This berm will be placed with a small dozer and will have 2H:1 V sideslopes. Once final bond release is granted, this berm will be regraded into the surrounding terrain. Alternative Sediment Control (SAE) South Half of North Pit Once the North Pit has been backfilled, topsoiled and seeded, approximately the south half of the North Pit will flow onto a grassed terrace prior to flowing offsite into undisturbed drainages. To verify the adequacy of sediment control, a calculation using the SCS Curve Number Method was employed. Based on the post mining topography map of the North Pit, approximately 2.9 acres will flow from the reclaimed area onto the terrace from the reclaimed North Pit. The volume of runoff from this area was calculated assuming precipitation of 2.4 inches from a 10 year-24 hour event, a curve number of 90, an S factor of 1. 11, accumulated runoff of 1.442, and accumulated runoff without abstractions of 2.400. The accumulated volume of runoff would be 0.47 acre-feet of runoff. The size of the terrace at the foot of the south half of the North Pit was calculated to be 300 feet long, 20 feet wide and adequate to control sediment from the area above. TR09, TR11, TR14, TR15 5-10 Revised 3/98, 8/02,12/06,1/08