2008-03-28_PERMIT FILE - C1981018A (3)

Home Browse Search

• It was also assumed that since the drainage basins are small, no sediment would be deposited until it reaches the sediment control ponds. The three-year sediment yield is calculated as follows: Sediment Volume = (AxArlTons/vr x 2,000 Iblron x 3 yr (Acre-feet) 80 Iblcu ft x 43560 sq ft/acre Sediment volume calculations for sediment ponds are included in Illustrations 4, 5, 6, 6A and 7. The sediment volume requirements are also shown on the pond drawings, Maps 34, 78, 79, 80 and 80A. Peak runoff flow rates were initially estimated using procedures set forth in SCS-TP-149 (Rev. 1973) "A Method of Estimating Volume and Rate of Runoff from Small Watersheds." In all cases, runoff curve numbers (RCN) were selected assuming antecedent moisture condition two (AMC II) prevailed. Disturbed areas were assumed to be the same as "farmstead" areas in Table 2 (NRCS-TP-149). Undisturbed areas were assumed to be the equivalent of "fair range without mechanical treatment", in Table 2 (NRCS-TP-149). Hydrologic soil groups were taken from the NRCS soil survey data previously discussed. • Total runoff volumes for 10-year, 24-hour storms were determined from Figure 3 of NRCS-TP-149 which is a solution of the runoff equation: Q = P-0.2xS z (P+0.8xS) where: Q =accumulated direct runoff P =accumulated rainfall (from design storm) S =potential maximum retention = 1000 - 10 RCN RCN =runoff curve number Runoff calculations for ponds designed before mine construction are included in Illustrations 4, 5, 6, and 7. The available pond capacities were determined by the average end area method from planimetered areas of contours shown on the drawings. All ponds were sized according to the requirements of Section 4.05.6. The peak runoff rates to the ponds for the 25-year, 24-hour and 100-year, 24-hour storms were • initially determined for preliminary design purposes from the graphics! procedures for determining Mid-Term Review 2002 (8/2002) V-10