2009-04-29_PERMIT FILE - C1982056 (3)

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6872 .307 1.711 6873 .354 2.041 6874 .4488 2.441 To of embankment The maximum sediment storage that will occur over three years is 0.009 ac-ft. With a three year sediment accumulation, the base elevation for water storage will have increased from 6862.0 ft amsl to 6862.32 ft amsl. The SEDCAD model accounts for the maximum sediment storage volume by recalculating the elevation-area-capacity table to show 0.000 ac ft capacity at the maximum sediment storage elevation. The following pond elevation-area-capacity data were input into SEDCAD for Pond 2 (Table 4): Table 4. Pond 2 Elevation-Area-Capacity L? Elevation (ft amsl) Area (ac) Capacity (ac-ft) References 6846.5 0.013 0.000 Bottom of and 6847 0.025 0.009 6848 0.051 0.047 6849 0.080 0.111 6850 0.106 0.204 6851 0.133 0.323 6852 0.159 0.469 6853 0.185 0.641 6854 0.212 0.839 Emergency spillway 6855 0.241 1.066 6855.5 0.256 1.190 To of embankment Because Pond 2 will only be operational during the construction of the intake shaft installation, the SEDCAD elevation-area-capacity table was not redefined based on sediment storage capacity for three years. Following is a list of SEDCAD runs that were completed to support this design. Supporting documentation for the SEDCAD runs is provided in the four attached SEDCAD reports. 10-year, 24-hour Shaft Pad During Construction - Pond 1 is assumed to contain three years of accumulated stored sediment and has a starting initial pool elevation at the invert of the principal spillway. Pond 2 has a starting initial pool elevation at the invert of the emergency spillway. This analysis was completed to show that given these conditions, discharge from Pond 2 will meet NPDES requirements for water quality. Requirements include that the peak total suspended solids (TSS) concentration not exceed 70 mg/l and that the peak settleable solids (SS) concentration never exceed 0.5 ml/l. From the SEDCAD model, TSS in runoff entering Pond 1 is estimated at 5,920 mg/1. Outflow concentrations from Pond 1 are 535 mg/1. Because Pond 2 receives runoff from additional areas as well as the outflow from Pond 1, the SEDCAD results indicate that runoff entering Pond 2 has an estimated peak TSS concentration of 1,347 mg/l, and an outflow concentration of 62 mg/l. Settleable solids concentrations entering the ponds never exceed the 0.5 ml/l requirement: the SEDCAD results show inflows to Pond 1 having a maximum SS value of 0.3 ml/l and inflows to Pond 2 having a maximum SS concentration of 0.32 ml/l. Outflow SS concentrations for both ponds are zero. Twentytmile Coal Company 18L Shaft Pad Drainage and Sediment Control System Design Water & Earth Technologies, Inc. March 2, 2009