PERMFILE135209

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 9/2/2004
• Doc Name: Attachment 1 to 9
• Section_Exhibit Name: Exhibit 7 West Pit Pond - 008A
• Media Type: D
• Archive: Yes

COLOWYO COAL COMPANY
OOOA -WEST PIT
SEDIMENTATION POND SUMMARY

i
ORIGINAL 1996 1998 2002 DIFFERENCE
CONFIGURATION STUDY STUDY DIFFERENCE STUDY 1998 TO 2002
WATERSHED AREA (acres):
Disturbed 0.0 0.0 0.0 0.0
Postmining Topsoiletl antl Seetled 80.6 90.4 9.8 60.9 -29.5
Postmining Undisturbed 22.7 0.0 -22.7 23.8 23.8
1-2 Year Reclamation 20.6 20.9 0.3 0.0 -20.9
3+Year Reclamation 379.9 446.0 66.1 151.2 -294.9
Total Area 503.8 557.3 53.5 235.9 -321.5
POND PARAMETERS:
Bottom Elevation 6820.7 6820.7 6820.7 0.0 6821.0 0.3
Principal5pillway Invert Elevation 6837.0 6835.5 6837,0 1.5 6837,0 0.0
Emergency Spillway Invert Elevation 6837.7 6837.7 6838.0 0.3 6838.0 0.0
Top of Embankment 6841.0 6841.0 6841.0 0.0 6841.0 0.0
Storage Volume Below Emergency Spillway (ao-ft) 13.02 13.02 12.86 -0.16 13.8 1.0
Storage Volume Below Principal Spillway (aoft) 11.62 10.43 11.62 1.19 12.6 1.0
Principal Spillway Parameters:
Type Drop Inlet Drop Inlet Pert. Riser Perf. Riser
Riser Diameter (in) 18.0 18.0 18.0 0.0 18.0 0.0
Riser Height (ff) 11.6 10.1 11.6 1.5 11.6 0.0
Barrel Diameter (in) 18.0 18.0 18.0 0.0 16.0 0.0
Elevation of Perforation(s) (ft) N/A N/A 6830.0 6830.0
Size of Perforation(s) (in) NIA N1A 4.0 4.0
Number of Perforations N/A N/A 3 3.0
Emergency Spillway Parameters:
Type Trapezoidal Trapezoidal Trapezoidal Trapezoidal
Diameter (in) N/A N/A N/A N/A
Bottom Width (ft) 20.0 20.0 24.0 4.0 24.0 0.0
Length (ft) 25.0 25.0 21.0 4.0 21.0 0.0
Side Slopes'z'H:1V (ft) 2.D 3.0 1.5 -1.5 1.5 0.0
HYDROLOGICMYDRAULIC PARAMETERS:
Base Flow Rates (cfs) 1.00 0.03 -0.97 0.03 0.0
70.Vear, 24-Hour Storm Event:
Precipitation (in) 1.8 1.8 0.0 1.8 0.0
Runoff Volume (aaft) 4.8 5.3 0.6 2.3 -3.0
Principal Spillway Peak Discharge (cfs) 12.2 2.4 -9.8 1.3 -1.1
Emergency Spillway Peak Discharge (ds) 0.0 0.0 0.0 0.0 0.0
Peak Suriace Water Stage Elevation (ft) 6837.6 6833.7 -3.9 6831.0 -2.7
25-Year, 24-Hour Stonn Event:
Precipitation (in) 2.3 2.3 0.0 2.3 0.0
Runoff Volume (ao-k) 8.1 9.1 1.0 4.8 -4.3
Principal Spillway Peak Discharge (cfs) 14.2 3.1 -11.1 1.9 -1.2
Emergenq Spillway Peak Discharge (cfs) 14.0 0.0 -14.0 0.0 0.0
Peak Sudace Water Stage Elevation (tq 6838.4 6836.1 -2.3 6832.4 -3.8
Pond Freeboartl (ft) 2.6 4.9 2.3 8.6 3.8
SEDIMENT LOADING PARAMETERS:
Storm Predicted Peak Se8lahle Concentration (ml/0 0.00 O.OD 0.0 0.00 0.0
Predicted Annual Sediment Loatling (ac-0) 0.17 0.49 0.32 0.24 -0.2
Available Sediment Storage Volume (ac-ft) 4.68 1.88 -2.80 2.18 0.3
Maximum Sediment Storage Elevation (ff) 6825.4 6825.4 6825.4 0.0 6825.4 0.0
Predicted Number of Storage Vears 27.5 3.8 -23.7 9.0 5.2
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