2010-05-07_PERMIT FILE - C2009087 ‎(24)‎

• Permit No: C2009087
• IBM Index Class Name: Permit File
• Doc Date: 5/7/2010
• Doc Name: Sediment Pond 002 Design Report
• Section_Exhibit Name: Exhibit 2.05.3-E2 Appendix 2.05.3-E2.A
• Media Type: D
• Archive: No

The sediment storage volumes for upstream impoundments included in the model (e.g., PECOCO
and Lower Coal Stockpile Sump [LCSS]) were user input values equal to the volumes calculated for
• the structures as part of the PECOCO and Coal Stockpile Sump designs (see Appendix 2.05.3 -E2.D
and 2.05.3 -E2.E, respectively).
The 25 -year, 24 -hour event demonstration also utilized a user input sediment storage volume. The
sedimentology subroutine calculates the R storm value as a function of the design storm rainfall
amount and distribution. Consequently, the 25 -year, 24 -hour storm event "over- estimates" the
"design" sediment storage requirements. To eliminate this bias a user input sediment storage value,
equal to the volume calculated by the 10 -year, 24 -hour storm event, was used as the calculated
sediment storage requirement for the 25 -year, 24 -hour event.
Storage Capacity: Results of the design SEDCAD demonstration show a designed pond capacity of
179.6 acre -feet. The top of sediment elevation (6,112.1 feet) corresponds to 159.1 acre -feet of
available storage, which equates to 125 years of sediment storage. The average settleable solids
concentration in pond discharge during the peak 24 -hour period is 0.0 ml /1. Consequently, the 10-
year, 24 -hour storm event demonstration shows there is sufficient storage capacity available, and that
the settleable solids effluent concentration is less than the applicable water quality standard.
Therefore, Pond 002 as designed and constructed meets the intentions of Rule 4.05.6(3)(a).
The following summarizes the storage capacity for the 10 -year, 24 -hour event SEDCAD
demonstration (see Addendum 2.05.3- E2.A -1).
Spillways: In accordance with Rule 4.05.9(2)(a), Pond 002 was designed and constructed with a
combination spillway. The single open - channel spillway is constructed in native material; in
addition, the channel is lined with vegetation and rock riprap - a Manning's roughness coefficient of
0.035 was assumed for the design. Survey data collected by Seneca Coal Company personnel
indicate the channel is a minimum of three feet deep; the roadside MSHA berm adds additional
freeboard. The calculated peak discharge from the 25 -year, 24 -hour storm event is 45.5 cfs (see
Addendum 2.05.3- E2.A -2). The 25 -year, 24 -hour event peak stage is 6,616.0 feet. The top of the
embankment is 6,618.0. The SEDCAD4 demonstration and as -built drawing (see Map 2.05.3 -E2.A,
Sediment Pond 002 As- Built) indicate there is a minimum of 1.0 feet of freeboard in the spillway as
40 the design event flows discharge through the spillway. Therefore, the pond as designed and
constructed meets the intentions of Rule 4.05.9(7)(d).
2.05.3 -E2.A4 12/09
Pond 002
Total pond storage capacity (at invert of combination spillway):
179.6 ac -ft
Predicted 10 -year, 24 -hour event runoff volume:
40.2 ac -ft
Combination spillway invert elevation (ft msl):
6,614.5 ft
Permanent pool elevation (ft msl):
6,614.8 ft
Predicted peak water elevation (ft msl):
6,615.2 ft
Predicted annual sediment loading:
1.0 ac -ft
Predicted maximum sediment storage elevation (ft msl):
6,612.1 ft
Predicted sediment storage volume:
159.1 ac -ft
Predicted number of storage years:
125 years
Spillways: In accordance with Rule 4.05.9(2)(a), Pond 002 was designed and constructed with a
combination spillway. The single open - channel spillway is constructed in native material; in
addition, the channel is lined with vegetation and rock riprap - a Manning's roughness coefficient of
0.035 was assumed for the design. Survey data collected by Seneca Coal Company personnel
indicate the channel is a minimum of three feet deep; the roadside MSHA berm adds additional
freeboard. The calculated peak discharge from the 25 -year, 24 -hour storm event is 45.5 cfs (see
Addendum 2.05.3- E2.A -2). The 25 -year, 24 -hour event peak stage is 6,616.0 feet. The top of the
embankment is 6,618.0. The SEDCAD4 demonstration and as -built drawing (see Map 2.05.3 -E2.A,
Sediment Pond 002 As- Built) indicate there is a minimum of 1.0 feet of freeboard in the spillway as
40 the design event flows discharge through the spillway. Therefore, the pond as designed and
constructed meets the intentions of Rule 4.05.9(7)(d).
2.05.3 -E2.A4 12/09