2009-09-04_PERMIT FILE - C1982056 ‎(4)‎

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 9/4/2009
• Doc Name: MAIN NORTH INTAKE SHAFT
• Section_Exhibit Name: Exhibit 08X 6-ain North Intake Shaft Drainage Designs
• Media Type: D
• Archive: No

The following sediment pond elevation- area - capacity data were input to SEDCAD:
Elevation (feet)
Area (acres)
Capacity (acre -feet) References
6,730.52
0.000
0.000 Bottom of Pond
6,730.69
0.150
0.008
6,730.80
0.162
0.026
6,731.69
0.270
0.216
6,732.69
0.422
0.559
6,733.69
0.461
1.000
6,734.69
0.523
1.492
6,735.60
0.562
1.986
6,735.68
0.566
2.031
6,735.69
0.566
2.036 Top of Embankment
The sediment storage (maximum that will occur over three years) is 0.003 acre -feet. With a three -year
sediment accumulation, the base elevation for water storage will increase from 6,730.52 feet to 6,730.58
feet. Therefore, the SEDCAD model conservatively assumes that the pond is at maximum sediment
storage, and redefines the SEDCAD Elevation- Area - Capacity Tables to show 0.000 acre -feet capacity at
the maximum sediment storage surface elevation, i.e.:
• The above data input table shows a capacity of 0.003 acre -feet at an elevation of 6,730.58 feet.
• The model output tables (in the attached SEDCAD reports) show a capacity of 0.000 acre -feet at
an elevation of 6,730.58 feet.
• SEDCAD Model Runs
The following five (5) different models runs were completed to support the design evaluation for the
6MN Pad Pond and Spillway (SEDCAD reports are attached):
10 -Year, 24 -Hour with Pond Empty with Erodible Perimeter Drainage Ditches — The sediment
pond is assumed to contain three (3) years of sediment storage with a starting permanent pool
elevation immediately above the top of the sediment storage elevation. The perimeter drainage
ditches are modeled as bare earthen channels. This analysis was completed to show that the
empty pond with 3 years of sediment storage will completely contain the 10 -year, 24 -hour runoff
volume with at lease 1.0 feet of remaining freeboard to the spillway invert.
• 10 -Year, 24 -Hour with Pond Full with Erodible Perimeter Drainage Ditches — The sediment pond
is assumed to contain three (3) years of sediment storage and have a permanent pool at the invert
of the emergency spillway at the start of the design storm. The perimeter drainage ditches are
modeled as bare earthen channels. This analysis was completed to show that flow velocity in
both temporary drainage ditches is less than the limiting velocity of 5 fps, and that discharge from
the sediment pond will comply with 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 the sediment pond is estimated at 2,547 mg /L and the SS inflow
concentration is 1.78 mL /L. Outflow sediment concentrations from the sediment pond are 43
mg/L and 0.00 mL /L, respectively.
TCC 6MN Intake Shaft Facility 4 Water & Earth Technologies, Inc.
Drainage and Sediment Control System Design April 24, 2009