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

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Sediment Pond Spillway Channel with Erodible Perimeter Drainage Ditches
The as -built downstream spillway channel will convey the 25 -year, 24 -hour peak flow of 6.99 cfs without
scour, maintaining at least 0.3 feet of freeboard. The discharge in the spillway channel does not exceed
the maximum permissible velocity. The spillway channel is trapezoidal shaped with a 12.0 ft bottom
width, and 2:1 sideslopes and a depth of 1.2 feet. The downstream spillway channel was modeled as an
erodible channel for stability, and a vegetated channel for capacity. When modeled as an erodible
channel, the downstream spillway channel will pass the 25 -year, 24 -hour peak flow of 6.99 cfs with a
maximum flow depth of 0.25 feet and at least 0.3 feet of remaining freeboard. When modeled as a
vegetated channel, the downstream spillway channel will pass the 25 -year, 24 -hour peak flow of 6.99 cfs
with a maximum flow depth of 0.74 feet and at least 0.3 feet of remaining freeboard.
Hydrologic Modeling
SEDCAD was used to model hydrology and sedimentology characteristics of the intake shaft facility.
The hydrologic SEDCAD modeling evaluated the stability and conveyance capacity of perimeter drainage
ditches, the pond spillway, and the downstream spillway channel. The sedimentology SEDCAD
modeling evaluated the ability of the pond to treat, retain and discharge runoff from the 10 -year, 24 -hour
storm event for conditions that varied from an empty pond to a pond that is full to the invert of the
combined principal and emergency spillway at the start of storm runoff. Table 1 lists precipitation
amounts for the two design storms used in the hydrologic analyses. SEDCAD's sedimentology routine is
based on the Revised Universal Soil Loss Equation ( RUSLE). RUSLE is a set of mathematical equations
that estimate average annual soil loss. RUSLE has five input parameters, R, K, LS, C, P. R is the rainfall
erosivity factor, and is a site specific factor based on local climatic conditions. K is the soil erodibility
factor, and is based on the classification of site soils (silt, clay, etc.). LS is the length -slope factor, and is
determined from representative slope lengths and gradients at the site. C is the cover management factor,
and is based on cover conditions at the site (vegetated, bare soil, etc.). P is the support practice factor,
and is based on effects of conservation practices (terraces, buffer strips, etc.). From a modeling
standpoint, the R, K, and LS factors have already been determined by the location and configuration of
the intake shaft installation. Reclamation, gravel surfacing, and other best management practices
(BMP's) utilized by TCC primarily affect the C factor used in SEDCAD's sedimentology routine and the
curve number (CN) used in SEDCAD's hydrology routine; a plan view showing the gravel surfacing and
reclamation is attached as Exhibit 2. Curve numbers (CN) and the soil cover factor (C) are shown in
Table 2. Benefits related to the P factor were not included in this analysis for conservatism. All of the
hydrology and sedimentology input parameters are shown in the attached SEDCAD output reports.
Table 1. Design Storm Information
Design Storm
Precipitation inches
10 -year, 24 -hour
1.8
25 -year, 24 -hour
2.1
Table 2. CN & C for Affected Areas
Soil Condition
CN
C
Reclaimed
81
0.003
Road Base
87
0.15
Facilities
98
0.00001
Pond
100
0.00001
TCC 6MN Intake Shaft Facility 3 Water & Earth Technologies, Inc.
Drainage and Sediment Control System Design April 24, 2009