2010-05-07_PERMIT FILE - C2006085 ‎(7)‎

• Permit No: C2006085
• IBM Index Class Name: Permit File
• Doc Date: 5/7/2010
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Exhibit V
• Media Type: D
• Archive: No

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3.4 Length -Slope (LS) Factor
The effect of topography on erosion is determined through the LS- factor. The LS- factor is calculated
from the representative length L, and the representative slope, S of a run -off area. SEDCAD4 uses the
Modified USLE (MUSLE) with the addition of the new S- factor developed for the RUSLE. MUSLE
enables the user to calculate the soil loss from a subwatershed based on USLE parameters, volume of run-
off, and peak flow. Input parameters to determine the LS factor were obtained from scaled maps of the
subwatershed areas. Values measured from the scaled maps are shown in the SEDCAD4 printouts
(Appendix B).
3.5 C Factor
The C factor represents the influence of cover material on or just beneath the surface of the soil. For
Subwatershed C on the Northfield Project site (see Site Drainage Map) where disturbances are proposed,
a C factor of 1.00 for bare soils under construction conditions (as provided by SEDCAD4 tables) was
used.
3.6 P Factor
The P factor accounts for specific support practices such as contouring, terracing, and deposition at the
base segment of a concave slope. It also accounts for sediment control barriers such as grass buffer strips,
straw bales, gravel, or filter barriers. Since the use of potential sediment control barriers will be
determined during site construction and their locations are not yet known, a P factor of 1.00 was used.
This P factor assumes that control practices are not used.
• 4.0 DIVERSION AND COLLECTION DITCHES
Design results for diversion and collection ditches can be found in the SEDCAD4 printouts located in the
Appendix B. In addition, a summary of the ditch designs is provided in Table 2.
The diversion and collection ditches on the north side of the facility are designed to safely pass the peak
flow from the 10 -year, 24 -hour storm event. However, the diversion and collection ditches on the south
side of the facility are upgradient and downgradient of the mine waste stockpile and have been designed
to safely convey the peak flow from the 100 -year, 24 -hour storm event. The South Run -on Diversion
Ditch is considered to be a permanent structure. Although the. South Run -off Collection Ditch gathers
flow from facility areas that do not include the waste rock stockpile, the entire ditch was sized to convey
the 100 -year, 24 -hour storm event in order to simplify the ditch design and to provide a conservative
design for ditch stability. The ditch dimensions were determined using the SEDCAD4 model for channel
flow and sizing.
Construction procedures will confirm that sizing and design of drainage and sediment control structures
are incorporated into the site grading plan with additional available geotechnical and detailed topographic
information. Once construction of site facilities and grading is completed, diversion and collection ditch
locations and slopes should be verified. It is anticipated that the ditches will be grass - lined; however,
armoring may be used depending on flow velocities. For diversion ditches routing run -off from
undisturbed areas, energy dissipaters shall be used at the ditch outlets to disperse flow and decrease flow
velocities. Energy dissipaters shall be sized and stabilized with vegetative cover or armoring (rip rap or
gravel), depending upon flow velocities. Depending upon final site construction and grading, diversion
and collection ditches may also include culverts to pass flow under haul roads or other facility structures.
Culverts should. be designed to safely pass the peak flow from the 10 -year, 24 -hour storm event.
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