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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TABLE 2
Northfield Project Ditch Design rnma
Flow
Slope
Side Slope
Depth of
Velocity
Freeboard
Total
Ditch
(cfs)
N
(H:1V)
Flow
(fps)
(ft)
Depth
Stabilization
ft
ft
North
Run -on
10.00
1.0
3.0
1.99
1.96
0.3
2.29
Grass Mix
Diversion
South
Run -on
37.34
1.0
3.0
2.67
3.38
0.3
2.97
Grass Mix
Diversion
North
Run -off
11.14
2.0
3.0
1.74
2.76
0.3
2.04
Grass Mix
Collection
South
Run -off
12.37
2.0
3.0
1.78
2.88
0.3
2.08
Grass Mix
Collection
For the North Run -off Collection Ditch to be able to discharge into the sediment pond, it will need to be
constructed on the fill of the constructed facility bench area. If the ditch was constructed at the toe of the
bench area fill, the site topography would not allow the ditch to drain into the sediment pond. Therefore,
the ditch will collect all run -off from the disturbed facility area, except for the initially disturbed area of
the bench fill slope. Following construction of the facility bench area, the slope will be revegetated for
stability and sediment control and best management practices will be used to control run -off. Following
revegetation, the slope will not be disturbed by mining activities.
All ditches associated with haul roads located inside or outside the disturbed area shall also be designed to
contain the flow from the 10 -year, 24 -hour storm event and control erosive velocities. Sedimentation
control measures such as rip rap and straw bales shall be installed, as needed.
5.0 SEDIMENT POND
The Northfield Project site sediment pond was designed to effectively treat the run -off from the 10 -year,
24 -hour storm event. The pond was designed utilizing the SEDCAD4 computer model. The input
parameters and results are shown on the SEDCAD4 printout located in the Appendix B. Sediment pond
SEDCAD4 results are summarized in Table 3.
The location of the pond and its corresponding subwatersheds are shown on the Site Drainage Map.
Details for the sediment pond are located on the Sediment Pond Details sheet attached. A simplistic
approach was taken in determining disturbed area contributing to run -off. All areas which could possibly
be disturbed during the life of the project were considered disturbed for the sediment pond design. These
areas included the acreage below the run -on diversion ditches and above the run -off collection ditches.
Additionally, no sediment control barriers were used in the sedimentology design of the pond. This
approach resulted in a conservative design for the pond sediment storage design.
The sediment pond contains a principal and emergency spillway. The principal spillway has a 3 -foot tall
perforated riser. The riser will be enclosed with a trash rack screen to prevent clogging and pass the
10 -year, 24 -hour storm event without use of the emergency spillway. The principal spillway will also
allow for approximately 0.22 ac -ft of dead pool storage (not counting unused sediment storage volume) to
. treat mine dewatering flows. The dead pool storage will allow for approximately 1.0 days of dewatering
flow retention. The emergency spillway consists of a trapezoidal channel with 3H: IV side slopes. The
spillway crest and outlet slope will be armored. The emergency spillway is designed to safely pass the