2023-12-14_PERMIT FILE - C1981019 ‎(17)‎

• Permit No: C1981019
• IBM Index Class Name: Permit File
• Doc Date: 12/14/2023
• Doc Name: Sediment Control System Analysis at End of Mining
• Section_Exhibit Name: Volume 18 Exhibit 7 Item 23 Part I
• Media Type: D
• Archive: No

Exh. 7-23I-3 Revision Date: 10/23/23
Revision No.: TR-161
reduce peak flows from larger storm events and provide a water source to support the post mine
land use.

A SEDCADTM model (100 Year 24 Hour Channel Demonstration) has been included which
evaluates the peak flow for each of the channel. The minimum channel configurations for Little
Collom Gulch are summarized below.

Little Collom Gulch
Station Peak Flow
(CFS)
Average
Slope (%)
Channel
Type
Side
Slopes
Minimum
Depth
(Feet)
Erosion
Protection
Station 0+00
to 10+00
(LCG-1)
41.51 16.6 Trapezoidal
15’ bottom 3H:1V 4.0 Riprap,
D50 = 9”
Station 10+00
to 25+00
(LCG-2)
55.67 4.9 Trapezoidal
10’ bottom 3H:1V 4.0 Riprap,
D50 = 9”
Station 25+00
to 55+00
(LCG-3)
81.69 4.7 Trapezoidal
12’ bottom 3H:1V 4.0 Riprap,
D50 = 9”
Station 55+00
to 85+00
(LCG-4)
37.30 7.8 Trapezoidal
10’ bottom 3H:1V 4.0 Riprap,
D50 = 6”
Station 85+00
(LCG-4) to
100+00
40.52 7.4 Trapezoidal
10’ bottom 3H:1V 4.0 Riprap,
D50 = 6”

Sage Draw, Grouse Draw, Sharpie Draw, Lower, Middle, and Upper Bobcat Draws

Sage Draw, Grouse Draw, Dusky Draw, Sharpie Draw are tributary drainages to the reconstructed
portion of Little Collom Gulch, while Lower, Middle, and Upper Bobcat Draws are tributary to
natural tributary of Little Collom Gulch. Contributory areas to these ditches are less than one
square mile which would make the required modeled storm event the 10-year 24-hour storm event.
For ease and conservatism, all seven channels have been modeled against the 100-year 24-hour
storm event.

Lower and Middle Bobcat Draw will be constructed inside the reclaimed footprint of the Section
36 and Middle Ponds. Therefore, in the 100-Year 24-Hour model, it is assumed the entire
watersheds reporting to both ponds have been approved for Phase II bond release and both ponds
are no longer necessary for sediment control. In lieu of the two sediments ponds post mine channel
configurations have been added to the model.

A SEDCADTM model (100 Year 24 Hour Channel Demonstration model) has been included which
evaluates the peak flow for each of the channel. The minimum channel configurations for Little
Collom Gulch are summarized below