2022-07-12_PERMIT FILE - C1981019A ‎(18)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 7/12/2022
• Doc Name: Sediment 26 Pond Sediment Control Post Mining
• Section_Exhibit Name: Volume 18 Exhibit 7 Item 23 Part J
• Media Type: D
• Archive: Yes

Exh. 7-23J-2 Revision Date: 6/27/22
Revision No.: TR-154
The as-built configuration for the Section 26 Pond is presented on Figure 7-23J-1, and the as-built
configuration for the Sidehill Pond is presented on Figure 7-23J-2. Figure 7-23J-3 provides the
breakdown of drainage areas and hydrologic conditions for all areas in the watershed reporting to
these two sediment ponds. The Sidehill Pond as part of the Section 26 Pond sediment control
system is utilized to reduced peak flows and provide additional sediment storage if need. All
discharges from the Sidehill Pond flow directly to the Section 26 Pond through the D-1 Ditch, and
the Section 26 Pond is the final discharge location.
The following pages present the results of the SEDCADTM models for the post mining condition.
At this stage all areas that have been disturbed by mining activities in the Section 26 watershed
has been reclaimed with the newest or younger (topsoil and seeded) reclamation is in the central
and northern areas of the Section 26 Pond watershed. The farthest northern portion of the
watershed is relatively undisturbed.
The SEDCADTM model herein provides the results of the 10-year 24-hour design storm and
demonstrates the Section 26 Pond will meet the applicable settleable solids standard under this
modeled storm event. The second SEDCADTM model demonstrates that the Section 26 Pond
emergency spillway elevation is capable of containing the 25-year 24-hour storm.
In summary, for the post mining case at the Section 26 Pond, the 10-year 24-hour storm produces
5.6 acre feet of runoff, and the peak settleable solids concentration is 0.00 ml/l. The 25-year 24-
hour storm event peaks at the 6,984.4 elevation, which below the emergency spillway elevation of
6,986.0’
Section 26 Pond Temporary Post Mine Channels
One temporary channel is necessary during the reclamation of areas within the Section 26 Pond
watershed. This temporary channel (D-6 Ditch) will route water from reclamation areas to the
upper reaches of the permanent post mine channel Bear Draw as shown on Figure 7-23J-3. Once
Phase II bond release has been achieved in the entire sub-watershed reporting to the D-6 Ditch, it
will be reclaimed.
A SEDCADTM model (25 Year 24 Hour Channel Demonstration model) has been included which
evaluates the peak flow for this temporary channel. The minimum channel configurations are
summarized below
Name
Peak
Flow
(CFS)
Average
Slope
(%)
Channel
Type Side Slopes
Minimum
Depth
(Feet)
Erosion
Protection
D-6 Ditch 12.09 6.0
Trapezoidal
5’ bottom 2H:1V 3.0
Vegetation