2025-05-05_PERMIT FILE - C1981010 ‎(24)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 5/5/2025
• Doc Name: AOC Variance Summary
• Section_Exhibit Name: Appendix B AOC
• Media Type: D
• Archive: No

11
The post-mining backfill configuration for the L Pit would decrease the depth of spoil placed at the head
of the slope and increase the depth of spoil placed at the toe of the slope to act as a buttress to any
movement. Shallower overall surface topography also would increase the stability of the backfilled spoil.
The stability analysis showed that the post-mining stability for the AOC variance has improved Safety
Factors when compared to restoring the L Pit to AOC.
In addition, post-mining drainages have been designed to mimic pre-mining drainages as part of the
AOC variance drainage design and would result in overall watershed improvements. For the L Pit, the
overall drainage area for Flume Gulch, which is the major drainage associated with the L Pit, would
increase by approximately 181.2 acres or 9.7%. The pre- and post-mining drainage densities would
remain the same and the overall drainage gradient would increase by 0.2%. Sediment yield would
decrease from 1.13 tons/acre to 0.81 tons/acre under the proposed post-mining configuration.
Table 7 summarizes the pre- and post-mining characteristics for the L Pit.
Table 7 Pre- and Post-Mining Characteristics for the L Pit
Condition Acreage
Slopes
(°)
Safety
Factors
Drainage
Density
(mi/mi2)
Gradient
(%)
Sediment
Yield Per Acre
(tons)
Pre-Mining 1,877.5 4-26 1.3 to 1.5 3.8 10.6 1.13
Post-Mining 2,114.3 4-14 1.58 to 2.14 3.6 10.3 0.74

The Ash Pit shows overall pre-mining topography with shallower slopes but does include the steeper
slopes of over 23° in the northern portion of the pit. Post-mining the slopes would range from 4 to 14°
and would blend with the surrounding topography.
The post-mining stability analysis for the Ash Pit showed that the AOC variance would significantly
improve Safety Factors when compared to restoring the Ash Pit to AOC.
In addition, post-mining drainages would result in overall watershed improvements. For the Ash Pit, the
drainage area for Johnson Gulch, which is the major drainage associated with the Ash Pit, would
increase by approximately 3 acres. The pre- and post-mining drainage densities would change slightly,
and the overall drainage gradient would decrease 1.5%. Sediment yield would decrease from 1.03
tons/acre to 0.56 tons/acre under the proposed post-mining configuration.
Table 8 summarizes the pre- and post-mining characteristics for the Ash Pit.
Table 8 Pre- and Post-Mining Characteristics for the Ash Pit
Condition Acreage
Slopes
(°)
Safety
Factors
Drainage
Density
(mi/mi2)
Gradient
(%)
Sediment
Yield Per Acre
(tons)
Pre-Mining 1,432 4-23 1.36 3.1 9.3 1.03
Post-Mining 1,435 4-14 1.75 3.2 7.8 0.56

Approval of the post-mining topography AOC variance also would improve the post-mining land use of
rangeland by allowing for cattle to graze on shallower slopes. In steeper areas cattle would tend to
congregate on shallower slopes and that could result in over-grazing in these areas. The shallower
slopes would provide for more dispersed grazing.
Finally, the landowners have approved the proposed changes in post-mining topography.