2023-04-19_PERMIT FILE - C1981010 ‎(76)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 4/19/2023
• Section_Exhibit Name: APPENDIX B
• Media Type: D
• Archive: No

9
The total acreage of the pre-mining Flume Gulch watershed is approximately 5,800 acres. The pre-mine
Flume Gulch total watershed acreage within the Trapper Mine permit area is 1,878 acres and increases
by approximately 205.7 acres post-mining to an acreage of 2,083.2, an increase of approximately 9.9%.
In order to understand the changes in the drainage flows and sediment generation as a result of the
increased acreages, Mattern also reviewed the pre- and post-mining drainage densities and sediment
generation. Table 4 shows the drainage densities pre- and post- mining and Table 5 shows the
predicted SEDCAD outputs.
Table 4 Flume Gulch Pre-Mine and Post-Mine Comparative Hydrology Characteristics
Sub-Drainage
Pre-mining
Drainage Density
(mi/mi2)
Post-Mining
Drainage Density
(mi/mi2)
Pre-Mining
Average Gradient
(%)
Post-Mining
Average Gradient
(%)
Grouse 4.8 3.9 8.3 8.2
Sage 3.4 6.5 9.1 10.0
Oak 3.8 3.1 11.4 11.9
West Flume 4.5 4.0 13.5 14.4
Middle Flume 2.8 3.5 10.4 11.4
East Middle Flume 4.4 4.1 10.8 9.4
East Flume 5.0 4.4 9.4 8.0
Overall Average 3.8 3.9 10.6 10.6

Table 5 Flume Gulch Pre-Mine and Post Mine Sediment Yield Projections for the L-Pit
AOC Variance

USLE Results
(annual tons/acre)
SEDCAD Total
Area (acres)
Overall Sediment
Yield (tons)
Sediment Yield Per
Acre (tons
Pre-Mining 2.44 771 872.8 1.13
Post-Mining 0.79 1,004 808.0 0.80

Based on the Mattern evaluation, the sub-drainage watersheds would be slightly larger overall. The
watershed characteristics would be very similar with the same overall drainage densities and similar
drainage gradients. The post-mining AOC variance topography shows an overall decrease in sediment
yield per acre based on the SEDCAD evaluation.
Drainage patterns would be designed to blend with surrounding drainage patterns and would not
represent a change from the surrounding drainage patterns. Figure 3 shows the proposed drainage
patterns for the AOC variance post-mining topography for the L-Pit area, the drainage patterns within
other reclaimed areas of the Flume Gulch drainage and the surrounding areas. As shown on the figure
the proposed post-mining drainage pattern blends with the surrounding drainage patterns.
There would not be any significant changes in the post-mining watershed areas that would impact
seasonal or flood flows from the post-mining reclaimed areas or adversely affect surrounding ecology or
any existing or planned use of surface water or groundwater.
4.2 Ash Pit Drainage Improvements
The 103.5-acre Ash Pit lies primarily within the Johnson Gulch watershed (65 acres) with the remaining
acres within the Pyeatt watershed. The Ash Pit represents only 1.5% of the 7,053-acre Pyeatt/Johnson
Gulch watersheds. The evaluation of post-mining drainage conditions completed by Mattern focused on
the Johnson Gulch watershed since it represents the majority of that drainage. Table 6 shows the