2023-10-31_REVISION - M1973021 ‎(2)‎

• Permit No: M1973021
• IBM Index Class Name: Revision
• Doc Date: 10/31/2023
• Doc Name Note: Attachment B - Final Phase III Drainage Report
• Doc Name: Request for Technical Revision
• From: Holcim - WCR, Inc.
• To: DRMS
• Type & Sequence: TR9
• Media Type: D
• Archive: No

Table 3-5. Diversion Channel Summary Table
Diversion 100- Side Bottom Flow Depth Design Velocity
Channel year Peak Slope Width (ft) (ft) (ft/s)
Feature Flow Rate (XH:1V)
(cfs)
South Quarry 24.5 2 2 1.36 3.96
Highwall
Diversion
Channel
Diversion 24.5 2 3 0.73 7.76
Channel
Chute
3.4 Diversion Channel Geotechnical Assessment
As discussed above, the southern end of the diversion channel will be constructed at elevation
of 7,110 ft on a reclaimed bench. A geotechnical assessment was completed to evaluate a) the
slope stability of the reclaimed surface for the diversion channel foundation; and b) the stability
of the reclaimed benches below the diversion channel should overtopping of the channel
occur.
For the diversion channel foundation, stability modeling was performed to support the design
of a channel slope that meets or exceeds a factor of safety of 1.3. The slope stability analysis
indicates that regrading the reclaim fill to slopes of 1.5 horizonal:1 vertical (1.51-1:1V) with 20-
foot-wide benches every 20 vertical feet results in an acceptable factor of safety for the
diversion channel foundation (refer to Appendix G).
To evaluate the potential for failure based on flow above capacity overtopping the diversion
channel, a geotechnical assessment determined that above capacity flow was not anticipated
to cause bench failure below the channel. This determination was based on the observation
that stormwater is currently flowing over the south quarry highwall, and the channel will reduce
or lessen these flows.
3.5 Water Quality Enhancement BMPs
Runoff is managed within the mine footprint and is subject to the COG500320 Stormwater
Management Plan (see Appendix C) which includes water quality enhancement BMPs.
3.6 Additional Permitting Requirements
Water is conveyed via the diversion channel to the to the existing south quarry drainage area
where it will be retained and pumped to Outfall 006. This water management approach is
consistent with COG500320. The current DRMS 1973-021 reclamation permit will be revised
to include this change to the quarry drainage plan.
No additional permits or permit modifications are anticipated.
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