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

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Exh. 7-23I-1 Revision Date: 10/23/23 Revision No.: TR-161 EXHIBIT 7, ITEM 23, PART I Section 25 Pond Sediment Control Post Mining Introduction Exhibit 7, Item 14 in Volume 2D describes the hydrologic methodology used in sediment pond, permanent post-mine channels, and temporary conveyance channel demonstrations. Runoff curve numbers assigned to the undisturbed and/or reclaimed lands in various stages of reclamation have been selected in accordance with Table 1 in the Introductory Text for Exhibit 7 in Volume 2D and are directly tied to Colowyo’s reclamation procedures and timing presented on Map 29B as required by Rule 4.05.6(3)(a). For channels protected by a riprap liner, selection of minimum riprap size is done using the Simons/OSM method in SEDCADTM. For channels to be protected by a vegetative liner, the permissible velocities are also determined using SEDCADTM routines. SEDCADTM demonstrations with two distinct time frames have been evaluated in detail for the Section 25 Pond. Exhibit 7, Item 23, Parts B addresses sediment control for the Section 25 Pond during the mining timeframe. Exhibit 7, Item 23, Part B also contain the design criteria for all temporary channels used to route surface runoff to the sediment pond, except for the temporary D- 5 and D-7 Ditches which will be constructed during reclamation activities. The design criteria for the D-5 and D-7 Ditches are contained below. This exhibit (Exhibit 7, Item 23, Part I) address sediment control during the post mining timeframe for the Section 25 Pond and provides the design criteria for the post mine channel configurations, the temporary ditch, and terrace ditches. The temporary conveyance channel will be retained until Phase II bond release has been achieved in the entire watershed reporting to particular pond the temporary channel is supporting. Colowyo has several types of mining disturbances shown in this demonstration and in the larger mining permit. The disturbance limit shown on maps and figures (a greenish colored boundary on maps) is the permitted disturbance limit. This is the disturbance limit that could potentially be disturbed but may not depending on mine plans and a reclamation bond has not been posted to date to disturb to this limit. Volume 20, Exhibit 13C provides the cumulative bond schedule disturbance limits. These disturbance limits are areas that Colowyo has permitted for disturbance and posted a reclamation bond to disturbed. However, given Colowyo’s current mine plans it may not have disturbed the entire area shown in Exhibit 13C. The disturbance or reclaimed areas shown in this hydrology demonstration for the Section 25 Pond are actual disturbance to date with planned future disturbances. Current mining plans do not anticipate any further ground disturbances beyond these areas, especially north of the temporary out of pit spoil pile (Map 29C), and on the west and end sides of the Collom Pit. Should mine plan changes occur, and additional disturbance is necessary, it will be permitted and bonded for accordingly, and this hydrology demonstration for the Section 25 Pond will be reevaluated prior to implementation of ground disturbance activities in the Section 25 Pond watershed. Section 25 Pond – Post Mining The location of the Section 25 Pond and its watershed is presented on Map 41B. The profiles of the associated permanent channels associated with the Section 25 Pond watershed are presented