2022-07-12_PERMIT FILE - C1981019 (17)

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Exh. 7-23M-1 Revision Date: 4/28/22 Revision No.: TR-154 EXHIBIT 7, ITEM 23, PART M Coal Stockpile Pond The location of the Coal Stockpile Pond is presented on Map 23B. The as-built configuration for the Coal Stockpile Pond is presented on Figure 7-23M-1, and Figure 7-23M-2 provides the breakdown of drainage areas and hydrologic conditions for this sediment pond. Exhibit 7, Item 14 in Volume 2D describes the hydrologic methodology used in sediment pond and channel assumptions. Runoff curve numbers assigned to the undisturbed and 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. 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. Coal Stockpile Pond The following pages present the results of the SEDCADTM model for the Coal Stockpile Pond. The Coal Stockpile Pond contains all runoff from the Collom crusher area and is a non-discharging structure during mining activities. The Coal Stockpile Pond will be used during reclamation as an additional sediment pond within the larger Section 25 Pond watershed. Please refer to Volume Exhibit 7 Item 23 Part I. During mining a conservative approach has been implemented in this model. Areas above the Collom crusher contain stockpiled coal, but at time may also not contain any coal. A more appropriate curve number of the coal stockpile area could be 74 or lower. However, for built in conservatism, a curve number of 85 was used throughout this model even though a lower curve number may be more appropriate in areas like the Collom coal storage area, or other disturbed areas where surface roughness would reduce surface water flows. It is Colowyo’s experience operating the Coal Stockpile Pond since it was constructed that it is typically dry or nearly dry. Therefore, in this model the initial pond level is set nearly to the pond bottom elevation of 7,211’. Two culverts are included in this model, culverts 45 and 64 respectively. Both culverts as installed are 36 inch. One other culvert (culvert 68) may appear to be within the Coal Stockpile Pond watershed on Figure 7-23M-2; however, it routes water from an upstream non-disturbed area back towards the Collom Pit. Therefore, it is not included in this model. A paved area where Colowyo’s highway haul trucks enter the Collom crusher area for loading was modeled with a curve number of 98. This curve number was selected directly from the SEDCADTM software for paved roads. Temporary conveyance channels shown on Figure Exh. 7-23M-2 are for the most part natural drainages areas that flowed water prior to mining commencing, or areas where a berm directs flow to the Coal Stockpile Pond. Two areas of short riprap channel exists where water is routed away from the truck loading area on the top of the Collom crusher area. These channels were too small for SEDCADTM to model effectively with a subwatershed, therefore the entire channel is shown