2016-08-08_PERMIT FILE - C1981019A (117)

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EXHIBIT 7, ITEM 25 COLLOM POSTMINE CHANNEL DESIGN The location of the Collom postmining drainage channels are presented on Map 41 B. The following figures have been prepared to show the individual subbasins: Exh. 7, Item 25, Figure 1 Exh. 7, Item 25, Figure 2 Exh. 7, Item 25, Figure 3 Exh. 7, Item 25, Figure 4 Exh. 7, Item 25, Figure 5 Exh. 7, Item 25, Figure 6 Exh. 7, Item 25, Figure 7 Little Collom Reach 1 and Trib. 3 Draws Little Collom Reach 2 and Trib. 4 Draws Deer Draw Bear Draw Badger Draw Marmot, Weasel, and Trib. 5 Draw Trib. 1 Draw Channel profiles for these structures showing channel lengths and elevation changes are presented on Map 33, Sheets 1 through 3. A SEDCAD analysis is attached for each of the subbasins. Table 1 provides a listing of the individual analyses. Given the size of the drainage area for the Little Collom reaches, a design storm of 100 year, 24 hours was used for the Little Collom Reaches 1 and 2. All other design events are based on the 10 year, 24 hour storm. All of the basins are assumed to be completely reclaimed with the entire drainage area contributing to flow. As such, a curve number of 75 was used for all basins. As noted in Volume 18, Exh. 7-23, "Methodologies and Assumptions for Sediment Pond Design Evaluations", this curve number assumes that about 50 percent of the area is reclaimed for less than 2 years and 50 percent of the corresponding drainage is reclaimed for three or more years. As such, this analysis is expected to predict long term flows on the reclaimed area. As noted on Map 29B, spoil grading and subsequent reclamation activities are expected to proceed southward over a number of years and it will generally take several years before an individual basin is contributory. All postmining channels listed will be riprapped using stone with a D50 at least as large as the size indicated for each channel on the respective SEDCAD analyses. This size criterion is based on SEDCAD's Simons-OSM method. The minimum D50 for each channel is included within the attached SEDCAD output files. Except as noted below, all channels will have a completed width of 12 feet, 3H:1 V sideslopes, and a total depth of 3.0 feet. The Little Collom Reach 2 and will have a completed width of 20 feet, 3H:1 V sideslopes, and a total depth of 4.0 feet. Exh. 7-25-1 Revision Date: 7/31/15 Revision No.: PR -04