2010-12-17_REVISION - C1981019 (89)

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EXHIBIT 7, ITEM 25 COLLOM POSTMINE CHANNEL DESIGN The location of the Collom postmining drainage channels are presented on Map 41B. 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 Exh. 7, Item 25, Figure 8 Exh. 7, Item 25, Figure 9 Exh. 7, Item 25, Figure 10 Exh. 7, Item 25, Figure 11 Little Collum Reach 1 and Trib. 2 Draws Little Collum Reach 2 and Trib. 3 Draws Little Collum Reach 3 Deer & Little Deer Draw Elk and Trib. 4 Draws Porcupine Draw Cougar Draw Badger, Marmot, & Weasel Draws Bear Draw Little Bear, Trib. 6, Little Fox 1, & Little Fox 2 Draws Red Fox & Trib. 5 Draws 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 Collum reaches, a design storm of 100 year, 24 hours was used. All other design events is based on the 10 year, 24 hour storm. All of the basins are assumed to be completely finished with the entire drainage area contributing to flow. As such, a curve number of 75 was used for all basins. As noted in Volume 2D, "Methodologies and Assumptions for Sediment Pond Design Evaluations ", as modified by TR- 73, 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 material size and design methods specified in Volume 2C, Exhibit 14H. 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 Collum Reach 2 and Reach 3 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: 03/25/10 Revision No.: PR -03