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EXHIBIT 7, ITEM 23, PART K <br />Sediment Control System For the Temporary Facilities Pond <br />Near the beginning of construction for the Collom area, a short term facilities area and laydown <br />yard will be built south of and adjacent to Moffat County Road 32. The yard will be used to <br />house temporary facilities, stage equipment and store materials used for the construction of the <br />Collom Haul Road and waterline. To control runoff from this area, the Temporary Facilities <br />Pond will be built to completely contain the runoff from the 10 year, 24 hour storm event. The <br />Temporary Facilities Pond, located in the SE/4 of Section 16, T4N, R93 W, will be evacuated by <br />pumping when needed to maintain adequate storage. This small, subgrade structure is temporary <br />in nature and is expected to be in place for a 2 to 4 year time period. <br />The pond has been designed with 3H:1 V side slopes and a 12 foot wide embankment crest. <br />Total depth of the pond is less than 10 feet. Exhibit 7-23, Figure K1 shows the location of the <br />disturbances, curve numbers, drainage areas, watershed names, structure numbers, and other <br />information. An open channel emergency spillway 20 feet in width with 3H:1 V sideslopes will <br />be provided. No other outlet will be provided. A small amount of fill will be needed to <br />construct that portion of the embankment above the emergency spillway. <br />Two SEDCAD models are provided for this impoundment. The first provides sedimentology for <br />the resultant 10 year, 24 hour storm event, while the second model evaluates the effectiveness of <br />the pond's spillway system under the 25 year, 24 hour storm. All of the undisturbed areas are <br />modeled at a curve number of 63 while all disturbances are modeled at a curve number of 85, <br />based on Volume 2D, Exhibit 7, Table 1 values. <br />Exh. 7-23, Figure K2 shows the design information for the Temporary Facilities Pond, which <br />will be built downgradient from the disturbed area. As planned, it will be built early in the initial <br />construction phase. Two collector ditches are associated with this design, these have been <br />designated as the North Ditch and the South Ditch. In total, there are about 112 acres included in <br />the watershed, which includes about 20 acres of disturbance. The North and South Ditches have <br />been modeled as triangular (V type) ditches with 2H:1 V sideslopes with a minimum depth of 3.0 <br />feet. <br />For the first SEDCAD model, it can be seen that the 10 year, 24 storm event with 1.8 inches of <br />precipitation resulted in about 1.39 acre-feet of runoff and a peak flow of 11.4 cfs. Since the <br />design capacity of the pond somewhat larger, the structure has approximately 0.9 acre-feet of <br />excess capacity. The annual (one-year) sediment yield from the entire watershed is <br />approximately 0.05 acre-feet. <br />For the second SEDCAD model, the 25 year, 24 hour storm event of 2.3 inches of precipitation <br />resulted in the a peak flow of 18 cfs and a total runoff volume of 2.70 acre-feet. Flow depth <br />through the emergency spillway is approximately 0.09 feet. <br />Exh. 7-23K-1 Revision Date: 3/3/17 <br />Revision No.: TR -108 <br />