2010-12-17_REVISION - C1981019 (95)

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EXHIBIT 24, ITEM 1 COLLOM ACCESS ROAD CULVERTS The location of the Collom Access Road is presented on Map 25E. Sheet 1 of that Map presents the general location of the road, and the individual culverts are presented on Sheet 3 along the profile and on Sheets 5 through 9 in the plan profile sheets. There are 21 individual culverts proposed for this access road. One of these is a large culvert installation near station 66 +85. This is designated as the Jubb Creek Culvert. All of the other culverts have been designated alphabetically (Culverts A through T). For the Jubb Creek Culvert, the upstream drainage area consists of the combined streamflows from both the West Fork and East Fork of Jubb Creek. Map 12B shows the boundaries of this drainage basin. At the proposed crossing point, the upstream drainage area is about 4,665 acres (7.29 square miles). As discussed with DRMS staff, it was agreed that an alternate flood prediction tool could be used because of SEDCAD's inability to accurately predict peak flows from such a large drainage area. A program known as Trihydro was chosen. This program has developed and maintained by WWC Engineering of Sheridan, Wyoming. It is based on the NRCS Triangular Unit Hydrograph method and has been in widespread use in the mining field for over 25 years. • Current regulations in Section 4.03 require that this crossing be designed for a 20 year -24 hour event given that the average end area of the culvert is expected to exceed 35 square feet (about 80 inches). For a conservative approach, the culvert has been designed for the 100 year event. Based on the drainage area, the stream length, elevation change and other parameters as shown in Figure 1, Trihydro predicts a peak flow of 268 cfs and a total runoff volume of 122 acre -feet. This is based on a curve number of 63 using values based on Volume 2D, "Methodologies and Assumptions for Sediment Pond Design Evaluations ", as modified by TR -73. Figure 2 provides a similar analysis except that a curve number of 70 was used to account for future disturbances that might occur to the upstream watershed. This resulted in a peak flow of 667 cfs and 215 acre -feet of runoff. This peak flow subsequently chosen as the design flow for the Jubb Creek culvert. Assuming a headwater depth over the top of the pipe of about 2.0 feet and a mitered entrance, the nomograph provided on Figure 3 shows that a 108 inch (9.0 foot) diameter corrugated metal pipe would be adequate. This its based on the assumption that the pipe operates in inlet control. A similar nomograph was checked to insure the pipe is not operating in outlet control, and this was verified based on in installed length of about 230 feet at the slope described below. The Jubb Creek culvert is specified to be 108 inches in diameter and be of galvanized steel with 3" x 1" corrugations with a galvanized thickness of 0.109 inches (12 gage). This culvert shall be mitered at the inlet to match the fill slope and have riprap placed at the sides and inlet to a height 4 feet about the top of the pipe. Exh. 24 -1 -1 Revision Date: 03/25/10 Revision No.: PR -03