2017-03-20_PERMIT FILE - C1981019A (14)

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EXHIBIT 24,ITEM 1 COLLOM HAUL ROAD CULVERTS Introduction The location of the Collom Haul Road is presented on Map 25E. Sheet 1 of that Map presents the general location of the road, and the individual culverts presented in this exhibit. As discussed with DRMS staff during initial permitting for the Collom Mine, it was agreed and subsequently approved that an alternate flood prediction tool could be used for these two larger watersheds because of SEDCAD's inability to accurately predict peak flows from such a large drainage areas. 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. The peak flows and total runoff volumes are based off of a curve number of 63 which can be found in Volume 2D, Exhibit 7, Table 1. Wilson Creek Crossings For the Wilson Creek culvert, the upstream drainage area consists of Wilson Creek for about 10 miles south to the ridgeline known as the Danforth Hills. Map 12B shows the boundaries of this drainage basin. At the proposed crossing point, the upstream drainage area is about 9,241 acres (14.43 square miles). Rule 4.03.1(4)(e) requires that crossings be designed for a 20 year-24 hour event if the average end area of the culvert is expected to exceed 35 square feet (an 80-inch diameter culvert). In spite of this, for a more conservative approach,the culvert at Wilson Creek has been designed for the 100 year event. Based on the drainage areas, the stream lengths, elevation changes and other parameters as shown in Figures 1, Trihydro predicts 100 year peak flows for Wilson Creek to be 404 cfs and the total runoff volume to be 242 acre-feet. Figure 2 presents a generally accepted FHWA nomograph for flow through culverts operating under the entrance control condition. The nomograph is marked up to illustrate acceptable pipe diameters for the Wilson Creek crossing. Acceptable solutions have been limited to diameters that can deliver the design flow with a Headwater/Diameter (HW/D) ratio of 1.5 or less. Pipe ends mitered to conform to the slope have been assumed, and the pipe will have riprap placed at the sides and inlet to a height 4 feet about the top of the crown. At Wilson Creek, it can be seen on Figure 2 that a 7-foot diameter pipe can pass the 404 cfs design flow at a HW/D of about 1.3. An alternate design of twin 66 inch diameter pipes would also be adequate. In the final road design however, it was determined that a 12 foot diameter pipe will be installed at this crossing which surpass the minimum requirements. This pipe size was selected as approximately 3 foot of fill will be placed in the pipe bottom for a natural channel surface to comply with other regulatory requirements. Exh.24-1-1 Revision Date: 12/30/16 Revision No.: TR-108