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Exhibit 7, Item 25C <br />Temporary Access Road <br />Small Area Exemption Demonstration <br />Colowyo will be constructing temporary access roads to support the development of the Collom <br />Pit and the temporary spoil pile. A portion of the Temporary South Access Road (Station 57+28 <br />to 64-72) and the entirety of the Temporary Tie -In Access Road (please see Exh. 7-23 Figure D4 <br />for locations) will need to be constructed to allow equipment to access the west side sediment <br />control system prior to and during construction. However, this area is outside of the watershed <br />contributing to the Section 25 Pond and will be necessary prior to installation of the west side <br />sediment control system. Therefore, in accordance with Rule 4.05.2(3)(a), this demonstration <br />provides an exemption from the requirements of 4.05.2.; and surface water runoff from these two <br />temporary access roads will be treated through a small area exemptions. <br />For the model demonstration, SEDCAD's grass filter routine has been used to demonstrate that <br />runoff from this area will meet the effluent standards of 0.5 ml/1 in accordance with Rule <br />4.05.2(7) (US EPA, 40 C.F.R. §434.52). Sedimentology parameters and curve numbers used are <br />defined in Exhibit 7 Methodologies and Assumptions for Sedimentation Pond Designs <br />Evaluations. <br />The two access roads have a combined disturbance of 1.6 acres, 128.5 acres of contributing <br />watershed, and a ground slope for the contributing watershed of 9.0%. A curve number of 85 <br />was utilized to reflect the disturbed condition of the temporary access roads and a curve number <br />of 47 was used to reflect the undisturbed condition of the contributing watershed. Please see <br />Figure Exh. 7-25C-1 for the location of the watershed utilized in the model demonstration. <br />This demonstration utilized the Manning's roughness coefficient of 0.0054, and a stem spacing <br />of 0.86 inches. These inputs are intended to reflect that the native grass cover density adjacent to <br />the temporary access roads. The 1.17 cfs flow was then run through a native grass filter <br />approximately 1,942 feet in length (road length) and 10 feet in width (direction of the flow <br />through the grass filter). <br />As shown in the attached SEDCAD output files, the calculated resulting peak settleable <br />concentration released and the 24 -hr weighted settleable solids concentration is 0.18 ml/1, below <br />the effluent standards of 0.5 ml/1, and a 85% trap efficiency. <br />Page Exh. 7-25C-1 Revision Date: 1/19/18 <br />Revision No.: TR -118 <br />