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Exhibit 7, Item 25D <br />Power Line Light -Use Road <br />Small Area Exemption Demonstration <br />Colowyo will be constructing a light -use road to support construction and long-term access to a <br />69kV power line. Portions of this light -use road are outside of the Section 25 Pond watershed, <br />and according to Rule 4.03.3(4)(a)(ii) sediment control is required for light -use roads. Since it is <br />not practical nor necessary to constructed sediment control from the section of the light -use road <br />that is outside of the Section 25 Pond watershed, the demonstration will in accordance with Rule <br />4.05.2(3)(a) provides an exemption from the requirements of 4.05.2. Therefore, surface water <br />runoff from this power line light -use road will be treated through a small area exemption. <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. Four subwatersheds report to the light -use road along the length of the area that is <br />outside of the Section 25 Pond watershed. For this SEDCAD demonstration the largest <br />subwatersheds (Area 4 on Figure Exh. 7-2513-1) has been modeled as the worst case scenario. <br />The other areas shown on Figure Exh. 7-2513-1 have much smaller subwatersheds and will <br />eventually be cut off from reporting water by the Collom Pit. <br />The light -use road has a total disturbance area of 2.1 acres, 51.8 acres of contributing watershed, <br />and a ground slope for the contributing watershed of 6.4%. A curve number of 85 was utilized to <br />reflect the disturbed condition of the light use roads and a curve number of 47 was used to reflect <br />the undisturbed condition of the contributing watershed. Please see Figure Exh. 7-25D-1 for the <br />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 light -use road. The 2.04 cfs flow was then run through a native grass filter approximately <br />2,000 feet in length and 10 feet in width (direction of the flow 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 0.01 ml/l, below <br />the effluent standards of 0.5 ml/l, also with an 87.3% trap efficiency. <br />Page Exh. 7-25D-1 Revision Date: 6/19/18 <br />Revision No.: TR -121 <br />