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Exh. 7-23N-1 Revision Date: 4/28/22 <br /> Revision No.: TR-154 <br />EXHIBIT 7, ITEM 23, PART N <br />Perimeter Area Sediment Control Post Mining <br /> <br />Introduction <br />Colowyo will have ground disturbance along the perimeter of the Collom Pit during reclamation <br />activities (post mining) that cannot effectively be routed to one of the existing sediment ponds. <br />Colowyo will implement a perimeter sediment control plan for these areas. Generally, this will be <br />ground disturbance on the west and east sides of the pit outside of the Section 25 and 26 sediment <br />pond watersheds as shown on Map 41B. This demonstration will, in accordance with Rule <br />4.05.2(3)(a), provide an exemption from the requirements of 4.05.2. Therefore, surface water <br />runoff from these reclamation areas will be treated through a grass filter or a combination grass <br />filter and silt fence. <br /> <br />For the model demonstration, SEDCADTM grass filter routine has been used to demonstrate that <br />runoff from this area will meet the effluent standards of 0.5 ml/l in accordance with Rule <br />4.05.2(7) (US EPA, 40 C.F.R. §434.52). Runoff curve numbers assigned to the undisturbed, <br />disturbed and reclamation areas have been selected in accordance with Table 1 in the <br />Introductory Text for Exhibit 7 in Volume 2D. <br /> <br />The grass filter physical parameters were taken as the defaults in the SEDCAD drop down tables <br />for a good stand of well established rye grass, except that the stem spacing was conservatively <br />increased from 0.67 inches to 2.0 inches, and the roughness coefficient reduced from 0.0121 to <br />0.008, as described Reference 1, page 568, for a "fair stand" of grass. Both of these revisions <br />produce a more severe loading on the grass filter by allowing the flow velocity to be higher than <br />in a "good stand". <br /> <br />Each specific reclamation area that will require a grass filter or grass filter and silt fence for <br />sediment control has been evaluated individually. Each area is shown on Figure 7-23N-1. These <br />areas for reclamation and timing are also shown on Map 29B. <br /> <br />For each individual grass filter or grass filter and silt fence demonstration included herein, it is <br />demonstrated that these areas will control sediment as required, and all SEDCADTM models <br />indicate effluent limits for each area is below 0.5 ml/l. <br /> <br />References <br />1) Applied Hydrology and Sedimentology for Disturbed Areas, Barfield, Warner and Haan, <br />1981 <br />