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Page 1 Revision Date: 8/15/22 <br /> Revision No.: TR-155 <br /> <br /> <br />Exhibit 7, Item 20 <br />Topsoil Pile 15G Grass Filter Demonstration <br /> <br />As shown on Map 12 and Figure Exh. 7-20-1, topsoil pile 15G and a small portion of a road <br />cannot have surface water runoff routed to the Section 3 Ponds for primary sediment control. <br />This demonstration will, in accordance with Rule 4.05.2(3)(a), provide an exemption from the <br />requirements of 4.05.2. Therefore, surface water runoff from this disturbed around will be <br />treated through a grass filter. <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 />and/or native lands in various stages of mining and reclamation have been selected in accordance <br />with Table 1 in the Introductory Text for Exhibit 7 in Volume 2D or as further described below. <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 />The results of the SEDCADTM model demonstrates that a grass filter 5 feet in length (direction of <br />flow) that is 300 feet long is adequate to manage sediment for this area. In summary, the grass <br />filter has a trap efficiency of 83.8%, and the settleable solid concretion is 0.14 ml/l, which is <br />below the 0.5 ml/l standard. <br /> <br />References <br />1) Applied Hydrology and Sedimentology for Disturbed Areas, Barfield, Warner and Haan, <br />1981