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i Northwest Sediment Trap <br />Rules 4.05.9(11) and 4.05.6(10) address certification requirements for sedimentation <br />ponds and impoundments. On June 1, 1994, Sun Coal's professional engineer <br />conducted a survey of the Northwest Sediment Trap and prepared a text description and <br />a drawing titled, "Reconnaissance Plan Map and Profiles As Functioning Northwest <br />Sediment Trap." This text description (pages 301b and 301c) and drawing represent <br />the "as-built" configuration of the sediment trap. The sediment trap is configured to <br />meet the designs approved in Appendix 2.05.3-10A of the approved permit application. <br />The Northwest Sediment Trap outlet spillway was originally designed and approved as <br />a single steel pipe opening into the Northwest Sediment Trap ditch which conveys flow <br />to the Explosives Bench Sediment Pond. The steel outlet pipe is 20" in diameter (I. D.) <br />and placed as depicted on the drawing, "Reconnaissance Plan Map and Profiles As <br />Functioning Northwest Sediment Trap." <br />Re-interpretation of the applicability of the design requirements for impoundments led <br />CDMG to require that the Northwest Sediment Trap outlet structure comply with the <br />design requirements of Rule 4.05.9(2) and by reference, Rule 4.05.6(3)(d). Rather <br />than reconstruct the sediment trap to impoundment design requirements, Sun Coal has <br />elected to modify the trap so that it does not impound water, while still functioning as a <br />• water control structure. <br />Sun Coal proposes to fill the bottom of the Northwest Sediment Trap to the inlet <br />elevation of the existing 20" I.D. culvert conveying runoff from the reclaimed area to <br />the Explosive Bench Sediment Pond. By doing so, the Northwest Sediment Trap will <br />no longer "impound" runoff water, thereby eliminating the necessity to wmply with the <br />impoundment design regulations. <br />Based on several SEDCAD+ simulations which used discharge volumes from the <br />current drainage area, the minimum size of the culvert leading from the former <br />sediment trap under the access road to the Explosive Bench Sediment Pond was <br />established. The existing culvert size was sufficient to pass all required design flows. <br />Modification to the Northwest Sediment Trap is as follows. Fill material will be placed <br />and compacted in the bottom of the Northwest Sediment Trap. The material will be <br />non-toxic non-combustible native rock, overburden and/or subsoil material obtained <br />from the existing sides of the sediment trap or other approved sources on-site. The <br />material will be placed and compacted to an elevation equal to the inlet elevation of the <br />former outlet pipe, now designated as a road culvert. This culvert extends under the <br />existing access road to the Explosive Bench Sediment Pond Ditch. <br />• Minor Revision 10 -301a- Rev. 12/94 <br />