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Response to DRMS Adequacy Review (5) β€” Cotter SR -13A Mine Reclamation Plan Amendment <br />K = (1 β€”sin20 /sin' +)°'S, where 0 = riprap side -slope angle from horizontal (@ 3 :1), <br />and + = riprap angle of repose (from horizontal, conservatively estimated at <br />1:1). The calculated velocity is 5.92 ft /sec (from the worksheet β€” confirming that <br />maximum slope of 12% and minimum roughness of n =0.035 was used) which = <br />1.80 meter /sec, d = 0.54 feet (from worksheet) which = 0.16 meter. Results of the <br />calculation indicate a D5o of 0.1031 meters, which = 4.06 inches. Therefore the <br />D50 of 8 inches that was specified is adequate. <br />6 Page ESWMP -6, section 7.4 paragraph and Retention Pond Drainage Design Plan (Sheet 5 <br />of 5). The response to this comment is adequate. <br />7. Page ESWMP -7, last paragraph. The DRMS appreciates the decision to breach the <br />embankment upon the cessation of mining. In order to further reduce the potential for <br />erosion and sediment problems, please commit to the following: <br />a. Provide a maximum slope for the channel through the embankment to avoid the <br />3H:1 V drop from contour 5616 to the daylight line at β€” 5614. <br />b. Provide a cross - section of the channel through the breach indicating a minimum <br />bottom width for the purpose of construction. <br />c. Flatten the inlet channel to competent rock or armor it to prevent it head cutting to <br />the east towards the toe of the reclaimed waste dump. <br />d. Commit to delaying the breaching of the embankment to after reclamation of the area <br />contributing runoff to Pond 30 is complete in order to minimize the discharge of <br />sediment below the pond. <br />a. The reclaimed channel has been regraded to provide a maximum slope of 10% <br />and eliminate the drop originally shown near the toe of the embankment. This <br />revision is indicated on sheet 3A of Attachment 3. <br />b. A cross - section of the channel is provided indicating a minimum bottom width <br />for this construction. <br />c. A note indicating the channel should be flattened to competent rock or to add <br />riprap to prevent head cutting has been added to the drawing. <br />d. Cotter commits to delay breaching the embankment until the final stages of <br />reclamation of the Pond 30 watershed to minimize discharge of sediment below <br />the pond. <br />8. Pages ESWMP -8 -10, hydrographs. The response to this comment is adequate. <br />9. Pages ESWMP- 11 -13, weighted CNs. The response to this comment is adequate. <br />10. Page ESWMP -18, pond volume, Drawing E -6 and ESWMP Drawings 2 and 3. The response <br />to this comment is adequate. <br />