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Response to DRMS Adequacy Review (2 drainage) — Cotter SM -18 Mine Reclamation Plan Amendment <br />4. Page ESWMP -6, second paragraph, "Haested FlowMaster " output. The selected <br />Manning's n values and estimate peak flow velocities are adequate, but the statement that <br />"These new calculations indicated that the required free board of 0 5 ft is provided by the <br />existing channels with no need to adjust channel depths. " is in error. The "Irregular <br />Channel Worksheets " in FlowMaster were used to perform hydraulic calculations. These <br />worksheets require X -Y data for channel geometry. Apparently the highest channel <br />elevation was used to calculate freeboard. The lowest "top of bank" should be used instead. <br />There are two channels (10 -5 and 10 -6) with less than the required 0.5 feet of freeboard and <br />channel 20 -3 is suspect. Based on the channel cross - section provided in Sheet 4 of 11 for <br />Section 20 -3, it appears the lowest bank elevation is approximately 53 79. 1 instead of <br />the 5379.59 as labeled on the drawing. <br />a) Sheet 3 indicates the slope for channel 10 -2 is 5.31 percent. The FlowMaster <br />worksheet for Section 10 -2 uses a slope of 5.301 percent. Please double check the slope <br />and make corrections as necessary. <br />As requested, the calculations for Section 10 -2 were redone using the correct value of <br />5.31% for channel slope (instead of 5.301 %). The results are provided in Attachment 1. <br />A copy of the original worksheet is also provided for the velocity analysis and shows the <br />differences to be negligible. <br />b) Please provide narrative and drawings) indicating how the freeboard will be <br />increased for channel sections 10 -5 and 10 -6 <br />Notes have been added to the plan (Sheet 1 of Attachment 7) and to the cross - sections <br />on Sheet 3. A schematic has also been added on Sheet 1. These notes and schematics <br />are intended to direct the operator to regrade the Offsite Basin 10 channel generally <br />from the beginning (high point) down to cross - section 10 -4 (where the existing channel <br />is adequate) by using a small excavator (skid loader, etc.) with a 5' width and adding a <br />few inches of excavation to create an overall channel depth of 18" (from lower top -of- <br />bank). This will lower the 100 -year water surface to an allowable level and provide the <br />minimum 0.5' of required freeboard. The worksheets in Attachment 2 show the results <br />(for capacity and velocity) and the redrawn cross - sections on Sheet 3 of Attachment 7 <br />illustrate the water surfaces at Qtoo. <br />c) Please double check the cross - section for channel 20 -3 and make the necessary <br />adjustments and/or corrections to ensure adequate freeboard for channel 20 -3. <br />The cross - section for channel 20 -3 has been regraded to provide adequate freeboard. <br />Proposed grading is now shown for the area between the Pond 20 outlet and section 20- <br />4 with erosion protection provided at the confluence of the pond discharge and the <br />Offsite Basin 20 channel. An additional cross - section has been added and analyzed for <br />2 <br />