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Response to DRMS Adequacy Review (2)- Cotter SR -I I Mine Reclamation Plan Amendment <br />the existing retention pond. These soils will be removed from the retention pond <br />area on a regular basis and placed on the existing soil stockpile. These soils will be <br />used during final reclamation as part of the final cover for the affected area prior to <br />re- vegetation. <br />c. Please include the existing berm and associated incised channels as BMPs, and label <br />these individual segments on Sheet 1. <br />The berms and channels have been individually labeled on Sheet 1. <br />2) Page ESWMP -4, Section 7.1 - Engineering Approach and FlowMaster analyses. The second <br />paragraph states BMPs are labeled on Sheet 1 of 4. As discussed in Comment 41, no BMPs <br />are labeled within Subbasin Onsite 30, nor are there hydraulic analyses for any BMP in <br />Onsite 30, except for the retention pond. <br />a. Please provide hydraulic analyses for the BMPs discussed in Comment 41. These <br />channels need to be designed to convey the peak flow resulting from the 100 year, 24- <br />hour design storm. For flow velocities exceeding 5.0 feet per second, appropriately sized <br />revetment or a demonstration that the channel is non - erodible is required. <br />Hydraulic analyses are now provided for the channels discussed in Comment #1 and <br />can be seen on the worksheets provided in Attachment #1. These calculations <br />indicate runoff velocities for the 100 -year event are all below 5 feet per second, <br />including the 10% chute below the broad crested weir proposed for the pond. <br />Riprap is proposed for the weir throat, flume, and chute, however, to control <br />erosion in that area. <br />b. Please provide rationale for selected roughness coefficients. Note: channel roughness is <br />seldom uniform, the DRMS requires channels be evaluated for both stability and <br />capacity, i.e., minimum and maximum expected roughness, as well as minimum and <br />maximum design slopes. For example, an excavated earth channel, after weathering <br />would be expected to have a minimum n = 0.018 (use to evaluate stability or maximum <br />expected velocity); and a maximum n = 0.025 (use to evaluate capacity). In addition, the <br />DRMS requires channel freeboard be evaluated.• channels shall be designed with a <br />minimum of 0.5 feet offre( 5oard unless the velocity head (V2 12g) is significant, then the <br />minimum required freeboard is half the velocity head, or v2 14g. <br />Our selection of CN and Manning's "N" roughness coefficients are derived from a <br />variety of possible values found in several reliable and respected resources such as: <br />tables from the SCS TR -55 Manual (Urban Hydrology for Small Watersheds), the <br />2 <br />