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Response to DRMS Adequacy Re ,.•iew (2) — Cotter SM -18 Mine Reclamation Plan Amendment <br />=l. Page ESWMP -6, second paragraph, "Haested FlowMaster" output, DDP Drawings 3 <br />and 4 of 7. It is stated the channels are "capable of transporting the 100 year flows ". The <br />DBMS could not find the FlowMaster analyses referenced. <br />a. Please provide the referenced FlowMaster analyses. When providing these analyses, <br />note that channel roughness is seldom uniform, the DRMS requires channels be <br />evaluated for both stability and capacity, i.e., minimum and maximum expected <br />roughness. For example, an excavated earth channel, after weathering would be <br />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, <br />the DRMS requires channel freeboard be evaluated for all engineered channels: <br />channels shall be designed with a minimum of 0.5 feet offreeboard unless the <br />velocity head (012g) is significant, " then the minimum required freeboard is half <br />the velocity head, or v? 14g. <br />b. Please provide a rationale for the selected roughness coefficients, and evaluate each <br />designated channellditch design slope for both capacity and stability. <br />c. Please design all engineered ditches with the appropriate freeboard and provide channel <br />design depths for construction. <br />d. Please note that channels expecting erosive channel velocities will need to be armored <br />with appropriately sized revetment or constructed in non - erodible material, such as <br />bedrock.- <br />FlowMaster worksheets are provided as Attachment 6 in this response. As <br />requested, the channels have been analyzed with two sets of Manning's "n" <br />values which, in our opinion, conservatively match the expected conditions <br />.produced by the existing channels we observed, and consequently by <br />proposed channels of similar construction. Our selection of n = 0.050 is a <br />conservatively high match for the rough channels with exposed rock which <br />we observed in the upper portions of the offsite channels (Sections 10 -1 <br />through 10-6 and 20 -1 and 20 -2 — see sheet 1 of Attachment 7). In the lower <br />sections, along the existing road ditch (Sections 20 -3 through 20 -5), a <br />Manning's n of 0.035 was chosen because of the more uniform surface. <br />These values were chosen to be conservatively high to use for analysis of <br />capacity. As suggested, however, we have also analyzed the channels for <br />velocity with lower "n" values (we selected n = 0.035 for the upper, more <br />rocky channels and n = 0.028 for the lower, existing roadway ditches) to give <br />a conservatively high velocity. Analysis was performed for both "n" values <br />at all of the original sections. These new calculations indicated that the <br />required freeboard of 0.5 ft is provided by the existing channels with no need <br />