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Response to DRMS Adequacy Review (2) -Cotter SR -11 Mine Reclamation Plan Amendment <br />National Engineering Handbook (Section 4, Hydrology), V.T. Chow (Open Channel <br />Hydraulics), and the Mesa County Stormwater Management Manual which <br />includes tables from many of these sources. Examples of tables utilized herein are <br />included in Attachment #2 of this response. <br />c. Please evaluate each designated channel /ditch design slopes (minimum and maximum) <br />for both capacity and stability. <br />As suggested, our analyses now include evaluations of the channels for both capacity <br />and velocity using a range of roughness coefficients believed to be applicable to the <br />conditions and materials proposed for the channels. We believe that for the site <br />conditions here, a range between 0.035 and 0.050 is reasonable and applicable. <br />Flows to these individual BMPs can be isolated by three similarly sized sub - basins <br />within Onsite Basin 30 (30A, 30B, and 30C). To simplify the analyses, each of the <br />three onsite sub - basins was assigned a 100 -year flow equivalent to the proportionate <br />flow of the largest sub -basin (Sub -Basin 30A). The total area of Onsite Basin 30 is <br />4.10 acres and the area of Sub -Basin 30A is 1.52 acres. The proportionate share of <br />the total 100 -year flow of 5.47 cfs is therefore equal to: (1.52 acre/ 4.10 acre) x (5.47 <br />cfs) = 2.03 cfs. This pro -rated flow was conservatively used in the channel analyses <br />for all sub - basins even though the others were slightly smaller in size. All sub - <br />basins could be estimated to have similar times of concentration as they all begin in <br />the upper portion of Onsite Basin 30 and all terminate at the collection pond. <br />Calculations of flow -depth (for capacity) and velocity (for stability) can be seen in <br />the worksheets included in Attachment #1. <br />d. Please design all the ditches with the appropriate freeboard and provide channel design <br />depths for construction. <br />With the exception of the proposed pond outlet channel, all ditches, on and off site, <br />are existing channels which were analyzed for capacity and velocity. Only the <br />interceptor channel (at Section 30 -2) has an indicated depth of flow with slightly less <br />than 0.5 feet of remaining freeboard (0.37 feet indicated). This is not a concern as <br />this ditch is only intended to intercept a portion of the flow occurring onsite in Basin <br />30 (Sub -Basin 30B) and route it directly to the pond. Should a portion of this flow <br />escape the ditch, it would flow overland across the area below and be collected in the <br />lower sub -basin (30A), still a part of Onsite Basin 30. It would continue to be routed <br />through the only collection pond proposed for the site, as originally indicated in the <br />initial drainage report. Regular inspections of the mine site accompanied by <br />maintenance of stormwater control structures should minimize any potential <br />problems associated with these facilities. <br />3 <br />