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Leadville Mill — Stormwater Comments <br />Page 3 <br />January 21, 2014 <br />ii. Hydraulic analyses evaluating the design capacity and stability of each <br />diversion and /or collection ditch (to include reaches with both the <br />maximum and minimum design slopes) to pass the 100 -year, 24 -hour <br />design storm peak discharge. This should consider rationale for the <br />selected roughness coefficients. Because channel roughness is seldom <br />uniform, the DRMS requires channels be evaluated for both stability and <br />capacity, i.e., minimum and maximum expected roughness. For example, <br />an excavated earth channel, after weathering would be expected to have a <br />minimum n = 0.018 (use to evaluate stability or maximum expected <br />velocity); and a maximum n = 0.025 (use to evaluate capacity). In <br />addition, the DRMS requires channel freeboard be evaluated for all <br />engineered channels: channels shall be designed with a minimum of 0.5 <br />feet of freeboard unless the velocity head (v2 /2g) is significant, then the <br />minimum required freeboard is half the velocity head, or v2 /4g. <br />iii. Erosion protection details and analyses if needed. <br />The DRMS recommends the Operator submit a drainage design plan, to include the required <br />analyses, certified designs and specifications for the engineered elements associated with the <br />environmental protection plan (EPP). The cover page, all drawings and specifications should be <br />stamped and signed by the responsible engineer. <br />If either you or the applicants have any questions regarding the comments above, please call the <br />at (303) 866 -3567, extension 8169. <br />m:VninUcIVninerals supporhm- 1990-057 union milling\strmwtrcmnts _mem21jan14.docx <br />