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The Union Milling Company <br />A" P.O. Box 620490 <br />Littleton, CO 80162 -0490 <br />+1.303.947.3499 <br />April 22, 2014 <br />Mr. Michael Cunningham <br />Environmental Protection Specialist <br />Division of Reclamation, Mining and Safety RECEIVED <br />Department of Natural Resources <br />1313 Sherman Street, Room 215 APR 2 2 2014 <br />Denver, CO 80203 <br />DivisionofReclamation, <br />michael.cunningham @state.co.us <br />Mining & Safety <br />RE: Leadville Mill; M1990 -057, Response to Amendment Review No. 4 Second Adequacy <br />for Stormwater AMC - <br />Dear Mr. Cunningham, <br />Following are Union Milling Company's (UMC) responses to Adequacy Review No. 4 Second <br />Adequacy for Stormwater queries from Tim Cazier. Also included with this letter is our updated <br />engineer's report which addressed the Division's queries in detail. <br />1. Pape 5, last paragraph. A CN of 79 is proposed for the outslope of the TSF, (assumed fair <br />grass cover). It is the Division's experience that tailings dams are seldom vegetated during <br />operations. Please revise the CN to a non - vegetated conditions (such as pervious, "newly <br />graded area" HSG Q CN = 91) or describe an aggressive revegetation plan for the TSF <br />outslope. <br />Response: <br />UMC plans on revegetating the outside slope of the TSF. Suitable Growth Material that <br />was stripped from the site and stockpiled prior to the construction of the TSF <br />embankment will be spread on the outside slopes of the TSF and planted with an <br />altitude appropriate seed mix. Silt fence will be installed at the toe of the slope during <br />the grow -in period. <br />2. Page 11, Table a With the exception of the "Berm Contact" and "West TSF Contact" open <br />channels, the maximum velocities are well above the Division's velocity limit for earth -lined <br />channels of five feet per second (5 fps). Drawing DR -2 indicates all the channels are lined <br />with 18 inches of D50 = 9 inch riprap. The Mannings' n used for the hydraulic capacity <br />analyses in Appendix B is 0.025. The Division views this as inconsistent. The purpose of the <br />hydraulic capacity check is to determine whether or not the channels are designed with <br />adequate capacity to pass the 100 year, 24 -hour peak flow. Mannings roughness values for <br />