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COLORADO <br />Division of Reclamation, <br />Mining and Safety <br />Department of Natural Resources <br />1313 Sherman Street, Room 215 <br />Denver, CO 80203 <br />October 22, 2014 <br />To: Jared Ebert <br />From: Jason Musick <br />RE: Trapper Mine, C- 1981 -010 <br />Permit Revision No. 7 (PR7) <br />Adequacy Review <br />Per your request, I have reviewed the submittal documents from Trapper Mining, Inc. (TMI) in response to <br />the Division's adequacy comment identified in the Adequacy Review No. 4 letter dated July 29, 2014. <br />The Division's July 29, 2014 adequacy comments requested: additional details for each postmining <br />drainage channel, to include the estimated discharge resulting from the design event, and a calculation of <br />whether the proposed drainage channel configuration will safely convey the design discharge with the <br />required 0.3 feet of freeboard (Rule 4.05.3(7)(b)) and requested that if Trapper elects to continue with <br />the generic channel design, the typical cross section provided on proposed page 4 -183b must indicate that <br />the depth will vary, and the minimum depth for each drainage must be provided.; design information for <br />the proposed check dams including calculation of flow velocity to demonstrate that the check dams will <br />reduce the flow velocity such that the channel is stable; and detailed design information (discharge, flow <br />depth, velocity, and channel lining) for all of the postmining drainage channels to be constructed, <br />completed, or reconstructed during the current permit term. <br />Based on a review of the proposed permit pages 4 -183 through 4 -183j, the report titled "Guideline for the <br />Design and Number of Check Dams and Dozer Basins for Erosional Control of Reclaimed Lands for Bond <br />Release Under a Variety of Channel Conditions Using SEDCAD Modeling ", and the proposed Appendices A <br />through H, submitted in response to the Division's concerns, the Division has the following comments: <br />In lieu of a detailed design plan submitted for each proposed postmining drainage channel, Trapper is <br />proposing to utilize a generic channel design based on the assumption that during the active mining stage <br />approximately one third of the watershed tributary to any given check dam within the channel would be <br />at a freshly topsoiled condition (CN 86), one third at 1 year's growth (CN 80), and one third at 2 years <br />growth (CN 67), all the while the grass lining in the channel is still immature. Hence, the curve number <br />selected for the channel design model was 78. Based on the report submitted, the SEDCAD models <br />presented in Appendices A through H are based on a model that demonstrates that Trapper's existing <br />postmining channel design and construction methods will meet bond release criteria. The models do not <br />represent proposed postmining drainage channels. The submitted study for the generic channel <br />construction and accompanying SEDCAD models present information based on the 78 CN. Although the <br />study and SEDCAD variables are accurate based on the assumptions made in the study, the Division <br />believes that the 78 CN utilized to determine the number of rock check dams in a generic drainage <br />OF' c <br />1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http: / /mining.state.co.us <br />John W. Hickenlooper, Governor I Mike King, Executive Director I Virginia Brannon, Director * t 6 <br />, *87 � <br />