2014-07-29_REVISION - C1981010 (3)

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Forrest Luke, TMI Page 7 7/29/2014 f. Trapper's Response #2: Updated Section 4.8.13 is enclosed. Revised pages i -13, 111-12, 4 -183, 4 -183a and 4 -183b are included, along with new pages 11-13 and 4 -183c through 4 -183j. Included in these pages are new Figures 4.8 -1a through 4.8 -1f. g. Division's Response: Trapper provided a description and general design for a generic channel, including check dam and dozer basin diagrams, representing proposed post - mining drainage channels. Trapper indicates (on proposed page 4 -183d) that supporting documentation for its proposed check dam configuration is available for review at the Trapper mine offices. In accordance with Rule 2.05.3(4)(a), this information must be submitted to the Division as part of the permit application package. Rule 2.05.3(4)(a) requires that "A general plan and detailed design plan shall be submitted for each sediment pond, impoundment, other treatment facility and diversion proposed to be constructed within the permit area." Please provide additional details for each postmining drainage channel, to include the estimated discharge resulting from a precipitation event with a 10 -year, recurrent interval (Rule 4.05.3(3)), and a calculation of whether the proposed drainage channel configuration will safely convey the design discharge with the required 0.3 feet of freeboard (Rule 4.05.3(7)(b)). If Trapper elects to continue with the generic channel design, the typical cross section provided on proposed page 4 -183b must indicate that the depth will vary, and the minimum depth for each drainage to be constructed, completed or reconstructed during this permit term must be provided. Rules 4.05.3(1)(e) and 4.05.3(7) address the requirements for stable channels, and require that channel linings shall be designed using standard engineering practices. Please provide design information for the proposed check dams including calculations of flow velocity to demonstrate that the check dams will reduce the flow velocity such that the channel is stable. For example, the required information for the Oak drainage, would include an estimate of the discharge resulting from the drainage area (310.7 acres) as a result of the 10 yr -24hr storm (23.2 cfs), taken from Appendix Q. Using the currently available maps for the drainage area, the channel slope can be estimated at 14 %. Using the SEDCAD channel utility, with the proposed channel dimensions, the flow depth in the channel would be 0.11 feet, so a minimum depth of 0.41' would be required; the design information would need to indicate that the Oak drainage channel will be at least 0.41' deep to provide adequate capacity to convey the design event. In addition, in the case of Oak drainage, the SEDCAD utility calculation also shows that a rip rap lining with a D50 of 6.0" would be necessary to ensure that the channel would remain stable. If Trapper elects to substitute check dams for rip rap lining, supporting calculations for the check dams would need to be included, demonstrating that the