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April 25, 2013 <br />To: Marcia Talvitie <br />From: Susan Burgmaier <br />Re: New Horizon North Mine (Permit No. C- 2010 -089) <br />Technical Revision No. 1 <br />Second Adequacy Review <br />I have completed my review of the responses provided by Western Fuels- Colorado <br />(WFC) to my adequacy concerns with Technical Revision No. 1 (TR -1), providing the <br />Pond 001 As -Built design at the New Horizon North Mine. WFC's responses to the <br />concerns I had were adequate, but I found an additional concern, as described in Item 9 <br />below. I have the following comments: <br />5. DRMS: Please include design information for the emergency spillway channel to <br />indicate the flow depth and demonstrate adequate embankment height to ensure <br />1' of freeboard, as required by Rule 4.05.9(7)(d). <br />WFC: The four SEDCAD runs for the as -built pond show no discharge through <br />the emergency spillway, therefore no emergency spillway flow is indicated for <br />use in calculating freeboard. The minimum freeboard resulting from there four <br />SEDCAD runs occurs during the 25yr -24hr seep holes closed case, where the <br />peak pool stage elevation is 5,676.62 ft. This provides for 2.88 feet of freeboard <br />to the top of the dam at 5,679.5 ft. The emergency spillway has been built to the <br />elevation of 5, 677.5, two feet lower than the top of the dam embankment. The <br />approved Pond NHN -001 design done by Arcadis included a 25yr -24hr SEDCAD <br />run assuming no flow through the primary spillway, resulting in a flow depth <br />through the designed 10 ft. wide, 2:1 side slope emergency spillway of 0.18 ft. <br />The resulting freeboard in such a scenario is 1.82 ft. In this worst case situation, <br />where the primary spillway would become blocked, the emergency spillway as <br />built would still have greater than the required 1.0 of freeboard. The emergency <br />spillway channel as -built is a rip- rapped trapezoidal channel, 10 ft. wide with <br />2h:1 v or flatter side slopes. <br />DRMS: The response adequately addressed our concern with <br />demonstrating adequate freeboard in the emergency spillway channel. The <br />design information provided, however, does not accurately reflect the as- <br />built configuration. Map 2.05.3(4) -6, the text on page 2.05.3(4) -6, and the <br />SEDCAD reports all indicate that the emergency spillway is an open <br />channel flowing southward over the embankment. Photographs of the <br />constructed spillway, taken on December 12, 2012, indicate that the <br />emergency spillway has been blocked on the south side, forcing flow <br />through a drop structure to the county road culvert. <br />