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TRI-STATE GENERATION AND TRANSMISSION ASSOCIATION, INC. <br />HEADQUARTERS: P.O. BOX 33695 <br />Mr. Brock Bowles <br />Environmental Protection Specialist <br />DENVER, COLORADO 80233-0695 303-452-6111 <br />October 13, 2017 <br />RECEIVED <br />Colorado Division of Reclamation, Mining and Safety <br />1313 Sherman Street, Room 215 OCT 16 2017 <br />Denver, CO 80203 <br />Division of Reclamation, <br />RE: New Horizon Mine (Permit No. C-1981-008) Mining & Safety <br />Technical Revision No. 80 (TR -80) <br />Adequacy Review Response <br />Dear Mr. Bowles: <br />Tri-State Generation & Transmission Association (Tri-State) has received your adequacy <br />review letter for TR -80, dated October 2, 2017 and provides the following responses to your <br />comments on behalf of the Elk Ridge Mining and Reclamation's New Horizon Mine. <br />Table 2.05.3.0-3 Permit Area Culvert Designs, 030-081, y,-. 33 & associated SEDCAD <br />For C134, Table 2.05.3-3 calls out 2-18 inch pipes. However, SEDCAD modeling submitted for <br />this culvert outputs a result of 1-18 inch pipe. Also, the submitted modeling appears to have <br />utilized the design flow of 10.6 cfs rather than the peak flow of 21.3 cfs for culvert sizing. <br />1. Given that the design event is a 100 yr-24hr event as required by County road designs please <br />either, explain your rationale for using the lower design event, or submit modeling utilizing <br />the peak flow value for this culvert. <br />Response: Since SEDCAD does not allow for modelling of multiple pipes, the following <br />methodology was used for sizing two culverts: (1) calculate peak flow for the watershed, (2) <br />use one-half of the peak flow as the design flow, (3) size one culvert based on one-half peak <br />flow and (4) indicate two culverts of the same size are adequate for the full peak flow. This <br />methodology has been verified and recommended by Pam Schwab with SEDCAD Support. <br />For C136, the submitted modeling appears to have utilized the design flow of 4.9 cfs rather than <br />the peak flow of 14.7 cfs for culvert sizing. <br />2. Given that the design event is a 100 yr-24hr event as required by County road designs please <br />either, explain your rationale for using the lower design event, or submit modeling utilizing <br />the peak flow value for this culvert. <br />Response: Since SEDCAD does not allow for modelling of multiple pipes, the following <br />methodology was used for sizing three culverts: (1) calculate peak flow for the watershed, (2) <br />use one-third of the peak flow as the design flow, (3) size one culvert based on one-third <br />peak flow and (4) indicate three culverts of the same size are adequate for the full peak flow. <br />This methodology has been verified and recommended by Pam Schwab with SEDCAD <br />Support. <br />AN EQUAL OPPORTUNITY /AFFIRMATIVE ACTION EMPLOYER CRAIG STATION ESCALANTE STATION NUCLA STATION <br />P.O. BOX 1307 P.O. BOX 577 P.O. BOX 698 <br />A Touchstone Energy' Cooperative CRAIG, CO 81 62 6-1 30 7 PREWITT, NM 87045 NUCLA, CO 81424-0698 <br />K970-824-4411 505-876.2271 970.864.7316 <br />