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~: <br />~~. <br />Terra Matrix <br />July 16, 1996 <br />Mr. Joe Dudash <br />Division of Minerals and Geology <br />1313 Sherman Street, Room 215 <br />Denver, CO 80203 <br />III IIIIIIIIIIIIIIII <br />999 <br />t <br />~~~t=1vED <br />JUL 18 1996 I' ~' <br />Dlvis~on of Minerals ~ Geology <br />Re: Hayden Gulch Mine, Permit No. C-80-003 <br />Technical Revision for Pond HG-8 Principal Spillway Structure Modification <br />Dear Mr. Dudash: <br />Enclosed, please find two copies of a Technical Revision (TR) request for H-G Coal Company's <br />(HGCC), Hayden Gulch Mine. HGCC is submitting a discharge structure design change which will <br />allow maintainenance of a permanent operating water level in pond HG-8. <br />The dischazge structure modification consists of installing a 4 inch HDPE pipe as the primary decant <br />orifice and removing the existing 18 inch gated valve and capping the existing 18 inch CMP outlet <br />barrel. The 4 inch decant pipe will be set with an invert elevation of 7,384.8 feet. SEDCAD <br />modeling was utilized to determine the elevation and size of the decant orifice to ensure that the <br />pond, with the proposed permanent water level, maintains adequate storage to detain 10-year, 24- <br />hour storm event inflows and that the inflow retention time exceeds 24 hours. <br />As part ofthis TR package, HGCC is including a revised edition of Figure 1, Pond HG-8 Watershed. <br />This map was initially submitted to the Division as part of TR-65, Riprap Evaluation. Subsequent <br />field truthing indicated that a ditch along the eastern boundary of the disturbed mine pit area had <br />inadvertently been omitted from the map. HGCC re-submitted the map in a transmittal dated June <br />7, 1996. During our phone conversation on June 18, 1996 you indicated that the revised map <br />submittal constituted a design change since TR-65 has been approved, and the revised map needed <br />to be submitted as a separate item or with a new TR. HGCC is re-submitting the revised map with <br />Englneering 6 Environmenfel Servltes <br />PO Bax 77xoiB <br />i~75 Pine Grove Road <br />Sreamboai Springs. Colorado 8aa77 <br />Phone 97o B79.6a6o Fax g7o.87q.goq0 <br />Sttamboat Springs Denver Fort Collins Seattle Santiago <br />