2009-01-26_REVISION - C1981019 (2)

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STATE OF COLORADO DIVISION OF RECLAMATION, MINING AND SAFETY Department of Natural Resources 1313 Sherman St., Room 215 Denver, Colorado 80203 COLORADO Phone: (303) 866-3567 Interoffice Memo DIVISION O F RECLAMATION FAX: (303) 832-8106 MINING SAFETY To: Jim Stark Bill Ritter, Jr. From: Kent Gorham Governor Harris D. Sherman CC: Sandy Brown Executive Director Ronald W. Cattany Division Director Date: January 26, 2009 Natural Resource Trustee RE: Adequacy review, TR-78, Colowyo Mine have completed my review of Technical Revision No. 78 (TR-78) for the Colowyo Mine, permit no. #C-81-019. My comments are as follows. General Comments The cover letter submitted with this revision states "The purpose of this revision is to comply with part five of NOV CV-2008-004 abatement to submit a complete Technical Revision to the Division detailing methods that will be immediately implemented in the Prospect watershed to reduce the peak flow to the Prospect Pond." Based on my conversation with you, Colowyo will also be submitting another revision by the end of March 2009 regarding additional reclamation practices and sediment control measures to be implemented in the Prospect watershed. This revision will propose important reclamation practices and sediment control measures to be implemented in the Prospect watershed during the period that Colowyo will use the internal ditches and temporary sump, as well as long-term measures to be taken to reduce runoff volume and limit peak flows to Prospect pond. I have no concerns with the SEDCAD modeling runs for the three internal ditches as proposed. Input used for the erodible particle size is consistent with a soil fitting a texture classification of a loam/silt loam and would likely be representative of spoil areas with moderate amounts of clay and fine particles. Hydrologic soil cover complex (CN) values used for the three ditch sizing modeling runs are appropriate for land in a significant disturbed condition. Ditch grades for the three ditches that will divert runoff to the East Pit are 1 %, except over short runs where the grade may be steeper. The operator has committed to use channel lining/protection, if necessary, where steeper grades exist to limit downcutting and erosion. More importantly, from a performance perspective, is that the ditches are maintained to have adequate capacity to carry the peak flows, especially the uppermost ditch, Ditch A, that may carry up to 56 cfs during peak flows. Breaching of Ditch A and the potential for overtopping of Ditches B and C downslope could be catastrophic to the Prospect pond. The operator has included two feet of freeboard on these ditches to account for additional flows in excess of what the modeling predicts. The fourth and final SEDCAD modeling run is for storm runoff and snowmelt that will be captured by the sump in addition to runoff from the remainder of the areas that drain directly to Prospect pond, i.e. not diverted into the East Pit via ditches A, B, or C. If approved, this Office of Office of Mined Land Reclamation Denver • Grand Junction • Durango Active and Inactive Mines