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Ryan Pawlowski <br />Colowyo Coal Company <br />0 5731 State Highway 13 <br />Meeker, CO 81641 <br />F. <br />® 3 September 2010 <br />Mr. James R. Stark <br />Environmental Protection Specialist <br />Colorado Division of Reclamation, Mining and Safety <br />Department of Natural Resources <br />1313 Sherman Street, Room 215 <br />Denver, CO 80203 <br />and s <br />RE: Technical Revision No. 87, East Pit Sediment Control Structures and Spoil <br />Grading Map Update, Colowyo Coal Mine, Permit No. C- 1981 -019. <br />Dear Mr. Stark, <br />Enclosed please find two copies of additional materials provided for Technical Revision No. <br />87. These changes result from the field inspection by Kent Gorham and Jim Stark on 18 <br />August 2010 in which all rock ditches constructed during the 2009 Reclamation season <br />were deemed insufficient. <br />• As noted on the index sheet in bold, the only changes to this submittal from the materials <br />previously submitted on August 10, 2010 are a result of the realignment of the lower portion <br />of the North Tributary, East Taylor Ditch. As discussed with Sandy and yourself, it was <br />decided that TR -87 is the appropriate vehicle for making this change in response to the <br />issuance of NOV- 2010 -004. <br />All SEDCAD modeling previously presented in Exhibit 7, Item 14, Part R was redone, even <br />though this change results in fairly insignificant changes. To aid in your review of the <br />hydrologic analysis, the following discussion describes the changes to the model. <br />The lowermost segment (approximately 600 feet) of the North Tributary, East Pit Ditch has <br />been realigned and its grade steepened. This segment is described as "Structure 8" in the <br />SEDCAD model. Its confluence point with the Final East Pit Ditch has also been moved <br />downstream about 200 feet. Overall, the ditch is about 316 feet shorter in length. Finally, <br />the two sub - watersheds immediately adjacent to this segment, formerly designated Sub - <br />watersheds E and F, comprising 23.9 acres of the 711.1 acre total watershed, were split up. <br />However, their combined acreage and assigned runoff Curve Numbers for the various <br />scenarios were maintained unchanged. Times of concentration details and sedimentology <br />parameters for the four new sub - watersheds were also revised to reflect the split. <br />As expected, the revision had an insignificant impact on the flow rates and quantities <br />reporting to Prospect Sedimentation Pond, as reflected in the attached revised introductory <br />text (Pages Exh. 7 -14R -1 to 3). The impacts were limited to flow changes in "Structure 8 ". <br />Specifically, the flows changes as follows for the three time scenarios evaluated: <br />10 Colowyo Coal Company, L.P. • 5731 State Highway 13 • Meeker, CO 81641 <br />T + 1 970 824 1578 • F +1 970 824 1598 - http:// prospect .riotinto.org /irj /portal <br />