2010-09-03_ENFORCEMENT - C1981019 (5)

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1� • The change in tributary acreage is simply the 6.7 acres of the new subdivided sub - watersheds E1 and F2 that are now assumed to report into Structure 8 as opposed to reporting just below it. In that regard, the total acreage of the watershed reporting to structures downstream of this point has not changed. While it can be observed that the flows in Structure 8 have changed, primarily because these 6.7 acres of the 23.9 acres of adjacent watershed now report through Structure 8, the flows are still very small in comparison to the capacity of the ditch, and they have no impact on its design. Should you have any questions or require additional information, please feel free to contact me at (970) 824 -1578. Sincerely, Ryan Pawlowski • Environmental Engineer Encl. 2 copies Cc: file CF 1.1.2.87 Colowyo Coal Company, L.P. • 5731 State Highway 13 • Meeker, CO 81641 T + 1 970 824 1578 • F +1 970 824 1598 • http: // prospect .riotinto.org /irj /portal Drainage Area, Acres Ditch Flow, cfs Time frame August 10 model Current model August 10 model Current model 2010 53.9 ac 60.6 ac 0.68 cfs 2.69 cfs 2012 63.4 ac 70.1 ac 1.28 cfs 2.00 cfs 2017 77.1 ac 83.8 ac 1.64 cfs 2.00 cfs The change in tributary acreage is simply the 6.7 acres of the new subdivided sub - watersheds E1 and F2 that are now assumed to report into Structure 8 as opposed to reporting just below it. In that regard, the total acreage of the watershed reporting to structures downstream of this point has not changed. While it can be observed that the flows in Structure 8 have changed, primarily because these 6.7 acres of the 23.9 acres of adjacent watershed now report through Structure 8, the flows are still very small in comparison to the capacity of the ditch, and they have no impact on its design. Should you have any questions or require additional information, please feel free to contact me at (970) 824 -1578. Sincerely, Ryan Pawlowski • Environmental Engineer Encl. 2 copies Cc: file CF 1.1.2.87 Colowyo Coal Company, L.P. • 5731 State Highway 13 • Meeker, CO 81641 T + 1 970 824 1578 • F +1 970 824 1598 • http: // prospect .riotinto.org /irj /portal