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Permit Revision 11(PR-11) <br /> Performance of the Trapper Mine West Buzzard Sediment Control System <br /> West Buzzard impoundments#1, #2 and #3 were originally certified during the year 1987.The design <br /> files from this time period indicate that a watershed area of 472.2 acres reported to this structure.With <br /> the completion of reclamation activities and Phase III bond release in the associated watershed,these <br /> impoundments were either reclaimed (West Buzzard#1 and#2)or left as a permanent impoundment <br /> (West Buzzard#3). <br /> As a part of the PR-11 submittal,Trapper Mine has applied to re-disturb and newly disturb portions of <br /> the West Buzzard watershed in order to access coal reserves in C pit and West I pit. Under the proposed <br /> mining plan, the worst-case disturbance acreage will total 155.6 acres.The total watershed area <br /> reporting to the West Buzzard ponds is 555.4 acres. <br /> In order to provide adequate drainage and sediment control to treat runoff from the proposed <br /> disturbed areas,Trapper will refurbish the prior West Buzzard#3 structure and add West Buzzard#4. <br /> For the worst-case year 2024,the areas and associated curve numbers for the watershed are as follows: <br /> Area Description Acres Curve Number <br /> Active Mining 155.6 87 <br /> Reclaimed—2+Years Growth 190.5 67 <br /> Undisturbed 209.3 49 <br /> Total 555.4 76 <br /> The composite curve number of 76 is a run-off weighted average. <br /> SEDCAD software was used to model the hydrology and sedimentology for this system. In order to <br /> comply with current regulations,the impoundment modeling must result in a 10-year,24-hour peak <br /> settleable effluent concentration less than 0.5 ml/I at the system sampling point(below West Buzzard <br /> #4). In addition,the freeboard resulting from the peak stage of a 25-year, 24-hour event must be at least <br /> one foot at each of the two impoundments. The model was run for a 10-year, 24-hour rainfall event and <br /> a 25-year, 24-hour rainfall event. <br /> A summary of the relevant results is shown below: <br /> West Buzzard#4, 10-year, 24-hour event peak outflow settleable concentration (mill) 0.00 <br /> West Buzzard#3 Top of Dam Elevation 6273.0 <br /> West Buzzard#3 25-year, 24-hour event peak stage 6271.5 <br /> West Buzzard#3 25-year, 24-hour event freeboard (ft) 1.5 <br /> West Buzzard#4 Top of Dam Elevation 6223.0 <br /> West Buzzard#4 25-year, 24-hour event peak stage 6221.5 <br /> West Buzzard#4 25-year, 24-hour event freeboard (ft) 1.5 <br />