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W <br />, - "" <br />COAL <br />. COMPANY �.� <br />�i'.pEOPLE PRODUCTYOK.1�� <br />Division Comment (Fourth Adequacy) <br />This response is adequate. <br />COMMENT #2 <br />Colowyo is correct in the assumption about the Division's error. In our July 2, 2012 <br />letter, we did mean to state `B soils" rather than "C soils." Thank you for pointing out <br />this error. <br />Colowyo states that in the past "SEDCAD modeling overestimated actual flows <br />witnessed at Colowyo Mine." What is this assertion based on? Does Colowyo have flow <br />data or other information to confirm this? It has been the Division's belief in the past <br />that the models have underestimated flows. This was part of the impetus for TR -73. <br />Colowyo's Response: <br />Colowyo's experience has been that under storm event conditions that are not short <br />durationlhigh intensity events, the model predicts a much greater volume of runoff <br />generation than is witnessed as inflow into the ponds. This suggests that Colowyo's <br />reclaimed areas are intercepting and holding a much greater volume of precipitation <br />than the modeling parameters assume. This phenomenon held to be true utilizing the <br />curve numbers in place prior to the approval of TR -73. Colowyo is required by <br />regulation to design sediment control /stormwater control structures to treat and process <br />the resulting runoff for various defined storm events for 10 year, 25 year, and 100 year <br />24 -hour events, depending on their purpose and intended use. There are many instances <br />when a set of storm conditions can generate stormwater volumes that exceed the design <br />capacities of these structures, and the modeling itself bears this out. This was known <br />prior to the incorporation of these regulations, and it was determined that these defined, <br />prescribed storm scenarios were protective. <br />Promulgated regulations concerning the design, structure, and function of sediment <br />control /stormwater control structures do not mandate or expect operators to design and <br />build structures that would be protective under all storm conditions. Colowyo believes <br />that the structures currently in place are protective to the public under the condition <br />assumptions utilized to design them. In all but two instances to date, the structures have <br />been protective to the public regardless of the size or intensity of storm events in the past <br />several decades. Colowyo contends this fact should be a source of confidence in <br />previous actions and support for reduction in the curve numbers recently increased, <br />rather than a justification for not considering their modest reduction on all areas covered <br />by SEDCAD modeling (with the exception of areas reporting to the Prospect Drainage). <br />Division Comment (Fourth Adequacy) <br />A mining property of Western Fuels- Colorado, A Limited Liability Company <br />