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COAL <br />COMPANY <br />4 <br />�•AEO"r -E PRODUCTtOK'E`y£ <br />Prior to modifications approved in TR -73, SEDCAD modeling <br />overestimated actual flows witnessed at Colowyo Mine under the accepted <br />10 year — 24 hour model hydrograph. As the Division is well aware, <br />Colowyo rarely receives a storm event that would conform to this <br />hydrograph. Historically, the only form of storm event that causes <br />structures to become overwhelmed are those that fall into the category of <br />50 to 100 year (or greater) events for six hour (or less) duration that do <br />not generate greater than the 1.8 inches of precipitation characterized by <br />a 10 year — 24 hour event as defined in Colowyo's PAP. Colowyo <br />believes that the Division insisted on utilizing model curve numbers <br />consistent with a pre -mine soils condition of Class C in order to require <br />Colowyo to add significant additional capacity to the Prospect Drainage <br />in order to manage significantly increased modeled peak flows assumed <br />under 10 year — 24 hour model hydrograph conditions. <br />Colowyo has completed construction of the last of five additional <br />"stockponds" that were required in order to add additional capacity, as a <br />result of the modification to the curve numbers approved in TR -73. <br />Colowyo has chosen not to modify the curve numbers associated with <br />reclaimed lands reporting to Prospect Pond through this permitting <br />action. Colowyo believes that the regulatory requirements for hydrologic <br />modeling and design recognize that limitations exist for the responsibility <br />of mine operators to manage stormwater flows generated from reclaimed <br />areas. Mandatory utilization of the 10 year - 24 hour, 25 year - 24 hour, <br />and 100 year — 24 hour hydrographs are meant to define that <br />responsibility. <br />Colowyo believes the information submitted to your office on June 25, <br />2012 is a reasonable and necessary modification that addresses the <br />Divisions original concerns and protects the public from potential impacts <br />of designed flows, both physically and from a water quality perspective. <br />3) DRMS Original Comment 3: <br />Additional information would provide better rationale for altering the <br />.CN values. This could include site specific infiltration tests and /or <br />calibration of existing models using rainfall and runoff data. <br />Colowyo's June 25, 2012 Response to Comment 3 above: <br />A Western Fuels - Colorado, LLC mining property <br />