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Interoffice Memo <br />Date: May 23, 2012 <br />To Rob Zuber <br />Cc: Dan Hernandez <br />From Kent Gorham PI <br />RE: Colowyo Hydrologic Curve Numbers <br />This memo provides my input to your inquiry regarding previous reviews of the curve numbers used in <br />hydrologic modeling at the Colowyo Mine. <br />Rob, you had asked in an email; <br />"While it is odd that Colowyo is proposing to update their CN values again (when they were just <br />changed recently), they do have some basis for the change. The attached files provide some <br />justification, as does the CSU report I mentioned. <br />Dan, my question to you is procedural — do we discourage operators from making frequent changes to <br />such things as model inputs, even if they have a basis? <br />Kent, I would like your take on the CN values themselves. <br />I still have no answer from them on why they want to do this. I will press them again for this answer. <br />Thanks, <br />Rob <br />The attached files referred to in the email text are hydrologic information from the Seneca and Trapper <br />Mines, Seneca located in Routt County and the Trapper Mine located some 20 miles north of Colowyo <br />in Moffat County. Additionally, Colowyo and /or their hydrology consultants had mentioned the (CSU) <br />report titled "Hydrologic and Erosional Characteristics of Regraded Surface Coal Mined Land in <br />Colorado, Striffler and Rhodes, 1981" This report is apparently part of the currently approved Colowyo <br />permit application. <br />Over the last five years, the Division has expressed great concern about the storm runoff situation in <br />the East Pit area of the Colowyo Mine as this large pit area was systematically reclaimed. This area <br />has been the subject of numerous storm runoff events that have caused significant erosion on the <br />disturbed and reclaimed areas of the East Pit, as well as storm runoff and sediment that has <br />jeopardized State Highway 13 below the Prospect pond. Through past permitting actions, notices of <br />violation, on -site meetings, and focused personal communication with Colowyo and their consultants, <br />5/23/2012 1 <br />• <br />