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COAL <br />� COMPANY �� <br />T%.pEOPL C E PAODU V10' ¢ <br />Colowyo's Response: <br />The Prospect Drainage is unique in that over the past 30 years it has been subjected to <br />two very short duration, extremely intense localized storm events that did not meet the <br />criteria for a 10 year 24 -hour storm event as defined by the SEDCAD program. These <br />events were responsible for offsite impacts from runoff generated from the areas <br />reporting to Prospect Pond. <br />Other drainage basins at Colowyo Mine have been subjected to similar or potentially <br />even more intense short duration storm events in the past and have adequately managed <br />the subsequent runoff without off -site impacts. <br />Division Comment (Fourth Adequacy) <br />This response is adequate. <br />Ordinarily, a change in a hydrologic model (say between two different watersheds) would <br />be justified by physical differences on the ground, such as differences in soil types or <br />vegetation, or by new information on the relationship between the inputs and outputs of <br />the model. This second justification could be rainfall and runoff data that allows the <br />modeler to calibrate his/her model. The Division understands that Colowyo's proposal to <br />change the model is not based on such physical factors or data. It is, rather, based on the <br />relative risks of pond failure between Prospect and other watersheds. The risk associated <br />with Prospect Pond failure is much greater than other ponds (in terms of public safety) <br />due to the proximity of Prospect Pond to State Highway 13. Please confirm if this <br />understanding is correct or incorrect. If correct, this is a point worth discussing. <br />Colow oy s Response: <br />Colowyo agrees with the Division, that in the event Prospect Pond should fail in the <br />future, the relative risk to the public would be greater due primarily to its proximity to <br />State Highway 13. <br />The noted justification for NOV CV- 2008 -04 referred to the failure of the structures to <br />properly treat and pass the 10 year 24 -hour event, as the total rainfall received from that <br />storm event was not greater than the 1.8 inches in a 24 hour period that Colowyo utilizes <br />for modeling purposes to define a 10 year 24 -hour event. Colowyo demonstrated during <br />the NOV assessment conference held between the Division and Colowyo, that the <br />intensity of the storm event generated a volume of runoff that exceeded the design <br />capacities for Prospect Pond and its associated structures. In order to approve TR -73, <br />the Division insisted that the curve numbers be raised based on the pre -mine designation <br />of a portion of Colowyo's surface as Class C soils. This led to the utilization of the curve <br />numbers ultimately incorporated into the TR -73 package, not actual on the ground <br />conditions. <br />A mining property of Western Fuels- Colorado, A Limited Liability Company <br />