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COAL COMPANY �o� •A * -pLE. VEk0DDC '0% The Prospect Drainage is unique in that over the past 30 years it has been subjected to two very short duration, extremely intense localized storm events that did not meet the criteria for a 10 year 24 -hour storm event as defined by the SEDCAD program. These events were responsible for offsite impacts from runoff generated from the areas reporting to Prospect Pond. Other drainage basins at Colowyo Mine have been subjected to similar or potentially even more intense short duration storm events in the past and have adequately managed the subsequent runoff without off -site impacts. Ordinarily, a change in a hydrologic model (say between two different watersheds) would be justified by physical differences on the ground, such as differences in soil types or vegetation, or by new information on the relationship between the inputs and outputs of the model. This second justification could be rainfall and runoff data that allows the modeler to calibrate his/her model. The Division understands that Colowyo's proposal to change the model is not based on such physical factors or data. It is, rather, based on the relative risks of pond failure between Prospect and other watersheds. The risk associated with Prospect Pond failure is much greater than other ponds (in terms of public safety) due to the proximity of Prospect Pond to State Highway 13. Please confirm if this understanding is correct or incorrect. If correct, this is a point worth discussing. Colowyo's Response: Colowyo agrees with the Division, that in the event Prospect Pond should fail in the future, the relative risk to the public would be greater due primarily to it's proximity to State Highway 13. The noted justification for NOV CV- 2008 -04 referred to the failure of the structures to properly treat and pass the 10 year 24 -hour event, as the total rainfall received from that storm event was not greater than the 1.8 inches in a 24 hour period that Colowyo utilizes for modeling purposes to define a 10 year 24- hour event. Colowyo demonstrated during the NOV assessment conference held between the Division and Colowyo, that the intensity of the storm event generated a volume of runoff that exceeded the design capacities for Prospect Pond and it's associated structures. In order to approve TR -73, the Division insisted that the curve numbers be raised based on the pre -mine designation of a portion of Colowyo's surface as Class C soils. This led to the utilization of the curve numbers ultimately incorporated into the TR -73 package, not actual on the ground conditions. A mining property of Western Fuels - Colorado, A Limited Liability Company