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Page 2 of 5 <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 />Colowyo '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 />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 />duration /high 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 />