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Colowyo has not provided precipitation or runoff data to back up the assertion that <br />the model overestimates runoff (peak flow or volume). It appears that Colowyo's <br />assertions are based on a general knowledge of the site and /or anecdotal <br />information, not on collected data. Please elaborate on Colowyo's experience that the <br />model overestimates runoff or retract the statement that the model overestimates <br />runoff. <br />Prospect pond was built in 1992. If the volume of runoff exceeded the capacity of <br />the pond twice in the time since construction (21 years), it is not obvious to the <br />Division that the return events for the associated storms were necessarily greater <br />than 25 years. Is Colowyo contending that these storms were likely larger (possibly <br />much larger) than the design storm (25 year event)? If so, please provide evidence. <br />Does Colowyo have any data showing that a storm with a return event of <br />approximately 25 years was successfully contained in one or more of the ponds? <br />Colowyo's Response (Fourth Adequacy): <br />Current pond design requirements do not require a mechanism for direct measurement of <br />inflow from reporting areas, and as such none of Colowyo's ponds are equipped with <br />such devices. Beyond flume measurements taken during a storm event (which would be <br />rare), Colowyo does not collect data that can be directly compared to precipitation <br />station measurements. A reasonable engineering estimate of maximum flow can and has <br />been used in the past to provide additional information when investigating instances <br />where failure or damage to structures has occurred in the past. <br />Based on the precipitation data readily on hand, Colowyo cannot identify a storm event <br />that demonstrates a 25 year 24 -hour event, especially not with the characteristics <br />described under the required modeling hydrograph. <br />After additional research, Colowyo can only document one instance of a storm event <br />runoff volume exceeding the capacity of Prospect Pond, August 9, 2008. There was an <br />incident that occurred on May 16, 1993 that impacted the South Collection Ditch, but <br />Prospect Pond was not impacted by this storm event. Engineering work completed after <br />the August 9, 2008 event estimated an actual peak flow between 210 and 400 cfs, much <br />greater than the anticipated 66 cfs resulting from the observed pond condition reflecting <br />complete plugging of the primary and secondary outlets, and partial plugging of the <br />emergency spillway. It is reasonable to conclude that the actual storm intensity above <br />Prospect Pond was locally greater than what was captured by the precipitation monitors. <br />COMMENT #3 <br />The condition of Colowyo's mine site will change drastically as reclamation progresses, <br />and compliance with water quality regulations in the past is not an assurance that future <br />conditions at the site will also result in compliance. Therefore, one cannot say that past <br />compliance is justification for lowering curve numbers. <br />A mining property of Western Fuels- Colorado, A Limited Liability Company <br />