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Page 3 of 5 <br />curve numbers in place prior to the approval of TR -73. Colowyo is required by <br />regulation to design sediment control /stormwater control structures to treat and process <br />the resulting runoff for various defined storm events for 10 year, 25 year, and 100 year <br />24 -hour events, depending on their purpose and intended use. There are many instances <br />when a set of storm conditions can generate stormwater volumes that exceed the design <br />capacities of these structures, and the modeling itself bears this out. This was known <br />prior to the incorporation of these regulations, and it was determined that these defined, <br />prescribed storm scenarios were protective. <br />Promulgated regulations concerning the design, structure, and function of sediment <br />control /stormwater control structures do not mandate or expect operators to design and <br />build structures that would be protective under all storm conditions. Colowyo believes <br />that the structures currently in place are protective to the public under the condition <br />assumptions utilized to design them. In all but two instances to date, the structures have <br />been protective to the public regardless of the size or intensity of storm events in the past <br />several decades. Colowyo contends this fact should be a source of confidence in <br />previous actions and support for reduction in the curve numbers recently increased, <br />rather than a justification for not considering their modest reduction on all areas covered <br />by SEDCAD modeling (with the exception of areas reporting to the Prospect Drainage). <br />Division Comment (Fourth Adequacy) <br />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 C'olowyo's ehperience 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 />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 />Colowyo's Response: <br />Probably the best justification for reduction in the current curve numbers, especially for <br />those associated with reclamation areas > three years and older is the performance of <br />the ponds below areas that have received Phase III bond release, namely Gulch A, <br />Streeter, and the West Pit Pond. With the exception of inj7ow from runoff caused by <br />