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Page 2 of 4 <br />known 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 build <br />structures that would be protective under all storm conditions. Colowyo believes that the <br />structures currently in place are protective to the public under the condition assumptions utilized <br />to design them. In all but two instances to date, the structures have been protective to the public <br />regardless of the size or intensity of storm events in the past several decades. Colowyo contends <br />this fact should be a source of confidence in previous actions and support for reduction in the <br />curve numbers recently increased, rather than a justification for not considering their modest <br />reduction on all areas covered by SEDCAD modeling (with the exception of areas reporting to <br />the Prospect Drainage). <br />Division Comment (Fourth Adequacy) <br />Colowyo has not provided precipitation or runoff data to back up the assertion that the <br />model overestimates runoff (peak flow or volume). It appears that Colowyo's assertions <br />are based on a general knowledge of the site and /or anecdotal information, not on collected <br />data. Please elaborate on Colowyo's experience that the model overestimates runoff or retract <br />the statement that the model overestimates runoff. <br />Prospect pond was built in 1992. If the volume of runoff exceeded the capacity of the pond <br />twice in the time since construction (21 years), it is not obvious to the Division that the <br />return events for the associated storms were necessarily greater than 25 years. Is Colowyo <br />contending that these storms were likely larger (possibly much larger) than the design storm <br />(25 year event)? If so, please provide evidence. Does Colojvyo have any data showing that a <br />storm with a return event of approximately 25 years was successfully contained in one or more <br />of the ponds? <br />Colowyo's Response (Fourth Adequacy): <br />Current pond design requirements do not require a mechanism for direct measurement of inflow <br />from reporting areas, and as such none of Colowyo's ponds are equipped with such devices. <br />Beyond flume measurements taken during a storm event (which would be rare), Colowyo does <br />not collect data that can be directly compared to precipitation station measurements. A <br />reasonable engineering estimate of maximum flow can and has been used in the past to provide <br />additional information when investigating instances where failure or damage to structures has <br />occurred in the past. <br />Based on the precipitation data readily on hand, Colowyo cannot identify a storm event that <br />demonstrates a 25 year 24 -hour event, especially not with the characteristics described under <br />the required modeling hydrograph. <br />After additional research, Colowyo can only document one instance of a storm event runoff <br />volume exceeding the capacity of Prospect Pond, August 9, 2008. There was an incident that <br />occurred on May 16, 1993 that impacted the South Collection Ditch, but Prospect Pond was not <br />impacted by this storm event. Engineering work completed after the August 9, 2008 event <br />estimated an actual peak flow between 210 and 400 cfs, much greater than the anticipated 66 cfs <br />