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GENERAL COMMENTS <br />Current Pond Design and Operation <br />Colowyo' s first sediment control pond was designed in 1980 with the hydrological modeling <br />performed by Tipton & Kalmbach, Inc. This pond was designed, constructed, and modeled as a <br />containment pond, providing adequate detention (retention) time of not less than 24 hours for the <br />water inflow or runoff from a 10-year, 24-hour precipitation event.' With approval from the CDMG, <br />Colowyo has performed the hydrological modeling and since permitted and constructed an additional <br />nine sediment control ponds all of which aze designed and constructed as containment ponds and <br />operated with manual head gates to meet the applicable effluent limitations.'- Colowyo's sediment <br />control ponds aze designed to provide adequate capacity to contain the inflow and sediment resulting <br />from a 10-year, 24-hour precipitation event or snowmelt of equivalent volume <br />(precipitation/snowmelt event). <br />Under Colowyo's currently permitted "containment" pond scenario, precipitation induced runoff <br />resulting from any size stomt event enters a sediment control pond where suspended solids in the <br />water aze settled out prior to dewatering the pond. Colowyo opens the manually operated headgate <br />to initiate dewatering of the sediment control pond. In concurrence with the WQCD Colowyo will <br />continue to operate and maintain the 10 sediment control ponds as containment ponds utilizing a <br />manually operated headgate to dischazge water.' <br />Pursuant to the conditions. of Colowyo's General Permit No. COG-850017, which Colowyo has <br />operated under and continues to operate under, Colowyo is eligible to claim the Alternate <br />Limitations. In order to claim the Alternate Limitations, Colowyo is subject to Burden of Proof <br />requirements to prove the occurrence of the dischazge and applicable precipitation/snowmelt event. <br />Waiving total suspended solids (TSS) and total iron limitations applies during <br />precipitation/snowmelt events less than or equal to the 10 yeaz, 24 hour event. Waiving TSS, total <br />iron, and settleable solids (SS) limitations applies during precipitations/snowmelt events greater than <br />the 10 yeaz, 24 hour event. All other discharges must meet the primary limitations. <br />As evidenced by Colowyo's 19-yeaz dischazge history and corresponding oversight by the CDMG, <br />Colowyo's sediment control ponds are designed as containment ponds utilizing manual head gates <br />for dischazging and have always been operated as containment ponds. The letter CDMG issued on <br />January 12, 1997 to the WQCD fails to acknowledge this basic fact even though Colowyo's CDMG <br />Permit No. C-81-019 ,under which Colowyo operates, as prescribed and detailed in the permit in <br />Section 4.05.7 Dischazge Structures plainly spells out the operation of containment ponds. <br />'CDMG Permit C-81-019, Volume 2B, Exhibit 7, Table 8 and Attachment 1 which <br />references the SCMRA General Design Criteria for sedimentation ponds. <br />'-A containment pond is a pond which is designed to fully contain the inflow from a <br />10-year 24-hour precipitation event (10-yeaz, 24-hour event). <br />'Letter to R. Atkinson (Colowyo) from R. Shukle, dated May 14, 1996 <br />