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• This application contains calculations used to determine mnoff volumes and Flow rates for a <br />theoretical 10-year, 24-hour precipitation event and subsequent sediment volumes. The <br />precipitation data were obtained from the NOA Atlas 2, Volume 3 for Colorado; soil types <br />were obtained from the Soil Conservation Service, and are shown on the Soils -South Map <br />(Map 6) and the Soils -North Map (Map 5). <br />The ongoing mining activities within each watershed of the permit area will create constantly <br />changing hydrologic conditions. The design models are generally based on a static, <br />theoretical scenario, utilizing SEDCAD+ Version III which considers a final reclamation and <br />pit closure scenario in each watershed. Refer to Drawing 41 -Watershed Boundaries for <br />Hydrologic Modeling for a delineation of the areas used for these modeling purposes as well <br />as the individual maps associated with each SEDCAD run. The dates indicated on Drawing <br />41 are for development of the worst case scenario for hydrologic modeling and are not a <br />definitive schedule for mining and reclamation activities. <br />It is Colowyo's contention that the proposed models represent nothing more than our best <br />hydrologic estimates for a described range. of conditions. According to Applied -Hydrology <br />and Sedimentology for Disturbed Areas, Barfield, Warner and Haan, 1981, "Hydrologic <br />calculations are estimates, with the error in these estimates increasing as the degree of <br />approximation increases or as the estimation procedure is applied beyond the range of <br />conditions for which it is intended." Accordingly, it would be inappropriate for these <br />modeling scenarios to be used or relied upon other than for their intended purpose: as an aid <br />in the design of sedimentation pond structures to predict compliance with applicable effluent <br />• standards. Colowyo believes it would be an inappropriate use of the SEDCAD models to use <br />them as an enforcement tool for such operations as topsoil stripping; backfilling, grading, <br />reclamation, etc. <br />The scenario used for the Work area pond and loadout ponds correspond to an active, <br />disturbed operation. In terms of ground water; Colowyo's pits have remained essentially dry. <br />Pumping of pit water (precipitation induced surface runoff) into sedimentation ponds is <br />unlikely but, nevertheless should it be necessary to pump, ponds 001, 002, 007; 008, 009 and <br />010 are designed to handle a small base flow. Colowyo's CDPS Discharge Permit (to be <br />renewed in 1998) will provide for pumping of pit water from both precipitation induced <br />surface runoff and groundwater sources should they be-encountered. Pit pumping rates will <br />be controlled so that there will be no outflow thru the emergency spillway from the combined <br />flows of pit pumping and up to the I Oyear, 24-hour storm event. Discharges from Colowyo's <br />ponds will remain in compliance with the applicable CDPS Discharge Permit requirements. <br />The use of flocculents in sedimentation ponds may also be used in accordance with the <br />provisions of the CDPS Permit. <br />Sediment will be removed from all sedimentation ponds on an as needed basis or when the <br />sediment level will not allow effective treatment of the runoff resulting from the 10-year, 24- <br />hour precipitation event in accordance with Rule 4.05.2. Quarterly inspections will note the <br />level of sediment in each pond. Ponds will typically be cleaned of sediment when water <br />levels are lowest, and the (east amount of precipitation is expected. The removed <br /> <br />4.05-6 Revision Date: 12/4/98 <br />Revision No.: TR-41 <br />