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RULE 4 PERFORMANCE STANDARDS <br />. 4.05.6 Sedimentation Ponds <br />The location, design parameters, and detailed sedimentation calculations of all planned sedimentation <br />ponds are presented in Erosion and Sediment Control Structures (Exhibit 7, Item 20). The design plans <br />and specifications for the sedimentation ponds are described in this section. All sedimentation ponds will <br />be located as close as practical to the aeeas to be disturbed. Steep terrain in the upper basins precludes <br />location of the ponds at the disturbance boundaries, necessitating down-valley locations. Other methods <br />of sediment control will be located on the reclaimed areas; these methods include the use of contour <br />furrowing, contour drainage ditches, chisel plowing, and revegetation. <br />This application contains calculations used to determine runoff volumes and flow rates for the theoretical <br />10-year, 25-year, and 100-year, 24-hour precipitation events and 50 percent of the probable maximum <br />precipitation (PMP), as well as subsequent sediment volumes. PMP information is required for State <br />Engineer's Office (SEO) requirements for Class II, small to moderate hazard dams. The precipitation data <br />were obtained from the NOAA Atlas 2, Volume 3 for Colorado; soil types were obtained from the Soil <br />Conservation Service, and are shown on the Soils -South Taylor (Map SC). <br />The ongoing mining activities within each watershed of the permit area will create constantly changing <br />hydrologic conditions. The design models are generally based on a static, theoretical scenazio, utilizing <br />SEDCAD 4, which considers the worst-case scenario wherein mine phasing has caused impacts to the <br />entire disturbance area and reclamation has not ye[ been attained for any areas. Refer to Map 41A for a <br />delineation of the areas used for these modeling purposes as well as the individual maps associated with <br />each SEDCAD run. The dates indicated on Map 41A are for development of the worst-case scenazio for <br />hydrologic modeling and are not a definitive schedule for mining and reclamation activities. <br />It is Colowyo's contention that the proposed models represent nothing more than our best hydrologic <br />estimates for a described worst-case condition. The intent of the modeling is to aid in the design of <br />sedimentation ponds to predict compliance with applicable effluent standards. A primary limitation of the <br />modeling and subsequent designs is the available existing topography, which is very coarse at a 25-ft <br />interval. Colowyo believes it would be an inappropriate use of the SEDCAD models to use them as an <br />enforcement tool for such operations as topsoil stripping; backfilling, grading, reclamation, etc. <br />Furthermore, more detailed topography must be obtained [o verify results prior to implementation. <br />The scenario used for the sedimentation ponds corresponds to an active, disturbed operation. In terms of <br />groundwater, Colowyo's pits have remained essentially dry. Pumping of pit water (precipitation induced <br />surface runoff) into sedimentation ponds is not anticipated. Discharges from the ponds will remain in <br />compliance with Colowyo's CDPS Discharge Permit. The use of flocculants in sedimentation ponds may <br />also be used in accordance with the provisions of the CDPS Permit. <br />• <br />South Taylor/Lower Wilson -Rule 4, Page 9 Revision Date: 11/30/06 <br />Revision No.: PR-02 <br />