2023-01-05_PERMIT FILE - C1981019 (13)

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RULE 4 PERFORMANCE STANDARDS ,Aqw . 'K"'Lt;R' ., r#er"d-tw.4t-fi-r;.rrw,W +ia4•'.#tra+:<xb{r.,r, r*.rr 'x'✓::n;^.�n,.r,;•.;;, n�.Ks a3n%/,4,K:4ttOn. 4%' 4.05.5 Sediment Control Measures Sediment control measures to be implemented are shown in Erosion and Sediment Control Structures(Exhibit 7 Item 20). These facilities, consisting primarily of diversion ditches and sedimentation ponds, will be located, constructed and maintained to avoid erosion and increased contribution of sediment load to runoff. Facilities to control sediment are typically installed in areas above and/or below the planned sites of disturbance. "Upstream" facilities, such as temporary diversion ditches and check dams upslope from the mining activities, serve to divert runoff away from the disturbed areas. Because South Taylor mining activities cover the top of the drainages, no upstream facilities are necessary. Temporary diversion ditches below the disturbed area will help collect runoff from disturbed areas and route it into the sedimentation ponds. During active mining, the mining areas will aid in retaining sediment within the disturbed areas by catching water in pits, small depressions and dozer basins, etc. This captured water and sediment will not leave the mining areas. Once reclaimed, the basins will drain as they did prior to mining activities (i.e., historic drainage patterns will be re-established). All temporary diversions will be removed and reclaimed when no longer needed for sediment control in accordance with the Operations and Reclamation Plan described in 2.05.4. Channel lining rock riprap and energy dissipaters will be used when necessary. As stated above, all temporary diversion structures will be seeded and revegetated. Colowyo does not anticipate that there will be any significant excess material resulting from the construction of diversion ditches. None of the diversions will drain into underground mines. 4.05.6 Sedimentation Ponds The location, design parameters, and detailed sedimentation calculations of all sedimentation ponds are presented in Erosion and Sediment Control Structures (Exhibit 7, Item 20). The design plans and specifications for the sedimentation ponds are described in this section. All sedimentation ponds will be located as close as practical to the areas to be disturbed. Steep terrain in the upper basins precludes location of the ponds at the disturbance boundaries, necessitating down-valley locations. Other methods of sediment control will be located on the reclaimed areas; these methods include the use of contour furrowing, contour drainage ditches, chisel plowing, and revegetation. This application contains calculations used to determine runoff volumes and flow rates for the theoretical 10- year, 25-year, and 100-year, 24-hour precipitation events and 50 percent of the probable maximum precipitation (PMP), as well as subsequent sediment volumes. PMP information is required for State Engineer's Office (SEO) requirements for Class II, small to moderate hazard dams. The precipitation data were obtained from the NOAA Atlas 2, Volume 3 for Colorado; soil types were obtained from the Soil Conservation Service, and are shown on the Soils—South Taylor(Map 5C). The ongoing mining activities within each watershed of the permit area will create constantly changing hydrologic conditions. The design models are generally based on a static, theoretical scenario, utilizing SEDCAD 4, which considers the worst-case scenario wherein mine phasing has caused impacts to the entire disturbance area and reclamation has not yet been attained for any areas. Refer to Map 12 for a delineation of the areas used for these modeling purposes as well as the individual maps associated with each SEDCAD run. The dates indicated on Map 12 are for development of the worst-case scenario for hydrologic modeling and are not a definitive schedule for mining and reclamation activities. South Taylor/Lower Wilson—Rule 4,Page 4 Revision Date: 12/20/19 Revision No.: TR-135