2022-07-12_PERMIT FILE - C1981019A (9)

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RULE 4 PERFORMANCE STANDARDS Collom – Rule 4, Page 6 Revision Date: 6/20/22 Revision No.: MR-242 4.05.6 Sedimentation Ponds The location, design parameters, and detailed sedimentation calculations of all planned sedimentation ponds are presented in Erosion and Sedimentation Control Plan (Exhibit 7, Item 23). The design plans and specifications for the sedimentation ponds are described in this section (Part C). 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 Collom Pit disturbance boundaries during the critical early phase of operations, necessitating down-valley locations downstream of the Collom Pit and temporary spoil pile footprint. 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. Colowyo has specificially provided information regarding Rule 4.05.9(7)(a-e) with respect to the construction of sediment ponds on the design drawings. Please see Exhibit 7-23C, Table 1 and Exh. 7-23 Figures C5 through C9. Exhibit 7, Item 23, Part C contains calculations used to determine runoff volumes and flow rates for the theoretical 10-year, 25-year, and 100-year, 24-hour precipitation events, as well as annual sediment volumes. 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 survey map (Map 5D). 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. Please refer to Exhibit 7, Item 23 for a delineation of the areas used for these modeling purposes, the presentation of the assumed worse case scenario, as well as the maps associated with the SEDCAD runs. The scenario used for the sedimentation ponds corresponds to an active, disturbed operation. In terms of groundwater, Colowyo’s pits have remained essentially dry. Discharges from the ponds will remain in compliance with Colowyo’s CDPS Discharge Permit. The use of flocculants in sedimentation ponds may also be used in accordance with the provisions of the CDPS Permit. Sediment will be removed from all sedimentation ponds on an as needed basis or when the sediment level will not allow effective treatment of the runoff resulting from the 10-year, 24-hour precipitation event in accordance with Rule 4.05.2. Quarterly inspections will note the level of sediment in each pond. Ponds will typically be cleaned of sediment when water levels are lowest, and the least amount of precipitation is expected. The removed sediment may be used as topsoil or subsoil if it meets the suitability criteria discussed under Section 2.04.9 or placed in the backfill of the pits. The Division will be notified of this determination if the material is selected as overburden material that can be substituted for or as a supplement to topsoil. All sedimentation ponds will be designed so that the minimum elevation at the top of the settled embankment is at least one foot above the elevation of the water surface in the pond with the emergency spillway flowing at design depth. Colowyo will design, construct, and maintain the sedimentation ponds to prevent short-circuiting to the extent possible. As a general rule, the inflow to the ponds will be at the opposite end from the outflow area. The constructed height of the sedimentation pond embankment will be designed to allow for settling. During construction, a registered professional engineer will ensure that the appropriate embankment height is accomplished. For all sedimentation ponds, the entire embankment, including the surrounding areas disturbed by construction, will be seeded after the embankment is completed, using the