2025-06-30_PERMIT FILE - C1981019 (5)

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RULE 4 PERFORMANCE STANDARDS Rule 4 Performance Standards 4-11 Revision Date: 4/9/25 Revision No.: MR-261 It is Colowyo’s contention that the proposed models represent nothing more than our best hydrologic estimates for a described range. of conditions. According to Applied -Hydrology and Sedimentology for Disturbed Areas, Barfield, Warner and Haan, 1981, “Hydrologic calculations are estimates, with the error in these estimates increasing as the degree of approximation increases or as the estimation procedure is applied beyond the range of conditions for which it is intended.” Accordingly, it would be inappropriate for these modeling scenarios to be used or relied upon other than for their intended purpose: as an aid in the design of sedimentation pond structures to predict compliance with applicable effluent standards. Colowyo believes it would be an inappropriate use of the SEDCAD models to use them as an enforcement tool for such operations as topsoil stripping; backfilling, grading, reclamation, etc. 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 will be used as topsoil or subsoil if it meets the suitability criteria discussed under Section 2.04.9. 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 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. The active upstream side of the embankment where water will be impounded will be riprapped or otherwise stabilized, where necessary. Areas in which revegetation is not successful or, where rills and gullies develop, will be repaired and revegetated. Sedimentation ponds that are subsequently approved as part of the post-mining land use will remain in place after the termination of mining activities. Colowyo will inspect the condition of each pond and submit a quarterly report. None of Colowyo’s ponds meet the size criteria of 30 CFR 77.216(a)(1989). A small area exemption for sediment control will be utilized for sediment control for the remaining reclamation area in the Gulch A watershed that has not achieved Phase II or Phase III bond release. This small area exemption allows for a grass filter to be utilized for sediment control for the small area exemption. Please see Volume 2C, Item 18 for the small area exemption demonstration (grass filter) for the Gulch A watershed.