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RULE 4 PERFORMANCE STANDARDS <br />It is Colowyo's contention that the models represent nothing more than the best hydrologic estimates for a <br />described range. of conditions. According to Applied -Hydrology and Sedimentology for Disturbed <br />Areas, Barfield, Warner and Haan, 1981, "Hydrologic calculations are estimates, with the error in these <br />estimates increasing as the degree of approximation increases or as the estimation procedure is applied <br />beyond the range of 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 in the design <br />of sedimentation pond structures to predict compliance with applicable effluent standards. Colowyo <br />believes it would be an inappropriate use of the SEDCAD models to use them as an enforcement tool for <br />such operations as topsoil stripping; backfilling, grading, reclamation, etc. <br />Sediment will be removed from all sedimentation ponds on an as needed basis or when the sediment level <br />will not allow effective treatment of the runoff resulting from the 10 -year, 24-hour precipitation event in <br />accordance with Rule 4.05.2. Quarterly inspections will note the level of sediment in each pond. Ponds <br />will typically be cleaned of sediment when water levels are lowest, and the least amount of precipitation <br />is expected. The removed sediment will be used as topsoil or subsoil if it meets the suitability criteria <br />discussed under Section 2.04.9. The Division will be notified of this determination if the material is <br />selected as overburden material that can be substituted for or as a supplement to topsoil. <br />All ponds will be designed so that the minimum elevation at the top of the settled embankment is at least <br />one foot above the elevation of the water surface in the pond with the emergency spillway flowing at <br />design depth. <br />Colowyo will design, construct and maintain the sedimentation ponds to prevent short circuiting to the <br />extent possible. As a general rule, the inflow to the ponds will be at the opposite end from the outflow <br />area. The constructed height of the sedimentation pond embankment will be designed to allow for <br />settling. During construction, a registered professional engineer will ensure that the appropriate <br />embankment height is accomplished. For all sedimentation ponds, the entire embankment, including the <br />surrounding areas disturbed by construction, will be seeded after the embankment is completed. The <br />active upstream side of the embankment where water will be impounded will be riprapped or otherwise <br />stabilized, where necessary. Areas in which revegetation is not successful or, where rills and gullies <br />develop, will be repaired and revegetated. <br />Sedimentation ponds that are subsequently approved as part of the post -mining land use will remain in <br />place after the termination of mining activities. <br />Colowyo will inspect the condition of each pond and submit a quarterly report. None of Colowyo's <br />ponds meet the size criteria of 30 CFR 77.216(a)(1989). <br />4.05.7 Discharge Structures <br />The sedimentation ponds at Colowyo are designed to treat the 10 -year, 24-hour storm event in accordance <br />with Rule 4.05.6(3)(a). As such, the general operation of the ponds will be a passive discharge system <br />where water is allowed to discharge automatically as necessary. Colowyo will sample discharges as <br />appropriate to remain in compliance with applicable CDPS Permit requirements. Pond dewatering <br />through a manual headgate may be performed as necessary to lower the water level depending on <br />operational requirements. Manual dewatering of ponds will meet applicable CDPS Permit standards. <br />Discharge from sedimentation ponds will be controlled by energy dissipaters and flow check devices <br />where necessary. All embankment ponds utilize separate principal and emergency spillways with the <br />emergency spillway located at a minimum of 1 foot above the elevation of the maximum water surface <br />during the discharge of the 10 -year, 24-hour storm event through the principal spillway. The principal <br />spillways are designed for the 10 -year, 24-hour storm event and the emergency spillways are designed to <br />Rule 4 Performance Standards 4-10 Revision Date: 1/31/18 <br />Revision No.: TR -119 <br />