2021-08-09_PERMIT FILE - C1981019A (5)

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RULE 4 PERFORMANCE STANDARDS 4,y 4..,.i^+ N+•:s>.rxwA ..4m'84 h.w,.:..�..;.,:a .;avoe urnv^2#"w`'a::: -,1, `trsGF.•`r^,;rako R,,A.^- &^r scenario in each watershed. Please see Map 12 for watershed boundaries used for hydrologic modeling. 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. The scenario used for the Work area pond and loadout ponds correspond to an active, disturbed operation. In terms of ground water; Colowyo's pits have remained essentially dry. Pumping of pit water (precipitation induced surface runoff) into sedimentation ponds is unlikely but, nevertheless should it be necessary to pump,ponds 001, 002, 007; 008, 009 and 010 are designed to handle a small base flow. Colowyo's CDPS Discharge Permit (to be renewed in 1998) will provide for pumping of pit water from both precipitation induced surface runoff and groundwater sources should they be-encountered. Pit pumping rates will be controlled so that there will be no outflow thru the emergency spillway from the combined flows of pit pumping and up to the 10year, 24-hour storm event. Discharges from Colowyo's ponds will remain in compliance with the applicable CDPS Discharge Permit requirements. The use of flocculents 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 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 Rule 4 Performance Standards 4-11 Revision Date: 12/20/19 Revision No.: TR-135