2015-10-06_PERMIT FILE - C1981019A (3)

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RULE 4 PERFORMANCE STANDARDS The "Fill Placement and Compaction" note in Exh. 7, Item 26, Fig. 1 requires thin horizontal lifts, and presents other generally accepted compaction specifications for earthfill dams. Minimum 1 foot of freeboard when discharging design flood The peak pool elevation when discharging the spillway design flood (25 yr.24 hr storm) is EL. 6643.28 as described in Table 7-23C-2 for "Initial Mining Phase. The crest of the dam is EL. 6649 as shown on Exh. 7, Item 26, Figure 1. The overall embankment height is about 30 feet above existing grade, and with the 95% Std Proctor compaction specified is not expected to settle more than a few inches. Combined settlement of the embankment and underlying foundation soils was estimated as 9 inches or less by Shannon and Wilson in their June 30, 2009 report (Included in Volume 20 of the application). This would leave 5 feet of freeboard while passing the spillway design flood flow. e. Combined upstream and downstream slopes no steeper than 5:1 The slopes shown on Exh. 7, Item 26, Figure I are 2:1 downstream and 3:1 upstream, satisfying the combined slope requirement for 5:1. Additional information regarding permanent post -mine "small impoundment" design can be found in Exhibit 7, Item 23. 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, 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, which considers the worst-case scenario. The worst-case scenario occurs approximately 6 months after the start of mining, when topsoil has been stripped and stockpiled for all of Little Collom X, and portions of Collom Lite, but no mining or sedimentation control installation has yet occurred in Collom Lite, and reclamation has not yet been attained for any areas. Refer to Exhibit 7, Item 23 for a delineation of the areas used for these modeling purposes as well as the maps associated with the SEDCAD runs. The 6 -month disturbance boundary indicated in Exhibit 7, Item 23 is for development of the worst-case scenario for hydrologic modeling and does not represent a definitive schedule for mining and reclamation activities. The scenario used for the sedimentation ponds corresponds to an active, disturbed operation. In terms of groundwater, Colowyo's pits have remained essentially dry. Pumping of pit water (precipitation induced surface runoff) into sedimentation ponds is not anticipated. 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 le iei 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 Collom — Rule 4, Page 8 Revision Date: 9/28/11 Revision No.: PR -03