2020-07-13_REVISION - C1981008 (2)

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embankment. During normal conditions, the valve will be left slightly open to provide 24- hour detention time for each pond during the large events. This is accounted for as weep holes in the SEDCAD design runs. For Ponds 012 and 013, SEDCAD analysis has shown that the effluent from a slightly open valve still meets effluent requirements. For these ponds,the operator can decide to leave the valve cracked or shut as described above. d. Appropriate combinations of principal and emergency spillways to safely contain or discharge the runoff from a 25-year, 24-hour or larger precipitation events, as required by the DRMS's regulations, will be installed. If a pond is designed to contain the runoff from a 25-year, 24-hour or larger event, an emergency spillway will still be installed. e. Pond freeboard will be a minimum of 1.0 feet from the highest level of water allowed in the pond(or emergency spillway)to the top of dam. f. The top of dam width will be at least 10 feet in all areas. g. All ponds except Pond 018 will have a keyway cut along the entire width of the dam. This keyway will be filled with competent material and compacted to the same standards as the dam foundation. h. All principal spillways will have 2 anti-seep collars installed within the dam which will be at least 2x the pipe diameter. All dam fill will be compacted around the anti-seep collars and principal spillway pipe to 90% of the maximum dry density of the fill material. i. Attachment 2.05.3(3)-15 demonstrates that the given materials on site have been tested for suitability in constructing pond embankments and that the long term slope safety factors exceed 1.3 in all cases, given the pond design parameters outlined above and in the Attachment. For this reason, no individual slope stability analyses are presented for the ponds. The exception to this being Ponds 016 and 018 for which individual slope stability analyses have been provided. Tables 2.05.3(3)-1 and 2.05.3(3)-2 list the values of CN and "k" for different soil and vegetation types. The tables have been revised to address new soils found in the area west of 2700 Road. The curve number(CN) and "k" factor for each of the ponds is determined as a weighted average. The undisturbed area curve number and "k" factor information is based on the soils and vegetation information found within Sections 2.04.9 and 2.04.10, respectively, of the New Horizon Mine permit application. The pond volumes are calculated by first using the Curve Number and Runoff Routine within the Hydrology module of Carlson Software and then using the USLE sediment storage routine within the Hydrology module of Carlson Software. The LS factors for the USLE are determined from the 3D CAD grid model of the drainage areas for each pond. The water volume is determined from the weighted average CN for the watershed based on the worst case conditions over the life of the mine and the appropriate design storm rainfall. Curve numbers used are from 78 to 90 based on the existing conditions, level of disturbance and the hydrologic soil type of each watershed. Once the required volume of sediment and storm inflow is known, these two are combined to get the Section 2.05.3(3) Page 7 July 2020 (TR-99)