2011-06-20_REVISION - C1981019 (4)

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TABLE 5 POND GEOMETRY LCX -IN (SUMP) LCX -DS POND Elevation (ft) Area (ac) Elevation (ft) Area (ac) 6705 1.29 6625 0.13 6710 1.83 6630 0.61 6715 2.05 6635 1.21 6720 3.02 6640 2.04 6725 3.68 6642 2.54 6735 5.11 6645 3.39 6737 5.36 6646 3.81 6740 5.70 6650 5.69 4.0 POND CLASSIFICATIONS AND SPILLWAY EVALUATIONS The site is located in a rural, agricultural area and meets Class A criteria for dams in the U.S. Department of Agriculture, Soil Conservation Service Technical Release No. 60 (210- VI -TR60, October, 1985). All proposed structures will be either constructed below existing grade or will have an above -grade storage volume of 20 acre -feet or less, with a dam height of 20 feet or less (measured from the upstream toe to the emergency spillway crest) and are therefore exempt from Mine Safety and Health Administration regulations under 30 CFR § 77.216. • The proposed above -grade structures will not be designed or operated for the purpose of impounding water above the natural surface of the ground other than flood detention and are therefore exempt under Rule 17 of the State of Colorado, Rules and Regulations for Dam Safety and Dam Construction. The Little Collom (LCX -D /S) Pond facility will provide adequate capacity to contain or treat the runoff or inflow entering the pond as a result of 10 -year, 24 -hour precipitation event, provide a nonclogging dewatering device or conduit spillway to remove water storage resulting from inflow, and an emergency spillway design to safely pass the 25 -year, 24 -hour precipitation event. The Little Collom Sump (LCX - IN in -pit sump) facility will be operated as an enclosed sump within the Little Collom X pit, and pumped into LCX -DS for polishing as water quality and pond drawdown allows. 5.0 RIPRAP SIZING Unless specifically stated otherwise, riprap sizes were determined using the SEDCAD 4.0 subroutine for designing riprap lined channels utilizing the OSM/Simons Method. SEDCAD also provides another method, the Pader Method, for selecting riprap sizes. For background, both methods are summarized below. The OSM /Simons Method determines the flow induced shear stress on the channel lining and compares this value to the permissible shear stress of the lining material. The permissible shear stress is defined as the lowest shear force which will cause incipient motion of the D50 particle. To ensure a stable channel, the minimum allowable D50 is selected by the program with an internally applied safety factor. The PADER method compares the flow velocity to laboratory determined permissible velocities for particle sizes. The PADER method assumes a specific gravity of 2.65 for riprap (Bureau of Land and Water Conservation, 1996), which is considered appropriate for materials used by Colowyo at the mine site. If the Pader Method were to be used, given the uncertainty in determining riprap sizes for steep Exh. 7 -23 -6 Revision Date: 3/25/11 Revision No.: PR -03