2010-12-17_REVISION - C1981019 (122)

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• EXHIBIT 7, ITEM 23, PART A COLLOM INITIAL SEDCAD MODELING Runoff from Little Collom Gulch will be treated in two facilities during the early, worst -case phase of operations. One, the Little Collom Sump (also referred to as the LCX -IN in the model), is an excavated sump within the Little Collom X pit, located at the bottom of the Collom syncline. The other, Little Collom Pond (also referred to as the LCX -DS in the model), is an embankment dam containing slightly less than 20 acre -feet of storage, and located downstream of the mine operations. It operates as a flow - through pond, drained by a perforated riser and floating siphon. The location of the Little Collom sedimentation ponds is presented on the included Exh. 7 -23, Figure Al. Channel designs of these permanent drainage channels are provided in Exhibit 7, Item 23, Part E. Exhibit 7, Item 23, Figure Al shows the breakdown of drainage areas and hydrologic conditions for the two sediment ponds. A design drawing for the impoundments is presented on Figures 1 and 2 of Exhibit 7, Item 26. The following pages present the results of three SEDCAD runs for the worst case hydrologic conditions. The results of the 10, 25, and 100 year, 24 hour storms are presented. The 25 -year storm is analyzed assuming a clogged principal spillway riser in LCX -DS, and a starting water surface at the emergency spillway invert, to provide a conservative spillway design. Further information regarding the design assumptions and the model inputs are documented in the introductory text of Exhibit 7, Item 23. Parts B and C of Exhibit 7 -23 provide a summary of the design parameters for the two sediment ponds. 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. Reclamation has not yet been attained for any areas. The 6 -month disturbance boundary indicated in Exhibit 7 -23, Figure Al is for development of the worst -case scenario for hydrologic modeling and does not represent a definitive schedule for mining and reclamation activities. After this initial 6 month period, the Collom Lite pit will be sufficiently developed to contain runoff within it, effectively reducing the drainage area being served. • Exh. 7 -23A -1 Revision Date: 12/15/10 Revision No.: PR -03