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2011-06-20_REVISION - C1981019 (4)
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2011-06-20_REVISION - C1981019 (4)
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Entry Properties
Last modified
8/24/2016 4:34:42 PM
Creation date
6/20/2011 11:20:00 AM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
C1981019
IBM Index Class Name
Revision
Doc Date
6/20/2011
Doc Name
Revised 3/28/2011 Exhibit 7 Item 23
Type & Sequence
PR3
Media Type
D
Archive
No
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EXHIBIT 7, ITEM 23, PART A <br />COLLOM SEDCAD MODELING <br />Runoff from Little Collom Gulch will be treated in two facilities during the early, worst -case phase of <br />operations. One, the Little Collom Sump (also referred to as the LCX -IN in the model), is an excavated <br />sump within the Little Collom X pit, located at the bottom of the Collom syncline. The other, Little <br />Collom Pond (also referred to as the LCX -DS in the model), is an embankment dam containing slightly <br />less than 20 acre -feet of storage, and located downstream of the mine operations. It operates as a flow - <br />through pond, drained by a perforated riser and floating siphon. <br />The location of the Little Collom sedimentation ponds is presented on the included Exh. 7 -23, Figure Al. <br />Channel designs of these permanent drainage channels are provided in Exhibit 7, Item 23, Part E. Exhibit <br />7, Item 23, Figure Al shows the breakdown of drainage areas and hydrologic conditions for the two <br />sediment ponds. Revisions in the mine plan that exclude mining in the SEA of Section 24 and the SWA <br />of Section 19 allow for a clean water diversion of undisturbed lands in the eastern unnamed tributary of <br />Little Collom Gulch is now included. This removes approximately 916 acres of watershed that would <br />otherwise be tributary to the downstream sediment pond. <br />A design drawing for the impoundments is presented on Figures 1 and 2 of Exhibit 7, Item 26. <br />The following pages present the results of the SEDCAD run for the worst case hydrologic conditions. <br />The results of the 10 year, 24 hour storm are presented. The 25 -year storm and the 100 year storm were <br />also analyzed, but the SEDCAD model output is not presented herewith pursuant to CDRMS instructions. <br />Further information regarding the design assumptions and the model inputs are documented in the <br />introductory text of Exhibit 7, Item 23. <br />Parts B and C of Exhibit 7 -23 provide a summary of the design parameters for the two sediment ponds, <br />the Little Collom Sump (LCX -IN) and the Little Collom Pond (LCX -DS). <br />The ongoing mining activities within each watershed of the permit area will create constantly changing <br />hydrologic conditions. The design models are generally based on a static, theoretical scenario, utilizing <br />SEDCAD 4, which considers the worst -case scenario. The worst -case scenario occurs approximately 6 <br />months after the start of mining, when topsoil has been stripped and stockpiled for all of Little Collom X, <br />and portions of Collom Lite, but no mining has yet occurred in Collom Lite. Reclamation has not yet <br />been attained for any areas. The 6 -month disturbance boundary indicated in Exhibit 7 -23, Figure Al is <br />for development of the worst -case scenario for hydrologic modeling and does not represent a definitive <br />schedule for mining and reclamation activities. After this initial 6 month period, the Collom Lite pit will <br />be sufficiently developed to contain runoff within it, effectively reducing the drainage area being served. <br />Once the Collom Lite Pit is active, and after the temporary spoil pile has been started, the upstream <br />watershed will at least in part be intercepted by the pit, and revegetation of the Little Collom X activities <br />will be in progress. Because the total direct runoff reporting to the sediment control ponds would be <br />reduced under these conditions, and the areas nearest these facilities would have some vegetative cover, <br />this phase is not considered to be a worst case condition from a sediment control perspective. <br />Later in the mine progression, once the Collom Lite pit has been backfilled and the temporary spoil pile <br />removed, the entire Little Collom Gulch watershed (except for the clean water diversion) will once again <br />be tributary to the Little Collom Sump and the Little Collom Pond. To reduce the direct runoff volume, a <br />series of small "near source" sediment ponds will be constructed in the reestablished drainages in the <br />reclaimed spoils as reclamation pit progresses southward. The generic design for these ponds envisions <br />Exh. 7 -23A -1 Revision Date: 3/25/11 <br />Revision No.: PR -03 <br />
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