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EXHIBIT 7, ITEM 20, PART A <br />SECTION 28 SEDIMENTATION POND <br />The location of the Section 28 Sedimentation Pond is presented on Map 41A. The profiles of the <br />associated permanent ditches (Section 28 East and West Tributary Ditch) are presented on Map <br />33B. Channel designs of these permanent drainage channels are provided in Exhibit 7, Item 2Q <br />Part E. Exhibit 7, Item 20, Figure lA-1 shows the breakdown of drainage areas and hydrologic <br />conditions for this sediment pond, and Figure lA-2 provides a diagram of the structures and <br />networking. A design drawing for the impoundment is presented on Figure lA-3. <br />To minimize the size of the structure needed, a variety of control features will be utilized. Two <br />short-term sediment traps will be utilized along the edge of the topsoil replacement limit. It is <br />expected that these sediment traps will be relocated as the topsoil replacement progresses. Thus <br />the life of these structures will generally be less than one year. These sediment traps will be <br />subgrade (excavated by dozing) and will have a maximum capacity of 2.0 acre-feet, The <br />sediment traps will typically be rectangular in shape with a maximum depth of 10 feet and a <br />maximum of 2H:1 V side slopes. They will be evacuated by pumping when needed. The design <br />also includes the construction of two permanent stock ponds in this drainage basin as shown. <br />Each will have a maximum capacity of 0.50 acre-feet and will be similar in nature to those <br />currently constructed. <br />. The following pages present the results of three SEDCAD runs for the worst case hydrologic <br />conditions. The first run provides the results of the 10 year, 24 hour design storm. This design <br />event generated about 37 cfs of peak runoff into the pond and a total runoff volume of about 3.5 <br />acre-feet under these conditions. Further information regazding the design assumptions and the <br />model inputs are documented in the introductory text of Exhibit 7, Item 20. Note that these <br />design assumptions are identical to the assumptions provided in at the front of Volume 2D <br />(Introductory text) for the 10 existing sediment ponds at the Colowyo Mine. The second model <br />run documents the results of the 100 yeaz, 24 hour design event, and the final model run shows a <br />maximum event (50% of Probable Maximum Precipitation) for adequacy of spillway design. <br />Table 4 of the introductory text of Exhibit 7, Item 20 provides a summary of the design <br />parameters for this sediment pond. <br /> <br />Exh. 7-20A-1 Revision Date: 3/29/07 <br />Revision No.: PR-02 <br />