2011-10-04_REVISION - C1981019 - Laserfiche WebLink

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There were no changes to the opening introductory text in pages Exh. 7 -23 -1 through 7 The changes to the remainder of Exhibit 7, Item 23, are briefly summarized below. Corresponding changes to the Table of Contents were also made, but are not described in detail here. Exh. 7, Item 23, Part A, "SEDCAD Modeling" Specifically, a paragraph was added on p. Exh.7 -23A -2 to highlight that two small areas are not permitted for disturbance at this time, but the runoff curve numbers in those two areas were left unchanged, that is conservatively assumed for modeling purposes to still have a disturbed (higher) runoff curve number. All references to the Lower Clean Water diversion were excised, and the Network Sketch 7 -23A was revised to show the watershed network without that diversion. The SEDCAD model results for the 2012 and 2017 time frames were re -done and re- submitted with the eastern watersheds no longer diverted, but reporting to the sediment pond system. Exh. 7, Item 23, Part B, "Little Collom (LCX -IN) Sump Design" was also revised to reflect that the absence of the clean water diversion requires the incised sump to be operated at a slightly lower starting pool elevation, at least in the initial (2012) time frame, to keep 10 year /24 hour storms from engaging the emergency spillway in the Little Collom Pond (LCX - DS) immediately downstream. These changes also resulted in revisions to the peak inflows, outflows and maximum storm surge elevation in the sump, which revisions were incorporated into the Exh. 7 -2313-2 tables in that section. Exh. 7, Item 23, Part C, "Little Collom Pond (LCX -DS) Design" also had to be revised to reflect the absence of the clean water diversion. All discussion of the LCW diversion was excised, and the watershed acreages discussed in the text were revised to now include the no longer diverted 958 acres of the eastern watersheds. As in the case of the sump described above, the changes in the runoff volumes, flow rates and peak pool elevations in the 2012 and 2017 time frames resulted in changes in the two Exh 7 -23C -2 tables in that section. There were no changes required to Exh. 7, Item 23, Part D "Sediment Control for Topsoil Stockpiles. Exh. 7, Item 23, Part E "Temporary Channel Design" was changed, essentially excising any mention of the LCW (Lower Clean Water) Diversion. Reference to that diversion in the trapezoidal channel summary table, Table 7 -23E -2 was also excised. The SEDCAD model run was also excised. No changes were required to Exh. 7, Item 23, Part F, "Geotechnical Analysis- Collom Sediment Ponds ". A small change was made on page Exh. 7 -23G -1 in Exh. 7, Item 23, part G, "Roadside Impoundment" to point out that the two small areas currently shown as not yet approved for disturbance would nevertheless be conservatively modeled by using their higher disturbed runoff curve number, a similar to the parallel comment in Exh. 7, Item 23, Part A. No other changes were required to Part G. No changes were required to Exh. 7, Item 23, part H "Small Area Exemptions." Hopefully this explanation helps in your review of the materials. Please feel free to contact me at any time should you have any additional questions or concerns. Thanks for your efforts Kurt Kurtis R. Blunt Environmental Supervisor Colowyo Coal Company LP Rio Tinto 5731 State Highway 13 Meeker, Colorado 81641 T: 970.824.1531 M: 970.326.8941 F: 970.824.1598 kurtis.blunt(c_riotinto.com www.riotinto.com