2015-08-07_REVISION - C1981019 (2)

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Mr. Rob Zuber August 3, 2015 Page 6 Response: Colowyo previously addressed a similar comment in an e-mail to Mr. Rob Zuber dated 4/21/15 (TR -105) with the response as repeated below. The values of 145 feet for "L" and 31 % for "S" were selected to approximate the combined LS factor that would apply to a compound slope, in this case a concave slope with a very steep (repose angle) upper part and a very flat lower part (bench). SEDCAD cannot handle a compound slope directly, but a method to do so is described in Chapter 5 of "Hydrology and Sedimentology for Disturbed Areas" by Barfield, Warner and Haan, 1981. The method uses the original 1965 Wischmeier and Smith USDA Handbook No. 282 formula, then adjusts each segment with a factor, and finally provides a weighted average for the compound slope. We applied this method to a 65 foot upper segment at repose angle (77%) followed by an 85 foot flat bench at 1%, resulting in a weighted average LS of 9.0. We then examined several possible combinations of a single L and a single S for input into SEDCAD to arrive at a similar LS factor. We found that a single line with L of 145 feet with a drop of 45 feet, producing a S of 31 % produces a LS of 10. 1, which is slightly higher than the calculated value of 9.0 for the actual compound slope. That line also happens to be close to a simple straight line from the crest of one bench to the crest of the next one lower down the face, so it has some physical relevance to the actual spoil pile face. The L and S of 145 feet and 31%, producing a LS factor about 12% greater than the actual compound concave slope calculated were then conservatively used in the modeling of all spoil pile faces. No changes have been made pursuant to this review comment. ITEM 21. In particle size distribution there is a discrepancy in grain size curve: is 33% less than 0.040 mm or 0.004 mm? There may be a typo on page 7 in Exhibit 7 (revised with TR -95). Please address this apparent error. Response: With regard to the issue of .040 mm appearing in the table on page 7 of Exhibit 7, Methodologies and Assumptions for Sedimentation Pond Design Evaluation, it is correct that it is a typographical error. It will be corrected to 0.004 in the next re -issue of that Exhibit. However, the correct value of 33% finer than 0.004 mm is and always has been used in the SEDCAD 'input model. ITEM 22. In design specs. "borrow " sources should replace "sorrow " sources. Response: This typographical error in the material specifications appeared in Exh. 7-23, Figures C5 through C9, the design drawings for the individual ponds. It has been corrected with this submittal.