2013-05-28_REVISION - M1977311 (3)

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Response to DRMS Adequacy Review (5) - Cotter SR -13A Mine Reclamation Plan Amendment Cotter Corporation (N.S.L.) ("Cotter ") submits this response to the April 17, 2013 letter from Dustin Czapla, Division of Reclamation, Mining and Safety ( "DRMS ") to Glen Williams, Cotter. The DRMS' comments are in italics and Cotter's responses are in bold. SR -13A Mine, File No. M- 1977 -311, Amendment (AM1) Application Adeg uacy Review (5) 1. Page ESWMP -3, first paragraph. The response to this comment is adequate. 2. Page ESWMP -3, third paragraph. The response to this comment is adequate. 3. Page ESWMP -5, section 7.2. The response to this comment is adequate. 4. Page ESWMP -5, section7.3, second paragraph. The response to this comment is adequate. 5. Page ESWMP -6, second paragraph and FlowMaster output pages. The response to may be adequate with some clarification. The FlowMaster output pages (Attachment 4, pp. 19 - 23) address Channel BB/Section 10 -1. Attachment #4, page 21, and Drawing 2 of 5 (Attachment #6, p. 32) indicate the maximum slope is 12 percent. Pages 22 and 23 (which are identical, save the page number) indicate that a 7 percent slope is the minimum for which rock armor is not required for erosion protection. There is no output for the 12 percent slope with the minimum expected Manning's roughness coefficient, which would be expected for sizing the riprap protection. Drawing 5 of 5 (Attachment #6, p. 35) indicates a 12 -inch thick riprap blanket with a D50 = 8 inches is adequate erosion protection. a. Please confirm Channel B B and Section 10 -1 are the same channel. b. Please provide some discussion as to how the selected 8 -inch DSQ riprap was determined (include methodology) to be adequate and confirm the condition representing the 12 percent maximum slope and the minimum roughness coefficient (n = 0.035) was used for riprap sizing. a. The flow calculation worksheet for the maximum slope (12 %) and minimum roughness coefficient (n = 0.035) was inadvertently left out of the last responses. This worksheet is now enclosed with these responses (see Attachment 1). It is labeled for Section 10-1, which is the same as Cross - Section B -B. b. Riprap was sized using the Federal Highway Administration HEC -11 methodology (Chapter 4: Design Guidelines for Rock Riprap). Copies of the relevant portions of this document are included in Attachment 2. In general, it provides a required D5o size for riprap in a straight channel with known side slopes, flow depth, and discharge velocity. The equation is metric and is as follows: D5o = 0.00594 (V)3 / d" Ki.s where V = channel velocity (meters /sec), d = normal depth (meters), and K is a variable dependent on the riprap side -slope and angle of repose.