2013-10-30_REVISION - M1977310 (3)

Home Browse Search

S � Whetston Cq N Whetstone CA Associates4 Technical Memorandum To: Stephanie Mitchel (DRMS) 4148A Glen Williams (Cotter Corporation (N.S.L.)) From: Susan Wyman, P.E., P.G. (Whetstone Associates) 114 Date: October 30, 2013 Subject: Response to DRMS Second General Stormwater Comments Permit Number M- 1977 -310 / AM-O:1 -6 Mine The Division of Reclamation, Mining and Safety (DRMS) prepared comments on the February 26, 2013 Drainage Design Plan for the JD -6 Mine in a memorandum from Tim Cazier to Stephanie Reigh dated March 22, 2013. The Drainage Design Plan was submitted as part of Cotter Corporation (N.S.L.) (Cotter) response to Adequacy Review #1 to the Mine Permit Application Amendment (AM -01) JD -6 Mip_e. The March 22, 2013, DRMS comments are in italics below and Cotter's responses are in bold. 1. The hydrologic method and analyses are very well presented with assumptions stated and substantiated. Sections 1.2 through 2.2.3 and 3.0 through 5.1 exempla what is expected of hydrologic /runoff analyses for EPPs. Cotter response: Comment noted. 2. Page S, section 2.3.1. Trapezoidal channels were assigned side slopes of 1.5H.•1 V or 2H.-1 V. Based on the photographic evidence presented in Figure 9 (p. 25), these values may be appropriate for the channels as constructed. However, steep side slopes are unstable and prone to sloughing that can lead to channel blockage and failure when constructed in erodible material. These channels require modification in design and construction (if already in place). The DRMS requires channel side slopes be no steeper than 3H:1 V, unless it can be demonstrated that the specific material will be stable under the design flow at 2.5H.-I V. a. Please redesign these channels as appropriate b. Please commit to constructing /improving these channels to meet the redesigned configuration to be approved by the DRMS. Cotter response: The existing channels were surveyed in May 2013 to verify the side slope ratios, local bed slope, conveyance capacity, and stability of the Upper, Middle, and Lower Diversion Structures. The survey results are presented in Attachment 4 of the revised Drainage Design Plan (DDP), and indicate that the average slide slope ratios are 2.0:1, 4.8:1, 3.4:1, and 3.1:1 for the UDS, MDS, LDS -E and LDS -N, respectively. Additional stormwater runoff modeling was performed to evaluate channel capacity and stability based on these existing side slope ratios. Additional runoff modeling was Whetstone Associates, Inc. 104 W Ruby Avenue ♦ Gunnison, Colorado 81230 ♦ Phone 970 - 641 -7471 ♦ Fax 970 - 641 -7431