2014-02-07_REVISION - M1979094HR

Home Browse Search

Whetstone Associatest Technical Memorandum To: Dustin Czapla (DBMS) Glen Williams (Cotter Corporation (N.S.L.)) From: Susan Wyman, P.E., P.G. (Whetstone Associates) Date: February 7, 2014 Subject: Response to DRMS Adequacy Review (4) 4149A 3dS V ONINIH NOI MV-133b :10 NOISIA10 301330 C1131A N01i0Nnr 0NV89 WZ G 0 834 Permit Number M- 1979 - 094 -HR / AM -01 JD -7 Mine GM1333U The Division of Reclamation, Mining and Safety (DRMS) prepared comments on the September 26, 2013, Drainage Design Plan for the JD -7 Mine in a memorandum from Tim Cazier to Dustin Czapla dated January 13, 2014. The Drainage Design Plan was submitted as part of the Cotter Corporation (N.S.L.) (Cotter) response to Adequacy Review #1 to the Mine Permit Application Amendment (AM -01) JD -7 Mine. The January 13, 2014, DRMS comments are in italics below and Cotter's responses are in bold. General Comments: 1. Stormwater runoff estimates or analyses. The DRMS reviewed the hydrologic analyses with respect to diversion channels and sediment ponds, providing the following comments. a. Maps delineating contributing subbasins. The response is adequate with the following exceptions: i. Diversion channels NEChl and NECh2 intercept a significant portion of subbasin NE that is not accounted for in the runoff analysis. Please re- delineate subbasin NE (reference Figure 3) or add a re- delineated Basin B4, and revise the runoff model to ensure the appropriate contributing area is included in the runoff analysis. Cotter response: The entire area of subbasin NE (140.91 acres) was included in the runoff model. The model assumes that runoff from the entire area of subbasin NE reports to the diversion ditch OPWRP_Divl at the upstream end, which conservatively overestimates flow at the upstream end of the OPWRP_Divl channel. As explained in the HydroCAD guidance: "Since the reach routing methodology assumes a single inflow at the upstream end, channels that drain a subcatchment along their length may be better handled by including them as a channel flow segment within the subcatchment time -of- concentration. " NEChl and NECh2 are natural channels, rather than constructed diversion ditches. They are modeled explicitly as reaches in the WinTR55 and HydroCAD2 models. 1 The area of subbasin NE decreases slightly in the revised modeling ( -1.2 acres or -0.8 %) to account for an expanded pit pool in the adjacent subbasin Dl. 2 The Pit Diversion West (PDW) and Open Pit Waste Rock Diversion (OPWRD) are modeled using HydroCAD in the revised modeling to accommodate to the pit pool design, pond storage, and routing. Whetstone Associates, Inc. 104 W Ruby Avenue ♦ Gunnison, Colorado 81230 ♦ Phone 970 - 641 -7471 ♦ Fax 970 - 641 -7431