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Whetstone <br />Associates Technical Memorandum <br />v. See attached Tables A -1 and A -2 for additional analyses needed for the ponds <br />Cotter response: The sedimentation ponds and pit pool have been re- designed as <br />requested. The modeling assumes the ponds are full at the start of the 100 -yr 24 -hr <br />storm. A scour analysis has been performed to demonstrate that the spillways will <br />provide adequate erosion protection as designed. The design drawings are provided as <br />Plate 2 and Plate 3 in the revised DDP. <br />b. Diversion Channels: The Drainage Design Plan attempts to demonstrate adequate <br />capacity by averaging cross - sections for given existing channels and providing stage <br />tables (e.g., Table 20). This is inadequate for several reasons: a) averaged cross sections <br />do not adequately represent the variability in the current channels (reference the 18 <br />percent difference in channel depth shown in Figure 14), b) Figure 15, along with the <br />aforementioned Figure 14, demonstrate the existing engineered channels are in need of <br />significant maintenance, and c) the Division is unable to adequately assess available <br />freeboard throughout the channel with the high variability in the condition of these <br />channels. For these reasons, the Division requires a redesign of all existing engineered <br />channels so as to conform to standard prismatic geometry and the Operator to commit to <br />the necessary maintenance and /or reconstruction of these channels such that adequate <br />capacity and stability for the 100 year, 24 -hour design peak flow can be assured. The <br />Drainage Design Plan also references the Urban Drainage and Flood Control District <br />Drainage Criteria Manual to justify a flow velocity of 7 ft /s for stability. Tables 22 (p. 49) <br />and 36 (p. 79) indicate the Froude Number (Fr) > 0.8 for all evaluated channels. The <br />UDFCD manual requires the Fr be less than 0.8 in order for flow velocities to be as high <br />as 7 ft /s (reference UDFCD manual Chapter 7; Sections 3.1.3.2, 3.2.3.1, and 4.1.1.1; and <br />Tables MD -2, and MD -3). The following shall be provided in the Operator's response: <br />L Drawing(s) (to scale) showing diversion channel locations, cross- section geometry for <br />construction (minimum design depth included). <br />Cotter response: Plate 3 in the revised DDP shows the engineering design for the PDW, <br />PDE, OPWR_Divl and OPWR_Div2 diversion channels, including channel locations, <br />cross - sectional geometry, and profiles. The existing engineered channels will be <br />maintained or improved to meet these specifications consistent with the JD -7 Pit Mine <br />Drainage Design Plan Implementation Construction Schedule to be issued by DRMS. <br />ii. Hydraulic analyses evaluating the design capacity and stability of each diversion <br />and /or collection ditch to pass the 100 year, 24 -hour design storm peak discharge. The <br />DRMS's criterion for maximum flow velocity in earth -lined channels is S ft /s. The DRMS <br />requires channel freeboard be a minimum of 0. S feet unless the velocity head (vl 12g) is <br />significant, then the minimum required freeboard is half the velocity head, or v1 14g. <br />Please provide the requisite hydraulic analyses for all pit and waste rock diversion <br />channels. <br />Cotter response: The revised DDP contains hydraulic analysis for improved diversion <br />ditches PDW, PDE, OPWR Divl and 0PWR_Div2. All segments are demonstrated to <br />4149A.140207 4 <br />