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Knight Piesold <br /> CONSULTING <br /> Environmental Department, Meg Burt, Senior Manager April 27, 2018 <br /> Cripple Creek and Victor Gold Mining Co. (Newmont) <br /> By way of summary, Knight Piesold generally used higher CNs for "grass" areas and lower CNs for <br /> "wooded" areas. There is always some subjectivity in selecting CN values from the standard tables. <br /> However, Knight Piesold does not agree with the following Steffens assumptions: <br /> • A single HSG classification of B for both "grass"and "woods". Steffens(2012)states that the NRCS soil <br /> survey was used to estimate this classification. However, a soil map was not found in the Steffens <br /> (2012) report to justify this. The soil map presented herein illustrates that soils with lower infiltration <br /> rates (i.e., HSGs C and D) are present in the region of the mine, in addition to HSG B. This is the <br /> reason for higher"grass"CNs herein. <br /> • A hydrologic condition of"poor" for "woods". Based on Knight Piesold's assessment, there is greater <br /> than 30 percent ground cover. This is the reason for lower"woods"CNs herein. <br /> In addition to the individual CN value discrepancies, the composite CN value discrepancies are likely due <br /> to varying assumptions on total contributing areas and delineations of cover type areas. Steffens (2012) <br /> does not appear to present a map of the cover type and HSG area delineations, so a comparison to <br /> Figure 2.4 herein could not be performed. <br /> The Steffens (2012) EMP design storage volumes are presented in Table 2.7 for comparison to the 2x the <br /> 10-year/24-hour volumes and currently as-built available volumes estimated herein. The following <br /> discrepancies are noted: <br /> • EMPs 9a-d (combined): The current as-built volume is larger than the Steffens (2012) design volume, <br /> but still insufficient based on the existing SWMP results herein. <br /> • EMPs 13, 16, 17, 18, 20, 21, and 22: The current as-built volumes are smaller than the Steffens (2012) <br /> design volumes. This is likely due to sediment accumulation over time. <br /> Knight Piesold could not locate information on design riprap D50 sizes, required freeboard depths, or <br /> required riprap stability safety factors in the previous documentation. Thus, comparisons between <br /> channel designs and Knight Piesold's evaluation of existing channels cannot be made. <br /> 3.0 MODIFIED (UPGRADED) DESIGN CONCEPTS <br /> Knight Piesold is currently in the process of developing upgraded designs for most of the structures that <br /> are stated to need such upgrades in the previous Section 2.0. However, the following structures that are <br /> stated to need upgrades in Section 2.0 will not be redesigned for the following reasons: <br /> • EMP 8c: The impoundment volume is stated to need an upgrade. However, that volume increase was <br /> estimated based on the current contributing area, which will be significantly reduced in the coming year <br /> due to the expansion of the Squaw Gulch Valley Leach Facility (VLF) 2. CC&V directed Knight Piesold <br /> to reevaluate this EMP based on the reduced area. Once VLF 2 construction activities commence, the <br /> existing storage and spillway conveyance capacities of EMP 8c will be sufficient to meet the design <br /> criteria. Furthermore,voids in this area could pose a safety hazard during expansion efforts. <br /> • EMP 9a-d: The impoundment volume and spillways are stated to need upgrades. The four spillways will <br /> be redesigned. However, the impoundment volume will not need an upgrade because Knight Piesold is <br /> designing a new diversion channel, denoted as DC-EMP8a, that will divert runoff from the upper part of <br /> the existing EMP 9a-d contributing area to EMP 8a (reference Figures 3.1 and 3.2). This channel will <br /> reduce the contributing area to EMP 9a-d such that its existing capacity will be sufficient and will <br /> increase the contributing area to EMP 8a,which is more amenable to expansion. <br /> • EMP 11: The impoundment volume and spillway are stated to need upgrades. However, CC&V has <br /> decided to reclassify this structure as a 'non-EMP' structure because it does not fit the definition or <br /> purpose of 'EMP' at the mine. It is located in a depression above the Arequa Gulch VLF, not at the <br /> perimeter of the mining infrastructure. Furthermore, the existing capacity is sufficient to store the runoff <br /> volume from a single 10-year/24-hour storm event, and voids in this area could pose a safety hazard <br /> during expansion efforts. CC&V will regularly monitor the sediment accumulation within this structure <br /> and perform sediment removal efforts as often as necessary to maintain adequate storage for the runoff <br /> 4 8 <br />