2018-04-27_REVISION - M1980244 - Laserfiche WebLink

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Knight Piesold CONSULTING Environmental Department, Meg Burt, Senior Manager April 27, 2018 Cripple Creek and Victor Gold Mining Co. (Newmont) Most of the EMPs and some of the Best Management Practices (BMPs) and diversion channels were inspected during the site tour. However, the ground was largely covered with snow at the time, which inhibited a thorough inspection. Because of this, the evaluation was largely performed based on information provided by CC&V, including survey data, photos, and previous documentation. 2.0 DETAILED REVIEW OF EXISTING SWMP LAYOUT 2.1 Facility Layouts The existing SWMP general arrangement is presented on Figure 2.1. In general, the diversion channels convey stormwater to the EMPs, which are located at the perimeter of the mining infrastructure. Knight Piesold understands that the EMPs do not collect process-affected or potentially-acid-generating (PAG) runoff and that sediment is the only constituent within the stormwater runoff. The day-to-day and month- to-month water that is accumulated in the EMPs (i.e., live volumes) infiltrates and/or evaporates, per CC&V, but a water balance has not been performed to estimate the quantities or timings of accumulated water volumes. Discharge via pumping does not occur from the impoundments. The accumulated sediment volumes are removed from the impoundments as necessary by CC&V. As part of this review, the following existing structures were evaluated to the maximum practical extent, based on survey information provided by CC&V(reference Figure 2.1): • EMPs and associated spillways (where existent): 6, 8a-c, 9a-d, 11, 13, 16, 17, 17a, 17b, 18,20, 21, and 22. • Diversion channels (DC): DC-EMP8b, DC-EMP13, DC-EMP16, DC-EMP17a, DC-EMP17b, DC-EMP18N, DC-EMP18W, DC-EMP20N, DC-EMP20S. Note that the names of the diversion channels signify the EMP that they discharge to. • East CressonNVild Horse Overburden Storage Area (ECOSA) facility toe berm. Note that this structure is not classed as an `EMP'and is thus, not subject to the same design criteria. However, the berm was evaluated to assess if sufficient storage capacity exists to detain the runoff volume generated from a single 10-year/24-hour storm event. 2.2 Design Criteria The following design criteria are set forth in the CC&V(2017) SWMP document: • Conveyance structures (i.e., channels, spillways): Convey (capacity) and withstand (erosion protection) the peak flow generated from the 1 00-year/24-hou r storm event. • Storage structures (i.e., EMPs): Store twice (2x) the runoff volume generated from the 10-year/24-hour storm event below the spillway inlet invert elevation. Neither criteria consider rain-on-snow events. Both the 10-year/24-hour and 100-year/24-hour storm events depths have historically been applied to unfrozen ground conditions without considering snow melt. Rain-on-snow events can produce significant runoff volumes and peak flows due to reduced runoff losses and contributions to runoff from snow melt. However, they do not necessarily produce greater runoff volumes and higher peak flows than rainfall-only events because the rainfall depths during rain-on- snow events are typically less than rainfall-only depths. Knight Piesold cannot speculate if rain-on-snow events would dictate the sizing of the structures at CC&V without analyzing the climatic/hydrologic data, but this issue is presented herein for CC&V's consideration. This subject was discussed with CC&V personnel, who directed Knight Piesold to consider unfrozen ground conditions only. The storage design criterion for the EMPs does not directly consider a specified dead volume for sediment accumulation. The dead volume for sediment is not a significant issue because CC&V removes the accumulated sediment volumes within the ponds as often as necessary. Furthermore, Steffens and Associates, Inc. (Steffens, 2012) states the following in the supporting documentation for the Mine Life Extension (MLE)2,Amendment 10: 2