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Knight Piesold CONSULTING Environmental Department, Meg Burt, Senior Manager October 8, 2018 Cripple Creek and Victor Gold Mining Co. (Newmont) 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: "Terminal Enhanced Management Practice detention ponds ("EMP Ponds") are constructed for twice the volume of the 10-year/24-hour storm. Implicit within those criteria are several factors to ensure that sediment and water release will be automatically controlled without administrative intervention. For example, the second storm "volume" provides assurance that inevitable accumulations of sediment from previous storm events will not hinder the ability to store the first 10- year storm." 2.3 Hydrologic Analyses Methodologies, Inputs and Assumptions 2.3.1 General Hydrologic analyses were performed to estimate the peak flow rates and runoff volumes that need to be accommodated by the SWMP infrastructure per the design criteria. The results of these analyses were compared to the results of the as-built hydraulic analyses to evaluate adherence to the design criteria. 2.3.2 Hydrologic Analyses Methodologies The hydrologic analyses to estimate the peak flows to the diversion channels and spillways were performed using the United States Army Corps of Engineers (USACE) Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS) precipitation-runoff model (USACE, 2017). The methods used within the model include the Natural Resource Conservation Service (NRCS, 2004) curve number (CN) runoff method and kinematic wave transform and flow routing techniques. The analyses to estimate the runoff volumes to the EMPs were performed using the NRCS (2004) CN runoff method. Modeling is not required to estimate the inflow volumes. 2.3.3 Hydrologic Analyses Inputs and Assumptions Storm Event Depths and Temporal Distribution The design storm event depths were procured from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 8, Version 2 estimates (NOAA, 2013). Specifically, NOAA storm depth estimates from the Cripple Creek Climatological Station were procured using the online Precipitation Frequency Data Server (PFDS). The Cripple Creek Station is located approximately two miles from the centroid of the mine and is thus considered representative of the mine. The following storm event depths were applied to these analyses(NOAA, 2013): • Conveyance structures(i.e., channels, spillways), 100-year/24-hour storm event depth: 4.1 inches. • Storage structures(i.e., EMPs), 10-year/24-hour storm event depth: 2.4 inches. Information downloaded from the NOAA (2013) Atlas 14, Volume 8, Version 2 PFDS is presented in Attachment 2. This information includes precipitation depths for various storm event recurrence intervals and durations. The 100-year/24-hour storm event depth was distributed over 24 hours according to the NRCS (1986) Type II rainfall temporal distribution to estimate the peak flows to the channels and spillways. A temporal rainfall distribution is not required to estimate the inflow volumes to the EMPs. Contributing Basin Areas The current contributing basin area to each existing structure was delineated by Knight Piesold based on 5-foot contour topography and structure layouts provided by CC&V in February 2018. The existing EMP basin delineations are presented on Figure 2.1. The existing diversion channel basin delineations are presented on Figures 2.2 and 2.3. These three figures also present the existing contributing basin area numerical values. CC&V reviewed and approved the basin delineations presented herein. 3