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might Piesold CONSULTING Environmental Department, Meg Burt, Senior Manager October 8, 2018 Cripple Creek and Victor Gold Mining Co. (Newmont) Runoff Curve Numbers Various NRCS (2004) runoff CNs were estimated and applied to represent the runoff potentials from the varying cover types (i.e., land and material types) present in the contributing areas. The CN values were estimated based on the following assumptions and information: • Antecedent Runoff Condition (ARC) II (i.e., average conditions). • Cover types and hydrologic conditions based on site tour observations and ground and aerial photos. • Hydrologic Soil Groups (HSG) based on information procured from the NRCS online Web Soil Survey and supplemented with site tour observations, ground and aerial photos, and soil gradations estimated by AMEC (2011). Information downloaded from the NRCS Web Soil Survey is presented in Attachment 3. CN estimates were made for the following cover types based on varying hydrologic conditions and HSGs, where applicable: • Natural ground- Herbaceous grass and oak-aspen woods • General disturbed areas(e.g., cut slopes, compacted stockpiles, etc.) and roads • Waste rock The estimated individual CN values are presented in Table 2.1, per NRCS (2004). As shown on Figure 2.4, the existing basin area to each structure was further delineated into sub-components corresponding to the applicable CN classifications presented in Table 2.1. Using the area-weighting technique, a single composite CN value was calculated per basin area. The results of this analysis for the existing EMPs are presented in Table 2.2. CC&V reviewed and approved the curve numbers presented herein. The composite CNs were applied to the hydrologic modeling to estimate the peak flows for the diversion channels and spillways and used to estimate the runoff volumes for the impoundments. In addition, direct precipitation (i.e., no losses) within the EMP impoundment footprints was accounted for in the modeling. 2.4 Hydraulic Analyses Methodologies, Inputs and Assumptions 2.4.1 General Hydraulic analyses were performed on the as-built infrastructure information provided by CC&V to estimate the existing capacities and erosion protection adequacies of the diversion channels, spillways, and EMP impoundments. The results of these analyses were compared to the results of the hydrologic analyses to evaluate adherence to the design criteria. 2.4.2 Hydraulic Analyses Methodologies The hydraulic capacity evaluations of the as-built diversion channels and spillway outlet channels were completed using Manning's equation for normal flow conditions (Chow, 1959). The as-built riprap erosion protection for longitudinal slopes less than 20 percent was evaluated using the shear stress method presented in the United States Federal Highway Administration's (FHWA, 2005) Hydraulic Engineering Circular (HEC) Number 15 based on the estimated 100-year/24-hour peak flows. The methodology proposed by Robinson et al. (1998) was used to evaluate the riprap for longitudinal slopes greater than 20 percent. The as-built spillway inlet weir capacities were evaluated using the standard trapezoidal weir equation. A discharge versus depth (head) rating curve was developed and applied to the HEC-HMS hydrologic model to evaluate the peak inflows to the impoundments, the flow routing effects within the impoundments (i.e., attenuation and lag), and the as-built spillway discharge capacities. It was assumed that each pond would be full to the spillway inlet invert elevation at the onset of the 100-year/24-hour storm event. 4