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Knight Piesold CONSULTING Environmental Department, Meg Burt, Senior Manager October 8, 2018 Cripple Creek and Victor Gold Mining Co. (Newmont) Discrepancies in runoff CN values were also found (Table 2.8). The major differences are in the natural ground CNs. Steffens (2012) considered the following two CNs for natural ground: • "Grassed existing areas and closure areas reclaimed with growth media and Revegetation methods, Curve Number 71" (Steffens, 2012). Per the NRCS (1986), this value corresponds to the following: - Cover type: Herbaceous("grass") - Hydrologic condition: Fair(30 to 70 percent ground cover) - HSG: B • "Existing wooded areas, Curve Number 66" (Steffens, 2012). Per the NRCS (1986), this value corresponds to the following: - Cover type: Oak-aspen ("woods") - Hydrologic condition: Poor(less than 30 percent ground cover) - HSG: B By way of summary, Knight Piesold generally used higher CNs for "grass" areas and lower CNs for "wooded" areas. There is always some subjectivity in selecting CN values from the standard tables. However, Knight Piesold does not agree with the following Steffens assumptions: • A single HSG classification of B for both "grass" and "woods". Steffens (2012) states that the NRCS soil survey was used to estimate this classification. However, a soil map was not found in the Steffens (2012) report to justify this. The soil map presented herein illustrates that soils with lower infiltration rates (i.e., HSGs C and D) are present in the region of the mine, in addition to HSG B. This is the reason for higher"grass" CNs herein. • A hydrologic condition of "poor" for "woods". Based on Knight Piesold's assessment, there is greater than 30 percent ground cover. This is the reason for tower"woods" CNs herein. In addition to the individual CN value discrepancies, the composite CN value discrepancies are likely due to varying assumptions on total contributing areas and delineations of cover type areas. Steffens (2012) does not appear to present a map of the cover type and HSG area delineations, so a comparison to Figure 2.4 herein could not be performed. The Steffens (2012) EMP design storage volumes are presented in Table 2.7 for comparison to the 2x the 10-year/24-hour volumes and currently as-built available volumes estimated herein. The following discrepancies are noted: • EMPs 9a-d (combined): The current as-built volume is larger than the Steffens (2012) design volume, but still insufficient based on the existing SWMP results herein. • EMPs 13, 16, 17, 18, 20, 21, and 22: The current as-built volumes are smaller than the Steffens (2012) design volumes. This is likely due to sediment accumulation over time. Knight Piesold could not locate information on design riprap D50 sizes, required freeboard depths, or required riprap stability safety factors in the previous documentation. Thus, comparisons between channel designs and Knight Piesold's evaluation of existing channels cannot be made. 3.0 MODIFIED (UPGRADED) DESIGN CONCEPTS Knight Piesold has developed upgraded designs for most of the structures that are stated to need such upgrades in the previous Section 2.0. The upgraded designs include the reductions of spillway inlet invert elevations for the EMPs that are currently classified as Jurisdictional Dams such that they will become non-Jurisdictional Dams once the upgrades are constructed. However, the following structures that are stated to need upgrades in Section 2.0 were not redesigned for the following reasons: • EMP 8c: The impoundment volume is stated to need an upgrade. However, that volume increase was estimated based on the current contributing area, which will be significantly reduced in the coming year 8