2018-04-27_REVISION - M1980244 - Laserfiche WebLink

Home Browse Search

Knight Piesold CONSULTING Environmental Department, Meg Burt, Senior Manager April 27, 2018 Cripple Creek and Victor Gold Mining Co. (Newmont) The ECOSA toe berm currently has sufficient capacity to store the runoff volume from a single 10- year/24-hour storm event. This was deemed acceptable and further analyses were not performed for this structure. 2.5.3 Comparisons to Previous Estimates This section presents comparisons of key parameters between estimates made by Steffens and Knight Piesold. Information from the Steffens (2012) document is compared to the information presented by Knight Piesold herein. The Steffens (2012) design report used a 10-year/24-hour (10-yr/24-hr) storm event with a depth of 2.7 inches, and a 100-yr/24-hr storm event of 3.5 inches. The storm events used in the Steffens report were sourced from the NOAA Atlas 2 Volume 3, which was last revised in 1973. More recent analyses have been performed in the NOAA Atlas 14 Volume 8, which supersedes the NOAA Atlas 2 Volume 3. NOAA Atlas 14 Volume 8 used increased data quality control and a longer period of record and reported a 10-yr/24-hr storm event depth of 2.4 inches and a 100-yr/24-hr storm event depth of 4.1 inches for the Cripple Creek station near the mine site. Knight Piesold was able to independently corroborate the storm depths using the records from the Cripple Creek Station, as well as with data from the on-site weather stations. Comparisons of contributing basin areas and runoff CNs are presented in Table 2.8. There are some significant discrepancies in the basin areas to the EMPs. The following potential justifications were made, which were largely confirmed by CC&V: • EMP 8a: Knight Piesold estimated a smaller area because the previous inflow diversion channel that Steffens considered was removed. • EMPs 9a-d (combined): Knight Piesold estimated a larger area because a new inflow diversion channel was constructed that Steffens did not consider. • EMP 13: Knight Piesold estimated a larger area because Steffens did not include the area up-gradient of the road within the catchment. • EMPs 16, 17, and 20: Knight Piesold estimated smaller areas because Steffens included the ECOSA waste rock area to these EMPs. CC&V directed Knight Piesold to assume runoff from the waste rock area will be detained by the toe berm. To this end, Knight Piesold estimated the required size (volume) of the toe berm herein. • EMP 21: Knight Piesold estimated a smaller area but a robust justification for the larger Steffens area could not be found.This could be simply due to differences in topography. 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