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Knight Piesold CONSULTING Environmental Department, Meg Burt, Senior Manager October 8, 2018 Cripple Creek and Victor Gold Mining Co. (Newmont) 4.2 Hydraulics Analyses The hydraulic analyses methodologies, associated input parameters, and design criteria that were used are described in Section 2.4. The detailed hydraulic sizing calculation results for the spillways' inlet weirs and outlet channels (i.e., chutes) are presented in Tables 4.2 and 4.3, respectively. The designs are summarized as follows: • Bottom width: 10 feet(both weirs and both chutes) • Side-slopes: 2.0 horizontal to 1.0 vertical (2H:1V, both weirs and both chutes) • Normal flow depths: 0.5-foot(both weirs), 0.3-foot(both chutes) • Freeboard depths: 1.0-foot(both weirs and both chutes) • Channel depths (normal + freeboard depths): 1.5 feet(both weirs), 1.3 feet (both chutes). Note that the chutes were designed with channel depths of 1.5 feet for consistency. • Riprap D5o: 6 inches(both weirs and both chutes) • Riprap placement thickness(2x D5o): 1.0-foot(both weirs and both chutes) • Geotextile: Non-woven 8 ounce-per-square-yard (oz/sy) layer required beneath all riprap placements to provide better long-term stability of the underlying subgrade material. 4.3 Civil Design The designs presented on Drawings 000 through 220 were completed to an FS-level and are not for construction. The pertinent design data are presented on the drawings themselves via tables and notes. In addition, the Amendment 12 permit boundary is presented on Drawings 050 and 200. The existing impoundment is denoted as EMP-22 on the design drawings. The new second impoundment, located downstream of EMP-22 and denoted as EMP-22b, has been designed to be constructed entirely in cut with side slopes of 2.011:1.0V. Plan and profile views of EMP-22/22b are presented on Drawing 200. The new spillway connecting EMP-22 to EMP-22b is also designed to be constructed entirely in cut. The spillway is to be constructed and lined with riprap and nonwoven geotextile to the bottom of EMP-22b to prevent erosion in the spillway and on the impoundment slopes. Plan and profile views of the upstream spillway, along with a design cross-section is presented on Drawing 210. The new spillway connecting EMP-22b to the downstream environment, designed entirely in cut, will be located at the northwest corner of the EMP-22b impoundment. The spillway will discharge into an existing channel that serves as a bypass to reduce the contributing area to EMP-22. The entirety of the spillway is to be lined with riprap and nonwoven geotextile. The plan, profile, and design cross-section are presented on Drawing 215. Knight Piesold recognizes that the longitudinal slopes for the new spillways are significantly steep and that the DRMS cautions the use of riprap underlain by non-woven geotextile on slopes greater than 10 percent to 12 percent because the geotextile-riprap interface could become unstable during higher flows. Although the DRMS states that no action is required at this time, Knight Piesold notes that the FHWA (1998) Geosynthetic Design and Construction Guidelines state that, in general, "special construction procedures will be required" to provide adequate geotextile-riprap interface stability on slopes steeper than 2.5H:1.OV, or 40 percent. It is Knight Piesold's experience that proper construction methods are critical in this situation, including progressively placing the riprap from the bottom of the slope to the top of the slope to create a buttress effect. Notwithstanding this, the DRMS recommendation to use granular filters instead of geotextile on steep slopes is noted and will be considered in future designs based on field observations. 10