2018-10-19_REVISION - M1980244 - Laserfiche WebLink

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Knight Piesold CONSULTING Environmental Department, Meg Burt, Senior Manager October 8, 2018 Cripple Creek and Victor Gold Mining Co. (Newmont) 2.4.3 Hydraulic Analyses Inputs and Assumptions The as-built hydraulic analyses were performed based on discrete cross-section surveys provided by CC&V. The procured cross-section data for each channel and spillway were assumed to be representative of the entirety of the respective structure. The following hydraulic criteria were applied by Knight Piesold: • Freeboard depth (above the design normal flow depth): Larger of 1.0-foot or 25 percent of the design normal flow depth, per the United States Department of the Interior Office of Surface Mining (1982). • Minimum riprap stability safety factor(SF): 1.20, per Knight Piesold standard practice. The Manning's roughness coefficients for the riprap erosion protection were calculated based on the median (D5o) riprap size and the normal flow depths associated with the peak flows for the design storm event in an iterative process. It was assumed that the entire lengths of the channels and spillways are lined with riprap, except where site observations and photos proved otherwise. Based on this information, the existing as-built capacities (flow and erosion protection) were estimated. The as-built flow capacity evaluation was conducted by assessing the maximum possible flow that could pass through the as-built sections, which was then compared to the estimated peak flows from the 100-year/24-hour storm event. The riprap adequacy was evaluated using the peak flows from the 100-year/24-hour storm event that were estimated herein. 2.5 Results and Identified Design Gaps 2.5.1 Diversion Channels and EMP Spillways The HEC-HMS hydrologic model flow routing diagram and peak flow results for the existing diversion channels and spillways are presented on Figure 2.5 and in Table 2.3, respectively. Note that the routing diagram is for the spillways and DC-EMP17a only; routing for multiple elements was not required for the other diversion channels. The as-built hydraulic flow capacity and 100-year/24-hour storm event erosion protection adequacy results are presented in Tables 2.4 through 2.6 for the existing diversion channels, spillway inlet weirs, and spillway chutes, respectively. The tables also present comparisons of the required flow and erosion protection capacities per the design criteria and the corresponding as-built flow and erosion protection capacities. The existing diversion channel results indicate the following (Tables 2.4 and 2.9): • The as-built flow capacity is greater than the required 100-year/24-hour storm event peak flow for the following diversion channels: - DC-EMP8b, DC-EMP13, DC-EMP17a, DC-EMP17b, DC-EMP18N, and DC-EMP20N • The as-built flow capacity is less than the required 100-year/24-hour storm event peak flow and thus, design modifications will be required for the following diversion channels: - DC-EMP16, DC-EMP18W and DC-EMP20S • The as-built riprap safety factor is greater than the required factor of 1.20 based on the 100-year/24-hour storm event peak flow for the following diversion channels: - DC-EMP16 and DC-EMP17b • The as-built riprap safety factor is less than the required factor of 1.20 based on the 100-year/24-hour storm event peak flow and thus, design modifications will be required for the following diversion channels: - DC-EMP8b, DC-EMP13, DC-EMP18W, and DC-EMP20S • The following diversion channels do not currently contain riprap to Knight Piesold's knowledge and thus, design modifications will be required: - DC-EMP17a, DC-EMP18N, and DC-EMP20N 5