2016-12-22_REVISION - M1980244 (3)

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ERIC CC&V Poverty Gulch Diversion Channel Sizing December 2016 Table 1—Hydrologic Parameters Watershed Longest Flowpath Average Composite Initial Lag Time Peak Area(acres) Length(ft) Watershed Curve Abstraction(in) (min) Flow(cfs) Slope(%) Number 74 2,020 24.7 70.8 0.83 8.8 97 Hydraulic Design Based on the provided topography data,ERC established an approximate 616-foot-long diversion channel alignment that will allow the limits of the diversion to stay between the Amendment 10 disturbance boundary and the planned extents of the Globe Hill Pit perimeter. The plan and profile of the channel alignment is provided in Appendix A of this memorandum.Table 2 describes the profile of the alignment. ERC sized the diversion by inputting this profile along with the peak flow obtained from HEC-HMS into USACE's river analysis program HEC-RAS(v.5.0.3)to size the diversion and evaluate armor requirements. A trapezoidal channel with a 4-foot bed and 2:1 side slopes was selected. Because of the steep slopes of the channel and the corresponding velocities and shear stresses,armoring is required at all locations. ERC determined the Manning's roughness coefficient of the channel and the median riprap rock size(D5o)using the iterative process outlined by the Federal Highway Administration (FHWA 2005). The Manning's roughness coefficient was found to vary between approximately 0.040 and 0.049 throughout the channel, depending on the slope and flow depth of each reach. ERC determined that a D50 of 18 inches will be adequate armoring at all stations of the diversion.A non-woven geotextile fabric filter is required below the riprap for filtering purposes(detail shown in Appendix A).Sizing results for the diversion channel are presented in Table 3.The design depth shown in this table includes%foot of freeboard at all locations. Table 2—Diversion Channel Profile Station Slope (ft/ft) 6+16 to 5+00 0.090 5+00 to 3+00 0.120 3+00 to 1+50 0.140 1+50 to 0+00 0.070 Table 3—Diversion Channel Sizing Results Station Channel Bottom Channel Side Design Channel Top Riprap Riprap Width(ft) Slopes(X:1) Depth(ft) Width(ft) Dso(in) Thickness(in) 6+16to 0+00 4 2 2 12 18 36 3