2018-06-25_REVISION - M2011001 (2)

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April 5, 2018 Page 3 spillways have concrete cut off walls near the entrance and exits and are lined with riprap for erosion protection. FINDINGS Impacts from Flooding The Iverson Mine has a significant risk for flood overtopping of the riverside berm and river capture of the reclaimed mine pit. The impacts from flooding at the site were evaluated using the FEMA HEC- RAS hydraulic model of the Cache La Poudre River. The model was modified to include as-built cross- sections based on the As-Built Survey dated 12-19-17 of the reclamation area and spillways. The model was used to evaluate the river water surface elevations during flood events, river velocities, the discharges in the spillways and the overtopping depths of the riverside berm. The peak discharge of the September 2013 flood, May 2014 flood and the May 2015 flood were modeled as well as the FEMA Flood discharges. The peak discharges for the flood events in 2013, 2014 and 2015 were taken from USGS gage records at the Cache La Poudre River near Timnath Station No. 06752280. The FEMA discharges were taken from the USACE General Investigations Study,Cache La Poudre River,Greeley Colorado,Appendix B-Hydraulics,dated 2014.The results of the modeling are summarized in Table 1. Table 1 -Results of Hydraulic Modeling West side of Project Site River Sta. 65220 Flood Peak Water Surface Inlet Riverside Description Discharge Elevation Spillway Berm CFS Feet Flow Overtopping NGVD 1929 CFS Depth Feet 2-Year 1780 4685.3 1.4 0 May 12 2015 2640 4686.1 56 0.7 5-Year 2940 4686.3 78 0.9- May 25 2014 3660 4686.6 121 1.2 10-Year 4100 4686.7 139 1.3 Sep 2013 7010 4687.4 246 2.0 50-Year 8360 4687.6 289 2.2 100-Year 11200 4688.2 405 2.8 Table 1 indicates that the riverside berm will be overtopped during all flood events greater than the 2- Year Flood. The hydraulic model indicates river velocities along the riverside berm are erosive ranging from 5 to 8 feet per second during the flood events. The overtopping depth of 2.0 feet during the September 2013 flood(slightly less than the 50-Year Flood)and 1.2 feet during the May 25,2014 flood (slightly less than the 10-Year Flood)breached the riverside berm. The May 12,2015 flood was slightly less than the 5-Year Flood and resulted in washout of the spillways.