2018-06-25_REVISION - M2011001 (2)

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April 5,2018 Page 4 Adequacy of Flood Hazard Mitigation Measures The reclamation improvements were evaluated for conformance to the UDFCD Guidelines per Comments 14 and 16 of the DMRS First Adequacy Review dated 4-1-2011. The evaluation was based on the As-Built Survey dated 12-19-17 and hydraulic modeling described in the previous sections. The following items do not adequately address the DMRS concerns in Comments 14 and 16. • The UDFCD Guidelines require riverbank protection in areas where river currents will have a tendency to erode the river banks and the riverside berm is less than 400 feet in width. 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 Cache La Poudre river at this location clearly has the potential for eroding the banks, altering the channel location and capture of the mining pit. The existing berm has widths as narrow as 150 feet on the west side, 95 feet on the north side and 180 feet on the east side. The UDFCD Guidelines require the both the riverbank and the pitside to be protected with riprap for riverside berm widths of 150 feet in areas of unstable (erosive)river conditions. The lack of erosion protection for the riverside berm is a major factor in the repeated breaching of the berm. • The spillways do not meet the size requirements of the UDFCD Guidelines for a 27.2 acres impoundment with a riverside berm 5,400 feet in length. The UDFCD requires a spillway width of at least 100 feet for the conditions at the Iverson Mine reclamation site. The existing trapezoid spillways have a bottom width of 34 feet on the inlet spillway and 26 feet on the outlet spillway. The discharge through the spillways as shown in Table 1 during flood events up to the 50-Year Flood are inadequate to equalize the water levels between the river and the lake in a reasonable amount of time. High differential heads between the river and lake result in erosive velocities of flows overtopping the riverside berm and possible breaching of the berm. The quality of the construction of the reclamation improvements was evaluated in view of the long term stability of the site. John Allis PE,of TZA Water Engineers,Alan Hill of Hill&Pollock,LLC and Tim Iverson, landowner, visited the project site on November 8, 2017 and observed the existing spillway structures and the site grading completed to date. A topographic survey was completed by Lamp Rynearson & Associates on December 19, 2017 to document the as-built condition of the site. The topographic survey is attached as Exhibit A. The following observations are made as a result of the inspection and survey. • The grading of the riverside berm at the northwest corner of the lake is not uniform and has several low areas. The general location of the low areas is shown in Figure 1. The low areas range in elevation from 4685.1 to 4685.4. This area was reported to have been breached during the floods of September 2013 and May 2014 and is highly subject to erosion during flood events. The 2-Year Flood elevation at this location is 4685.3 which is approximately equal to the ground elevation at the low points. Flood events greater than the 2-Year Flood will overtop the riverside berm and could result in additional breaches at the northwest end of the lake.