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Two Rivers Gravel Pit Mine <br /> Riverside Berm Failure Analysis <br /> and Flood Control Mitigation Plan <br /> 7ech"olooicess January 22,2020 <br /> TWO RIVERS GRAVEL PIT MINE <br /> BERM FAILURE ANALYSIS <br /> AND <br /> FLOOD CONTROL MITIGATION PLAN <br /> January 15, 2020 <br /> ABSTRACT <br /> Varra Companies, Inc is considering gravel mining on its Two Rivers property (Site)located <br /> between the Big Thompson and South Platte Rivers just upstream from the confluence. As part <br /> of the extraction plan, a riverside berm width needs to be approximated that is sufficient to <br /> prevent head cutting/erosion from capturing the river. Site-specific analyses were performed that <br /> adopted a dam-breach erosion model (WinDAM C)for estimating head cutting/erosion from a <br /> 100-yr flood including head cut length, depth, width, and head cutting time progression. <br /> A conservative philosophy was applied throughout the study. Because of the many <br /> "uncertainties" when evaluating natural occurrences - analytical approaches, scenarios, and <br /> modeling input parameters were biased so that results are conservative (tend toward the most <br /> head cutting and erosion). <br /> Study results indicate that the Northwest and Northeast Pits riverside berms adjacent to the Big <br /> Thompson River have sufficient width(approximately 100 ft)to prevent complete riverside berm <br /> failure and river capture, and the Central Pit riverside berm adjacent to the South Platte River of <br /> approximately 100 ft(per the site plan) also has sufficient width to prevent complete riverside <br /> berm failure and river capture. However for conservatism, additional berm widths of 150 —and <br /> 200 ft were also evaluated so that decisions can be made as to the best berm width to prevent <br /> river capture in addition to maximum extraction of gravel. <br /> Additional "analytical assurance," was performed via an HEC-RAS 100-yr flood water surface <br /> profile analysis for the South Platte River along Central Pit. That analysis indicates that only <br /> about 40% of the riverside berm along Central Pit is susceptible to overtopping, head <br /> cutting/erosion thereby decreasing the chances of river capture. <br /> Information presented in this study is intended to assist design of ultimate pit extents and <br /> design of berm head cutting/erosion protection should study results indicate the necessity. <br /> pg.4 <br />