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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 /> VIL SUMMARY AND Figure 9. Central Pit CONCLUSIONS <br /> River stations for Figure 8. <br /> Varra Companies, Inc. is interested in mining Two <br /> Rivers property - located between the Big Thompson and South Platte Rivers just upstream from <br /> their confluence - for gravel mining. As part of the extraction plan, site-specific analyses were <br /> performed that adopted a dam-breach erosion model (WinDAM C)for estimating head <br /> cutting/erosion of riverside berms from a 100-yr flood including head cut length, depth, width, <br /> and head cutting time progression. <br /> WinDAM C model input data was based on site-specific soils testing, pit extraction plan, <br /> existing site topographic maps, riverside berm width assumptions, a historic flood hydrograph <br /> for the Big Thompson River, the September 2013 flood for the Big Thompson and South Platte <br /> Rivers, and FEMA Flood Insurance Study peak discharges. <br /> Study results indicate that the Northwest and Northeast riverside berms adjacent to the Big <br /> Thompson River have sufficient width(approximately 100 ft)to prevent complete berm failure <br /> and river capture. The Central pit riverside berm of approximately 100 ft(per the site plan) has <br /> sufficient width to prevent complete riverside berm failure and river failure. However for <br /> conservatism, additional riverside berm widths of 150— and 200 ft were evaluated so that <br /> decisions can be made as to the best riverside berm width to prevent river capture combined with <br /> 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 earthen berm along Central Pit is susceptible to overtopping thereby decreasing <br /> the chances of river capture. <br /> Should flooding occur that results in head cutting/erosion of a riverside berm, Varra Companies, <br /> Inc. will immediately restore the damaged area. <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 /> Head cutting/erosion analysis - as with any computer modeling of natural processes -is not an <br /> exact science. However, results provide an estimate and relative comparisons of potential for <br /> head cutting and erosion for design, planning, and decision making so river capture is <br /> minimized. <br /> pg. 23 <br />