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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 /> 3.1 Develop a Representative Head cutting/Erosion Scenario <br /> Because a large flood will completely inundate the site, most any location around a pit rim is <br /> susceptible to head cutting/erosion. For the sake of head cutting analyses, a representative <br /> riverside berm head cutting/ erosion scenario will be applied to Central Pit, and Northwest Pit. <br /> The site was divided into two distinct berm failure scenarios— South Platte and Big Thompson <br /> Rivers. Although the complete site can experience significant flooding, floodplain flows would <br /> be of different magnitudes adjacent to the South Platte and Big Thompson Rivers. Although head <br /> cutting/erosion mechanics are essentially the same, the amount of flood flow from each river <br /> would vary which impacts the head cutting/erosion process and also the pit fill times. <br /> As with any floodplain, flows through site are 2-dimensional and would and would need a 2 —D <br /> river model such as HEC-RAS to determine flow paths and quantities. Such a modeling effort is <br /> very data and time intensive, and beyond scope of this study. Rather, engineering judgment was <br /> applied as needed. <br /> For the South Platte River scenario, only Central Pit was evaluated. And for the Big Thompson <br /> River scenario, only Northwest Pit was evaluated although Northeast Pit is also located along the <br /> river. That is because the failure mechanics for Northwest Pit can be applied to Northeast Pit. <br /> And because Northeast Pit has a much smaller ultimate extraction volume than Northwest Pit, <br /> then if Northwest Pit doesn't have riverside berm failure issues (per the methods and <br /> assumptions of this analysis) neither will Northeast Pit. <br /> 3.2 Perform Head Cutting Analyses to Predict Berm Head Cutting, Length,Width, and <br /> Failure Time (Time From Initial Head Cutting On Pit Side, to Riverbank On Riverside) <br /> Based on the existing extraction plan, riverside berm widths vary from about 100—to- 150 ft. To <br /> demonstrate the riverside berm width adequacy, a head cutting/erosion analysis was performed. <br /> As explained below in Section V, Hard Armoring, the pit locations most susceptible to head <br /> cutting will be armored, but analyses was performed with the intent to demonstrate that site plan <br /> riverside berm widths are sufficient to prevent river capture. <br /> 3.2.1 Head cutting and Erosion Analysis Methodology <br /> A special study was performed to evaluate head cutting progression (length, width, and time) and <br /> erosion for site-specific conditions. The riverside berm was treated as an earthen dam with the <br /> riverside berm width representing the crest and the pit side representing the face. Modeling will <br /> use site-specific soil physical parameters, flood inflow hydrograph, and dam breach state-of-the- <br /> art head cutting/erosion analyses. Analyses only considered bare ground without vegetation. <br /> The National Resource Conservation Service (MRCS)WinDAM C (Windows Dam Analysis <br /> Modules) (National Resource Conservation Service, 2016.) is the model of choice for this study. <br /> WinDam C is a modular software application for the analysis of overtopped earth embankments <br /> (adapted for gravel pit riverside berms in this case) and internal erosion. The model addresses <br /> pg. 11 <br />