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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 /> I. INTRODUCTION <br /> Varra Companies, Inc is interested in mining the Two Rivers property (Site), located between the <br /> Big Thompson and South Platte Rivers just upstream from their confluence, for gravel mining. <br /> As part of the extraction plan, a riverside berm width needs to be determined that is sufficient to <br /> prevent head cutting/erosion from capturing the river, but not in excess so that extraction can be <br /> maximized. Site-specific analyses were performed that adopted WinDAM C (National Resource <br /> Conservation Service, 2016), a dam-breach erosion model, for estimating head cutting/erosion <br /> from a 100-yr flood including head cut length, depth, width, and head cutting time progression. <br /> II. SITE DESCRIPTION <br /> 2.1 Location and Size <br /> The Site - approximately 232.7 acres in size—is located between the Big Thompson and South <br /> Platte Rivers just upstream from their confluence (Figures 1 & 2). It consists of three fields— <br /> Central, Northwest, and Northeast(154.3, 54.9, and 23.5 acres, respectively)that will be mined <br /> in succession. Planned extractions volumes for Central, Northwest, and Northeast Fields are <br /> 5,665, 1,648, and 659 acre-ft, respectively. <br /> pg.6 <br />