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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 /> water surface elevation intersects head cutting elevation, the sooner it ceases. Refer to <br /> Subsection 3.2.1 for the WinDAM C head cutting/erosion computational process. <br /> Due to uncertainties of head cut/erosion modeling, and per Subsection 3.5, additional riverside <br /> berm widths of 150 ft and 200 ft were also modeled for comparison Figures 5 and 6). Options <br /> are then available for decision making of the best riverside berm width for site planning. Of <br /> course the widest riverside berm width is safest, but may be uneconomical for mining due to less <br /> available gravel. <br /> Assumed 150 ft-wide riverside berm width for Central Pit results summarized in Figure 5. Figure <br /> 5 indicates that Best soil parameter case results in head cutting/erosion approximately 4 feet from <br /> the pit bottom with a remaining riverside berm length of 120 feet from the river. <br /> Assumed 200 ft-wide riverside berm width for Central Pit results summarized in Figure 6. Figure <br /> 6 indicates that Best soil parameter case results in head cutting/erosion approximately 5 feet from <br /> the pit bottom with a remaining riverside berm length of 171 feet from the river. <br /> Based on the methodology, assumptions, and data of this study, none of the riverside berm <br /> widths evaluated breach through to the river. Of course a 200-ft riverside berm is the most <br /> robust, but a 100-ft riverside berm is still robust yet most practical for maximum extraction. <br /> Modeling results are intended to provide confidence that the riverside berm widths selected will <br /> not head cut/erode to such a degree that a riverside berm will breach. <br /> pg. 16 <br />