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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 /> 3.5 If head cut length is not sufficient to prevent river capture assuming a 100-ft-wide riverside <br /> berm, assumed width will be increased and then repeating steps (2)—(4)until a sufficient <br /> riverside berm width is determined. Refer to Section IV, Results. <br /> 3.6 Provide hard armoring in locations most susceptible to head cutting/erosion during <br /> reclamation as an additional measure of safety. This will be discussed in a separate report by <br /> Varra Companies, Inc. Refer to Section V, Mitigating Measures. <br /> IV. RESULTS <br /> Study results are presented below. It is important to note that results apply to a typical pit <br /> riverside berm because it is unrealistic to try and model detailed sections on all riverside berms. <br /> Due to the large amount of possible combinations of parameters for WinDAM C, results are <br /> presented for the "Weakest, Best, and Strongest" soil parameter cases for Northwest Pit. That is, <br /> critical head cutting/erosion parameters are input such that the weakest case results in the most <br /> head cutting/erosion, strongest case results in the least, and best case results are in-between. <br /> Results for Central Pit are presented for Best soil parameter case scenario only because three <br /> different riverside berm widths were evaluated and results would be unnecessarily "busy." That <br /> is because for that scenario, analysis indicates possibility of the riverside berm breaching. Due to <br /> such, riverside berm widths of 150 and 200 ft were also analyzed. Presenting results for all three <br /> scenarios including weakest, best, and strongest case is confusing and not necessary, and results <br /> for best soil parameter case are adequate. <br /> Again, it is important to note that the results presented below have an additional level of <br /> conservatism because they do not consider flood warning time. In an actual flood event, a flood <br /> threat would likely be issued by CWCB as discussed above in Sub-section, 3.3.2. Upon that <br /> issuance, Varra Companies, Inc plans to stop dewatering of active Two Rivers pits by shutting <br /> off dewatering pumps. Without dewatering— and any other inflow—the pits would fill in a lesser <br /> time period. Thus if a flood threat is issued— say 3 days in advance of the flood -the pits would <br /> partially fill thereby decreasing head cutting/erosion and chance of capturing of the river. <br /> 4.1. Central Pit (South Platte River Side) <br /> Assumed 100-ft-wide riverside berm width for Central Pit results are summarized in Figure 4. <br /> Figure 4 indicates that Best soil parameter case results in head cutting/erosion approximately 3 <br /> feet from the pit bottom with a remaining riverside berm length of 35 feet from the river. <br /> The large amount of head cutting/erosion can be explained by two reasons: (1) Central Pit will <br /> have a large volume upon ultimate extraction (5,665 acre-ft)but the inflow hydrograph has <br /> relatively low flows at the beginning for about 3 hours (see Appendix)while the pit is filling, and <br /> (2)the flows are sufficient to result in head cutting but not sufficient to result in the water surface <br /> in the pit to be higher as head cutting is occurring. As previously mentioned, the faster the pit <br /> pg. 15 <br />