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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 /> V. MITIGATING MEASURES <br /> Study results presented above indicate that head cutting/erosion will not progress through the full <br /> length of a 100-ft riverside berms. It is important to note that should a flood occur that results in <br /> head cutting/erosion of a riverside berm, Varra Companies, Inc. will immediately restore the <br /> damaged area to pre-flood conditions. <br /> As mentioned, this analysis conservative and riverside head cutting/erosion is based on the 100- <br /> yr flood. There is a small probability that such a flood event will occur during extraction and <br /> when the pit is dewatered. <br /> VI. WATER SURFACE PROFILE ELEVATION FOR SOUTH PLATTE RIVER <br /> ALONG CENTRAL PIT <br /> As an extra measure of confidence that the likelihood for head cutting into Central Pit will be <br /> small, the 100-yr water surface profile was evaluated via HEC-RAS (HEC-RAS). The 100-yr <br /> discharge was obtained from the Weld County FEMA Flood Insurance Study, (Weld County <br /> FIS). Earthen berm elevations were obtained from the Varra Companies, Inc. existing topo <br /> survey (Existing topo), and river cross-section data obtained from Google Earth and existing <br /> topo survey. <br /> Results are presented in Figure 8. Per Figure 8, approximately only 3,600 ft(Station 5200 to <br /> 8800) of the total 8,800 ft(40%) of the pit length along the river would be subject to overtopping <br /> from the 100-yr flood. Thus in addition to WinDAM C head cutting/erosion analysis, the <br /> riverside berm length overtopping analysis indicates that much of the riverside berm would not <br /> overtop thereby minimizing the susceptibility to Central Pit capturing the river. <br /> HEC-RAS program input and output data are presented in the Appendix. <br /> pg. 21 <br />