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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 /> routing of a flood over the dam and evaluation of bare earth. It can also evaluate the potential for <br /> vegetation or riprap to delay or prevent failure of the embankment(riverside berm in this case). <br /> The model is a three-phase erosion model: <br /> 1)Phase one of the erosion or failure process is failure of the vegetal cover(or, lack <br /> thereof) and development of concentrated flow. <br /> 2)Phase two is downward erosion in the area of concentrated flow, resulting in head cut <br /> formation. <br /> 3)Phase three is downward and upstream movement of the head cut, potentially <br /> breaching the dam (or, riverside berm in this case). <br /> Each phase is described by a set of threshold-rate relationships based on the process mechanics. <br /> A head cut erodibility index describes the resistance of the exposed geologic materials to erosive <br /> attack during the third phase of the process. <br /> For a complete erosion estimate, the geotechnical composition must be characterized. Typical <br /> parameters are representative particle size, percent clay fraction, plasticity index, total unit <br /> weight, undrained shear strength, and critical shear stress. Specific to WinDAM C software, the <br /> head cut erodibility index and detachment coefficient also need to be estimated. <br /> 3.2.2 WinDAM C Parameters <br /> Model parameters are listed in the Appendix. Riverside berm width ("dam crest") of 100 ft was <br /> applied for three different scenarios. The scenarios, Weakest, Best, and Strongest involve <br /> changing a critical parameter which would result in the model to predict weakest(most) head <br /> cut/erosion potential, strongest(least)head cut/erosion potential, and best which is in-between <br /> weakest and strongest. All parameters remained constant for the three scenarios except for the <br /> erodibility factor, "Kd" which is the most sensitive parameter. For this study, Kd had a range of <br /> two orders of magnitude from 100 for Weakest, to 0.1 for Strongest. Such a range of magnitude <br /> was chosen to cover a very large range of head cutting/erosion possibilities - due to the many <br /> uncertainties of such modeling -with the intent that the actual values would fall somewhere <br /> within the range of model results. <br /> Although some vegetation may exist on both the riverside and lateral berms during mining <br /> operations, only bare soil parameters were input for the sake of conservatism. If vegetation were <br /> considered, the modeling results would indicate less head cutting/erosion. <br /> pg. 12 <br />