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C'rA1\ SEIti' LI1.TA\ A ASSOC'IATES <br />Hydrogeoloaical and Environmental Consultants <br />Where: v= velocity (ft/s) <br />R= mean depth (ft) <br />S= slope (dim) <br />The average velocity from the rating curve is 2.41 ft/s and the average mean depth is 1.40 <br />ft. The slope was calculated to be 0.0016 using the total linear distance of the river <br />channel and the difference in elevation between the up and downstream boundaries of the <br />model. Solving the equation for n, n = 0.031. <br />This "n" value is lower than the one (0.035) used in the Deere and Ault model but is <br />closer to the value (0.030) used in the FIS study. However, the "n" used in the GL&A <br />model for the overbank values is 0.040 and was considered conservative compared to the <br />overbank values used in the FIS model that ranged from 0.045 to 0.120. A lower <br />roughness coefficient gives a lower water surface elevation. Given the gravelly nature of <br />the overbank material and abundant vegetation along the banks of the river channel that <br />includes large cottonwoods, an "n" value of 0.040 appears more appropriate than 0.035 <br />that was used in the D&A model. <br />Model Boundary Conditions <br />Boundary conditions are necessary to establish a starting water surface elevation at the <br />upstream and downstream ends of the river system. In a subcritical flow system, <br />boundary conditions are only required at the downstream end of the river. It was <br />assumed that subcritical conditions exist in the river reach for this project. The slope of <br />the river along this reach is low enough (0.0016) to preclude high velocity flows and <br />supercritical flow. Under these model conditions, it was assumed that the river flow <br />regime is at subcritical conditions and the critical depth was calculated by the model <br />during model simulations. <br />Model Results <br />A summary of model results are presented in Table 1 and complete model output results <br />are presented in Tables 2 and 3. HEC -RAS cross-sections of model results are shown in <br />Figures 6 and 7. These results indicate that existing mining conditions in 2008 (based on <br />the 2008 aerial survey conducted by L.G. Everist) result in a maximum change of <br />approximately 2 feet in the water surface elevation compared to pre -mining conditions <br />for the 100 -year flow event. At cross-section 3193.73, which transects the Bachofer <br />property, there is 1.86 -foot change in water surface between pre -mining (1978) and <br />post -mining (2008) topographic surfaces for the 100 -year flood event. <br />-6- <br />