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Mr. Clay Kimmi <br />December 9, 2008 <br />Page 3 <br />Downstream Boundary Condition <br />Weld County Road 18 was used as the downstream boundary condition for both pre-mining and <br />existing conditions. Flow at this cross section is split between a portion overtopping the roadway <br />crown on the left (west) overbank and a portion remaining within the channel banks. Critical <br />depth was assumed to occur at this cross section for both pre-mining and existing conditions. An <br />n value of 0.025 was used for the overbank portions of this cross section where roadway <br />overtopping would occur. A survey of portions of the road centerline was used to supplement <br />road topography developed from the aerial survey. <br />MODELING RESULTS <br />The HEC-RAS modeling results for both pre-mining and existing conditions are presented in <br />Attachment A and summarized in Table 1. <br />Table 1 <br />HEC-RAS Modeling Results <br /> Pre-Mining To ogra h Existing To o ra h Increase in Elevation <br />Cross <br />Section Water <br />Surface <br />Elev. Energy <br />Grade <br />Elev. Water <br />Surface <br />Elev. Energy <br />Grade <br />Elev. Water <br />Surface <br />Elev. Energy <br />Grade <br />Elev. <br />10 4872.86 4873.57 4872.86 4873.57 0.00 0.00 <br />20 4873.66 4873.70 4873.66 4873.70 0.00 0.00 <br />30 4873.70 4873.79 4873.70 4873.79 0.00 0.00 <br />40 4873.90 4873.97 4873.89 4873.96 -0.01 -0.01 <br />50 4874.23 4874.37 4874.17 4874.27 -0.06 -0.10 <br />60 4873.89 4875.06 4873.90 4874.98 +0.01 -0.08 <br />70 4876.28 4876.43 4876.24 4876.43 -0.04 0.00 <br />80 4876.72 4876.81 4876.71 4876.78 -0.01 -0.03 <br />90 4876.93 4877.06 4876.86 4877.06 -0.07 0.00 <br />100 4877.29 4877.70 4877.42 4877.82 +0.13 +0.12 <br />These results indicate that the greatest increase in water surface and energy grade elevations <br />occurs at Cross Section 100. These increases of approximately one-tenth of a foot are <br />significantly less the allowable one foot increase. At many cross sections through the subject <br />property, the existing conditions have actually lowered the water surface profile compared to <br />pre-mining conditions.