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The model was terminated at I-25 because the estimated peak flow rate at I-25 due to a clear-day <br /> breach of Highland No. 3 Dam is significantly less than the 100-year flood. <br /> 2.4 Results <br /> The modeling performed in this study shows that a clear-day failure of the Highland No. 3 Dam <br /> with the water surface at the spillway crest will result in a peak outflow of 4,800 cfs. The time it <br /> takes to achieve this peak outflow will be 26 minutes from the onset of the breach. The flood wave <br /> will travel south across mostly open farmland and will enter Saint Vrain Creek just upstream of I- <br /> 25. By the time the peak flood wave reaches I-25 it will have traveled about 1.5 miles and <br /> attenuated to a peak flow rate of 4,700 cfs. <br /> A summary of the HEC-RAS results at critical cross sections is shown below in Table 4, and a <br /> summary of the full HEC-RAS model results is attached in Appendix D. The HEC-RAS model is <br /> included on the Project CD. <br /> Table 4. Highland Reservoir No. 3 Dam Failure <br /> Summary of HEC-RAS Results at Critical Sections <br /> HEC-RAS WSEL(ft) Ave Vel Chan Vel Top Width <br /> Critical Section Station NAVD 88 Q(cfs) (ftls) (ftls) (ft) <br /> Mead High School 7332 4933.3 4,700 3.0 3.1 1,800 <br /> 1-25 423 4829.1 4.700 4.1 4.1 300 <br /> 3.0 INUNDATION EXTENTS <br /> 3.1 Detail of Mapping <br /> The water surface profile from the HEC-RAS hydraulic modeling was exported to ArcGIS, and the <br /> inundation extents were delineated using the 2011 USDA 10-meter DEM. <br /> All bridge crossings in the study reach (with the exception of I-25 over Saint Vrain Creek) were <br /> assumed to be significantly overtopped. For this reason, ground surveys were not necessary at <br /> bridge crossings or roadway embankments. A detailed as-built plan set was obtained for the I-25 <br /> frontage road bridge, and the crossing was modeled as described in Section 2.3 of this report. <br /> The inundation maps associated with this report are provided in Appendix E. <br /> 3.2 Description of Inundation Area <br /> The flood from a clear-day piping failure of Highland No. 3 Dam will reach a peak flow rate of <br /> 4,800 cfs at the breach, and will flow across mostly open farmland along a natural drainage path for <br /> approximately 1 mile until converging with Saint Vrain Creek upstream of I-25. In this reach the <br /> athletic fields at Mead High School and several small structures are expected to be inundated. <br /> Velocities calculated in this reach generally range from 3 to 7 feet per second, and depths average <br /> 1.5 to 2.5 feet. The flood wave will then follow Saint Vrain Creek and will pass under the I-25 <br /> HIGHLAND NO.3 DAM -4 - June 15,2012 <br /> INUNDATION MAPPING REPORT <br />