12000000053 Highland No. 2 Dams Inundation Report

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Summaries of the HEC-RAS results at critical cross-sections are shown below in Table 7 and Table 8 for the North and South Dams, respectively. Summaries of the full HEC-RAS model results are attached in Appendix E and Appendix F for the North and South Dams, respectively. The HEC- RAS models are included on the Project CD. Table 7. Highland Reservoir No.2 North Dam Failure Summary of HEC-RAS Results at Critical Sections Critical Section HEC-RAS 1 WSEL(ft) Q cfs Ave Vel Chan Vel Top Width Station NAVD 88 ( ) (ftls) (ftls) (ft) Larimer Co. Rd. 15 36952 4981.1 17,000 4.8 6.9 1,500 1-25 26293 4867.7 12,400 8.7 8.7 200 Hwy 60 13264 4763.7 9,400 7.6 7.9 300 South Platte River 0 4671.6 6,100 6.5 6.5 700 Table 8. Highland Reservoir No.2 South Dam Failure Summary of HEC-RAS Results at Critical Sections HEC-RAS WSEL(ft) Ave Vel Chan Vel Top Width Critical Section Station NAVD 88 Q(cfs) (is) (ftls) (ft) Hwy 66 236536 5052.9 9,500 2.6 1.9 2.100 3rd Ave. 218600 4957.0 7,300 1.8 1,9 1.700 Hwy 119(Boulder Co.) 212542 4918.7 7,200 3.2 3.6 900 Hwy 119(Weld Co.) 185288 4854.6 4,800 1.7 2.0 2,500 1-25 170796 4828.4 3,900 4.0 4.0 300 3.0 INUNDATION EXTENTS 3.1 Detail of Mapping The water surface profiles from the HEC-RAS hydraulic modeling were exported to ArcGIS, and the inundation extents were delineated using the best data available. The majority of the inundation mapping was based on data from the 2011 USDA 10-meter DEM. Delineation of the South Dam breach flood extents along Saint Vrain Creek between 3rd Avenue and County Line Road was based on a 2008 floodplain study by Anderson Consulting Engineers. All bridge crossings in the North and South Dam study reaches (with the exception of I-25 over Saint Vrain Creek) were assumed to be significantly overtopped. For this reason, ground surveys were not necessary at bridge crossings or roadway embankments. A detailed as-built plan set was obtained for the I-25 frontage road bridge, and the crossing was modeled as described in Section 2.3.2 of this report. The inundation maps associated with this report are provided in Appendix G and Appendix H for the North and South Dams, respectively. HIGHLAND No.2 NORTH AND SOUTH DAMS - 6 - June 15,2012 INUNDATION MAPPING REPORT