12000000053 Foothills Dam Inundation Report

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A summary of the HEC-RAS results at critical cross sections is shown below in Table 4, and a summary of the full HEC-RAS model results is attached in Appendix D. The HEC-RAS model is included on the Project CD. Table 4. Foothills Reservoir 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) (ftls) (Ws) (ft) Hover 230649 4993.6 51,100 3.1 3.3 3,200 Hwy 287 220233 4953.3 36,600 2.2 6.1 4,200 Hwy 119(Boulder Co.) 212542 4922.5 31,000 4.7 5.8 1,400 Hwy 119(Weld Co.) 185288 4856.2 16,500 2.2 2.2 4,800 1-25 170796 4833.7 11,500 4.7 4.7 2,900 Hwy 66 135020 4784.8 7,300 1.3 2.3 2,800 South Platte River 97254 4743.1 4,200 4.4 4.4 500 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 flood extents along Saint Vrain Creek between Airport Road and County Line Road was based on a 2008 floodplain study by Anderson Consulting Engineers. All bridge crossings in the study reach (with the exception of 1-25 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 of this report. The inundation maps associated with this report are provided in Appendix E. 3.2 Description of Inundation Area The flood from a clear-day piping failure of Foothills Dam will reach a peak flow rate of 92,200 cfs at the breach, and will flow through mostly open farmland along a natural drainage path for approximately 1 mile until converging with Saint Vrain Creek just west of Longmont. Velocities calculated in this reach exceed 14 feet per second in some places, and depths often exceed 8 feet. The flood will follow Saint Vrain Creek through Longmont and is expected to overtop every bridge crossing until it reaches I-25. The channel velocity in this reach generally ranges from 4 to 7 feet per second, and maximum depths along the stream can exceed 17 feet. Based on the hydraulic modeling in this study the two main 1-25 bridges and the frontage road bridge are not expected to overtop. FOOTHILLS DAM - 4 - June 15,2012 INUNDATION MAPPING REPORT