12000000053 McIntosh Dam Inundation Report

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3.0 INUNDATION EXTENTS 3.1 Detail of Mapping The water surface profiles from the HEC-RAS hydraulic modeling were exported to AreGIS, 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 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 1-25 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 McIntosh Dam will reach a peak flow rate of 12,200 cfs at the breach, and will flow along a natural drainage path for approximately 1 mile until converging with Saint Vrain Creek near Airport Road. In this reach, residential structures just downstream of the dam,portions of the Westview Middle School property, and the residential areas surrounding the west and south edges of Twin Peaks Golf Course are expected to be inundated. Velocities calculated in this reach generally range from 3 to 8 feet per second, and maximum depths average 4 to 5 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 3 to 5 feet per second, and maximum depths along the stream can exceed 15 feet in the city. Based on the hydraulic modeling in this study the two main I-25 bridges and the frontage road bridge are not expected to overtop. As reported in the I-25 frontage road as-built plan set, the 100-year flood at this location is 17,290 cfs—significantly greater than the 4,800 cfs estimated at I-25 due to a clear-day breach of McIntosh Dam. For this reason, the inundation mapping extent was terminated at I-25. MCINTOSH DAM -4 - June 15,2012 INUNDATION MAPPING REPORT