12000000053 Highland No. 1 Dam Inundation Report

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3.0 INUNDATION EXTENTS 3.1 Detail of Mapping The water surface profile from the HEC-RAS hydraulic modeling was exported to ArcGIS, and the inundation extents were delineated using the 2011 USDA 10-meter DEM. All bridge crossings in the study reach (with the exception of I-25 over WCR 32) were identified in aerial photography as small culverts and were therefore assumed to be significantly overtopped. For this reason, ground surveys were not necessary at the culvert locations. However, a site visit to the I-25 overpass was conducted to estimate the effective opening and to inspect the embankment because the 10-meter DEM is not detailed enough to accurately depict roadway embankments. During the visit a low point in the I-25 profile was identified approximately 1,000 feet north of WCR 32. Though this local depression was not included in the hydraulic modeling, it was considered during the delineation of the flood extents. The inundation maps associated with this report are provided in Appendix F. 3.2 Description of Inundation Area The flood from a clear-day piping failure of Highland No. 1 Dam will reach a peak flow rate of 850 cfs at the breach, and will flow through mostly open farmland along a natural drainage path for approximately 0.5 miles until reaching the I-25 bridges over WCR 32. Velocities calculated in this reach average 2 to 3 feet per second,but may achieve speeds up to 5 feet per second. Depths in this reach are generally less than 2 feet, but can potentially be much greater in the vicinity of the I-25 embankment (over 6 feet deep). The flow through the overpass opening will be nearly 4 feet deep with a velocity over 8 feet per second. Slight overtopping of I-25 at the low point in the embankment 1,000 feet north of WCR 32 is expected,but overtopping can become more severe if the overpass opening clogs with debris. After passing I-25, the flood will cross a field and enter a natural drainage channel which it will follow to Thomas Reservoir. Depths in this reach will range from 1 to 3 feet, and velocities will often exceed 5 or 6 feet per second due to the narrow channel width. The flood will then enter Thomas Reservoir where it will be fully contained within the reservoir's surcharge volume. For this reason, the detailed inundation mapping extent was terminated at the inlet to Thomas Reservoir. HIGHLAND NO. I DAM -4 - June 15,2012 INUNDATION MAPPING REPORT