12000000063 Hyatt Dam Inundation Report

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Table 4. Hyatt Lake 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) (ftls) (ft) Ward Rd. 31110 5471.6 11,900 8.5 11.5 700 Kipling St. 21723 5373.3 11,300 5.6 4.2 900 Wadsworth Blvd. 10204 5304.3 7,800 5.2 5.2 900 Sheridan Blvd. 2442 5254.9 7,000 6.0 6.0 200 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 extent was delineated using the 2012 USDA 3-meter DEM. All bridge crossings in the study reach were assumed to be significantly overtopped. For this reason, ground surveys were not necessary at bridge crossings or roadway embankments. 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 Hyatt Dam will reach a peak flow rate of 12,700 cfs at the breach, and will flow along the unnamed drainage below Hyatt Dam for approximately 1.25 miles to Ward Road. In this reach, several residential structures east of Eldridge Street are expected to be inundated, and a small unnamed reservoir between Zang Way and Ward Road will be overtopped. Velocities calculated in this reach average 10 feet per second and can exceed 16 feet per second in some areas. Maximum depths calculated in the reach range from 4 to 15 feet. The flood will overtop Ward Road and continue southeast along the unnamed drainage to Van Bibber Creek. Some of the flood water may also spill out of this unnamed drainage at Ward Road, and it will inundate several structures near the intersection of Ward Road and West 58th Avenue. However, the amount of flow spilling in this area is expected to be minimal, and the velocities and depths of flow will likely be low. Though this side spill area was not modeled in HEC-RAS, it was included in the inundation mapping. The flood will then follow Van Bibber Creek east to its confluence with Ralston Creek. The channel velocity in this reach will range from 2 to 14 feet per second, and maximum depths along the stream can reach nearly 20 feet. Numerous structures in this reach will be inundated, and nearly the entire length of West 58th Avenue between Ward Road and Garrison Street will be overtopped. The flood will continue east along Ralston Creek at a high velocity, and will inundate several businesses and residences until entering Clear Creek just upstream of Sheridan Boulevard. The flow velocities calculated in this reach range from 4 to 15 feet per second, and the maximum depths can reach up to 16 feet. HYATT DAM - 4 - June 21,2012 INUNDATION MAPPING REPORT