12000000063 Hyatt Dam Inundation Report

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Table 3. Hyatt Lake Dam Failure Summary of HEC-HMS Results Downstream of Hyatt Dam Estimated Time of Estimated Time to Estimated Location Arrival from Start Peak from Start of Peak Flow of Breach Breach Rate(cfs) Development Development Hyatt Dam 0 hr 0 min 0 hr 46 min 12,700 Ward Rd. 0 hr 22 min 0 hr 59 min 11,900 West end of Lutz Sports Complex 0 hr 44 min 1 hr 12 min 11,600 Oak St. 0 hr 57 min 1 hr 20 min 11,500 Kipling St. 1 hr 8 min 1 hr 27 min 11,300 Carr St. 1 hr 55 min 2 hr 14 min 8,500 Wadsworth Blvd. 2 hr 17 min 2 hr 35 min 7,800 Ralston Rd. 2 hr 36 min 2 hr 55 min 7,200 Sheridan Blvd. 2 hr 48 min 3 hr 5 min 7,000 2.3 Hydraulic Modeling in HEC-RAS The USACE's water surface profile modeling computer program HEC-RAS was used to create a steady-state hydraulic model of the area downstream of Hyatt Dam using the flow rates and locations shown above in Table 3. Cross-sections of the flow path were developed from a 2012 USDA 3-meter digital elevation model (DEM) using the HEC-GeoRAS extension in ArcGIS. No bridges or culverts were modeled in the study reach. However, the 3-meter DEM contained enough detail to show most embankments and top of road elevations, so cross-sections were placed along the roads that cut perpendicularly through the breach flow path. The model was terminated at the Sheridan Boulevard overpass just downstream of the confluence of Ralston Creek and Clear Creek. This location was selected because the estimated peak flow rate at Sheridan Boulevard due to a clear-day breach of Hyatt Dam (7,000 cfs) is less than the 10-year flood reported in ICON Engineering's April 2007 Flood Hazard Area Delineation(FHAD) for Clear Creek(8,600 cfs). 2.4 Results The modeling performed in this study shows that a clear-day failure of Hyatt Dam with the water surface at the spillway crest will result in a peak outflow of 12,700 cfs. The time it takes to achieve this peak outflow will be 46 minutes from the onset of the breach. The flood wave will travel east along the unnamed drainage below the dam, and it will enter Van Bibber Creek near Ward Road. The flood will then flow east along Van Bibber Creek and then into Ralston Creek, which it will follow to the confluence with Clear Creek just upstream of Sheridan Boulevard. By the time the peak flood wave reaches Sheridan Boulevard it will have traveled a total of 6.5 miles and attenuated to a peak flow rate of 7,000 cfs. 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. HYATT DAM - 3 - June 21,2012 INUNDATION MAPPING REPORT