13000000095 Continental Dam Inundation Report

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CONTINENTAL DAM - 5 - July 11, 2013 INUNDATION MAPPING REPORT Table 4. Continental Reservoir Dam Failure Summary of HEC-RAS Results at Critical Sections Critical Section HEC-RAS Station (miles) WSEL (ft) NAVD 88 Flow Rate (cfs) Avg Vel (ft/s) Chan Vel (ft/s) Top Width (ft) Downstream of Continental Dam 175.07 10249.6 283,400 32.2 34.5 300 Clear Creek-Rio Grande Confluence (Hwy 149) 160.46 9003.5 269,500 14.3 16.8 2,800 Creede (Airport Rd) 138.67 8610.6 146,300 9.5 11.9 2,300 South Fork 115.92 8185.2 92,600 11.4 8.4 1,000 Del Norte (Oak St / Hwy 112) 97.22 7881.8 54,400 3.0 4.0 4,600 Monte Vista (US 285) 80.85 7665.5 34,800 2.3 0.9 8,100 Alamosa (US 160) 48.25 7542.8 9,600 6.9 6.9 300 Conejos / Costilla County Line 33.31 7510.2 4,100 0.8 0.8 3,000 Lasauses 21.06 7477.3 3,200 1.9 2.2 500 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 USGS 10-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 G. 3.2 Description of Inundation Area A clear-day failure of Continental Dam with the water surface at the spillway crest will result in a peak outflow of approximately 283,400 cfs. The time it takes to achieve this peak outflow will be 65 minutes from the onset of the breach. From the dam the flood wave will flow through a steep and narrow canyon along North Clear Creek for approximately 9 miles to the confluence with South Clear Creek. At that point the two streams merge to become Clear Creek which continues down for another 5 miles until it enters the Rio Grande just downstream of the Highway 149 crossing of Clear Creek. These reaches of North Clear Creek and Clear Creek contain very limited development, but a few structures and bridges including Highway 149 are expected to be inundated. The average velocity in this reach will be about 23 feet per second, and the maximum channel depth will be up to 82 feet. From the Clear Creek confluence with the Rio Grande to the Town of Creede the maximum channel flow depths will range from 22 to 61 feet, and the average flow velocity will be roughly 13 feet per second. Multiple structures in this reach are expected to be inundated, as well as the Highway 149 crossing of the Rio Grande south of Creede.