13000000095 Santa Maria Dam Inundation Report

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SANTA MARIA DAM - 5 - July 11, 2013 INUNDATION MAPPING REPORT Table 4. Santa Maria 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 Santa Maria Dam 165.33 9470.9 298,400 10.2 13.5 700 Clear Creek-Rio Grande Confluence (Hwy 149) 160.46 9003.9 293,000 14.7 17.1 2,800 Creede (Airport Rd) 138.67 8615.1 209,400 9.8 12.0 2,500 South Fork 115.92 8186.6 119,800 12.6 8.8 1,600 Del Norte (Oak St / Hwy 112) 97.22 7882.8 75,200 3.3 4.1 4,900 Monte Vista (US 285) 80.85 7666.0 48,900 2.6 1.0 8,300 Alamosa (US 160) 48.25 7543.3 19,500 0.8 4.7 13,200 Conejos / Costilla County Line 33.31 7511.4 9,300 0.9 0.9 4,500 Lasauses 21.06 7480.1 7,400 2.1 2.6 700 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 Santa Maria Dam with the water surface at the spillway crest will result in a peak outflow of approximately 298,400 cfs. The time it takes to achieve this peak outflow will be 86 minutes from the onset of the breach. From the dam the flood wave will flow west along the Santa Maria Reservoir outlet channel for about 1 mile where it will then flow into Boulder Creek. Boulder Creek converges with Clear Creek a short distance downstream from that point, which will convey the flood south for 4 miles through a steep and narrow canyon until it enters the Rio Grande just downstream of the Highway 149 crossing of Clear Creek. These reaches of Boulder 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 19 feet per second, and the maximum channel depth will be up to 87 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 63 feet, and the average flow velocity will be approximately 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.