12000000065 Fuchs Dam Inundation Report

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Table 3. Fuchs Reservoir Dam Failure Summary of HEC-HMS Results Downstream of Fuchs Dam Estimated Time of Estimated Time to Estimated Location Arrival from Start of Peak from Start of Peak Flow Breach Development Breach Development Rate(cfs) Fuchs Dam 0 hr 0 min 0 hr 26 min 6,600 Burro Creek confluence 0 hr 37 min 0 hr 51 min 6,400 W.Fk.Pinos Creek confluence 0 hr 51 min 1 hr 1 min 6,300 Bennett Creek confluence 1 hr 30 min 1 hr 33 min 5,400 Pinos Creek Rd. 2 hr 35 min 2 hr 38 min 3,800 U.S.Hwy 160 3 hr 25 min 3 hr 28 min 3,100 Rio Grande confluence 5 hr 22 min 5 hr 36 min 1,700 Oak St./Hwy 112 5 hr 41 min 5 hr 55 min 1,600 Rio Grande split 5 hr 52 min 6 hr 10 min 1,500 W.County Rd.5n 8 hr 4 min 8 hr 15 min 1,100 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 Fuchs Dam using the flow rates and locations shown above in Table 3. Cross-sections of the flow path were developed from a 2012 USDA 10-meter digital elevation model (DEM) using the HEC-GeoRAS extension in ArcGIS. No bridges in the study reach were modeled and all were assumed to be overtopped. The model was terminated approximately 3.5 miles east of the town of Del Norte because the estimated peak flow rate at that location due to a clear-day breach of Fuchs Dam (1,100 cfs) is significantly less than the 10-year flood (7,701 cfs)reported in the effective FEMA FIS dated September 2, 2011. 2.4 Results The modeling performed in this study shows that a clear-day failure of Fuchs Dam with the water surface at the service spillway crest will result in a peak outflow of 6,600 cfs. The time it takes to achieve this peak outflow will be 26 minutes from the onset of the breach. The flood wave will travel northeast along Pinos Creek for about 17.5 miles until it turns east shortly after overtopping U.S. Highway 160. The flood will then parallel U.S. Highway 160 for about 3.5 miles, and it will enter the Rio Grande near the western end of the town of Del Norte. The flood will then follow the Rio Grande southeast past Del Norte to the downstream end of the study reach near West County Road 5n. By the time the peak flood wave reaches West County Road 5n, it will have traveled a total of 28 miles and attenuated to a peak flow rate of 1,100 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. FUCHS DAM - 3 - June 15,2012 INUNDATION MAPPING REPORT