12000000053 Foothills Dam Inundation Report

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Table 3. Foothills Reservoir Dam Failure Summary of HEC-HMS Results Downstream of Foothills Dam Location Estimated Time from Start Estimated Peak of Breach Development Flow Rate(cfs) Peak outflow at breach 0 hr 28 min 92,200 Saint Vrain Creek 0 hr 52 min 79,700 Hover St. 2 hr 3 min 51,100 Hwy 287(Main St.) 3 hr 2 min 36,600 Hwy 119 in Boulder Co. 3 hr 49 min 31,000 Hwy 119 in Weld Co. 6 hr 49 min 16,500 1-25 9 hr 29 min 11,500 Hwy 66 14 hr 50 min 7,300 Confluence with South Platte River 22 hr 27 min 4,200 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 Foothills Dam using the flow rates and locations shown above in Table 3. Cross-sections of the flow path were developed from a 2011 USDA 10-meter digital elevation model (DEM) using the HEC-GeoRAS extension in ArcGIS. The 1-25 main and 1-25 frontage road bridges over Saint Vrain Creek were modeled as a single bridge with opening,bridge deck, and channel dimensions taken from a 2001 as-built plan set for the frontage road bridge. The plan set also provided bridge hydraulics and 100-year flood data (17,290 cfs). Based on email correspondence received from the Colorado Department of Transportation (CDOT) and the Weld County Public Works Department, it is known that this 100- year event safely passes under the two main bridges and the frontage road bridge. Based on this information, the 1-25 and frontage road bridges over Saint Vrain Creek are not expected to be overtopped due to a clear-day breach of Foothills Dam, which would result in a peak flow of 11,500 cfs at the I-25 crossing. All other bridges in the study reach were not modeled and were assumed to be overtopped. The banks of the South Platte River are expected to contain the flood from a clear-day failure of Foothills Dam, so the model was terminated at the Saint Vrain Creek confluence with the South Platte River. 2.4 Results The modeling performed in this study shows that a clear-day failure of Foothills Dam with the water surface at the spillway crest will result in a peak outflow of 92,200 cfs. The time it takes to achieve this peak outflow will be 28 minutes from the onset of the breach. The flood wave will travel east along Saint Vrain Creek through the City of Longmont, and then follow Saint Vrain Creek northeast to the confluence with the South Platte River. By the time the peak flood wave reaches the South Platte River, it will have traveled a total of 31 miles and attenuated to a peak flow rate of 4,200 cfs. FOOTHILLS DAM - 3 - June 15,2012 INUNDATION MAPPING REPORT