12000000053 McIntosh Dam Inundation Report

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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 McIntosh 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 McIntosh Dam, which would result in a peak flow of 4,800 cfs at the 1-25 crossing. All other bridges in the study reach were not modeled and were assumed to be overtopped. The model was terminated at I-25 because the estimated peak flow rate at I-25 due to a clear-day breach of McIntosh Dam is significantly less than the 100-year flood. 2.4 Results The modeling performed in this study shows that a clear-day failure of McIntosh Dam with the water surface at the spillway crest will result in a peak outflow of 12,200 cfs. The time it takes to achieve this peak outflow will be 1 hour and 49 minutes from the onset of the breach. The flood wave will travel south into Saint Vrain Creek, and will then follow the creek east through the City of Longmont. The flood will then continue northeast along Saint Vrain Creek to the I-25 overpass. By the time the peak flood wave reaches 1-25 it will have traveled a total of 14 miles and attenuated to a peak flow rate of 4,800 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. Table 4. McIntosh Reservoir Dam Failure Summary of HEC-RAS Results at Critical Sections HEC-RAS WSEL(ft) Ave Vel Chan Vel Top Width Critical Section Station NAVD 88 Q(cfs) (ftls) (ftls) (ft) Hover 230649 4991.8 10.900 3.3 3.3 3,000 Hwy 287 220233 4949.8 10,100 4.3 5.7 2,400 Hwy 119(Boulder Co.) 212542 4919.3 9,300 3.4 4.1 1,000 Hwy 119(Weld Co.) 185288 4854.8 6,200 1.7 2.1 2,800 1-25 170796 4829.2 4,800 4.1 4.1 300 MCINTOSH DAM - 3 - June 15,2012 INUNDATION MAPPING REPORT