12000000053 Highland No. 1 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 the Highland No. 1 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 overpass at Weld County Road (WCR) 32 was modeled as a single bridge with opening and bridge deck dimensions estimated from aerial photo measurements and an on-site visual inspection. All other bridges and culverts in the study reach were not modeled and were assumed to be overtopped. Based on the storage-elevation curve for Thomas Reservoir from a 1978 plan set by McCall- Ellingson&Morrill, the flood from a breach of Highland Reservoir No. 1 will not cause the water surface in Thomas Reservoir to surcharge from the service spillway crest to the emergency spillway crest. Thomas Reservoir has a surcharge capacity of 950 acre-feet, and the total volume that would be released from a breach of Highland Reservoir No. 1 would only be 350 acre-feet. For this reason, the model was terminated at the inlet to Thomas Reservoir. The storage elevation curve is provided in Appendix D. 2.4 Results The modeling performed in this study shows that a clear-day failure of the Highland No. 1 Dam with the water surface at the spillway crest will result in a peak outflow of 850 cfs. The time it takes to achieve this peak outflow will be 125 minutes from the onset of the breach. The flood wave will travel northeast across open farm land to I-25, and it will mostly pass under the 1-25 bridges at WCR 32. It is also expected that there will be some overtopping of I-25 at a low point in the roadway approximately 1,000 feet north of WCR 32. The flood wave will then travel east along a natural drainage path and into Thomas Reservoir which will contain the full flood volume. By the time the peak flood wave reaches Thomas Reservoir it will have traveled a total of 1.6 miles and attenuated to a peak flow rate of 840 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 E. The HEC-RAS model is included on the Project CD. Table 4. Highland Reservoir No. 1 Dam Failure Summary of HEC-RAS Results at Critical Sections Critical Section HEC-RAS WSEL(ft) Q(cfs) Ave Vel Chan Vel Top Width Station NAVD 88 (Ws) (Ws) (ft) 1-25 6473 4958.8 840 2.4 2.3 _ 1,300 Thomas Reservoir 122 4883.1 840 2.1 2.3 400 HIGHLAND NO. 1 DAM - 3 - June 15,2012 INUNDATION MAPPING REPORT