12000000053 Highland N. 3 Dam Inundation Report

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Table 2. Highland Reservoir No. 3 Dam Failure Comparison of HEC-HMS Piping Simulation Results Empirical Method Time to Peak Peak Outflow (minutes) (cfs) GROUND-LEVEL BREACH Froehlich 169 2,700 Von Thun and Gillette 83 1,600 USBR 6* 760 BREACH INTO OUTLET CHANNEL Froehlich 95 4,500 Von Thun and Gillette 26 4,800 USBR 6 2,400 *Time of failure predicted in empirical equation was less than 0.1 hours. Failure time was rounded up to 0.1 hours(6 min). Of the six methods that were analyzed, the Von Thun and Gillette method with a failure into the outlet channel resulted in the greatest predicted peak outflow of 4,800 cfs with a time to peak of 26 minutes. This hydrograph was routed downstream using HEC-HMS through representative reaches using the Muskingum-Cunge method. The reach descriptions,routing times, and predicted flow rates are summarized below in Table 3. The HEC-HMS output is attached in Appendix C, and the full HEC-HMS model is included on the Project CD. Table 3. Highland Reservoir No.3 Dam Failure Summary of HEC-HMS Results Downstream of Highland No. 3 Dam Location Estimated Time from Start Estimated Peak of Breach Development Flow Rate(cfs) Peak outflow at breach 0 hr 26 min 4,800 Mead High School 0 hr 36 min 4,700 Saint Vrain River/1-25 0 hr 59 min 4,700 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. 3 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 I-25 main and I-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 I-25 and frontage road bridges over Saint Vrain Creek are not expected to be overtopped due to a clear-day breach of Highland No. 3 Dam,which would result in a peak flow of 4,700 cfs at the I-25 crossing. HIGHLAND No.3 DAM - 3 - June 15,2012 INUNDATION MAPPING REPORT