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Table 5. Highland Reservoir No. 2 North Dam Failure <br /> Summary of HEC-HMS Results Downstream of Highland No. 2 North Dam <br /> Estimated Time from Estimated Peak <br /> Location Start of Breach Flow Rate(cfs) <br /> Development <br /> Peak outflow at breach 0 hr 10 min 21,800 <br /> Little Thompson River 0 hr 23 min 21,300 <br /> Larimer Co.Rd. 15 1 hr 26 min 17,000 <br /> 1-25 3 hr 20 min 12,400 <br /> Weld Co. Rd. 17 5 hr 4 min 10,200 <br /> Hwy 60 5 hr 59 min 9,400 <br /> railroad bridge north of Milliken 6 hr 25 min 8,900 <br /> confluence with South Platte River 10 hr 17 min 6,100 <br /> Table 6. Highland Reservoir No. 2 South Dam Failure <br /> Summary of HEC-HMS Results Downstream of Highland No. 2 South Dam <br /> Estimated Time from Estimated Peak <br /> Location Start of Breach Flow Rate(cfs) <br /> Development <br /> Peak outflow at breach 0 hr 11 min 11,400 <br /> Longmont(Hwy 66) 1 hr 31 min 9,500 <br /> south end of city(3rd Ave.) 4 hr 7 min 7,300 <br /> Hwy 119 in Boulder Co. 4 hr 34 min 7,200 <br /> Hwy 119 in Weld Co. 9 hr 4 min 4,800 <br /> 1-25 12 hr 41 min 3,900 <br /> 2.3 Hydraulic Modeling in HEC-RAS <br /> The USACE's water surface profile modeling computer program HEC-RAS was used to create <br /> steady-state hydraulic models of the areas downstream of the Highland No. 2 Dams using the flow <br /> rates and locations shown above in Table 5 and Table 6. <br /> 2.3.1 Hydraulics Downstream of Highland No. 2 North Dam <br /> Cross-sections extending from the Highland No. 2 North Dam to U.S. Highway 287 were developed <br /> from a 2011 USDA 10-meter digital elevation model (DEM) using the HEC-GeoRAS extension in <br /> ArcGIS. These cross-sections were then appended to an existing HEC-RAS model from the <br /> September 2011 Ish Main Dam #3 Inundation Mapping Report by Tessara Water. The tie-in <br /> occurred downstream of U.S. Highway 287 on the Little Thompson River. The Ish model covered <br /> the reach from the tie-in location to the confluence with the Big Thompson River, and then to the <br /> confluence with the South Platte River. <br /> U.S. Highway 287 over the Little Thompson River was modeled as an inline structure with bridge <br /> deck data taken from a Little Thompson 100-year flood HEC-RAS model. The Little Thompson <br /> model was provided by the Larimer County Engineering Department and was created by Ayres <br /> Associates in 2005. <br /> HIGHLAND No.2 NORTH AND SOUTH DAMS -4- June 15,2012 <br /> INUNDATION MAPPING REPORT <br />