12000000053 Highland No. 2 Dams Inundation Report

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3.2 Description of Inundation Area 3.2.1 Inundation Downstream of Highland No.2 North Dam The flood from a clear-day piping failure of Highland No. 2 North Dam will reach a peak flow rate of 21,800 cfs at the breach, and will flow across mostly open farmland for approximately 1 mile until converging with the Little Thompson River upstream of Larimer County Road 21 (CR 21). The majority of the flow in this reach will parallel CR 21 until entering the Little Thompson. Velocities along this stretch will reach 10 feet per second and maximum depths will range from 4 to 8 feet. It is also possible that there will be some lower velocity, shallower"sheet flow"traveling north from the western section of the North Dam. The amount of water and the flow characteristics in this region depend on where the breach occurs. Modeling of this "sheet flow" region was not performed in this analysis. After entering the Little Thompson River the flood wave will then follow the drainage east to the confluence with the Big Thompson River, and then continue to the South Platte River. All bridges in this reach are expected to be overtopped. Depths along the Little and Big Thompson Rivers can reach 30 feet above the channel bottom, and velocities will range from 1 to 18 feet per second. As reported in the effective FEMA Flood Insurance Study, the 100-year flood in the South Platte near the Big Thompson confluence is approximately 32,500 cfs—significantly greater than the 6,100 cfs estimated due to a clear-day breach of Highland No. 2 North Dam. For this reason, the inundation mapping extent was terminated at the Big Thompson confluence with the South Platte River. 3.2.2 Inundation Downstream of Highland No. 2 South Dam The flood from a clear-day piping failure of Highland No. 2 South Dam will reach a peak flow rate of 11,400 cfs at the breach, and will travel south along a natural drainage across mostly open farmland. Some of the flood waters will enter Terry Lake along its western shore. This will likely cause the lake to spill through the primary emergency spillway located at the west end of the dam and through the auxiliary spillway at the east end of the dam. The flood will continue south into a proposed regional detention pond which is to be located between Vermillion Road and Highway 66, and east of 95th Street. The capacity of the pond is small in comparison to the volume which would be released from Highland Reservoir No. 2 causing the proposed detention pond to be overtopped. The flood wave will then flow south along the drainage and will enter a heavily developed area of Longmont beginning at Highway 66. Velocities in the reach north of Highway 66 can exceed 9 feet per second, and depths will range from 2 to 6 feet. The flood will follow an urban drainage path south through Longmont where it is expected to inundate many of the residential structures bordering the drainage. Velocities will range from 2 to 8 feet per second, and depths may reach up to 6 feet. The flood waters may also spill out of the main drainage channel into parallel drainages to the west between 17th Avenue and Mountain View HIGHLAND NO.2 NORTH AND SOUTH DAMS - 7 - June 15,2012 INUNDATION MAPPING REPORT