12000000053 Highland No. 2 Dams Inundation Report

Home Browse Search

The model was terminated at the South Platte River because the estimated peak flow rate in that area due to a clear-day breach of Highland No. 2 North Dam (6,100 cfs) is significantly less than the 100-year flood reported in the effective FEMA Flood Insurance Study(32,500 cfs), dated September 1999. 2.3.2 Hydraulics Downstream of Highland No. 2 South Dam Cross-sections of the Highland No. 2 South Dam breach flow path were developed from a 2011 USDA 10-meter DEM using HEC-GeoRAS. 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 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. 2 South Dam, which would result in a peak flow of 3,900 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 1-25 due to a clear-day breach of Highland No. 2 South Dam is significantly less than the 100-year flood. 2.4 Results The modeling performed in this study shows that separate clear-day failures of the Highland No. 2 North and South Dams will result in peak outflows of 21,800 cfs and 11,400 cfs, respectively. The time it takes to achieve these peak outflows will be 10 minutes from the onset of the breach for the North Dam, and 11 minutes for the South Dam. The flood wave for the North Dam will travel north across mostly open farmland and will enter the Little Thompson River just upstream of Larimer County Road 21. It will travel along the Little Thompson River to the confluence with the Big Thompson River, which it will then follow to the confluence with the South Platte River. By the time the peak flood wave reaches the South Platte River it will have traveled about 27 miles and attenuated to a peak flow rate of 6,100 cfs. The flood wave for the South Dam will head south through mostly open farmland until it reaches Highway 66. From there it will continue south through Longmont along an urban drainage until it enters Saint Vrain Creek just upstream of Highway 119 in Boulder County. It will then follow Saint Vrain Creek east to 1-25. When the peak flood wave reaches I-25 it will have traveled about 16 miles and attenuated to a peak flow rate of 3,900 cfs. HIGHLAND No.2 NORTH AND SOUTH DAMS - 5 - June 15,2012 INUNDATION MAPPING REPORT