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I I I I I I I I I I I I I I I I I I I 5 Toll Gate Creeks caused extensive damage in the Aurora vicinity and contributed additional flood volume to the South Platte River north of Denver. Nearly every highway and railroad bridge crossing Sand and Toll Gate Creeks was either damaged or destroyed. About 750 acres of land adjoining Sand Creek and 120 acres of land adjoining Toll Gate Creek were inundated. Flooding along the downstream 10 miles of Sand Creek spread to widths ranging between 500 feet to 1,500 feet. On the down- stream 5 miles of Toll Gate Creek, the flood width ranged from 300 feet to 1,000 feet. Damages from floOding on Sand and Toll Gate Creeks are tabulated in Table 2. TABLE 2 FLOOD OF 16 AND 17 JUNE 1965 DAMAGES Platte River downstream from Denver, that had been overtopped and flanked by the 16 and 17 June flood, were unable to prevent flOOding from the lesser floods of July. As this flOOding was less severe than the June flood, no specific damage estimates were made." - HYDROLOGIC AND HYDRAULIC DETERMINATIONS - Stream Rural Urban Transportati on Total Sand Creek $ 94,000 $316,000 $2,107,000 $2,517 ,000 Toll Gate Creek 51,000 169,000 468,000 688,000 Total Damages $145,000 $485,000 $2,575,000 $3,205,000 ~ydrologic Analysis Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for floods with recurrence intervals of 2, 5, 10, 25, 50, 100 and 500 years for present and two projected future basin conditions. Future development in the Sand Creek basin should follow the growth pattern of Aurora. Growth trends in the area indicate that the population in the Sand Creek basin in the forseeab1e future will be approximately 900,000 people. The future basin hydrologic analyses were completed using projected basin populations of 600,000 and 900,000 people. The 100-year flood plain in this report was developed from the hydrology for a future basin population of 600,000 people. The hydrologic analysis of Sand Creek was based on the development of a sur- face runoff model of the Sand Creek basin USing the runoff block of the Environmental Protection Agency's Storm Water Management Model. Discharge hydrographs developed from the model were then routed along the main stem of Sand Creek by use of an unsteady flow routing procedure developed by James A. Harder and modified by the Missouri River Division Office of the Corps of Engineers. Discharge probability relationships were developed by inserting precipitation-frequency val ues prepared for the State of Colorado by the Special Studies Branch, Office of Hydrology, Environmental Science Services Administration, National Weather Service, dated October 1967. A brief description of the model development and parameters used in the study is presented in the following paragraphs. "Floods of July 1965. Unstable weather continued in the Denver region subsequent to the June 1965 floods. Rainstorms of varying intensity occurred during the period from 20 to 28 July. As of 25 July, Denver had received 15.98 inches of rain, more than 1 inch above its average-annual precipitation. During July, eastern Colorado rainfall totaled about 7 inches. Sand Creek basin, during July, experienced repeated flood runoff, resulting from a series of thunderstorms. These storms hampered recovery operations and caused additional damages, particu- larly to temporary stream crossings constructed to accommodate traffic after the bridge destruction in June. On 24 July, flash flows on Sand Creek destroyed the Peoria Street temporary bridge. Other flows on 25 July damaged the left abutment of a major highway bridge at Colfax Avenue. Flash flooding occurred in Aurora, and a number of basements were flooded. Private agricultural levees on the South