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I I I I I I I I I I I I I I I I I I I -2- age and Flood Control District or the Colorado Water Conservation Board to determine if the information in this report has been amended. STUDY AREA DESCRIPTION ACKNOWLEDGMENTS This report was prepared by Wright Water Engineers, Inc., Consulting Engi- neers, Denver, Colorado, at the request of the Urban Drainage and Flood Con- trol District. DRAINAGE BASIN CHARACTERISTICS The study area includes the reach of the South Platte River from Sand Creek to Oxford Avenue. Jurisdictions included in the study area are Adams Coun- ty, Commerce City, Denver, Englewood and Sheridan. Assistance was obtained from Sellards and Grigg Engineers. McLaughlin Water Engineers of Denver provided data concerning the Third Avenue Dam. The South Pl atte River drainage bas in covers approximately 4,000 square miles of land at Sand Creek extending from the Continental Divide in the Rocky Mountain Range to the high plains and foothills of eastern Colorado as shown in Figure 1. The characteristics and hydrologic impact of the moun- tainous region and the high plains and foothills regions are quite differ- ent. The mountainous western portions of the basin produce heavy snow melt runoff during late spring but contribute little significant runoff during the remainder of the year. The high plains and foothills portions of the basin are more susceptible to high runoff during summer thunderstorms. The South Platte River has as its source the rrountains of the Continental Di- vide. Tributary basins begin as narrow valleys and canyons in the mountain- ous areas and increase progressively in width towards the lower basin areas on the eastern plains. Basin elevations range from 14,200 feet at the Con- tinental Divide to approximately 5,100 feet mean sea level at Sand Creek. Soils in the basin range from mountainous rock outcrops to dense clays, al- luvial sand and gravels. The various soil types and vegetated covers have been incorporated into the hydrologic analyses performed. As mentioned, two distinct runoff sources exist within the basin: one, snow melt; and two, thunderstorm activity. The rrountain and high foothill areas are generally responsible for peaks in the spring associated with snow melt while the high plains peak runoff is usually associated with summer thunder showers. The average annual precipitation at the Grant gage in South Park is 14.6 inches and the average annual precipitation at the Oenver gage is 15.51 inches. The average annual temperature at the Grant gage in South Park is 38.8.F, and at the Denver it is 49.3.F.