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III-l SECTION III HYDROLOGIC AND HYDRAULIC SUMMARY The metropolitan Denver area is heavily developed and outlying areas are sparsely developed. The mountainous areas are generally unsuited for dense development and should remain sparsely developed. The foothill and high plains areas are more likely to develop but not to the extent of the exist- ing metropolitan area. Existing development in the metropolitan area con. sists primarily of residential and comnercial areas with minimal industrial regions. I NTRODUCTI ON The South Platte River drainage basin through the Denver metropolitan area has been studied numerous times in the past 10 years. These studies define the hydrologic response of the basin, and when merged, result in a discharge profile for the 100-year runoff event from the Chatfield Dam to the Adams/ Weld County line. The information contained in these reports will be con- solidated and presented herein. Soils in the basin range from mountainous rock outcrops to dense clays to alluvial sands and gravels. The various soil types and vegetative covers have been incorporated into the hydrologic analyses performed previously. Previous Studies The U.S. Army Corps of Engineers completed studies of the South Platte River and its tributaries through metropolitan Denver in 1974 and 1977. These re- ports presented hydrologic results and discussions of various factors influ- encing flood flows, In 1977, Gingery Associates, Inc. prepared a Flood Hazard Area Delineation for the South Platte River through Adams County. This study used hydrology prepared by the U.S, Army Corps of Engineers. In 1983 a hydrologic study of the South Platte River between Chatfield Dam and Sand Creek was completed by Merrick and Company. Also in 1983, Michael A. Stevens completed a Stream Stability Investigation for the South Platte River which summarized much of the previous hydrologic analyses. Climatological Data As mentioned, two distinct runoff sources exist within the basin: (1) snow. melt; and (2) thunderstorm activity. The mountain and high foothill areas are generally responsible for peaks in the spring associated with snowmelt while the high plains peak runoff is usually associated with sumner thunder. showers. Table 111.1 illustrates the climatological difference by comparing the average monthly precipitation and temperature for two gages, the Grant gage in South Park and the Denver gage located in the mountainous region and in the high plains respectively. Basin Description The South Platte River drainage basin covers approximately 4,850 square miles of land extending from the Continental Divide in the Rocky Mountain Range to the high plains and foothills of eastern Colorado as shown in Fig- ure III-I. The characteristics and hydrologic impact of the mountainous re- gion and the high plains and foothills region are quite different. The mountainous western portions of the basin produce heavy snowmelt runoff dur- ing late spring. On the other hand, the high plains and foothills portions of the basin are more susceptible to high runoff during summer thunder- storms. RESERVOIR RIVER CONTROL Numerous reservoirs have been constructed in the South Platte River basin for various reasons. While all exert some localized impact on the river, in most cases, their impact with the exception of major flood control facili- ties, is minimal on the response of the entire system. The reservoirs are grouped into three categories for brief discussion: (1) water supply reser- voirs; (2) flood control reservoirs, and (3) minor detention facilities. The location of the most significant facilities of categories 1 and 2 are shown on Figure 111.1. Water Supply Reservoirs Four major water supply reservoirs are located within the study reach of the South Platte River: (1) Cheesman; (2) Antero, (3) Eleven-Mile Canyon, and (4) Strontia Springs. All are owned by the City and County of Denver and