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I II-4 three comprise 98.8 percent of the total annual import. Water for municipal use is diverted south of downtown Denver and returned upstream of the Minor on-site detention facilities required of developers by many municipal- ities may result in less localized runoff, however, the basin wide impact is Henderson gage. Current average annual streamflow has remained constant uncertdin due to inconsistent design, implementation and maintenance .....+ +-h.... VI l..1I1;;: since 1925. Prior to that, average annual flows were approximately 90 cfs higher. facilities. Channel improvements along presently unimproved reaches may re- duce overbank storage and, therfore, increase flood peaks. Although this scenario was not evaluated in the previous hydrologic studies increases in peak flows due to additional channelization are expected to be only a few percent of the current peaks. This study and the previous hydrologic studies are based on existing channel and floodplain conditions. Major Tributary Basins The South Platte River in this study reach has several major tributaries among them: (1) Big Dry Creek, (2) Bear Creek, (3) Cherry Creek, (4) Sand Creek, and (5) Clear Creek. Figure 111-2 shows the locations of these major tributaries and Table 111-4 summarizes peak discharges for selected tribu. tary basins. The peak discharges into the South Platte River are not directly additive as a result of varying time of peak runoff. Tributary Dutch Creek Big Dry Creek Be ar Creek Little Dry Creek Cherry Creek Clear Creek First Creek Sand Creek Peak 100-Year Inflow (cfs) 7,400 / 12, 600 8,200 7,100 9,700 23,000 7,400 30,500 SOUTH PLATTE RIVER FLOOD HYDROLOGY The hydrologic response of the South Platte River from the Chatfield Dam to the Adams/Weld County line has been analyzed in the aforementioned reports. Figure I 1I-3 conso 1 idates thi s i nformat ion and presents a di scharge profile for the study reach, The 10-year and 100.year discharges for the entire reach and the 50-year di scharge for the Adams County reach are obtained directly fran the reports. The 50-year discharge profile from Chatfield to Sand Creek has been estimated by plotting the 10-year and 100-year peak discharges on probability paper and interpolating between the 10-year and 100. year events at various locations. The 2-year discharge profile was extrapolated in the same manner. Figures III.4 and III.5 show typical hydrographs at various locations along the river. Table 111-5 presents the estimated peak values at various points. T ABL E I I I .4 TRIBUTARY BASIN PEAK INFLOW TABLE 111-5 SOUTH PLATTE RIVER FLOOD HYDROLOGY PRESENT VERSUS FUTURE BASIN DEVELOPMENT The flood discharges developed in the past studies reflect future basin development. Inherent in this is the assumption that future development can be predicted and no major features which change anticipated basin response are developed. Until ultimate development is attained, peak flow discharge on the river for any given frequency event will most likely be less than the estimated peak. Discharge (cfs) Recurrence Interval Locat i on L-Year 1O.Year ~o- Year 100. year D/S Dutch Creek * * 5,060 6,400 S. Platte @ Littleton * 3,100 6,170 7,940 D!S Big Dry Creek * 4,350 9,600 12, 600 n /(" I"; ++1.... n"". ,...............1" * t: 7t:f1 ,? ?fln ,!; non U/J Ll LLIC UI, ~I cc~ "',f.,JV .1.'-, L..VV .l.V,,,,,\,,,,.,, D/S Cherry Creek 3,580 9,700 18,000 22,500 S. Platte at 19th Ave. 3,580 9,700 18,000 22,400 IJ/S Sand Creek 3,580 9,700 17,400 21,700 D/S CI ear Creek 3,550 13,000 28,600 38,000 D/S Fi rst Creek 3,550 12,700 27 ,500 36,000 S, Platte @ Henderson 3,400 12,300 27,000 35,500 Baseline Road 3,130 11,400 25,500 33,600 * Snowmelt and Dam Releases Predominate