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<br />I I1-3 <br /> <br />The most recent facility is the Bear Creek Dam which was closed in 1977. <br />This reservoir is located on Bear Creek b miles upstream of its confluence <br />with the South Platte River and controls approximately 236 square miles of <br />mountain, foothill and high plain basin. <br /> <br />obtained from the recently updated Urban Storm Drainage Cr'iteria Manual <br />(USDCM), which were also based on NOAA Atlas 2 for Colorado. <br /> <br />Minor Detention Facilities <br />Numerous small detention facilities have been constructed for irrigation and <br />municipal water supply and local flood control. These do not significantly <br />affect the flood peaks on the South Platte River because of their small size <br />or relatively small tributary area and small flood storage volume. <br /> <br />Table 111-3 compares the design rainfall events for the two studies men. <br />tioned earlier by relating the values representing the percent of the 6-hour <br />storm for each 30 minute period. The distributions are different and are <br />based on the respective source frO'll which the infonnation was obtained. <br /> <br />Snowfall <br />Snowfall in the mountains and higher foothills results in runoff peaks in <br />the late spring, however, these peaks are often reduced by Chatfield Dam. <br />Sno\>lTlelt in the lower foothills and high pl ains is not normally associated <br />with large flood peaks. <br /> <br /> TABLE IlI.3 <br /> RAINFALL TEMPORAL DISTRIBUTION <br />Time % of 6.Hour Rainfall <br />(Hours) USACE 1977 Merrick & Co. 1983 <br />0 - 0,5 2 28 <br />0.5 . 1.0 4 23 <br />1.0 . 1.5 4 15 <br />1.5 2.0 5 10 <br />2.0 . 2.5 9 5 <br />2.5 - 3.0 1U 3 <br />3.0 . 3.5 40 3 <br />3.5 - 4.0 1U 3 <br />4,0 - 4.5 6 3 <br />4.5 - 5.U 4 3 <br />5.0 - 5.5 4 3 <br />5.5 . b.O 2 2 <br /> <br />PRECIPITATIUN PATTEkNS <br />Significant precipitation over the South Platte River basin consists of <br />snowfall in the mountainous region and thunderstorm activity in the high <br />plains and foothills portion. <br /> <br />Thunderstorms <br />Most major flood events on the South Platte River have been associated with <br />thunderstorm activity. Recurrent major storm tracks have been identified <br />which cross the South Platte River watershed and generate most flood flows. <br /> <br />Two sources of rainstorm design infonnation were usea in the previous re. <br />ports to generate flood hydrographs. The National Oceanic and Atmospheric <br />Administration (NOAA) Atlas 2 for Colorado was used by the U.S, Army Corps <br />of Engineers (USACE) to develop rainfall-frequency relationships in their <br />hydrologic evaluation of the South Platte through Adams County. <br /> <br />TkIBUTARY FLOOD INFLOWS <br />There are two primary sources of inflows into the South Pl atte River: (1) <br />transbasin diversions, and (2) tributary drainage basins. <br /> <br />Merrick <br />t hro ugh <br /> <br />and Company in their evaluation of <br />Denver to Sand Creek used rainfall <br /> <br />the South Pl atte River hydrology <br />intensity.frequency relationships <br /> <br />Transbasin Diversions <br />These flows consist primarily of Denver and <br />to supplement their municipal water supply. <br />an average of 154,000 acre-feet per year to <br /> <br />Aurora's efforts to import water <br />Three major facilities import <br />the South Platte River. These <br />