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<br />III - 5 <br /> <br />directly into Boulder Creek following the historical channel. <br /> <br />Peak flows for the 1990 conditions show little increase over existing <br />conditions since the urbanization of the bas in will be limited according <br />to the Boulder Valley Comprehensive Plan. <br /> <br />The hydrographs of the 5-, 25- and 100-year return interval storm <br />at the three study points are shown in Figures 2 through 10 at the end of <br />this report. Note that the shape of the hydrograph changes cons iderably <br />as the flood moves downstream. <br /> <br />Flood control facilities as evaluated herein, were designed to re- <br />duce the flooding produced by a 100-year return interval flood. It must <br />be recognized that floods of greater magnitude can occur, and their <br />effects must be considered. The Corps of Engineers has developed an <br />estimate of flood discharge, referred to as the Standard Project Flood, <br />for South Boulder Creek which can be used to evaluate performance of <br />hydraulic structures under very severe flood conditions. A 1969 report (I) <br />by the Corps of Engineers indicated a maximum discharge of 9,400 cfs <br />for the Standard Project Flood. This was used with the Commons di- <br />mensionless hydrograph to develop the hydrograph shown on Figure II <br />at the end of this report. Certain of the facilities proposed here in were <br />designed to operate with low risk of failure during a Standard Project <br />Flood as noted in the section on Preliminary Design. The Maximum Pro- <br />bable Flood (MPF) for South Boulder Creek was determined to be 41,000 <br />cfs. This was developed from data presented in the U. S. Weather Bureau's <br />Technical Paper no. 40 and from analytical techniques presented in a Bureau <br />of Reclamation Publication. (2) The mode of failure under MPF conditions <br />was analyzed and is discussed in Section VI. <br /> <br />BACKWATER ANALYSIS <br /> <br />Previous flood studies on South Boulder Creek were handicapped <br />by the lack of detailed topography. The U. S. Geological Survey maps <br />have contour intervals ranging from 10 to 40 feet. The new mapping <br />obtained for this study covered the flood plain at a contour interval of <br />one foot. This high degree of accuracy was necessitated by the extremely <br />flat lower sections of the Creek. <br /> <br />(I).. . . Flood Plain Information Boulder Creek and South Boulder Creek <br />Volume II, U. S. Army Corps of Engineers, Omaha, Nebraska _ <br />August 1969. <br />(2). . . . Design of Small Dams, U. S. Bureau of Reclamation. <br /> <br />