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<br />Frequency-discharge relationships for Dry Creek Basin and <br />Sprinsr Gulch were computed using the U. S. Army Corps of <br />Engineers HEC-l Flood Hydrograph package (Reference 10). <br />Synthetic flood hydrographs computed by this ~ethod re- <br />flect the effects of precipitation characteristic of the <br />basin,. ground cover, slope, drainage area, and other phy- <br />sical characteristics of the drainage basin. Where <br />available, hydrologic data were compared with other stud- <br />ies completed in the area (References 11 and 12). The <br />effec1:s of detention storage near State Highway 66 and <br />at Longs Peak Darn on Spring Gulch were studied (Reference <br />11) and found to be insignificant for the magnitude of <br />the floods considered in this study. That portion of the <br />Spring Gulch Basin located north and east of Terry Lake <br />is considered to be contained completely by Terry Lake. <br /> <br />The major portion of the Dry Creek Basin is located out- <br />side the limits of the study area. Runoff from this area <br />contributing to peak discharges within the study reach <br />is limited by the Colorado and Southern Railroad, which <br />diverts most of the upstream runoff north to the St. Vrain <br />River. <br /> <br />Rainfall data for the synthetic hydrologic analysis was <br />taken from the Rainfall/Runoff Information Report (Ref- <br />erence 13). The discharges computed using the HEC-l pro- <br />gram were verified using the Plains Region equations de- <br />veloped by the U.S. Geological Survey (Reference 14). <br />Peak discharges were also verified by the Soil Conserva- <br />tion Service runoff prediction method (Reference 15). <br /> <br />Discharges for the 500-year floods of all streams were <br />checked by straight line extrapolation of frequencies <br />previously determined using the procedure of the U.S. <br />Geological Survey (References 5 and 6), and compared to <br />the U.S. Army Corps of Engineers Standard Project Flood <br />data when available. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of streams in <br />the community were carried out to provide estimates of <br />elevations of floods of the selected recurrence inter- <br />vals along each stream studied in detail. <br /> <br />Water-surface elevations of floods of the selected re- <br />currence intervals were computed through the use of the <br />U.S. Army Corps of Engineers, HEC-2 water-surface pro- <br />files computer program (Reference 16). Starting water- <br />surface elevations for Left Hand Creek, Dry Creek, and <br />Spring Gulch correspond to the computed water-surface <br /> <br />16 <br />