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<br />Discharge magnitudes for Pawnee Creek Overflow floods were <br />developed using the Massachusetts Institute of Technology <br />Catchment Model (MITCAT). The Pawnee Creek basin was divided <br />into 60 subbasins. Rainfall data utilized were derived from U.S. <br />Weather Bureau Technical Paper No. 40 (Reference 5). Maximum 3- <br />hour rainfall values used for the 10-, 50-, 100-, and 500-year <br />floods are 1.65, 2.28, 2.60, and 3.32 inches, respectively. An <br />infiltration rate of 0.60 inch per hour was used. The standard <br />deviation estimated for Pawnee Creek by the MITCAT model compared <br />favorably with that determined in an adjacent basin having stream <br />gage data. It should be assumed that a Sand Creek flood will <br />peak within approximately 4 hours from the onset of heavy <br />rainfall. This compares with a lag time of approximately 15 <br />hours for a Pawnee Creek Overflow flood to peak at Sterling. <br /> <br />Peak discharge-drainage area relationships <br />River, Pawnee Creek Overflow, and Sand <br />Table 1. <br /> <br />for the South Platte <br />Creek are shown in <br /> <br />TABLE 1. Summary of Discharges <br /> <br />Flooding Source <br />And Location <br /> <br />Drainage Area <br />(Square Miles) <br /> <br />Peak Discharge(cfs) <br />10-Year 50-Year 100-Year 500-Year <br /> <br />South Platte River <br />At Crook <br /> <br />19,238 <br /> <br />17,000 <br /> <br />46,500 <br /> <br />68,000 <br /> <br />152,000 <br /> <br />Pawnee Creek Overflow <br />At Mouth <br /> <br />40 <br /> <br />1,250 <br /> <br />2,400 <br /> <br />5,000 <br /> <br />11,000 <br /> <br />Sand Creek <br />At State Highway 138 <br /> <br />40.5 <br /> <br />1,250 <br /> <br />5,250 <br /> <br />6,880 <br /> <br />15,000 <br /> <br />3.2 Hydraulic Analysis <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />riverine sources studied were carried out to provide estimates of <br />the elevations of floods of the selected recurrence intervals. <br /> <br />Cross sectional data and topographic mapping prepared at a scale <br />of 1: 2 ,400, wi th a contour interval of 2 feet, were establ ished <br />by photogrammetric methods from photographs taken in December <br />1976 by Blevins Mapping Company (Reference 6). These data were <br />furnished by Logan County in cooperation with the Colorado Water <br />Conservation Board. Additional cross section data were furnished <br />by the COE. U.S. Geological Survey 7.5-minute topographic <br />quadrangle maps at a scale of 1:24,000, with a contour interval <br />of 10 feet (Reference 7) were also used. <br /> <br />Bridge and roadway survey data were furnished by the Colorado <br />Water Conservation Board. <br /> <br />6 <br />