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<br />. <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />8 <br /> <br />Vthe Corps of Engineers. The discharges used in the prepara-v <br />tion of this study are considered "final" by the Corps. A <br />J detailed description of the hydrologic analysis used to esti- <br />mate the flood discharges is available from the Corps of Engi- <br />neers. The flood discharges are tabulated in Tables I and II. <br /> <br />Ii <br /> <br />The 100-year flood elevations computed as part of this report <br />are different from those published in the Preliminary Adams <br />County FIS. The reason for differences are the larger dis- <br />charges and the modified and added cross sections. <br /> <br />J.. <br /> <br />Hydraulic Analysis <br /> <br />A detailed, technical appendix with all hydraulic calculations <br />is available at the offices of the Urban Drainage and Flood Con- <br />trol District. <br /> <br />Hydraulic analyses were completed to determine the water sur- <br />face elevations for the 100-year storm event. The elevations <br />were computed using the Corps of Engineer's HEC-2 Water Sur- <br />face Profiles computer program. Valley cross-sections were co~- <br />piled from three sources: (1) 18 valley cross-sections sur- <br />veyed by M & I, Inc., of Fort Collins, Colorado for the Omaha <br />District of the Corps of Engineers, (2) 31 valley cross-sections' <br />surveyed by Gingery Associates, Inc. of Englewood, Colorado and <br />(3) 30 supplemental valley cross-sections measured on the 1"= <br />200' scale, 2-foot contour topographic mapping provided as a <br />part of this report. The field surveyed cross sections were \ <br />/ <br />modified to be consistent with the mapped topography. Estimates_/ <br />of channel and overbank roughness factors were made after a field <br />investigation of the study area. Typical roughness or Manning's <br />"n" values used in this study are: <br /> <br />FUTURE FLOODS <br /> <br />Flood Frequency and Discharge <br /> <br />Floods of the same and greater magnitude as those that have oc- <br />curred in the past will occur in the future. With development <br />in the basin, flooding will occur more frequently. The dis- <br />charges reported in Table I for the 100-year flood frequency <br />represent the relative extent and impact of this flood event. <br />The discharge information is usable not only for flood plain <br />regulation but also for planning, engineering and flood plain <br />management. <br /> <br />Channel <br /> <br />0.035 to 0.040 <br />0.060 to 0.070 <br /> <br />The 100-year flood event, which can be expected to occur at <br />any time in a given area, based upon recorded historical pre- <br />cipitation and other valid data, has a one percent chance of <br />being equaled or exceeded during anyone year. <br /> <br />Overbank <br /> <br />The computed flood profiles are plotted on the Flood Hazard Area <br />Delineation profile sheets included in the Drawings Section of <br />this report. Also, the computed flood elevations for the South <br />Platte River are tabulated at each reference point in Tables I and <br /> <br />The 100-year flood event is considered by the Urban Drainage <br />and Flood Control District, the Colorado Water Conservation <br />Board and the Federal Insurance Administration as the flood <br />magnitude for which flood plains should be designated for regu- <br />latory and improvement purposes. In Colorado, the 100-year <br />flood plain represents an area of state interest as defined <br />in H.B. 1041. <br /> <br />II. <br />