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-8- the U.S. Army Corps of Engineers HEC-2 water surface profiles computer pro- gram. Valley cross sections were developed using digitized cross sections obtained from aerial photography flown on April 16, 1983. The channel bot- tom e 1 evat i on shown on these cross sect ions was actually the water surf ace elevation on that day, rather than the ,actual stream thalweg. Since there was a significant flow in the South Pl atte River on that day, it was be- l ieved that in certain cases as much as two-feet of difference could exist between the water surface elevation on April 16, 1983 and the actual stream thalweg. In those areas where the water surface profiles for the lOa-year flood showed that overbank floodin9 would occur, the thalweg was surveyed in the field or adjusted by assuming a 1.5 foot depth of flow. In addition, in cases where the channel bottom profile obtained from the April 16, 1983 map- ping varied significantly from the channel inverts used by the Army Corps of Engineers in their floodplain analysis in 1977, surveys were performed to confirm the correctness of the cross section information used in this re- port. Sellards and Griqg Engineers have performed a detailed hydraulic study for the reach from Cherry Creek to Colfax Avenue. As part of that de- tai led study, they surveyed river cross sections from approximately Cherry Creek to Interstate 25. These surveyed cross sections were incorporated in- to the digitized cross sections obtained from the April 16, 1983 mapping. In general, the areas where the digitized cross sections were modified are from York Street to Frankl in Street, from 23rd Avenue to 15th Street, from Cherry Creek to Interstate 25, at the 3rd Avenue Dam, and at the drop struc- tures near Mississippi and Florida Avenues. Engineers in their 1977 study. The results of this study were consistent with the results of the Corps study except in areas where there were differ- ences in discharges and stream thalweg elevations. Starting elevations were obtained from the "Flood Hazard Delineation, South Platte River, Adams County" completed in September 1977. A detailed de- scription of the hydraulic analysis is included in the Technical Addendum. A floodway was also defined for the study reach of the South Platte River. A fl oodway represents the ch anne 1 and adj acent fl oodp 1 a in wh i ch is requi red to pass the 100-year flood without raising the flood profile roore than an acceptable amount. It also represents that part of the floodplain roost haz- ardous to personal safety and welfare. In this study, the floodway defines a flow area required to convey the laO-year flood wi thout increasing the 100- year energy grade 1 i ne, and, therefore, the water surf ace e 1 ev at i on by more than 1.0 feet. Floodway widths, which represent the maximum limits of encroachment into the floodplain, are tabulated in Table 1. The locations of the field surveyed cross sections, the digitized cross sec- t ions from the topographic maps, and the assumed inverts are shown on the flood hazard area delineation maps. All hydraulic analysis was done on the basis of existing channel conditions. In Table I the rise in the 100-year flood water surface elevation is greater than 1.0 feet at sections 780 to 860. This occurs because under existing conditions part of the laO-year flood flow is carried in a separate channel on the east side of Interstate 25 from Eighth Avenue to Speer Boulevard. By the time the east channel reaches Colfax Avenue it carries almost 20 percent of the total flow. When this flow is confined to the main channel, the water surface elevation is increased roore than 1.0 feet, even without any encroachment. Estimates of channel and overbank roughness factors were made by field in- spection and use of photographs. Typical roughness, or Manning's "n" val- ues, used in thi s study are: for the channel .03 to .035, for the overbank .04 to .07. The lOa-year flood elevations comouted for this study were com- pared to the IOO-year flood elevations developed by the U.S. Army Corps of