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HAZARDS OF LARGE FLOODS The extent of damage caused by a flood derends on the topoq- graphy of the area flooded, depth and duration of flooding, velocity of flow, rate of rise, and developments in the flood plain. An Intermediate Regional or standard Project Flood on the Cache la Poudre River would result in the inundation of mainly agricultural and commercial properties in the study area. Deep floodwater flowing at high velocity and carrylna floating debris could create conditions hazardous to persons and vehicles attempting to cross flooded areas. In general, floodwater two or more feet deep and flowing at a velocity of three or more feet oer second could easily sweep an adult person off his feet, thus creating definite danger of Injury or drowning. Rapidly rising and swiftly flowing floodwater may trap persons in homes that are ultimately destroyed, or in vehicles that are ultimately submerged or floated. Isolation of areas by floodwater would create hazards in terms of medical or fire emergencies. are shown on the Flooded Areas, Profile, and Cross Sections for correla- tion between drawinns. Table 8 provides data at reference point locations, includinq distance alonn the main channel center I ine, elevations of the stream bed, and elevations of the Intermediate Reoional and Standard Project Floods. The Flooded Areas show the area that would be flooded hv the IRF and SPF. The flood limits were located at each cross section and the intervenino flood outlines were drawn based upon avai lable topqnraphic mapping, engineering judnment, and field observations. It is therefore possible that more or less floodinn should be shown on the Flooded ^reas. For a specific situation where accuracy of flooded area is required, the flood I i m i ts can be more accurate I y estab I i shed by deterlT'i nino V1ater sur- face elevation from the Profile, Cross Sections, or Reference Table and then locatinn that elevation by survey on the flood plain. Flooded areas and flood damanes - Water surface profiles for the Intermediate Regional and Standard Project Floods were developed for the study reach using a backwater computer prooram employing the Standard Step t4ethod. Head losses at bridges were computed usino data in "Hydraul ics of Bridge \~aterways" publ ished by the U.S. Department of Transportation. The computations were based on channel and flood plain conditions as represented by surveys conducted in 1972 and 1973 and as supplemented by later field Investigations. A reference line is shown on the 'Iooded Areas for the Cache la Poudre ~iver. This line is used for reference onlv and does not necessari Iv coincide with the existinn river channel even thounh it may he refern"d to as such in the report. The Profile on plates q throuqh 12, shows the elevation of the water surfaces and water depths relative to thp. stream bed. The water surface elevations were used to determine flood limits on the cross sections and topographic maps and in turn the flood boundaries were transferred to the aerial photoqraphs. The results are shown on the Flooded Areas, plates 3 through 8. An index to the flooded areas plates is on plate 2. Representative cross sections are illustrated on plates 13 throuqh 19. The cross sections show around elevations across the valley on both sides of the channel and depths of overbank floodinq. Reference points, coincidinq with the locations of the cross sections, The flood elevations shown on +he Profile Dlates and in table R apDly lateral Iv from the channel over most of the flood riair widto. Ilowever, road crossinqs and other toronraohic features can alter thp. lateral flood elevations. nependino on whether the features divert or dam flows, the flood elevations at the edne of the flood plain nay be hinher or lower than at the channel. This condition is ii lustratnd by the cross sections. As an examole, cross section 3-3 on plate 15, shows that if the area west of Intersta+e Hiohway 7.5 is develoned. consideration should be niven to not onlv the water surface elevation of the flood- flow fol lowing the Cache la Poudre River channel at reference nolnt Q9, but also to the floodwaters which are diverted from the Cache Ie Poudre River channel by interstate Hinhway 25 at reference point q7.. This 27 28