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FLOOD02864
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Last modified
1/29/2010 10:11:50 AM
Creation date
10/4/2006 11:16:30 PM
Metadata
Fields
Template:
Floodplain Documents
County
Larimer
Weld
Community
Fort Collins, Greeley
Stream Name
Cache La Poudre River
Basin
South Platte
Title
Floodplain Information Report
Date
10/1/1975
Prepared For
Larimer-Weld Regional Planning Commission
Prepared By
US Army Corps of Engineers
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />HAZARDS OF LARGE FLOODS <br />The extent of damaqe caused by a flood derends on the topoq- <br />graphy of the area flooded, depth and duration of flooding, velocity of <br />flow, rate of rise, and developments in the flood plain. An Intermediate <br />Regional or standard Project Flood on the Cache la Poudre River would <br />result in the Inundation of mainly agricultural and commercial properties <br />In the study area. Deep floodwater flowinq at high velocity and carryina <br />floatlnq debris could create conditions hazardous to persons and vehicles <br />attemptlnq to cross flooded areas. In general, floodwater two or more <br />feet deep and flowing at a velocity of three or more feet oer second could <br />easily sweep an adult person off his feet, thus creating definite danger <br />of Injury or drowning. Rapidly rising and swiftly flowing floodwater may <br />trap persons In homes that are ultimately destroyed, or in vehicles that <br />are ultimately submerqed or floated. Isolation of areas by floodwater <br />would create hazards in terms of medical or fire emerqencies. <br /> <br />are shown on the Flooded Areas, Profile, and Cross Sections for correla- <br /> <br /> <br />tion between drawin~s. Table 8 provides data at reference point locations, <br /> <br /> <br />includinq distance alono the main channel center I ine, elevations of the <br /> <br /> <br />stream bed, and elevations of the Intermediate Regional and Standard Project <br /> <br /> <br />Floods. <br /> <br />The Flooded Areas show the area that would be flooded hv the <br />IRF and SPF. The flood limits were located at each cross section and <br />the intervening flood outl ines were drawn based upon avai lable topqoraphic <br />mapping, engineering iudoment, and field observations. It is therefore <br />possihle that more or less floodino should be shown on the Floorlerl Areas. <br />For a specific situation where accuracy of flooded area is required, the <br />flood I i m i ts can be more accurate I y estab I i shed by deterlT'i n i ng V1ater sur- <br />face elevation from the Profile, Cross Sections, or Reference Table and <br />then locatinq that elevation by survey on the flood plain. <br /> <br />Flooded areas and flood dama~es - Water surface profiles for <br />the Intermediate Reqional and Standard Project Floods were developed for <br />the study reach usinq a backwater computer program employing the Standard <br />Step t4ethod. Head losses at bridges were computed usino data in "Hydraul ics <br />of Bridge \~aterways" publ ished by the U.S. Department of Transportation. <br />The computations were based on channel and flood plain conditions as <br />represented by surveys conducted in 1972 and 1973 and as supplemented by <br />later field investigations. <br /> <br />A reference line is shown on the 'Iooded Areas for the Cache <br /> <br /> <br />la Poudre River. This line is used for reference onlv and does not <br /> <br /> <br />necessari Iv coincide with the existin~ river channel even thouoh it may <br /> <br /> <br />he referr'ed to as such in the report. <br /> <br />The Profile on plates 0 throuqh 12, shows the elevation of the <br />water surfaces and water depths relative to the stream bed. The water <br />surface elevations were used to determine flood limits on the cross <br />sections and topographic maps and in turn the flood boundaries were <br />transferred to the aerial photoqraphs. The results are shown on the <br />Flooded Areas, plates 3 through 8. An index to the flooded areas plates <br />is on plate 2. Representative cross sections are Illustrated on plates <br />13 throuqh 19. The cross sections show around elevations across the <br />valley on both sides of the channel and depths of overbank floodlnq. <br />Reference points, coincldinq with the locations of the cross sections, <br /> <br />The flood elevations shown on the Profile elates and in table <br />R apely lateral Iv frOM the channel over most of the flood piair width. <br />Ilowever, road crossinqs and other torooranhic featurns can alter thn <br />lateral flood elevations. Depending on whether the features divert or <br />dam flows, the flood elevations at the edne of the flood plain may be <br />hiqher or lower than at the channel. This condition is il lustratnd by <br />the cross sections. As an examnle, cross section 3-3 on plate 15, shows <br />that if the area west of Interstate Hiohway 7.5 is develoned. consideration <br />should be oiven to not onlv the water surface elevation of the flood- <br />flow fol lowing the Cache la Poudre River channel at reference noint 09, <br />but also to the floodwaters which are diverted from the Cache Ie Poudre <br />River channel by Interstate Hiqhway 25 at reference point 07.. This <br /> <br />27 <br /> <br />28 <br />
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