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. total of 278 croes sections were also dete~mtned by photog,ammotr!", methods. These cross seetions Were placed at close intervals above a~ helo...bridgcsandculvertsinorderrocompueethcsign1flcantbaekIJ"ter effccesofthesesrrl.lctures. The srrearnbed elevation dcslgnated on rhe water surfece profiles Is actually the low-water prOfile on 20 December 1977. Ho"ever. the flow was estimated as minimal. Bridge cross sec- t1onsandotherstr""tureswcreUeldsurveyed. The locaUons of the cross seetions are shown 00 ". flooded area nosps. Plate , " index "' which s~ows th... location 0' ", flooded area maps, plates 5 through 21. Tl,ecross section locations m also designated on the flood profiles, plates 22 through 57. Manning's "n"values ""re .O)Oto.OS5forthechanneland.040to .085 for the overbank a1'l<l were "stiO!ated by Held inspeet1cm. Statting water surface elevations at the mouth of Fossil Creek were based on t.OSS1! ~r"e~ r100~lng wttn a C01nC10ent Dase IIOw oI 1,u0u CUOlC Ieet per second (e.f.s.) in the Cache la Poudre Rlver. The wat"r surface elevaUonof the Cache lapoudre River was determlned byanalyzlng Hage-dlacharge relationshlpa for the Cache laPoudre River fro.. Flood P1alnlnfor1llatlon, Cache laPoudreRlver, Colorado,VolumeIII,Fo rt COIUM - Greeley, Ladmer -Weld County, p"bUshed by the Olllaha 01strict Corps of (ngineers ln October 1975. Upstrea.. froO! rossl1 Creek Reservolr, the stsrting water surface elevation was deterOlined by reSer- voir routings for each flood frequency. On the tributaths, the starting "ater surface elevation waS at COincident flow on the recelvlng stream. For example, for a 50-year flood on a tributary, computations would start at the 50-year water sur!acee1evatlononthe'>ainstrea.,. \late. sudace elevations on Fossil Creek and ita tdbutades were e01lll'uted by the Co.ps of Engineers' standard steplxlc\<l<nter cOllll'uter I'rog.am, IlEC-2. The effect of bridges, culvcru, and roadways upon . " . floodw$ter surface elevations wcre determlned f.om atage-dlscharge rating curves of combined culvert flow and weir flow or by using bridge $nalys1$ techniques contained in the HEC-2 computer program and a publication entitled Hydraulica of B.1dKe Watcr"ays, published by the U.S. Oepartment of Tranaportation in 1970. On Fossil Creek upstream from the bridge at Sect1onZ75,llIosr of the water flows over the csst- west road into the Cache laPoud.eRiver. Veryl1ttlewaterflowa downstream in Fossil Creek from this point; therefore, the discharges have been reduced to compensate for thid. The reach of Foasl1 Creek between Fouil Creek nam and the Fossil Creek Dam spillway channel confluence with Fossil Creek 1a inundated only by backwatet fr01ll the spillway channel and local drainage. No flood reconstitution was COm- puted beeauseofa lack of flood hiatory. Asd1scusaedinthehyd.ologicanalys1s,theflooddischargeswere eOlllputed assurling the existingdaM$ are in place and that road e.oss- ings are in place w1th culverts unobstructed. Flood elevations drawn on the profiles are based on open channel conditions fre"of debria0 r ie". The flood elevations arc, therefore, considered valid only if hydrauiie structures, in general, re~a1n unobstructed. Since so~e obst.uction is cOl'lm,ondudng flooda, flood conditions could be worse than shown. All e1cvations a.e referenced to mean sea level from the National Geodetic Vereical Datu~ of 1929. FINDINGS OF STUDY . Informat1onl'cgarding thel:loce frequent flood". sucha$the 10-year and 50-year floods is useful for deaign of minor englneering work" or for land use planninewhece a hlgh fal1ure l'1sk is econ01llic ally feasible and ha"-arda to life and prope.ty are low. The 100-yea. flood 1.s oft..n us~d for design wh"-n a l"",er risk of fa1.lu.c is deatr"d. Its most important usc is as a standa.d for flood plain designation and flood plain t,,-gulation. The 500-ye&r flood is useful to re~ind the " I