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<br />probability adjust~ents to raInfall were made ror length of record. The
<br />tfme dIstrIbutIon of rafnrall wIthfn the I-hour stor~ was developed frn~
<br />lllJAAerlterla. InrUtratIon ratcs were obtained fr0lll3 Weld County Soll
<br />Mnp and SolI Resources of Color~do, Region 2-Larimer and Weld Counties,
<br />published by the CoLorsdo State Urdvcrslty Experiment Station and the
<br />SCSin1976.AvalueofO.2:1nchwas"sedfordelentionatorage.
<br />
<br />sections were placed at close intervals upstream and downstream fro~
<br />bridges and culvcrrs in order tocOllIpute the significant backwater
<br />effects of the,. ,,_.,,",... "" ,
<br />.u r uge cross sect ons Were fleld surveyed
<br />to determlne elevation data and structure geo~try. The locations of
<br />the cross "ectiona are shown on the flooded area maps. Plate 4 ia an
<br />index which ahowa the location of the flooded area mapa, plates)
<br />through 16. The cross aection locations are also designated on the
<br />flood profiles, which are plates 17 chrough 28.
<br />
<br />The effect of future urb3nhation. edating smltll dams, road atruc-
<br />turas, and irrigation canals was considered. 1t was aasumed that
<br />irrieetlon canals do not provIde dependable flood control. Si~ilarly,
<br />since the sma.lldalllsw!tich exist in the baainserve itrigation, the
<br />assumption waS made by local intereats that these dams would not re"",In
<br />if the ar..a were utbanbed. Roadways that cross che Sheep Draw valley
<br />are relatively high and act to tetatd floodflows. aeca"se of theae
<br />factors and assumptions, the deciaion waa ~de at the local level to
<br />e~lude the effect of the irrigation 'anals and Che small irrigation
<br />da~a ft~ this atudy under both existing and urban1zed conditions but
<br />to 1nclude the affact of the roadways.
<br />
<br />The t.yclra"':'~,, ""alyals w."b cc,.nd"ctecl vu Gheep 0.."" to <leu,c,"t""
<br />
<br />~nning'a "n" values...ere eatImated by field inapection to be
<br />0.035 co 0.040 for the channel and 0.040 - 0.100 fot the overbank.
<br />Startlqs ...ster sutface elevationa at tbe moutb of Sheep Draw were baaed
<br />on Sheep Draw flooding with a coinCident base flow of 1,100 cubic feet
<br />per second (c.f.a.) in the Cache la POudre River. The water aurface
<br />elevation of the Cache Is Poudre River wa~ determined by analyzing
<br />stage-disch~rge relationshipa for th~ Cache la Poudr.. River from FLood
<br />Plain Information, Cache Is Poudre River, Colorado, Volume II, Greeley,
<br />Weld County, puhlhhed hy th"O....hs DiatrtctCorps of EOllineers in
<br />March 1974. Water surface elevatIons on Sheep Drsw were computed by
<br />the Corpo of Eng!neer~' atnndard step baCkwater computer progr~.
<br />HEC-2. II tf
<br />le e . ect of bridges, culverts, snd roedways upon Sheep Draw
<br />n~od w;Iter sutf"co elev..ti"u~ ~,'" d"te.rmlned by using bridge analysh
<br />~echniqueo conta:loed in the llEC-2 compucer program and a publication
<br />entitled HYdraulics of Bridl(eWaterw..yo, publiShed by tho U.S.
<br />D"p..H....nt of Traosportation in 1970. Flood reconatitution waa not
<br />conducted because of a lsck nf flnod hiscnry.
<br />
<br />HYDRAULIC ANALYSIS
<br />
<br />the water surface elevation of the 10-, 50-, lUU-, and 5UO-year floods.
<br />Topol(rapblc mapping consisted of orthophoto maps ..t a scale of 1,2400
<br />and a contour interval of 2 feet and USCS ] .~-l1inute Quadrangle lIIll.pp1ng
<br />haviOR n scale of 1:24,000 and n contour interval of 10 feet, The
<br />.>rthophoto"",ppingwasprep'ltedfnrlle1dCounty,Colorado.andthe\;WCB
<br />
<br />by 11&1 Consulting Engloeers of
<br />was taken on 20 Decembec 1977.
<br />
<br />Fort Collins, Colorado. The photography
<br />The mspping was prepared :In Dece~ber
<br />
<br />"'
<br />
<br />All flood elevations are based upon open
<br />debrla or ice. The flood elevations shown
<br />
<br />channel conditiona free
<br />
<br />1978. A total of 136 erosa sectIons were taken by photog<smmetrle
<br />
<br />are, therefore, COn-
<br />sidered valid only If hydraulIc structures, in general, remain
<br />unob.tructed. Since '0''''' ob"-.,,,," , "
<br />'u S cOllllOonuurl11g floods, flood
<br />Aa dlac088"d In the hydrologic
<br />...nalysis, the floud dhcharg"6 f~r both edstfng and urbanized
<br />
<br />onethods. The streambed elevatIon derived from the cro.s sectIons f.
<br />aetually .-h" lo......"aur proflle. II"",,,vec, atre".. flow'ms min1<...l at
<br />the tt,." tlw I'h<Hosraphy tor the ""'ppI"8 "a. ta~<'n. Slrealll ~ro.a
<br />
<br />conditions could h~ worae than shown.
<br />
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