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<br />probability adjuat~ents to raInfall were made for length of record. The
<br />time dIstrIbution of rainfall wIthin the I-hour stor~ was developed frn~
<br />lllJAAerIteria. InftltratIon ratcs were obtained fr0lll3WeldCountySoll
<br />Mnp and Soil ResQurces of Color~do, Region 2-Larimer and Weld Counties,
<br />published by the Colorsdo State Urdvcrsity Experiment Station and the
<br />SCStn1976. A value of 0.2:1nchwas"sed for deleotion atorage.
<br />
<br />sections were placed at cloae intervals upstream and downstream fro~
<br />bridges and culvcrra in order tocOlllpute the significant backwater
<br />effects of the,. ,,__,,",... "" ,
<br />.u r uge cro"" aect ons Were fleld surveyed
<br />to determlne elevation data and structure geo~try. The locations of
<br />the Cross sectiona are shown on the flooded area maps. Plate 4 ia an
<br />index which ahowa the location of the floodcd area mapa, plates)
<br />through 16. The cross sectio~ locations are also designated on the
<br />flood profiles, which are platea 17 through 28.
<br />
<br />The effect of future urb3nhation, edating s_ll dams, road struc-
<br />tures, and irrigation canala was consider~d. It was assumed that
<br />irrielltion canals do not provide dependable flood control. Si~ilarly,
<br />lIince the sma.ll dallla which exiat in the baainseeve irrigation, the
<br />assumption waS made by local intereata that these dama would not rema.in
<br />if the ar..a were urbanbed. Roadwaya that cross the Sheep Draw valley
<br />are relatively high and sct to retard floodflows. Secause of these
<br />factors and assumptions, the decision was ~de at the local level to
<br />e~lude the effect of the irrigation 'anals and tha small irrigation
<br />da~s fr~ this atudy under both existing and urbanized conditions but
<br />to include the effect of the roadways.
<br />
<br />The t.y<lrau':'~c ""a1fsls w."b cc,nJucted <:>n Gheep 0.."" tu ':eU't'"~""
<br />
<br />Manning'a"n"vsluell...creestImatedbyf1eldinapectiontobe
<br />0.035 to 0.040 for the channel and 0.040 - 0.100 for the overbank,
<br />Scartiqs ...nter aurface eIevationa at the mouth of Sheep Draw were baaed
<br />on Sheep Drsw flooding with a coinCident base flow of 1,100 cubic feet
<br />per second (c.f.s.) in the Cache Ia Poudre River. The water surface
<br />elevation of the Cache la Poudre River OlliS determined by analyzing
<br />stage-discharge relationahips for the CaChe la Poudre River from Flood
<br />Plain Information, Cache la Poudre River, Colorado, Volume II, Greeley.
<br />Weld County, publhhed hy theOmahs DtanictCorpa of Engineers in
<br />March 1974. Water aurtace elevatIons on Sheep Dcaw were computed by
<br />the Corps of Eogineers' atnndard ~tep batkwater computer progr~,
<br />HEC-2. Tl tf
<br />'e ~ . ect of brid~es, culverts, snd r08dwsys upon Sheep Draw
<br />n<,od W;1ter sllcf:lcQ ~lev..ti,,"~ ~,'" d"te.rmined by using bridge analysfa
<br />techniques contained in the llEC-2 computer program and a publication
<br />entitl..d HYdraulica of BridKeWaterways, publiShed by tho U.S.
<br />D"part!llentofTransportationinI970. Flood reconatituttonwas not
<br />conducted becauae of a lack of flOOd hiatoty.
<br />
<br />HYDRAULIC ANALYSIS
<br />
<br />the water surface elevation of the 10-, 50-, 100-, and 500-year floods.
<br />TopoKraphic mapping conaisted of orthophoto mapa at a scale of 1,2400
<br />and a contour interval of 2 feet and uses ] .~-l1inute Quadrangle lII8pping
<br />haviOR n scale of 1:24,000 and a contour interval of 10 feet. The
<br />.>rthophoto "",pping waa prep,..ed for Weld C(lU~ty, Colorado. snd the (;WCB
<br />
<br />by 11&1 Consulting Engineers of
<br />was taken on 20 December 1977.
<br />
<br />Fort Collins, Colorado. The photography
<br />The mapping was prepared :In Dece~bec
<br />
<br />"'
<br />
<br />All flood elevations are baged upon open
<br />debrla or ice. The flood elevationa ahewn
<br />
<br />channel conditions free
<br />
<br />1978. A total of 136 crosa sections were taken hy photogremmetrie
<br />
<br />are, therefore, COn-
<br />sidered valid only it hydraulic structurea, in genecal, remain
<br />unobstructed. Since '0''''' ob"--,,,," , "
<br />.u S CO"".on uUrillg floods, flood
<br />As discuna"d in the hydrologic
<br />analysis, th~ fluud dhcha'g,'a fur both edatfng and urbsnlzed
<br />
<br />;nethods. The streambcd elevatIon deriv~d from the cro"s sectIons i.
<br />8etually the low-w8ur profile. II""'ever, str..".. floww"s minll.al at
<br />the t t,." tlw ph<Hog.aphy fur the 10.81'1'1"8 "'as ta~<'n. Slrealll ~ro~s
<br />
<br />conditions could h~ worss than shown.
<br />
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<br />
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