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<br />probability adju~tmcnta to rainfall were made for length of record. The
<br />ti~ distribution of rainfall vithln the I-hour storm vaa developed from
<br />NOAA crit",ris. Infiltration rates wc_re obt,dned from.. Weld County SoU
<br />
<br />sections Were placed at close
<br />bridges and culverts in order
<br />effecta of these sttuctures.
<br />
<br />Intervals up~treaill and do..nstream frOlll
<br />
<br />to compute the significsnt
<br />Bridge Cr08~ sections ..ere
<br />
<br />b..ckvater
<br />
<br />!1a~ and Soil
<br />publJ.shedby
<br />SCSin1976.
<br />
<br />Resources of Colorado, Region 2-J,sri..er and Weld Counties,
<br />the Colorado State University P.x~rimenr Starion and the
<br />A value of 0.2 inch WaS used for detention storage.
<br />
<br />todeterrllineelevationdatannd t .
<br />s ruc_ure geometry. The lOcations of
<br />the cross sectiona are shown on th ., ,
<br />e uooued sre.. """ps. Plate 4 1s an
<br />index which shows the location of th~ flooded area ~aps, plates 5
<br />through 16. The cross sect!on locationa are also designated on the
<br />flood profile~, Which are plates 17 through 28.
<br />
<br />fieldaurveyed
<br />
<br />The effect of future urbanif-ation. existing small dama. road struc-
<br />turea, and irtigation canah was considered. It was aosumed that
<br />irrigationcanalo donoc provide dependable flood control. Stm.ilarly,
<br />aince the s~ll dams which exist in the basin serve irrigation, the
<br />assumption waa made by local interests th..t. theae dams would not reM&in
<br />if the area Were urbanized. Roadways thnt crosa the Sheep Draw v..lley
<br />arerelativelyh!ghandacctoretardfloodflowa. Secauseofthese
<br />factors and assumptiona, the decision was ~de at the local level to
<br />exclude the effect of the irrigation 'anals and the s....ll irrigation
<br />dama from this study under both exiatiog snd urbanized conditions buc
<br />to include the effect of the roadways.
<br />
<br />Manning's "n" values wer"esU_red by field inspectton to be
<br />0.035 to 0.040 for th.. channel and 0.0'0 00
<br />- .1 O. for the overbank.
<br />5t..ttlO& water aurface elevations ._ ...
<br />-"" lllOuthof Sheep. Dral.1 were l>aaed
<br />On. Sheep Draw flooding with a coincident 0'". ""w " 0
<br />~~ v 0 .1 0 cubic feet
<br />per second (c.f.a.) in the Cache la Poudr.. Rivet. The water aurface
<br />elevation of the Cache la Poudre River was determined by ..nalyzing
<br />atage-disch..rge relacionships for the Cache la poudre River from Flood
<br />Plain Infornecion, Cache In Poudre River, Colotado Vol~ IT ~ ,
<br />. , ~, ...ree~ey,
<br />Weld County, puhlished by the Omaha Discrict Corps of ~ngineers 1n
<br />March 1974. Water surface ..lev..tioo", ..
<br />~ On ~"eep Draw "ere computed by
<br />
<br />th"COE'psof
<br />
<br />HYDRAULIC ANALYSIS
<br />
<br />Engineers' stand~rd. step backwacer com,...~"
<br />~.~. progJ;IldI.,
<br />HEC-Z. The..ffecc of bridges, clllv"rts, and ro,.dwaysuponSheepDraw
<br />f1.oo,1""le" tf 1
<br />6U ace e evaciona WaS determined by using bridge ..nalyeis
<br />techniques cont..!ned in the HEC-Z cO"'-puce" progr.... and a publkation.
<br />entitled Hydrsulics of Bridge Waterways, published by the U.S.
<br />Department of Transporcation In 1970. Flood reconstitucion was nOt
<br />conducted because of a lack of flood histoty.
<br />
<br />The hydrauilcst1aiysi$ Waa conduct,,-d on Sheep Or"w to dete.",ine
<br />che water surface ele~ation of the 10-, 50-, 100-, and 500-year flood...
<br />Topographic mapping consisted of otchophoto ~ps at a scale of 1:2400
<br />and "- "ontour interval of 2 feet and USCS 7.S-Minute Quadrangle ,""pping
<br />having .~ Bcsle of 1'24,000 and a ,-,o"temr interval of 10 f..ec. The
<br />orthophotolll8pptngwasprepar..dforlieldCo"nty, Color"do, <lnd th "CIICS
<br />by M&l Consulting Engineers of fort Collins, Colorado. The photography
<br />"n" taken on 20 December 1977. The mapping "aa prepared in December
<br />1979. A total of 136 cro"s sections vere taken by photogr........tric
<br />""'thods. Thest.reslIlhedet..vat10nderivedfromthecrosaaections19
<br />~ctually the lo....water profUe. 1J0wever, stre.,m flow was minlmal at
<br />the ~lllle the photography for the wapping waS taken. Stream cross
<br />
<br />All flood elevations are based upon open
<br />of debri" Or lce. The flood elevation.. Shown
<br />
<br />channel ~"ndltfo=~ ~ree
<br />
<br />are. therefore, COn-
<br />s1dered valid only if hydraulic structutea. 1n g~neral, reOlSi~
<br />'lIlobstructed. s,- .
<br />.,ce some Ouatruccion is CoSmon dur!ng floods, flood
<br />condit10ns could be ~O~"e than shown. .
<br />~s discusaed io the hydrologic
<br />'''l<llysh. the flood discharges for both exlu1ng and urbanbed
<br />
<br />"
<br />
<br />"
<br />
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