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<br />CltMnRV
<br />fiYDROLOGIC A/;ALYSIS
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<br />A. G~ner,,1
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<br />The U.S. Army Corps of Engineers, Omaha District, has ~o"'pleted a.
<br />hydrologic investiga.tion for the entire S~. Vrain Creek Basin which includes
<br />the study reach. Discharges for the S-, 10-, ';0-, 100-, .iUel ~M-ycar fLooels
<br />"ere developed by th" Corps for St. Vr<lin Creek froOl the ~onf1uen~e of North
<br />and South St. Vrain Creek. near Lyons, Colorado ~o the mouth. Peak flows
<br />at different peints fer both the existing and year 2000 basin development
<br />conditions were comp~ted. Year 2000 urbanization reflected information
<br />provided by the City of LOngmont and the Boulder COllntyCornprehens iv"l'lan.
<br />
<br />t, revie" of the Corps hydrologir. inform.Hion "d~ '-''-'ll<lucleu by the COnSel-
<br />ltilntasprovidedinthescol"'of",ork. Th"inform<<cionapplienbl"toth<'
<br />stlldy reach "as sel,"",,,rized and ~abulated tor input to tm> hydra" I ic "n" 1 y.is
<br />portiollof tile stl1dy.
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<br />b. Basin Modelin~
<br />
<br />Rainfall-runoff chara~teti$tic" for th~ ~t. Vrain basin Wetc modeled
<br />by th" Corps of Engineer~ lltilizing the EPA Storm ~'<Jter Managc"",nt Madel
<br />(SIIMM) a"d the Missouri Rive. Division version of H<ltder's diffusion rOllting
<br />"ood~l. Th~ t(>lJting m<ld~l ~xt~"deJ fro", Lyon" to tbe' ",uuth of St. Vrain t.:r~~I:.
<br />Calibration of th~ mod..l< to ('(}ndiri<>ns !:>,'f<>r.> K"tt"" ~<>,'l<: tJam W~" "~~r~tj~~al
<br />
<br />"as ~ccomplish('d from a disch~tge probability analysis for St. Vrain C,<,ck at
<br />the !J.S .G.S. gaging "tat ion" located at Lyon., aod ~t tbe mOeltb near I'l~ttpvi 11~,
<br />Colorado. The analytical m~tllOd" pr"senled in BuliN in No. I J publi"h"d I>y
<br />~he Wa"cr Resoelrces COlln"il ~r~ l1s~d for "ach andly~is.
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<br />.35-
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<br />The gage at Lyons is located below the confluence of North and South
<br />St. Vrain Cree~s and has records available from IS88 to the current year.
<br />The U.S. Geological Survey, in a 1939 field investigation of the 31 May
<br />1894 flood,estimated a peak discharge of 9,SOO cubic feetperseco "dior
<br />thi. event, which was treated as a high outlier. Since Beltton Rock Dam
<br />has controlling effect On peak discha~ges at the Lyons gage, only the p"riod
<br />of record preceding its full time op~tation ",as llsed in the discharge proba-
<br />bility analy~is. Button Rock Reservoir ",as in the process of filling during
<br />th~ 7 May 1969 flood and an cstimate of the "ncontrolhd peak "'as made for
<br />use in the frequeacyanalysis. The 2,9M cubic feet pet second peak di~charge
<br />recorded at the gage "'as adjusted to 6,000 cubic feet per ~econd by using
<br />unit hydrograph ratios.
<br />
<br />The gage at the mc"th near Platteville, has fifty-three years of reco~d.
<br />The records for 1905, ]906, "nd 1917 through 1911 were ~sed in th.. frequency
<br />analy.ds. The )969 ev~nt was adjusted from 10,300 cubic feet per ,econd to
<br />13.400 cubic feet per secood in order co reflect <In uncontrolled cOn diti.on.
<br />I'~ak disoharg~s since 1969 appear to hav~ been !(cneraced from th~ lower
<br />portion of the basin and did nvt req"ir~ ..dju"tm~"t tu reflect effccts fro",
<br />!Iutton Rock Reservoir. The estim~tcd ]894 !leak disch<1nc of 15,S(JO r.ubic
<br />feet per second was treated a~ an historic event.
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<br />Rainfall vallIe" for the 10-, 10_, and lI)()-year 6-hOuc events ",ere
<br />obtained from tbe "I're~ ipitation-F~cquency .~t las of the Western United
<br />States, Atl"s 2,Volu:ne III,Colorado," publi"bpd by the National Oceanic
<br />and Atm<lsphe~ic Adrninistr,,"ion in 191.1. In each cas.., tbe .s-year and 5UO-yca~
<br />
<br />,,~~.~c "'''~ csci=atcd by c"~r",;:o~atin3 ~k .cl"~t~~ Atl~~ v~l"~~ Ull ~~.,"-
<br />
<br />logarithmic pap~~. Adjllstments foc depth-area ~er~ based 00 the dopth-are"
<br />relac ionship pcescn~~d in the /;OA,\ pllbl ic.:!c ion. Ad]ll' tlIK,nt, fer lengtb '"
<br />cpcord"'He made on lOj\3rithmi.c probability p"per <Isinj\ Be"cd'., tahl.' pf
<br />~xpected probahility "djllstmen(S 'Or varin", l""gt~< c>f Hcord. Th,' "pati~.l
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