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r: Pe~k floodin<;1 from a single r~infall eveM will usuallv OG.GJr "lthi nahal f day after the r~ in, 'with the <luretion of Iloooi"9 13st~ ing from 3 few hours to a ha If day. The duration of flooding may be significantly irx;r"",sed due to an extended period of "et weather and snowrneltrunotf. Forex31l\ple, i"June 1949 St. Yr3inCreek was report- edly out of its banks during most 01 the !'lOnth. Gommercial ouildingsandappurtenantfeaturessuchasfencesandstor. age tanks also i~pede ano divert overbank Ilows. The effect of these Obstructions is to slow the flowir<q water, croateabackwateraff eet, and raise water surfaces on the upstream side 01 the constricted pas- ~ges. The effecT at the bridges Is shown on the profile views on plates 5 and 6, aoo I n table 5, F 1<:JC>d PI~ I n RclerenGO Data, on page 28. Individual rainfall eventsmaycausefloodstodeveloo In differentways,,,s illustrated by accounts of two floods In the St. IIralnvalley.lnl94larainstormwhichoccurredmostlyoverthe South St. Vrain Creek basin produced a short duration flood having a high peak discharge of 10,500 cubiC feet per second (c.f.s.l ~t Lyons gage. Do~nstream of Lvons a portion of the flood ~aters spread over the flood plain and traveled downstream at II slower rate than the water in the channel. Thefioodp"dkw<)sreducaJ,lJOCdusar><ortofthd/low w"sstoredonthcfloodpiain,sothatthemaximumdischargeatthe Ptattevillegagewasonlyl,740c.f.s. In 1957 a general rain occurred over the entire basin. The trIbutary inflow was large and occurred over a longer period of tine so the fiood plain storage affects were overcome. The peaK diSCharge incroased from 3,060 c.f.s. at the Lyons 9age to 9,450 c.f.s. at the Plattev;lle \pg<l. ~1~9. 9.1l~E-e~r~d!:.c!.i2n..!"~,,~u!:.e.s. - Button ROCk Dam, located six miles west of Lyons on North St. YrainCreek,wascomplefed in 1969 just In time to receive the bulk of runoff from the 104 souare Il\llesof upstroamNortn St. VrainCreek dur/~ the 1969 flood. This OlOst-tortunatecoincldenc:e probably averted much'flOre serious flood damag<l to Lyons and Cityo/Longmentto little storage for ered in the study. one mile downstream Longmontthanoccurred. Theaamwasl>uiltbythe store municipal water supply and normally has flood runoff. The effect of this dam was consid- Longmont Oa,", a diverSion for hydroelectric power from 8uttom Rock Dam, and Beaver Reservoir, near Factors Affoctlng Floodi"'J and Its Impact Ob~.t~u::.t.!..o!!.s_t9. !..I~ !..I~w~ - Eight rood bridges <.1nd one r~llroa~ ~ridge cross ST. Vr~in Creek within tho study r~~h. At Pratt Parkway another crossing was made by buildinq a road bed over threalargeculverts. Elesidosthepossibleobstructlonotfloodflows by the cro~sing structures, r~ised rQ<1d surfaces across the flood plain obstruct overbank flows. Th~re are three diversion structures in th<l study reach; thesaw.11 heve littl",ettectonfloodllows. Plant growth such as trees bordering the stream impedes overhank flows <In:! occasionally 1s torn loose to join other floating debris that .ofteo catches on bridqes downstream. In Lon~mont, resideMial and tM South St. ~r"ln hMdwators. were considered to hav<l a negligible affoct on peak dlsGharges in the study re~ch. There is presently no city Or counfy ordinance providing flood Dlain regulation in the study r<311ch. There Is "" evidence of flood proofing of structures in the flood plain. No <3~tenslve flood conTrol works have been construct~ at Lornjmont. There is evidence of occasional Ghannel straightenin~ of St. Yrain Creek such as fram Sunset Street to the Colorado and Southerntrldge. Thoro are short stretGhesot iOGallybuiltlevees <.1iongbothbanks, such as can be found aiong th",rlght bank of the aLove menTioned rOdch. Q.t!>.e~ .!.ast2'"~ ~n~ !h!i;: .!..'"Ra::.t~ - Left H;lnd Cn~k <HId St. Yrain Creek share a COMmOn fiood olain near the juncture of the two streams. The common flood plain can be tl~oded by eithor stream or acombir>atlonoft>othstreams. Atuture largo fiood on St. Vraln Creek at Lo~mont Should involve the Gommunity in ~ergency flood , Ie I