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SU~MMARY OF FLOOD SITUATION Cr~uks. They ~re the Internealate Regional and Standard Project FlOOdS and are fully defined in th9 Glossary of Terms On page 28. Boulder CreeK Is a mountein s~ream drainln~ a 440 square mile "rea on the eastern slope of the Rocky Mountilins in north central Colorado. South Boulder Creek. a principal trIbutary draining 125 squaremtles, joins Boulder Creek twomlles east of the front ran<)e. Both streilmS flow eastward from the Continental Divide, draIning por- tions of tne mountains and portions of the high plains east of the mountains. As these streams emerge from the mounhins, their valleys widenandabrO<ld fiat flood plain Is formed. The flOOd plains <Ire rocky. Prlncipill uses Include p<lstur<lge, gravel mlni~, and urban devolopment. The City of Boulder is centered on tha Boulder Creek flood plllinwherethestreamem"'rgesfromthemountlllns. Initsgrowth,the City Is expanding eastward occupying more and more of the Boulder Creek floodplain. In addition, various widely separated urban developments are sprlngi~ up On both the Boulder and South Bould~r flood plains. This reoort defines the flood prone ilreas so t~at stops may 00 taken toav.".-tunwisoconstruction in these areas. , , . FLOOD DAMAGES whiCh would result from a recurrence of known flOOdS wO<.lld bc 'flOrO extensive Mcausc of rocent developrlents i., t~e flood plain. Thedilmagepotential.'illcontlnuetolncreascunlessthe fioodtnreat Isoroperlyrecognized in planning future construction. . . , FLOODED AREAS covered by the Interme~iate Regional Flood average 2,000 feet wide on Boulder Creek and 300 feet wide On Soutn 30ulder Creek. The Sfandard Project FlOOd averages 2,500 feet wide for both streams. , . . DURATiON OF FLOODS is relatively short because of the steee slopes whiCh speed the floodwaters down the valleys. Flows at or near the peak Iilst from I to 4 hours during a flooding period of from 6 to 24 nours. . . . . , , HAZARDOUS CONOITIONS would occur during large floods as a result of rapidly risl~ streams, high velocifies, and deep flows. Velocities greater than 3 feet per second combined with depths of 2 feet or crreaterarogenorallyconsldcredhaZilrdous. Allovcrbonkflowson Boulder and ~outh Boulder Croeks muy be dangerous and sl~uld be avoided or..pprQ<lcllfflJwlthc(,lution. THE CAUSE OF FLOODS ,sprimarily intenserainfali during thesprln'l and sUCllTlor montlls. Runoff from tho mountain snow pack cont...1 ~utes to ",ost floods b'Jt >:loes not cause ~eri01JS t lood I 'l;) by itself. . . . , , . FLOOD CONTROL AND OEFE~SE on eoulder and South Boulder Creeks has not been developed. The Environmental Science Services Ad~inlstr~tion We~ther Bureau issues war~ings of possible heavy rains or ~evorc thunder5tor~5 from its Denver office via various mass news media. These warnin<JsandforeCilsts, which may Include state"'ents on flaSh floodlnq are baseo on general storm cond;tlons without reference to specific creoks. f,n improvua storm warning service snould I>e aVilllable by about 1971 since 80ulder County will be int.1eeffectivllra,'gcofthll plannod weather radar at Limon, Colorado. GREAT FLOODS have been reported on Boulder Creek OVer tho pest 100 years. The first large flood wnlCh was reporteo caused heavy damages througnout the valley in June 1864. The largest flood occurred in June 1894. Studies of that floodmado sever(l; yo<.:rslatorestl",atcdll peak d;schilrgo of II,ODOcubic teet pe"'second le.l.s.l. Tllisflood produced particularly htlavy da",aglls along Fourmile Crtltlk and infloulJer. The second largest flood originated on South Boulder Creek where it had a peak dischar<Je of 7,390 c.f.s. in Scp'tOfilber 193A. Two potential floods are used to Illustratethellood prolllemoniloulderandSo.Jth30uldor 2